7" f/12 iSTar fiolded refractor 23: Cell division.

*

My fellow, folded refractor ATM, has suggested that the folding mirror cells need very fine adjustment. Two possible options immediately present themselves:

Finer adjustment threads on the mirror collimation screws. Or greatly increased size of each mirror cell's backboard. Tripling the size, of the mirror's initial triangular footprint, would be the equivalent of making the screw pitch 1/3 finer. M6 = 1mm/3 = 0.3mm. Which is finer than most commonly available fine pitch screws. [0.5mm] While the larger triangular form still allows a wide range of normal fasteners.

The greatly increased size should provide much better collimation stability since the loads on the screws and springs will also be reduced to one third. On the original backing disk there was only 7cm between screw centers. On the new triangle the distance between centers is 22cm. Though the screw spacing is slightly narrower across the triangle's base where sideways tilt is adjusted. Still more than adequate to the task.

The top screw will compress a soft, rubber tap washer as before. Providing further stability [compared with a long spring and loose bolt] and a degree of built-in tilt due to the springs on the lower screws having much greater length. Packing on the adjustment screws can easily be provided if need be. The length of the springs themselves may not be enough to achieve the necessary mirror tilt for optical folding.

Finer screws and considerably enlarged cell back plates would provide micro-adjustment of mirror tilt. The problem might be finding the fine screws and matching butterfly nuts. I was unable to find any wing nuts in fine threads online nor even coach bolts. The expense of buying new taps and dies with very doubtful qualities does not suggest I follow this route.

The image of the plywood mock-up shows a rough idea of the potential for a greatly increased size of cell backplate for the 1st mirror. The dimensions of the screw adjustment triangles is now three times greater as should be self evident.  I used a beam compass to ensure the collimation screw spacing was correct.

The increased weight of an alloy collimation backplate could be much reduced by perforation with hole saws or skeletonizing. A triangular form is assumed for minimum starting weight.

The 2nd mirror is rather less accommodating in its position to allow a greatly increased adjustment triangle. The lower pivot point may only be extended so far before the back plate obscures the light cone from the back of the objective lens. Though the triangle could be pivoted from the light baffle for maximum extension. Arranging the initial 2nd mirror tilt would be a matter of chosen screw length.

Both mirrors could be mounted on wedges to provide initial tilt. The triangular, collimation adjustment plates would then lie almost flat with the framework. Allowing the use of much shorter bolts and springs. This arrangement would also have removed the need for skewed screws in their holes. Though alloy plates could be easily bent at both ends to make the screws perpendicular to their plates and the framework plate.

By a happy coincidence the tilt on the 1st mirror backplate was close to that required. I held the camera roughly in the center of the objective aperture and saw the target reflected in the 1st mirror. The red cross marks the center of the 2nd mirror as seen in the shaving/make-up mirror used as a 1st mirror mock-up.

The hairy twine is just to mark a line from the top center of the objective aperture and runs parallel with the framework. This helped to indicate the lowest point allowed for the 2nd mirror backplate without causing obscuration of the conical light beam. The camera is obviously slightly off center but easily done when the lens is not remotely symmetrical to the camera body.

Further measurements and trials suggested I needed to lower the 1st mirror. Particularly because of its slope and raised mounting of the reflecting surface which caused some upwards displacement.  The center of the mirror surface must lie on the axis of the objective and that axis must be parallel with the framework. Having tried long springs on 100mm coach bolts I am now even more in favour of mirror support wedges and flat collimation plates.

Click on any image for an enlargement.

*

7" f/12 iStar folded refractor 22: Focuser & 1st folding mirror cell.

*

Yet another rainy afternoon so I cut the 155mm hole for the Vixen 2" focuser base. I chose the highest position which would not impact on the framework tubing.  Having used a large pair of dividers to scribe two lines, with slightly different radii, I sawed out the hole slightly undersized with an electric jigsaw. Oiling the blade and plate made cutting very much easier. I then filed to the line and the focuser dropped in with no play.

I have re-used the plywood clamping ring which I had employed on the earlier, straight tubed refractor, to allow focuser rotation. I may dump the plywood ring and turn an alloy ring instead. A quick "sand" with an electric orbital sander cleaned up all the marking on the backplate.

I discovered that there was no room for straight handles on the front plate with the dewshield in place. It would take cranked handles to clear the very large dewshield at the base. I shall have to see what is available or rethink the dewshield support method.

The next morning was sunny and reached 42F by lunchtime. So I was able to work out of doors in good light and perfect comfort.

Firstly I fitted the objective and tipped up the OTA to bring the sun's conical beam onto the back plate. This provided a 100mm 4" circle of bright light. I then measure down to the center of the objective and marked the backplate to match. This would be the center of the 1st mirror. The objective was then removed and returned to safety indoors.

A simple hinge was bolted to the 1st mirror's, plywood backing disk. I then sighted through the backing disk's central hole for the pencil marks and then marked the top of the hinge onto the backplate. A spare hinge was offered up to the line and the central screw hole marked and drilled. A single screw fixed the hinge and backing disk in place. The mirror shell was slid over the backing disk and a flat shaving mirror laid into the shell.

This mirror represented the real one without risking damage to the high precision surface. I then tipped up the OTA to shine the sunlight through the objective aperture to check the required tilt angle on the 1st mirror. No great precision was necessary at this stage as I centered the 1st mirror cell in the circle of light.

I was merely checking that I hadn't overlooked anything or that the hinge wasn't being jammed at the required tilt. All seemed to be well. I shall need long bolts passing through the backplate with compression springs inside the OTA to allow collimation by adjusting the 1st mirror's tilt. The single bolt through the hinge and backplate will provide rotation if necessary.

I then measured down to the center of the focuser and checked where that point lay inside the framework's shoulder.

By a happy coincidence the center of the 2nd folding mirror lay midway on the Porsa tubing flange. The 2nd mirror's support hinge could be applied to the face of the tube or even underneath for greater neatness.

Whoops! As you were! A helpful contact has pointed out that hinges would not allow for sideways tilting. Attempting to tilt the cell/mirror sideways, via the other sprung collimation screws, would have little effect. The hinge would need to bend midway along its length. Rotation on the single screw would not be in the correct plane.

I have therefore returned to conventional three [sprung] screw collimation. With the exception of using a soft, rubber tap washer under the top screw instead of a traditional spring. The two springs on the other two screws will supply the necessary pre-tilt. While the tap washer will flex enough to allow lateral and vertical tilt when the other screws are tightened individually against their firm springs. The thin tap washer also avoids adding unnecessary depth to the mirror support. Having now dumped the hinge idea I shall have to add a plate inside the OTA shoulder [arm pit?] for the 2nd mirror cell collimation screws. 

With reasonable luck my measurements should ensure that the light path follows parallel lines as it is folded back and forth between the objective and the focuser. It does seem as if there is a lot of fresh air around the folding mirrors. But the minimum dimensions of the framework are set by the diameter of the objective and its focal length. I also had no desire to compromise the design with stray light shining directly from the objective into the focuser. So I opened out the angles of reflection at the folding mirrors to help avoid this. Baffles will also be applied to prevent stray light spoiling the contrast.

Click on any image for an enlargement.

*

7" f/12 Istar folded refractor 21: Objective 'bayonet' plates completed.

*

I spent a rainy day working on the bayonet plates. These are for mounting the objective on the front of the Porsa framework. The previous domed screws were first replaced with countersunk screws. This was to ensure no screw heads would interfere with the sliding action.

