10" f/8 Pretty as a picture? [Nearing completion.]

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An image of the secondary cell with the two finders now fitted. The primary cell looks rather distant in this oblique view.

Another view below of the secondary cell with 4-vane spider, Vixen 2" focuser and small finder. With a larger 8x50 finder on the opposite side. The larger finder came with a secondhand 6" F8 Celestron CR150HD refractor OTA but was probably not the original finder. It could be a Skywatcher I suppose.

The bottom of the primary cell still just clears the ground despite the balance being moved up the OTA by the extra weight of the finders. Such a long lever increases the moment of any additions.

The balance around the polar axis has changed slightly. Requiring the addition of an extra [5lb] counterweight. [Not shown here]

The MkIV mounting is holding up rather well cosmetically despite its years living outside under a cheap tarpaulin. The Hammerite paint, stainless steel replacement shafts and all stainless steel fasteners help.

I have yet to paint the cell "innards" matt black to kill reflections. Or I could use thin black foam glued on instead. Both will require warmer weather for best results. The foam might affect thermal radiation to the cold night sky.

The low plinth is just a temporary construction from scrap, slotted angle iron. Without knowing the balance point in advance it seemed a waste of materials to make anything smarter. Rocking the OTA when pointing at the Pole Star shows that the base is not stiff enough. The angle iron pyramid is actually flexing. I could clad it in plywood or perhaps build a plywood box to replace the angle iron completely.

The Moon was high in a cloudless sky at 7pm so I set up the telescope. It seems the collimation does not survive a tour across the garden with the OTA on the trolley wheels.

First I used the Cheshire to align the primary and then I messed about with the finders for a while using the moon as an easy target for alignment. Some more mechanical work to do there in daylight.

Fullerscopes MkIV mounting and the trolley wheels for moving the OTA around the garden.

The Moon and Jupiter were clear enough at 100x, at first, but Jupiter soon disappeared completely behind the clouds. The Moon was much higher still visible through the clouds but considerably darkened and slightly softened. It was 5C/41F and probably gusting to 20mph which was enough to make the image shudder occasionally. Gave up and packed everything away again.

Added a new rubber band to the alignment ring of the 50mm finder. Then I found an old pill bottle to make an absent dewshield. The finder would regularly dew over when mounted on the large, Vixen focuser of the 6" refractor. Penny pinching or just ignorance by the makers? The make the same mistake with many Asian refractors.

The Moon was high again before it got dark. I set up the telescope and re-collimated. Misty high cloud rather spoilt the view. Taking the edge off the sharpness. For a while it looked quite promising and worth bringing out the VFO, the drive paddle and connecting up to the mains.

Later, after dark, there were better moments and I pushed the power up to 300x just for a bit of fun. Best view though was at 133x to 200x when the sky cleared briefly. I was then able to glimpse the central crater in Plato. The clarity did not last. Cloud rapidly covered the sky just before 7pm. I had a quick look at Venus and Jupiter. Both were still visible but rather low on opposite horizons.

I need a second clamping bolt for the primary cell. There is some slight sag when the OTA is reversed from east to west of the polar axis. I had fitted the clamping bolt too far up the cell to avoid the primary mirror. I need not have worried. There was plenty of room in front of the mirror surface.

Another clear, late afternoon but with annoying stripes of thin, high cloud. It cleared completely by 6.30pm CET so I set the telescope up again. Slight collimation was necessary to the primary to bring the secondary absolutely central in the focuser.

I started with Jupiter. Which is rising higher with each day that passes. Not spectacular, but with two clear belts and the four Galilean moons were easily visible. Being small and bright, including one very close to the planet. I moved well up and to the right to the gibbous Moon. The difference in clarity from last night was very obvious. I homed in on Plato but struggled to see the central crater due to wind shake.

It felt bitterly cold at 37F with a 20mph, gusty wind which was visibly shaking the telescope. Not helped by the base rocking on a bump on the lawn! I worked my way up to 200x and the image stayed sharp and very bright but wobbly. Yet again the cloud crossed the sky like a lid just after 7pm. Packed everything away.