The first image shows the location 'keyholes' in the OTA plate for the projecting, 'bayonet' socket head screws. The screw heads go easily though the large holes at the top of the keyhole slots. When the bayonet plate is lowered, the screw heads cannot pull back out through the much narrower slots. The square shoulders under the screw heads provide a firm grip and ideally fixed location without any 'slop.' [Any unwanted movement might affect collimation.] The screw heads are 8mm in diameter so I made the large holes 9mm to allow easy fitting and removal. Trying to be mean with the top hole sizes of the keyholes will only make extraction of the 'bayonet' plate screws foolishly difficult. Moreover, they have no effect [at all] on locking the 'bayonet screws safely in the lower end of the grooves.

Then I decided to pack out the bayonet plate to produce a fixed distance from the OTA plate. Self-adhesive, hard felt, furniture protectors proved to be the ideal solution. These have no "give" in them so provide repeatedly accurate location of the objective via the bayonet plate.

These packing pads also offer a wonderfully smooth sliding action. Thereby avoiding the likely grating which would occur between two, soft, aluminium surfaces. Adding further pads will ensure that the 'bayonet' plate cannot sag nor compress the existing pads. As a side effect the pads also allow room for lock nuts on the ends of the collimation 'pull' screws. Thus offering a secondary level of security in the unlikelihood of a thread stripping in the bayonet plate.

Here the bayonet plate is laid on the OTA plate with the screw heads in the keyholes. The plate is not yet slid downwards to lock the screw heads in the lower ends of the slots.

The ordinary hex nuts on the 'bayonet' screws will be replaced with stainless steel. Zinc plated fixings soon rust.

Note the eccentricity of the two 8" holes until the 'bayonet' plate is slid down relative to the OTA plate.

I had to snatch pictures between showers or I would have orientated them more logically.

In this image the bayonet plate screws are fully home in the bottoms of the narrow slots. Where the square shoulders of the undersides of the bayonet screws ensure secure location.

The objective will be collimated on the 'bayonet' plate when they are both seated on the mounted OTA. After that there should [ideally] be no need for re-collimation between removal and refitting of the objective. The cell will be attached to the 'bayonet' plate via its collimation 'push-pull' screws as if attached permanently to the OTA

In fact the bayonet plate acts just like a counter-cell in normal, "straight tube" refractor practice. Except that it slides vertically downwards to fix and locate the objective on the front of the OTA framework.

I chose the plate size to provide a safety margin beyond the 198mm / 8" holes necessary for the objective cell clearance. The OTA plates rests on the recessed, Porsa tube flanges for neatness. I thought this far better than mounting the plates on the surface of their plain, square tubing. 

The objective was then assembled to the 'bayonet' plate with the collimation 'push-pull' screws. Only to find the projecting rear of the cell obstructed the OTA plate! This required a combination of doubled sticky felt pads and deeper adjustment of the 'push' screws. The cell then clears the OTA plate nicely and slides very smoothly on the hard felt pads.

The 'bayonet' plate makes for much safer carriage of the lens indoors and out but feels rather sharp on the bare hands. Carrying the bare cell always felt really insecure due to its considerable weight and shape. I thought I might add some U-shaped, metal drawer handles to the top and bottom of the 'bayonet' plate. This will give a comfortable and secure grip when handling the objective and its plate. I will need to ensure the handles do not interfere with the 10" diameter dewshield and its fixing. If I re-use the saucepan base, to hold the dewshield, I may be able to attach handles to that.

The necessary gaps to ensure rear cell clearance are obvious in this picture.

The pull screws are only temporary until I can obtain some more stainless steel, socket head fasteners of the correct length.

It will be important that the lens in its cell is treated gently between attachments to the OTA to avoid spoiling the collimation. The lens, in its cell, will remain permanently attached to the bayonet plate. Being brought out only when the bare OTA is already placed on the mounting. The quick release 'bayonet' system will save me having to carry the 10lb burden of the objective while still attached to the OTA. That extra weight is not insignificant when the entire OTA must be carried in and out. Not to mention struggling to lift it onto the mounting.

Click on any image for an enlargement. 

*

7" f/12 iStar folded refractor 20: Porsa OTA framework completion.

*

Assembling the Porsa framework held some hidden hurdles. There is a minimum length of square tube before the joints butt up against each other leaving a visible gap between the tube and the joint's cubic center.

I wanted to reduce the height of the step [or shoulder] on the framework but was unable to go beyond a certain point. The unexpected arrival of a gap at the joint meant I needed to saw the ends off the joints. I found I could take the joints apart again, if necessary, but it was quite a struggle.

Before sawing the OTA to length I tried a number of full size, paper, light cone, folding options with the frame on the floor and the mirror shells in position. [Image above] Finally I decided that an overall length of 95cm would do the job. If the 1st mirror cell proves to be too deep, in practice, I can shorten it.

Cutting the partially assembled frame to length was rather unkind to the black finish. It was also difficult not to scratch the adjoining members with the saw blade in the miter frame. I propped the framework on one folding workbench while the miter saw was clamped to the other.

The images show the nearly completed Porsa framework. It seems rather bulky but would be difficult to make any smaller given the dimensions of the lens, its cell and the focal length. I needed 183cm from the cell mounting plate to the surface of the backplate where the focuser will sit. This was the dimension taken from the former straight-tubed design.

The frame is remarkably stiff now it has been closed at the rear. Any thoughts of dropping an upright tube from the shoulder have been set aside for the moment. Though such a tube might have added useful structure just where the Dobsonian altitude bearings will be attached. The advantage of the Porsa system is that such changes are still possible. Albeit with some effort required to separate the framework at the joints. The flanges on the tube can provide a strong attachment to a plate of aluminium or plywood. Even a sandwich between inside and outside of the flanged tubing. There is no shortage of room within the structure.  

The third image shows the objective's 'bayonet' plate resting on top of the fixed OTA plate sitting in the front recess. The OTA is still easily manageable without the objective lens in place. The next job is to fix the front OTA plate firmly to the framework with CSK screws. The backplate can also be given the same treatment. I have chosen to use firm, furniture protection pads between the two objective mounting plates to allow room for the protruding T-nuts for the objective 'pull' screws.

I had no small CSK screws to hold the front and back plates onto the Porsa framework. So I have used round, cross-head screws for the moment. I made the screw holes 8mm from the edge of the plate to avoid them getting too near the edge of the 15mm wide tube flanges. I shall try to obtain a small metalworking countersink while I am shopping for screws.

The OTA, as seen in the image, now weighs exactly 6kg or 13lbs. To which must be added the focuser, folding mirrors, their cells and baffles. The objective will only be added once the OTA is safely mounted for each observing session. This will save having to lug another 10lbs around outside. The present balance point is about 8" behind the 'shoulder.' This tendency will still be well to the rear once the folding mirrors and focuser are added. The 10lb objective should bring the balance closer to the middle again. There is a clear advantage with the moment arm of such a short OTA. The tube is much less sensitive to changes in eyepiece weight compared with a 'classical' straight tube. Hopefully no balance weight will be necessary for the much shorter, optically folded 'tube.'

*

7" f/12 iStar folded refractor 19: Collimation and 'bayonet' interface.