One other potential problem has proved to be groundless. More by luck than intent, the fixed eyepiece position has not proved to be significant so far. Observing to the East, South or West is not remotely difficult nor uncomfortable. It is only a matter of reaching the eyepiece on such a long OTA. A plastic beer crate has provided all the necessary extra height so far. Note the turnbuckle to maintain and effortlessly adjust polar altitude.

Reaching the finders is proving slightly more of a challenge at higher pointing altitudes. It might be better to have an elbow finder instead of the present "straight through" 8x50. And sights instead of [or in addition to] the small Vixen finder. Which I find lacks enough eye relief. It is also much too small to offer any useful sighting potential along the top of its tiny tube.

Click on any image for an enlargement.

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10" f/8 Arachnophobia 5 [Finders spiders.]

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Further progress this afternoon: I soldered a new secondary mirror retaining tab. Then replaced a plywood plate with a nice bit of Tufnol for much better stiffness in clamping the secondary cage. By leaning the OTA at 45 degrees against the workbench, re- collimation with the Cheshire eyepiece is now a piece of cake. All thanks to the new spider/secondary holder. When an adjustment is made it is absolutely positive and without any "spring." Adjusting the curved spider was just too vague for my liking. I'm hoping for the sky to clear but it is mostly overcast. Now I must fit sights and/or a finder. No shortage of finders but they have dovetail base fittings. I am still unsure whether I shouldn't have finders or sights half way up the spars for easy location from the ground.

In the end I found an incredibly simple way of adding two finders without modifying or even defacing the dovetail castings. The dovetail based, stand-off stalks are hollow castings. So I just drilled a 4mm hole inside the middle of each dovetail and inserted a screw from inside the stalk itself. Both castings have a small tongue extension on the base at the closed end of the dovetail. For the larger 8x50 finder I filed a small slot in the cell for the tongue. For the smaller Vixen finder I just added a thin packing piece under the dovetail instead. Though I could just as easily have filed a slot in the cell for the small tongue and may do so anyway. After tightening a couple of nuts to lock the dovetail bases onto the cell the finders were perfectly solid. The finders have added some weight at the top of the OTA so the balance point will have moved upwards slightly. The weather was far too foul to put the OTA onto the mounting to check. When the weather is kinder, in daylight, I shall take some better pictures.

Click on any image for an enlargement.

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10" f/8 Arachnophobia 4 The sequel: [Spider in a pot!]

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An update: CSK screws fitted to the shroud. By countersinking the threaded holes in the plastic, behind the brass shroud, I was able to pull the screws in quite tightly. I may redo the countersinking to sink the screws heads in further out of the light path. I lost the soldered mirror retaining tab while truing up the old shroud into a more perfect cylinder. It will have to be replaced.

The new spider fitted in place. The offset vanes only look twisted because of the distortion caused by the close camera shot. I was very careful to mark out the fixing holes accurately using a square, dividers and centre punches.

The holes were then drilled with a small drill and then opened out with a larger drill to size. This is much more accurate than trying to drill full size. There isn't usually enough room for the nose of a larger drill to start well in a centre punched hole. Boring the smaller hole first ensures that the larger drill runs true and accurately centred without wandering.

An oblique view of the new secondary mirror support in place within the cell. Even with the tension nuts only finger tight the stiffness at the hub is absolutely amazing.

The flange formed by the bottom of the cooking pot is highly resistive to radial compression forces. Ensuring that the walls of the cylinder are not pulled in. Though it is probably unnecessary I may fit packing pieces under the vane legs to avoid shifting from shocks while rolling the OTA on its wheels across the garden to its mounting.






Here I have fitted small pieces of beech packing under the feet of the spider vanes. The screws only just pinch the feet up against the packing pieces when tightened. The stiffness of the vanes themselves is thus maintained by the tension forces applied.

The distance between the centre of the tube and the outer face of the focuser sets the range limits of the minor axis of the elliptical secondary mirror. This distance is 260mm according to the tape measure. [Or about 10.2"]  According to online sources this sets the minimum secondary size as 1.3" for 0.25" diameter illuminated field. The maximum suggested is 1.69" for a 0.5" illuminated field. So I could reduce my secondary size [from the present slightly oversize 1.81" m.a.] for purely planetary and lunar work. Though I would then lose some off-axis illumination for low powers in 2" long focus eyepieces.

 http://www.loptics.com/ATM/diagonals.html

Alternatively, I could lower the focuser height by purchasing an upmarket, low profile focuser. Preferably one with slow motion focusing. A not inexpensive purchase for the optical return on the heavy investment. Or, I could reduce the diameter of the secondary cell. This automatically reduces the distance of the focal plane from the optical axis of the main tube. 

The effective secondary diameter [m.a.] is a moveable feast. To reduce the secondary size would involve a completely new secondary holder and spider. It would also involve the expense of a premium quality secondary to match my very high quality primary. The main advantage of an oversize secondary is that the edge of the secondary mirror is much less likely to be involved in high power image forming. The edge of the secondary is usually the weakest point optically because it may be turned down from heating effects during polishing. The glass expands and is then polished off. As the glass cools the edge shrinks below the rest of the optical surface.

As the magnifying power of the instrument increases, so the fully illuminated, field diameter at the focal plane  shrinks. A smaller secondary makes most sense if one never uses low powers and demand the highest potential image quality. [If the seeing conditions allow it!] In the best seeing conditions the image will be free from the deleterious effects of diffraction from using an over-large central obstruction. 

Further progress this afternoon: I soldered a new secondary mirror retaining tab. Then replaced a plywood plate with a nice bit of Tufnol for much better stiffness in clamping the secondary cage. By leaning the OTA at 45 degrees against the workbench, re- collimation with the Cheshire eyepiece is now a piece of cake. All thanks to the new spider/secondary holder. When an adjustment is made it is absolutely positive and without any "spring." Adjusting the curved spider was just too vague for my liking. I'm hoping for the sky to clear but it is mostly overcast. Now I must fit sights and/or a finder. No shortage of finders but they have dovetail base fittings. I am still unsure whether I shouldn't have finders or sights half way up the spars for easy location from the ground.

In the end I found an incredibly simple way of adding two finders without modifying or even defacing the dovetail castings. The dovetail based, stand-off stalks are hollow castings. So I just drilled a 4mm hole in the middle of each dovetail and inserted a screw from inside the stalk itself. Both castings have a small tongue extension on the base at the closed end of the dovetail. For the larger 8x50 finder I filed a small slot in the cell for the tongue. For the smaller Vixen finder I just added a thin packing piece under the dovetail instead. Though I could just as easily have filed a slot in the cell for the small tongue and may do so anyway. After tightening a couple of nuts to lock the dovetail bases onto the cell the finders were perfectly solid. The finders have added some weight at the top of the OTA so the balance point will have moved upwards slightly. The weather was far too foul to put the OTA onto the mounting to check. When the weather is kinder I shall take some better pictures.

Click on any image for an enlargement.

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Fullerscopes 6" F:6 on eBay

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Telescope, Fullerscopes 6" Newtonian | eBay

 

A 6" Fullerscopes up for auction on eBay[UK]. The seller's images were of high quality so well worth sharing here.  

This is a shorter focus model than the more usual F:8. It is amusing to think that Fullerscopes classed their F:6 as a rich field telescope. These days an F:6 is considered quite normal.

The clean, but unpainted, MkII mounting is equipped with 6" wormwheels. Their drive worms are fitted on smartly shaped alloy plates to an obviously high standard. Flexible hand drive stalks to both axes are included. Though it does lack a synchronous drive motor to the polar axis. Probably obtainable from Beacon Hill for quite a modest sum if they were told the wormwheel tooth count. 

The drives may even be Beacon Hill's own work. In which case one could even buy one of their reversible motors for the Declination axis and their variable frequency speed control box and paddle. Listed at £170 [GBP] @ 2015 prices. The motors are £40 and £49 respectively for the synchronous RA and the reversible Declination drives. The synchronous drive motor is well worth fitting to allow the observer to relax at the telescope. Without running the risk of losing the object while changing eyepieces or showing a friend the view of the Moon or a planet.

The instrument has some nice original details in the "Export" white GRP tube with tube stiffening end rings and the "slip" ring. The latter allows easy tube rotation without the tube slipping down through the felt-lined tube holding rings. This allows the eyepiece to be brought to a comfortable position regardless of where the telescope is pointed. The substantial tube rings make a complete mockery of many modern examples!