 *

Having alloy plates instead of plastic alters the potential for collimation screw fixings. The collimation 'pull' screws are vital because they support the considerable weight of the objective lens. Failure of the 'pull' screw threads in the relatively thin [4mm] and soft aluminium would be catastrophic. The 'push' screws only align the lens axis to ensure optical collimation. Failure of the 'push' screw threads would be merely irritating due to loss of collimation.

Needing a strong 'pull' screw thread suggests I use T-nuts instead of tapping threads directly in the aluminium. The anchor spikes on the T-nuts could still be useful to prevent unwanted rotation. Small holes could be drilled in the aluminium for these spikes and the spikes themselves filed to provide a more suitable anchor in harder materials like aluminium. The spikes could be reduced to fit in very small holes since they have no need to resist torque in a relatively soft, wood-based material. 

Until now I had been assuming metal to metal contact for the 'bayonet' plates. Though even this is not strictly necessary as long as collimation is maintained between repeated removal and refitting of the 'bayonet' objective lens mount. So plastic standoffs in the form of washers or even plastic nuts could be fitted to the 'bayonet' screws. The 'bayonet' screw heads would still project and provide the secure location and retention as before. But now with a fixed gap between the two plates tightly controlled by the plastic packing material and the underside of the bayonet screw heads. The 'bayonet screws would be adjusted and then locked to provide the minimum of slop between the packing and underside of the screw heads.

I only suggest plastic spacers to avoid burring or wear on the mating aluminium OTA plate from metal fixings. There are further advantages to using spacers. The collimation 'pull' screws might project beyond the T-nuts and the T-nuts themselves will have some thickness. The 'bayonet' acceptance plate on the OTA must not suffer from any projections or it will not allow the lens cell to drop to the [lower] anchored position in the keyhole slots.  Having spacers allows the 'bayonet' system to function cleanly while still maintaining the closely fixed distances between the lens [cell] and the OTA. It also moves the rear of the cell clear of the OTA location plate. Otherwise the collimation screws would have to push the 'bayonet' plate a full centimeter away to clear the OTA plate. Which again increases the loads on all the screws due to increased overhang. Not to mention lengthening the OTA to compensate for this increased overhang.  

However tempting it might be to cut down the objective's bayonet plate this might reduce the quality of relocation. Widely spaced 'bayonet' screws make the most sense to avoid rocking once in place. With an astronomical telescope the OTA is normally pointing upwards so that the heavy lens rests naturally against its mounting plate. Thereby relieving loads on the 'bayonet' lens mounting system.

Difficulties might arise if the telescope was set horizontally for testing or collimation on a distant object. The heavy lens in its cell would then have a much greater tendency to sag forwards on the 'bayonet' mount. A smaller' footprint' of the bayonet screws would tend to suffer greater loads and magnify any play in the location system. The larger, full plate 'bayonet' footprint should be far more tolerant of 'bayonet' screw maladjustment. The footprint could be seen as the degree of leverage the lens can place on its location system. A large plate and well separated screws has mechanical disadvantage [in the physics of levers sense] rather than in the English sense. 

*

7" f/12 iStar folded refractor 18: Heavy metal.

*

The Polypropylene cutting boards from the supermarket were never meant to be other than bird baths. They are far too curved to be cutting boards let alone precision mating surfaces for optical components.

I have recently discovered two potential sources of aluminium. Where I can hopefully obtain enough for the two, matching bayonet plates and the backplate. I had better check the plates are flat if I am offered anything. Aluminium is three times more dense than PP so there will be a weight penalty. Thankfully a large circle is cut out of both objective supporting plates. This will reduce the weight considerably. The backplate will only have a 55mm hole for the focused light beam to reach the focuser. Perhaps I can get away with only 3mm aluminium for the backplate? It will be supported on all four sides by the Porsa tubing flange.

I may be able to lessen the overall load by employing a suggestion from a fellow ATM. He too has folded an 8" f/9 objective using a tube from a Fullerscopes reflector. He helpfully suggested that I cut down the baking tin, mirror shells to only the depth required to house the mirror blanks and a backing disk of plywood. I can potentially save up to 40mm or 1.5" per cell. By 're-tuning' the folding the overall length of the OTA framework could be shortened by as much as 3" or 75mm. Thereby saving even more weight. The baking tin, mirror shells are not particularly heavy. Though the rolled rim is arguably the heaviest of what remains after cutting out the bases to leave only a narrow, mirror retaining lip.

The extra joints were dispatched yesterday by Porsa so hopefully I will see them today. Though it is even colder now with the day starting at -6C, 21F, albeit in bright sunshine. Not ideal for handling bare metal and tools. My feet felt the cold yesterday while working outside at -1C. I shall have to wear better footwear if I do any ATM today.

After a 12 mile ride I was redirected to another firm several miles further on which had loads of aluminium in stock. I was going to get the sheet over-sized and saw it up myself but they had a huge guillotine with digital readouts. So they cut it to size on the spot for a very reasonable sum. It's good to know they have all that stock and friendly service if I do need more aluminium.  The extra Porsa corner joints have arrived in the post.

I spent the afternoon outdoors cutting out the 20cm disks from the 'bayonet' and OTA front plates. It was lucky I tried the first hole on the objective because it was rather loose. So I scribed the next one 2mm smaller. Then spent literally ages filing the hole to fit the rear of the objective. The close fitting one will be used as the collimation and bayonet plate. While the other can become the matching, OTA support plate. With the disks removed the surrounding plates are very much lighter.

24cm x 28cm = 672 cm^2. Disk = Pi x r^2 = [3.142 x 100] = 314 cm^2.  672 - 314 = 358 cm^2.
So, very nearly half of the area [and weight] was in the unwanted disks.

After a short chain drill to make a starting hole for the jig saw I found that ordinary bicycle oil made sawing almost effortless. Run dry, there was a lot of resistance as the blade teeth immediately became coated in melted swarf. I drew an arc of oil on the surface of the plate and that was enough to keep the saw blade nicely lubricated at modest sawing speeds. The results of my labours can be seen in the image above. The plates themselves will be smoothed and lightly grained with abrasive paper on a sanding block. Then they might be painted matt black.

*

7" f/12 iStar folded refractor 17: A major rethink.

*

Having looked at the pros and cons of various folded layouts I decided I was not willing to sacrifice baffling for compactness. I would also have preferred a projecting 'nose' for the objective. Not for any other reason than cosmetic appeal to the more traditional appearance. I have had to order four more, 3-legged, corner joints from Porsa to allow the indent above the objective. I can build the front end of the framework while I wait for the joints to arrive. Leaving the final length of the framework to the backplate to be decided by the 1st reflection. When the mirror and focuser are both mounted on the backplate the OTA's length can be adjusted in a mock-up on the bench.

The new design will [probably] use a 30cm Porsa tube height for the objective area. Though I could reduce this slightly to [say] 28cm to make the nose more square. [Not ignoring the need for ample bayonet slide to ensure safe locking.] The backplate has to support both the larger 1st folding mirror cell but also the focuser. That is why I have chosen a nominal 40cm for the height of this area. Though this too could be reduced slightly to match the reduced 'nose' height. As the objective framework shrinks in height then so does the rear of the main 'box' section to match. Porsa jointing does not readily support angles and is meant to remain perfectly square. Which means opposing frameworks must all be identical. 

I badly wanted to open up the angles of reflection at the flat mirrors without increasing the area of usage unduly. By making the rear of the framework 40cm high I can push the 2nd mirror higher under the new shoulder. This helps to avoid [stray] light passing straight from the objective to the focuser and thence to the eyepiece. Which would seriously reduce contrast unless very well baffled. This was the major compromise with having a simple, rectangular framework only 35cm high. Only by refolding the full sized, paper model of the light cone could I see the potential for problems and how to overcome them. The tendency with all folded refractor builders is to want them as small as the straight-tubed version. This demand for maximum compactness leaves precious little room for the folded mirrors or the folded light beam.

All crossbars will be cut by hand on my miter saw to 24cm to match the polypropylene objective and backplate board's width. There is really no reason to make the OTA any wider and the original 30cm tube length was very generous. Particularly after adding the width of the tubes and joints to make an overall width of 14". The new width will be 24 + 5 = 29cm outside dimensions. The rest of the framework's measurements are shown on the image above. I really do believe I have reduced the compromises to a minimum with this new layout. It seemed to take an eternity to get there but I think I have now achieved the best design I can come up with for my folded refractor.

First cut: The objective board was cut down to 28 high x24cm wide. The Porsa frame was then cut to a generous size to give some clearance. The miter saw was used to cut the tubing neatly to length. Followed by mitering of all the flanges at 45 degrees. The assembly was then hammered together using a wooden board and rubber hammer  to avoid marring the finish. Care must be taken to ensure that joints can be inserted during assembly. It would be very easy to find it impossible if opposite sides weren't assemble first. Then the sides added in parallel.

The flanges needed care to avoid conflicts as the joints are closed. They must also be oriented as required before the joint has a chance to get a grip. I discovered that the joints were removable provided there was room to hammer and the frame had resistance. Best achieved by clamping in the workbench. The raised shoulder needed some thought as there was only one flange and several possible arrangements. The joints needed quite a wallop with the rubber hammer but thanks to the wooden board and timbers on which I rested the frame for resistance they all closed neatly. Even without the rear frame the assembly is quite stiff at the ends of the tubes. Once the frame is closed it should rival any tube for stiffness.

One thing I discovered today was that the PP boards bow very badly in the cold! I have laid them under the woodburning stove to see what happens with gentle warmth. If they remain bowed they will be a waste of time because I need flat, mating surfaces for the bayonet system to work properly. I may need to obtain some aluminium plate instead. I cut the height of the shoulder tubes down to 4" to make room for the 2nd folding mirror cell. This will lower the backplate by 2."
 
Trimming of the plastic burrs just before final closure of the joints kept things tidy. Simply rubbing with my gloved fingers was usually enough. Possibly because it was so cold and the plastic of the joints slightly more brittle than usual. The temperature outside is 30F, -1C and windy from the north. Requiring this ATM wrapped up warm in a large, down jacket. Thin rubber work gloves helped to avoid chilling my hands on cold metal without being bulky.

I'm afraid you'll have to ignore the almost bare soil where the permafrost and my construction activities have temporarily removed the grass. It will soon return in spring. More telescope making materials await their turn for attention.

Click on any image for an enlargement.

*

7" f/12 iStar folded refractor 16: Changed construction details.

*

After failing to find suitable aluminium plate to make the optics supporting plates I came cross some 'kitchen cutting boards' in a supermarket. Choosing the darkest grey option gave me four panels of up to a maximum of 30cm x 24cm each if I exclude the cut-out handles and the spare material beyond the oval hole. [Which will be very handy for practicing cutting, drilling and tapping this material.] The 5mm thick finely textured material is described as Polypropylene on the manufacturer's label. Which can probably be considered as an engineering plastic similar to ABS. A lucky find and not at all expensive at the discount 'offer' price.

The density of aluminium is three times as high as polypropylene. So I am still saving weight even with the 5mm plastic @ nearly 1lb per [full] cutting sheet compared with 4mm aluminium. The PP is remarkably stiff in this thickness and should be easily capable of performing 'bayonet' objective mounting duties as far as strength and stability is concerned. This plastic should not be exposed long term to UV but I won't be leaving it outside in sunshine for very long anyway.

The downside with PP are my doubts about its ability to take a tapped thread. I shall have to do some research into this matter. [Resulting in much reading.]

If the PP is going to be used as a 'bayonet' plate it will need to accept the collimation 'pull' screws of the heavy [~10lb] objective cell. Clear 'through' holes could be backed up by recessed and modified T-nuts. The reduced anchor points will need to be located somehow to avoid accidental rotation against the relatively hard, plastic material.  Pre-drilling dimples, for the T-nut anchor points, might work provided there is no local stress on the plastic. The screws are always in tension which helps maintain the T-nuts location anyway.

The collimation 'push' screws can have a Nyloc nut attached to avoid local loading on the face of the plastic 'bayonet' plate. Just as I did with my earlier, straight tube, counter-cell. Which worked well to stop the 'push' screws from slowly boring their way through the relatively soft, plywood counter-cell surface.

A standard engineering thread has the wrong thread flank angles [60 degrees] for holding well in thermoplastics. Though a variety of special, self-tapping screws are available for plastics with low flank angles [48 degrees with sharp, raised threads relative to the core diameter.] 5mm PP should achieve a reasonable level of strength using these specialist self-tapping screws. Arranging eight 'bayonet' screws on a square at 45 degree angles will increase the strength considerably. Spreading the load so widely will avoid local stressing of the plastic material. 

The 'keyholed' OTA support plate can be easily fixed at intervals to the Porsa flanges with small CSK head screws in countersunk holes. With washers backed up by Nyloc retaining nuts on the inside of the flanges. The screw heads must not protrude from the OTA plate or they will spoil the two plate's, flat location against each other to maintain collimation between removal and refitting. Access for tightening the nuts is easy with the Porsa framework completely open at the sides. Cutting more numerous keyhole slots neatly in 5mm PP may require some patience.

Can I trust my full-sized, paper, light cone in placing of the objective, folding mirrors and focuser back plate? Or should I build an optical bench with the optics supported in V-blocks before I begin construction? Perhaps the real question is whether I can measure the components on the optical bench as well as the the simple folds on the paper model? Building the "front" of the OTA would fix the objective and second mirror positions. Leaving the rear of the framework untouched until I have established the positions of the 1st folding mirror, in its cell, and the focuser backplate. The framework can be laid on its side while these components are moved about on blocks. The 2nd mirror can be easily packed away from its supporting frame if it should prove necessary. As can the 1st mirror cell.

Only the focuser backplate position is really critical to avoid falling short. So that no eyepiece can be brought to focus. A middle of the focusing range with the 2" star diagonal in place is optimal. 'Straight through' viewing is only a matter of adding extension tubes to the focuser.

I don't believe it! I laid out the folded paper, light cone on the floor and then laid the Porsa tubing on top.  If I cut the PP boards to 24cm wide x 30cm high I can have a simple rectangular framework 100cm long x 30cm high x 24cm wide [inside measurements.] The joints will add 5cm, 2" to each dimension. This moves the 2nd mirror support to 40cm from the objective support board. 91cm to the 1st folding mirror face and 39cm between the two mirror faces. The focuser then sits comfortably on the outside of the tailboard.

The new framework is longer, but narrower and lower than originally intended. I also seem to have a lot of Porsa bits I probably didn't need! Though I can use two of the unwanted 40cm lengths to reach the joint at the 2nd mirror vertical platform. Which will be reduced in height to only 4" instead of 6". The 75cm lengths can be cut down to reach the backplate as originally intended.