The overall appearance is very tidy indeed. Requiring no obvious restoration to bring it up to date. Many painted components from this era would normally be looking much rougher than the smart alloy castings seen here. 

There is no reason why the instrument couldn't have a synchronous drive motor fitted and be thoroughly enjoyed for another 50 years. It depends on personal taste whether such a "retro" telescope appeals. One certainly gets a very sturdy mounting and very decent optics for the asking price.



An excellent elbow finder and Telrad are useful additions.


The MkII mounting is sitting on an original and sturdy, Fullerscopes wooden tripod base. No problems reaching the eyepiece with this telescope for most users. The legs could even be replaced by longer ones for a taller user. Note the substantial axis locking knobs.

An original Fullerscopes rack & pinion  focuser is fitted. Plus a camera adapter.



Made in England by Broadhurst Clarkson and Fuller this is a traditional Newtonian  f6 Reflector equipped with manual controls in declination and right ascension. The telescope has been well used and is in excellent condition, accessories have been fitted to a professional standard. The high quality optics have yielded detail on the Martian surface as well as other planet and countless deep sky objects, see pictures.

 Glass  fibre tube dimensions  230x920mm 
 1.25” (32mm) rack & pinion focuser 
 50mm aperture Poyser finding scope with diagonal and 
 40mm Kellner eyepiece 
 Telrad Reflex sight 
 Fullerscopes MkII Equatorial Mount 
 6” drive wheels for manual tracking

 Camera adaptor to T mount

Eyepieces:

40mm Kelner (on finder)
18mm Orthoscopic (main tube)

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  Original Fullerscopes catalogue pages for the various 6" models.

  Note how they always used real people [with original bell-bottom trousers] to suggest the true scale of their instruments.

  
  
  Click on any image for an enlargement. 
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10" f/8 Arachnophobia 3 [Final cut.]

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I eventually decided that it would be much easier [and quicker] to use a cheap mitre saw rather than use the lathe. It would have taken far longer to set up properly than to make the actual cut on the vertical slide. With the workshop being so cold I didn't want to spend any more time out there than strictly necessary.

Here is the set up: Collimation nuts tightened against the spring for stability. Only a thin sliver was required to true the 45 degree face. Clamping the spider down into a vice with a G-cramp ensured stability and accuracy of cut. It is difficult to see here but the wedge is very thin at the far end.

 Here is the spider with the sawn face now at a true 45 degrees. The woodcutting blade worked well on the machining quality nylon.

The secondary adjustment allows for collimation regardless of the true angle on the face. Though this is likely to result in a larger obstruction than necessary if the holder is badly skewed to compensate for errors in the 45 degree angle.   

 This image shows the spider vane hub being reduced in the lathe to a diameter of 48mm.

The surface is now turned clean all over the entire circumference. With the vane fixing V-slots still deep enough for perfectly adequate stability.

Any burrs or rough edges are easily cleaned up with abrasive paper.

The final image is of the secondhand 50.5mm secondary support reduced to 48mm diameter. With the secondary holder cylinder further reduced to 47mm to match the elliptical secondary mirror. 

The newly-bare plastic will eventually be sprayed matt black to kill any reflections. Though this will have to wait until the weather is warmer.

I am reliably informed that this is/was a Beacon Hill spider.

I spent some time adjusting the thin brass shroud to match the secondary evenly all round. Then bored and tapped and countersunk an M4 thread for the three radial fixing screws. Then I couldn't find any suitable CSK screws. They were all cheese head.

As if to egg me on there was a brief glimpse of Jupiter, Venus and a very fine crescent Moon at dusk after a horrible day of cold, wintry and windy weather. A soon as I took the 8x42 binoculars outside the sky quickly turned strangely misty and it clouded over again.

Click on any image for an enlargement.

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10" f/8 Arachnophobia 2 [Director's cut.]

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The next stage was to turn down the 51mm secondary holder components to match my 47mm secondary minor axis. The reduction in diameter will only be about 3mm. Or roughly a 1.5mm cut below the existing surface. This will not affect the v-shaped slots in the sides of their vane mounting hub. They can easily be deepened if desired to bring the screw heads out of the light path.