Downside is the potentially greater risk of stray light from the objective entering the eyepiece. A length of tubing with a ring baffle inside the focuser position will help here. As will a baffle behind the 2nd mirror. Pushing the 2nd mirror as high as possible will also help the cause. The 25mm height of the square tubing aids mirror cell clearance. The image shows an uncut board on the left with a 20cm diameter lid placed on top for scale. This diameter is close to that of the 198mm protruding rear of the objective cell, though the clear aperture is only 180mm or 7".

I started to cut off the handle sections of the PP boards with a standard fine toothed, wood tenon saw. It seemed to be going rather slowly so I shifted to a coarser, hard toothed example. This went more quickly and I soon had two plain rectangles 24x30cm. I used a small block plain with the blade set low to clean the cuts though there was not much of a burr anyway. A final rub with fine sandpaper took off the sharp edges.

No pictures? It didn't happen. Capturing the true shade of grey is difficult. It is very like photographer's neutral density grey. The shading is purely an artifact of darkening the image to more realistic shades of grey.  Never knew you could have so much fun with two bits of plastic.

So far so good. Now I need to cut large [8"] holes in both boards. Which will reduce their weight considerably. Positioning the overlapping holes on the objective board will need care. There is the matter of the Porsa flanges and the dewshield fitting. The 'bayonet' slots will also need careful placement.

I think I shall try a normal metal cutting blade at the slowest speed in an electric jigsaw first. The problem with cutting plastics is the tendency to weld the blade to the material due to rapidly reaching the very low melting point. Acrylic is notorious for this problem and jams the blade almost before you turn on the power tool. At least PP does not seem to splinter like acrylic and PP feels quite a bit softer and more forgiving. Both make coarse swarf when being sawn but PP does not stink when heated by the cutting action of the hand saw. Blowing [with the mouth] is the quickest way to remove the dust from gloves and clothing.

*

7" f/12 Istar folded refractor 15: Porsa built OTA.

*

The image shows the latest arrangement of optical folding using two flat mirrors. The optical path easily fits inside a box 95cm long x 30cm wide by 40cm high. The image is taken from a photograph so some distortion is inevitable. I have propped up the mirror shells in the correct places to give a sense of scale. The shells will be used full depth and each hinged via a short piece of plywood cut to match their own circumference. The outer white rim suggests the 1" square framework. With a dropped frame for second mirror cell support. Note the wider angles of reflection at the flat mirrors. This is to avoid stray light from the objective coinciding directly with the last reflected parallel 'leg' of the folded light beam. However tempting it might be to make the instrument as compact as possible there are unavoidable downsides if taken too far.

My Porsa OTA tubing package has arrived promptly. There seems to be an awful lot of it for such a simple framework. It feels a bit heavier than I had hoped but the quality and care with which is was packed promises success.

The metal reinforced joints are really quite a bit heftier than I could ever have imagined despite knowing their weight beforehand. There lies the secret to the Porsa system. The tubing is not some flimsy, thin wall stuff with a high price tag. The joints are easily strong enough to avoid flexure and require no triangulation to remain rigid. The black finish is excellent and unmarked on first inspection. Never having handled the Porsa Building System in advance I am really quite impressed. At a quick glance one might think the tubing was raggedly cut but, in fact, it has a distinctive internal profile and the cuts are perfectly free of any burr.

Porsa really has this product very finely developed. Note the chamfers on the solid plastic joints to aid the beginning of the forced insertion. Then there is a relieved section to reduce friction followed by the full size again. If the legs were continuously full sized there would be much higher leverage forces at the joints. It all bodes well for very stiff and stable joints once any framework is competed. The cast-in, aluminium reinforcing section is just visible in the narrowest part of the joint.

The basic idea is to drive the joints home with a rubber hammer to avoid cosmetic damage. The slight interference fit provides the necessary adhesion to avoid the joints pulling out. This, of course, requires that the frame is exactly as desired before it is hammered together.

It might be possible to separate the joints again but I can't imagine it is very easy. Doing so without cosmetic damage only adds to the problems. Perhaps blocks of wood and a heavy weight to resist the hammer blows would help if a mistake were made during assembly.

Since I am aiming for a stepped design I must be careful to ensure the opposing tubes are all of the same length. The joints each add an inch to any run of tubing. This must be taken into account including the corner joints of a box frame. The tubing is sold in stepped lengths in cm with nicely cut ends. If one used [say] 50cm lengths of tube then the finished item would be 50cm + 2 x 2.5cm corner joints. Or 55cm in total length. Not the original 50cm length for which one had planned. This joint addition must be kept firmly in mind for any design using Porsa tubing and joints.

I have purchased the Porsa tubing with the optional longitudinal flange. This flange not only provides considerable extra stiffness but allows easy fixing of component plates or even a flush covering of the framework. The flanged tube is slightly more expensive than plain tube and adds to the weight but seemed like a easy option for enclosing my OTA later. The flange is slightly recessed to allow a flush or slightly recessed panel to taste. Which I considered a thoughtful touch.

Plain tube, as well as a variety of other flange forms and finishes are all available. White, silver and black options are offered in both joint and tubing colours. Porsa may seem quite expensive but offers genuine furniture quality finishes and results. Its most popular use is probably for aquarium support. The visual sense of lightness and cleanliness of the 1" square tubes is a far cry from heavy slotted [and rusting] angle iron held together with ugly bolts.

My original plan was for a simple frame 100cm x 30cm x 40cm. However, the addition of the corner joints adds 5cm to each length. Then there is the small dropped frame for the second mirror support. This too adds another inch [2.5cm] to the overall length but only on the 'top' panel. So 2.5cm [1"] must be sawn from the top frame somewhere to match the unbroken bottom rails.

Hack-sawing the tubes to length freehand seems a little crude unless I use a miter box to try and ensure my cuts are as neat as Porsa's own. It would be a shame to scratch the tubing by carelessness. I have a cheapo miter handsaw frame with swiveling base but it takes deep, 53cm long saw blades rather than  the standard 30cm hacksaw blade. This saw is really intended for cutting neat miters in wooden picture frames. So the small, hardened teeth may object to being [ab]used on aluminium. Perhaps I should buy a fine metal blade for my bandsaw? I have just bought some good quality 24tpi hacksaw blades so I'll see how I get on with the hacksaw and miter box first.

By sheer luck I was able to purchase a metal cutting saw blade for my adjustable miter saw. The blade was slightly too long but could drilled and clipped to the proper length. It has very fine teeth with a wavy edge to avoid jamming in the cut. So now I am equipped to cut the tubes neatly at right angles.

After more folding of the full scale, paper, light cone I discovered a slightly different layout. Where one square section is moved along relative to the other. Construction options exist for T-shaped joints in the adjoining rails. Or even two complete rectangular box frames fixed together with bolts or clamps. I think this arrangement has a more attractive or more traditional[?] appearance than a simple 100x40x30 rectangular box.

Allowing one complete leg to slide over the other provides greater flexibility in design to allow the flat, folding mirrors to be placed optimally. If their positions have already been confirmed then a fixed arrangement using four extra Porsa joints is probably lighter. It rather depends on whether cross bars are considered necessary at the junctions of the legs. If the two square 'tubes' were kept separate but overlapping then the overlapped lengths are duplicated. I doubt there is much difference in weight between these two options. Though both forms are heavier than a simple, rectangular box because there is no central bar dividing the sides of the simple box. The 30cm long section in front of the second folding mirror and above the objective is superfluous anyway. So could be "cut away" from the box to better define the objective section. While leaving the back of the OTA flat.

I read the Porsa instructions online and apparently the cut edge must remain sharp to achieve a cutting action as the plastic joint is driven in. The plastic burr is then trimmed with a sharp knife just before final closure of the joint. To get a close fit for a perfect cosmetic result. The inside of the tube must not be filed to aid the joints insertion. In case you need to put some Porsa tubing together without instructions the smallest section protruding from one end of the joint should point along the vertical axis of a floor standing frame. For an OTA one presumes that the "tongue" should point along the long axis to achieve maximum stiffness.

After failing to find suitable aluminium plate to make the optics supporting plates I came cross some cutting boards in a supermarket. Choosing the darkest grey option gave me four panels of up to a maximum of 30cm x 24cm each if I exclude the cut-out handles and spare material beyond it.  The 5mm thick material is described as polypropylene which can be considered an engineering plastic similar to ABS. A lucky find and not expensive at the discount price. The density of aluminium is three times as high as polypropylene. So I am still saving weight even with the 5mm plastic @ nearly 1lb per sheet compared with 4mm aluminium. The PP is remarkably stiff in this thickness and should be easily capable of performing 'bayonet' objective mounting duties.

The downside is some doubts over its ability to take a tapped thread. I shall have to do some research into this matter. [Resulting in much reading and only a little learned.] If the PP is gong to be used as a bayonet plate it will need to accept the collimation 'pull' screws of the heavy [10lb] objective. Clear holes could be backed up by T-nuts. Though the points will need to be anchored to avoid accidental rotation. Pre-drilling for the T-nit anchor points might work provided there is no stress on the plastic. The 'push' screws can have a Nyloc nut attached to avoid local loading. Just as I did with my earlier plywood counter-cell.

A standard engineering thread has the wrong thread flank angles  [60 degrees] for strength in thermoplastics. Special, self-tapping screws are available for plastics with low flank angles. [48degrees]  Dare I trust my precious lens to self-tapping screws alone? At the cost of a little extra work I could use multiple [self tapping] bayonet screws. 5mm PP should be thick enough to get some serious holding power if I follow pre-drilling recommendations. The OTA support plate can be easily fixed to the Porsa flanges with CSK head bolts in CSK holes and Nyloc nuts.

Click on any image for an enlargement.

*

7" f/12 iStar folded refractor 14: Square tube objective bayonet fitting.

 *

"Round" is far too narrow a definition of the meaning of a lens bayonet.

For a square tube there is absolutely no need for cell rotation. Simple, vertical 'keyholes' are cut in the permanently mounted, OTA, objective support plate instead. See image alongside for a rough idea of what I mean. Choose your own dimensions and layout to suit your own circumstances, materials, skills and OTA construction.

A second, 'bayonet' plate is attached to the objective cell to deal with both collimation and attachment to the square OTA. This plate [or ring] acts as an intermediary device to hold the projecting 'bayonet' screws as well as provide collimation. [Just as it did with the round tube, bayonet form.]

Most larger refractors will attach the objective cell via a counter-cell and collimation screws. This arrangement may not readily lend itself to a bayonet fitting due to the small flange size involved. The socket head, 'bayonet' screws must project equally from the back of the bayonet plate completely independently of the collimation screws.

Exactly as with the 'round' rotating bayonet system, the bayonet screw heads pass straight through the larger, round part of the vertical keyholes. These keyholes are cut into the square OTA's objective support plate. Then the entire objective assembly is gently lowered until stopped by the bayonet screw shanks reaching the bottom of their respective slots. Now the screw heads are safely retained by the narrower dimensions of the OTA slots. Only by lifting the entire objective assembly will the screws pull freely out of the round tops of the keyhole slots.

If you are prepared to use larger headed screws ensure that they have flat undersides to their heads. Definitely NOT conical countersunk screws which is just asking for trouble! You must also ensure that you have enough 'meat' in the OTA support plate to accept much larger keyholes without weakening the plate. I chose socket head screws primarily because they offer neat, round heads, often with small chamfers to easily pass through the round 'tops' of the vertical keyholes. They have easily enough shoulder width, under the heads, not to be easily pulled out though the narrow slots. Your mileage may vary, depending on your own skills in making reasonably accurate slots. Milling is unnecessary and filing or fret sawing will probably do for aluminium if care is taken in marking out the slots first.

The bayonet screws could be reversed so that they project from the OTA's front plate but I strongly advise against it. The jutting screws could very easily scratch the back of the objective glass while fumbling with a heavy objective assembly in the dark. Only if one could guarantee that the bayonet screws can never reach the glass would I ever consider this arrangement. A suitably thick bayonet plate might ensure safety in this matter. But! Do exercise great care if you do choose to have the screw heads projecting from the OTA.

It is also vitally important that the objective cell does not project behind the bayonet plate. Otherwise the large hole in the OTA objective support plate would need to be made oval. The projecting rear of the cell would stop the objective cell from dropping down if it was sitting inside a large round hole. Which would deny the bayonet screws the chance to safely enter their narrower, locating slots.

In the case of the iStar 7" objective the rear of the lens cell extends by about one centimeter beyond the rear face of the collimation screw flange. The length of the collimation screws and thickness of the square bayonet plate must be arranged to allow the rear of the cell to safely clear the OTA lens mounting plate.

A ring could be attached to the bayonet plate for the cell's collimation push-pull screws to act against. With that ring then fixed securely to the square bayonet plate behind it. A ring is lighter than a full thickness square backplate since both should ideally be made of metal to accept screw threads. Those with the skill may use a round ring for the bayonet plate despite it being fitted to vertical keyholes. The smaller dimensions are of a ring may allow slight rocking compared with a larger, square plate.

The 'pull' collimation screws, which hold the objective cell to the bayonet plate, should not project behind the bayonet plate either. Or they will bind against the OTA's lens support plate. Instead of allowing the square bayonet plate to lie flat before sliding safely downwards into the keyholes and thence the locating slots.

Add a locking thumbscrew between the bayonet plate and the OTA plate and the lens will never shift in use. This is a far simpler system, better suited to square tubes and altazimuth mountings. The rotational bayonet, discussed earlier, is much better suited to round tubes and equatorial mountings. A suitable arrangement should be provided for supporting a proper dewshield in both forms of bayonet lens mounting.

The cell and its bayonet ring [or square bayonet plate] can be kept safely indoors in a sealed and padded, plastic food container with a snap-on lid for safety. When the telescope is going to be used the food container and its precious cargo are brought out to the telescope. Only then is the lens removed from the container and fitted to the OTA.

Carrying a 'naked' and heavy objective from indoors is not really sensible in the dark.  Particularly if there are steps, changes in level and doors to be opened and closed [often with elbows] on the journey out to the telescope. [And back again later when it clouds over.]

The heavy glass lens may need to cool to the outdoor temperature before it performs well. So early fitting to the telescope is often advised to allow it to cool naturally from the higher indoor temperature. Experience will suggest how long your own lens needs to cool under different conditions.

When the observing session ends the food container is, once again, used to safely carry the precious lens back indoors. Dealing with dew might require the container be left open, once indoors, to allow the moisture to evaporate. It may be that the food container seals well enough not to allow dew to form when the cold lens is brought back indoors. Only direct experience will dictate how you should deal with the dew problem. Silica gel is sometimes used to absorb moisture in sealed spaces but do check regularly that the lens isn't deteriorating inside its own 'lunch box.' Moisture can attack the lens coatings sometimes and can even lead to acid etching fungus! The lens must be dry before long term storage.