The image shows the progress so far. It was [almost] freezing in my unheated shed at 33F, +1C! First I tightened the collimation screws against their central spring until everything was only just tight. I was relying entirely on the central screw which holds the secondary holder assembly to the 19mm aluminium stalk. If this screw had stripped or broken then the secondary parts would have flown off the lathe chuck. So I chose a low speed and very fine feed to reduce the impact on the screw. Fortunately there is a small spigot on the alloy stalk to locate the collimation disk. This must have taken the brunt of the forces during the cutting in the lathe. 

The entire assembly proved to be wildly eccentric when first turned in the chuck by hand. This required some careful thought and adjustment before I dared to start the lathe under power. The cutting tool wanted to take the most material away from the shortest section of the cylinder just below the 45 degree mirror mounting slope. [i.e. The curved surface facing the camera in this image.]

So I adjusted the collimation nuts again to produce a perfect witness mark along the longest section of the secondary holder cylinder. I then wound the tool out and rotated the cylinder until the shortest section was facing the cutting tool. I could now feed in the tool until I produced a very light cut. From that point on I tool only took a couple of thousands of an inch per cut using the finest feed. I kept this up until the entire cylinder was machined all over. This was achieved at about the 49mm diameter.

From that point on, every time a cut was completed I would turn the chuck by hand to bring the longest section of the secondary holder cylinder to face the tool. I would then run the tool along the cylinder without any tool adjustment. Just to ensure the witness mark was perfect each time. Had the line been broken at one end or the other I would immediately know that the assembly was moving out of perfect alignment. Fortunately it did not.

Having completed the machining of the secondary cylinder down to 47mm I moved onto the collimation disk This was also very eccentric at first but was cleanly cut all over the circumference by the time I reached 49mm diameter. I stopped at 48mm as the washers under the adjusting nuts were just beginning to strike the cutting tool. Which, by the way, was a small, diamond-shaped ceramic bit clamped into a CNC type holder. The tool cut the plastic very cleanly. Leaving a smooth surface and producing a fine and continuous thread of plastic swarf. My wife commented on the strong smell during machining but my nose was so cold I could not smell anything.

Cleaning the sharp edges left by the machining was a simple matter of running some emery paper along. The grey colour suggests PVC bar but I have no real idea. [An update: According to a reliable source the spider assembly is by Barrie Watts at Beacon Hill telescopes and the material is machinable nylon.]     

The next stage is to turn down the vane disk to a concentric 48mm diameter. Then make a new brass shroud to hold the secondary mirror gently onto the 45 degree slope of the secondary holder. My thinking is that the extra 1mm in diameter, over the 47mm m.a. of the secondary mirror itself, will help to provide a clean obstruction disk. Any screws sticking out beyond this disk would have produced their own diffraction effects. So masking them with a larger disk is likely to be more beneficial than worrying about achieving the absolute minimum obstruction.

The trend these days is to glue the secondary to a smaller mounting block. So that the secondary forms its own circular obstruction. I prefer to hold the secondary with a shroud for greater security. This will inevitably be of slightly large diameter than the mirror itself. There must also be fixing screws on the circumference of the shroud. The 48mm diameter of the collimation disk should help to mask them out of the light path. There is certainly logic to making the secondary support quite a bit smaller than the mirror itself. This means that any misalignment of the support components does not increase the size of the secondary's shadow on the primary mirror.

I checked the old shroud on the secondary holder and found the angle was completely wrong. A quick check with a 45 degree set square proved the point. So the secondary holding cylinder will have to go back in the lathe on the vertical slide to have the 45 degree slope milled true. At this point I haven't a clue whether the angle was wrong to start with or I have made it wrong by turning the cylinder down at a slight angle. Though it really isn't important at this stage as it is easily rectified.

Click on any image for an enlargement.

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10" f/8 Arachnophobia [Fear of oversized spiders!]

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The high frequency vibration in the curved secondary support suggested that it might be subtly vibrating all the time. Which would obviously spoil the view through the telescope. 

Some commentators also suggest that the curved secondary is a poor fix for reducing diffraction effects from the vanes. Instead of being concentrated in the familiar spikes the diffraction would be smeared all over the image.  