Anything cold will automatically attract condensation when subjected to the typically warm and moist conditions indoors. Storage in a secure shed or garage might then be tried if you enjoy 'tropical' central heating but live in a cold climate. The problem now is that the lens might actually be colder by lagging behind the outdoor temperatures. Causing the lens to instantly dew up on on all four surfaces when suddenly exposed to slightly warmer conditions. Safely warming and drying the objective enough to get rid of the dew can present quite a problem. A hairdryer would need to be used very gently indeed not to shock some ED and other exotic glasses into actual breakage!

My first ATM f3.8, 8.75" mirror lived permanently and snugly in a round food container with snap-on lid at the bottom of the skeleton OTA. Food for thought, methinks? ;)

Click on any image for an enlargement.

*

7" f/12 iStar folding refractor 13: Round tube bayonet objective fitting.

*

Observing life would be so much easier if I had no serious lifting or carrying to do. Or, the heaviest and most fragile bits could be easily removed for safe, indoor storage. So, what about a bayonet fitting for the objective cell on the OTA?

No need to carry that extra 10lbs along with the already heavy OTA. Just bring the lens out separately from safe storage and fix it straight onto the OTA without further ado, or fiddling about. An instant 10lb reduction in the carrying weight of the OTA. Read on:

Push in, twist and then safely lock in place with a separate thumbscrew. Socket screw heads would project on a fixed radius from a dual purpose ring sitting behind behind the objective cell. This added ring would provide collimation AND attach itself to the OTA via the projecting screw heads and the keyhole shaped slots.

The rough drawing gives some idea of what is needed on the end of the OTA. The keyhole slots would need to be true arcs of the correct width and radius to match the shanks of of the bayonet screws. Jamming between the screw shanks and the radiused slots must be avoided. There must also be unquestioned, fail-safe security of  objective lens on the OTA. The undersides of the screw heads need only project  by the thickness of the OTA ring plus just enough clearance for easy rotation. Flatness of the OTA ring obviously helps here. Warping between the rings must be avoided.

The keyhole slots could be cut in a fixed OTA ring or a modified counter-cell. The screw heads go straight through the larger round holes at the start of the arcing 'keyholes' on the OTA. Then the entire objective cell and its attached bayonet ring are rotated. This rotation brings the screw heads under the much narrower sections of the arced slots.

To aid rotation of the cell, into the locking position, the collimation pull screws could be seated in deep cups [used as handles] to give the user better grip on the cell. And to aid later removal, of course.

It is strongly advised that the round holes of the keyholes are made suitably over-sized to allow easy entry of the protruding 'bayonet' screw heads. There is absolutely no need for a tight fit on the screw heads. All that will do is make initial location of a heavy and fragile object exceedingly, and quite unnecessarily, difficult! Particularly in the dark!

It is the larger heads of the screws passing behind the narrower slots on the OTA which do all the real work of locating the objective assembly safely in place. With at least one locking screw for absolute security! An equatorial might eventually put rotational forces on the lens cell which the altazimuth mounted OTA would never experience. Consider a bayonet locking device absolutely essential in both types of mounting to avoid catastrophic disasters! Just carrying the OTA around with the bayonet mounted cell in place, particularly when you are tired, might dislodge an unlocked objective cell.

Once the locating screws reach the limits of their clockwise travel [in the slots] a simple thumbscrew in the bayonet ring is screwed in. This action must safely lock the bayonet ring to the permanently fitted OTA plate or ring. Most refractor counter-cells will not have remotely enough enough room for arced, keyhole shaped slots. The normal counter-cell normally provides threaded holes for the collimation 'pull' screws. So rotation is impossible without the addition of the added, cell backplate ring.

Several screws could be used to hold the bayonet ring firmly against the OTA plate to ensure exact collimation but this adds further complication. It is assumed that the socket head screws are chemically locked into their screw threads in the sturdy bayonet ring. Stainless steel screws seem appropriate to avoid wear problems and metal dust from the usual zinc coated fixings. Not to mention rapid rusting of the cheaper option on repeated exposure to moisture.

With reasonable care in handling, the objective collimation should remain undisturbed between the objective cell and its 'bayonet' ring. If the OTA's objective support ring is stiff enough [sturdy aluminium plate or brass plate] there is no reason for the collimation to change between removal and refitting of the objective.

Plywood would probably be too soft and changeable to act as a bayonet ring. It might also require a metal rear surface to prevent the bayonet screw heads from literally pulling out through the much softer slots in the plywood. It could work with harder birch plywood if much larger screw heads were chosen. Though this would require matching, over-sized holes for the large bolt heads to pass through.

Great care must be taken to avoid the lens, in its cell, form literally falling off the OTA. Which might occur if the telescope was pointed horizontally, or even downwards. The latter behaviour is quite commonplace with larger refractors when checking for objective dewing on the front surface of the glass. Which is usually caused by far too short a dewshield.

A bayonet fitting would probably require that the lens was replaced in the same orientation each time. [Unless the slots were perfectly matched radially.] The bayonet ring must fit flat against the OTA keyhole plate. No need for "This way up" markings with the iStar name and lens details neatly printed on the front ring of the 10lb objective cell. Always assuming you have enough light to see the lettering!

Click on any image for an enlargement.

*

7" f/12 iStar folded refractor 12: Porsa System OTA?

*

The area of plywood sheet needed to make my OTA box exceeds the baffle area of Jim Chung's skeleton design. Until I read the article and discovered the finished weight of his OTA I was quite tempted to copy his basic style. Now I really have to completely rethink my own design to make it easily transportable.

I checked the weight of the Vixen 2" focuser at 2.5 lbs. Add 10 lbs for the objective and then more for the two, folding flat mirrors and I now need sheet Unobtainium to build a manageable OTA! Moreover, I have no idea where to buy aluminium sheet other than ridiculously overpriced, cosmetically challenged, roof flashing materials. I could have used four 1" square tubes for stiffening and pop-riveted lightweight sheet metal onto the sides, top and bottom. Without access to affordable aluminium sheet that idea is a non-starter.  

There is a  Danish company called Porsa which sells 1" square tubing for a frame furniture building system. A large variety of tough plastic joints with aluminium reinforcement are offered. The basic idea being to use a soft rubber/plastic hammer to force the joints into place with an interference fit. The tubing is internally ribbed to achieve even greater resistance to coming apart. The system can support large or multiple aquaria without further reinforcement so must be remarkably strong and stiff. The tubing is available as plain or with a variety of "fins" along its length for adding shelves, baffles, or OTA end plates.etc. The images give a basic idea of what is on offer. Search for Porsa  for much more information and the extensive range of tubing and joints. English is available on their website with a click on the flag.

My OTA construction is obvious: Build a simple, rectangular, box frame and fix the optics to that. It should require no stressed sheet covering to add extra weight. The problem is discovering the weight of the finished framework. No weights are listed online against the various components.

I once thought I might make an OTA for my 10" f/8 mirror using this Porsa stuff. But back then I was thinking solely of using an equatorial mounting. The square format did not readily lend itself to tube rotation. Though it made perfect sense for a Dobsonian the cost of building the square tube was rather high when all the components were added up. There is a quarter round [quadrant section] option which might be marginally lighter and cosmetically more attractive at the slight loss of some stiffness.

Now I need an even lighter assembly to avoid mobility problems with the 7" folded refractor. It would still need some flat panels to support the optics and some more to act as baffles. Though the latter need be no more than the thinnest aluminium sheet. Tempting as it might be, to break the basic 'box' unit up into sub-assemblies, the extra joints required would each add their own weight. The minimum of eight corner joints become twelve or sixteen and each has its own weight penalty. Twelve times anything solid is going to quickly add up!

On request I have just been sent a list of weights for the components of the Porsa system. I should mention that the tubing is available in a large range of lengths from the manufacturer. With careful choice of length this should achieve the highest standard of cosmetic finish at the joints. Not to mention improved structural integrity over any likely variations of hand sawn lengths.  

A basic, rectangular, Porsa framework of 90cm x 30cm x 25cm = 5.8m of tube.
[i.e. Four long edges and two rectangular end frames at each end.]
1 meter of plain square section tube weighs 373grams.
Adding one edge flange adds about 75g per meter.
Each, reinforced, square corner joint weighs 140grams.
Tube material = 5.8  x .373 = 2.2kg = 5lbs.
8 joints = ~1kg. or about 2lbs. Tube frame total = 3.3 kg or  ~7.25 lbs.

Partitioning the overall length into two, adds four more joints and another four sided frame to support the second folding mirror. This would add about 1kg extra or 2.2lbs. The second optical flat would need a firm support board anyway. The flanged framework would allow a much lighter local support for the folding mirrors and objective. Without requiring the usually massive boards or heavy alu. plates for stiffness.

So [say] about 9.5-10 lbs total with the partition framing and four additional joints.
Rough estimate of the total price = 1000dkk, £100 or $150US.
Quite pricey, perhaps, but it offers a very stiff, attractive and convenient framework on which to hang the optics. 
  

The 5" and 4" optical flats weigh 2lbs and 1lb respectively. = 3lbs.
The mirror shells and 12mm plywood backing disks add another 2lbs. This could be reduced a little with backplate "ventilation" holes.
The 7" iStar objective in its cell weighs 10 lbs.
The 2" Vixen refractor focuser = 2.5lbs.

So, the essential optical/mechanical components weigh 17.5lbs before the supporting tube is even considered. Helium balloons would only catch the wind and probably end up in the trees.

A round tube is likely to need heavy plywood disks to support the objective and folding mirrors. An entire 12mm [1/2"] plywood box would not be light. Carbon fiber is too expensive to even consider. Though carbon fiber disks to close each end would be ideal from a weight point of view. Hard [phenolic] paper tubes will still need optical and mechanical supporting disks of 12" diameter to be practical. That would mean buying more tube rings and they get very pricey with increasing size. Closing disks might be substituted with baking pans/saucepans but the weight difference is really not that great. I can only speak from my own experience on my straight tubed OTA and the 10" f/8 reflector.  I did look longingly at the builder's straight edges but they do not suggest themselves for any folded refractor layout. Perhaps I just lack enough imagination but I'm thinking about the weight of the required baffles and boards.    

Assuming no other weight gains a 9lb Porsa tube framework means a total of 26.5 lbs before the mirror cells, baffles and rear and front optics supporting boards are added. Thin aluminium for baffles and slightly thicker end plates would do here. That's getting close to 3/4 of the present weight of my straight OTA! And, way, way over the remarkable 16lbs of Tom Gideon's superbly lightweight instrument. At least the completed OTA framework will be easily carried at about 3' long + focuser. The dewshield can be attached after mounting but doesn't weigh much anyway. I am assuming an altazimuth Dobsonian mounting which can be reached without difficulty to place the folded refractor OTA in its trunnions.;)

A refold of the light cone produced a new layout with 80cm to the first reflection, then 48cm and finally 58cm to the focuser backplate. I intend to house this arrangement in a rectangular Porsa frame 100cm long x 40 cm high x 30cm wide. The tubes size can easily be trimmed to a smaller size if it seems beneficial. I am trying to avoid too near a coincidence of the reflected light beams and that coming from the objective. See next "chapter" for a brand new idea on saving weight.  

Click on any image for an enlargement. 

*

7" f/12 iStar folded refractor 11: Layout.

*

After further folding of the full scale, paper light cone I found a suitable arrangement. One where the positions of the folding mirrors made best use of their available diameter. The resulting design would fit comfortably into a plywood box 82cm long x 30cm high x 20cm wide. Very different from the 20cm diameter straight tube with a length of 183cm plus lengthy dewshield and focuser. The straight OTA weighed 40lbs with tube balance weight, handles and finder. Well beyond my ability to lift it safely above my head into the mounting rings.

The folded design utilizes 120mm of the 125 mm first flat mirror and 80mm of the 100mm. The tilt and retaining lip do not allow much more than this on the 1st flat. The 2nd fold is much more generous but such odd diameters are unavailable. 

The image gives a rough idea of the optical layout but is not to scale. Space has been provided behind the folding mirror blanks for their collimation cells. The '?' merely suggests that this portion of the box could be cut way without affecting the optical arrangement. A generous length of lightweight, aluminium dewshield, 25cm in diameter, will be borrowed from the straight-tubed original OTA.

Once the plywood OTA is completed and the optics mounted, the longitudinal balance point can be found. Allowing Dobsonian altitude bearings to be fixed. The advantage of greater compactness must be weighed against the addition of the folding flats and their cells. The intention of the plywood construction is to confirm the layout actually works reliably. Hopefully without loss of collimation in normal handling and use. A lighter, more sophisticated construction might, by then, have suggested itself.

 A contact has kindly pointed out that for a little extra length in the OTA "box" I could use rather less of the 125mm flat. Then, if the flat does have any degree of turned down edge, it should not affect the final image.  If I reduce the active diameter to [say] 115mm at the first flat it will be spaced at 80cm from the objective rather than the original 72cm. Which only adds a little over 3" extra to the OTA's length. Another way of looking at it is that the greatest diameter of the folding flats is only used at relatively low powers. The field lens of higher power eyepieces is proportionally smaller so will not accept a wider beam. So any flat mirror TDE is unlikely to affect the image when it matters most.

While searching endlessly for folded refractor inspiration I came across various references to Jim Chung's [D&G] 8.5" f/12.5 skeleton folded refractor. This used click-lock telescoping alloy poles and 1/2" plywood baffles. It looked rather amazing but weighed 50lbs. The objective lens weighed 10lbs so the plywood baffles must have been the major problem. When a similar instrument came up for sale in the UK I contacted D&G to be told that they have never made 8.5" achromats. The seller then admitted the objective had been damaged. Be very wary of distance buying anything online without seeing lots of images to ensure exactly what is on offer!

Planning a folded open 8.5" f/12.5 refractor - ATM, Optics and DIY Forum - Cloudy Nights

A much lighter 6" f/15 skeleton, folded refractor design by Tom Gideon used lightweight aluminium angle sections with HDPE baffles. This weighed only 16lbs and replaced a much heavier, conventional, tubular design. Both of these skeleton designs offer inspiration but Tom Gideon's wins easily on weight. He went on to clad his instrument in thin, transparent plastic to give an even more modern appearance. High density polyethylene is a form of plastic unaffected by moisture.

Stellafane Telescope Gallery: Tom Gideon

Click on any image for an enlargement. 

*