The need for a stiff, but curved spider vane requires it be rather thicker than desirable.

I decided to make a 4-vane spider instead but quickly discovered that the construction materials were difficult to find. 

Fortunately my search online for inspiration produced an advert for a secondhand spider which suited my needs perfectly for a relatively small sum. 

It was found on Astrobuysell the popular, UK, amateur astronomical equipment, small ads website. I paid for the spider and a few days later it arrived. [Thank you, John!] 

 U.K. Astronomy Buy & Sell 

It is decades since I last handled a commercial spider/secondary support and. I imagined they would still  be made of solid aluminium alloy bar. It was not the case. The spider I had bought was mostly made from a tough, engineering plastic. Only the adjustable stalk was of solid aluminium. The difference in weight from the use of this material was readily apparent. Had the entire thing been made of solid alloy it would probably have weighed twice as much.

Not that spider weight is a serious issue with most people. Provided they don't need to carry their telescope/OTA very far. When light weight is a priority everything which reduces weight is worthwhile. The elderly amateur may no longer have the physical strength to carry anything far!

The secondary mounting block and collimation plate were a smidgen over 50mm. Which suited me perfectly as I can turn these down in the lathe to  to suit my 47mm secondary. This reduction in diameter will have no effect on the mechanical needs of the spider and its adjustment. 

I plan to replace the solid 19mm alloy stalk with a tube to save some weight. 

I shall probably make another shroud to hold the secondary mirror in place. The present one is a bit short of a complete wrap. Leaving a larger gap between the meeting edges than desired. I am not a fan of the present silicone glue attachment. Unless perfectly cleaned off  the silicone can [apparently] off-gas during re-aluminizing.  

These images show my purchased spider completely assembled and with it increasingly dismantled for clarity. 

The offset vanes have the potential for reduced diffraction effects in comparison with perfectly radial vanes. Though this may be at the expense of more smearing. At least they won't allow vibration! I may make some longer collimation nuts to provide easier [i.e. no-tools] adjustment of secondary alignment. If they will not clear the vanes then I may fit wing nuts instead of plain nuts. Though another possibility is to use a socket wrench I don't fancy using one in the dark. Not with the primary necessarily exposed below where I am working on optical collimation.  

Click on any mage for an enlargement.

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10" f/8 In defence of pots and pans for OTA construction:

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A note to those who sneer at my "pots and pans" construction: My cooking pot cells have a strong, integral base. Which naturally reinforces the tubular section of the body. Making a far stiffer and stronger unit than any rolled tube of remotely similar weight of material. This is still true even when the base has a large circle cut out of it. Most ATMs [amateur telescope makers] have to add plywood or metal rings to reinforce their tubular structures. Or make them thick [and heavy] enough to be stiff in themselves. My "tubular structures" include forged alloy rings as a standard feature of the unitary construction. Which sounds like the sort of empty hype you see in adverts. But is merely a flowery description of the fixed base of the cooking pots. It may be that these "cooking pots and pans" are spun rather than forged. But the base reinforcement argument still holds. 

Meanwhile the spars, or beams, are very lightweight, hollow structures with internal, stiffening ribs. Being duplicated and firmly spaced apart by the large, alloy, channel sections adds enormous lateral and vertical stiffness. The spars easily provide the necessary stiffness required for the widely spaced primary and secondary cells to maintain optical collimation in all pointing attitudes. There is normally a serious risk of flexure with such a long OTA. [10" F:8] A wooden structure would need at least four lengths of substantial timber at the corners of a nominal tube to achieve the same degree of stiffness. Greatly adding to the overall weight. Steel is heavier, rust prone and ugly without priming and regular painting.

The entire construction of my OTA is of aluminium in low cost, familiar structures. These are easily obtained and can be replaced in the event of damage or a desire for change to suit updated components. So, instead of using plywood, with all the accurate cutting work that entails. Or, spending literally hundreds of Dollars/Pounds/Euros for a manufactured OTA and all its components: My design is readily applicable to a range of apertures and focal lengths. 

The telescope builder simply chooses the size of pot or pan to match the clear aperture required. Which is fixed by the optics to give suitable clearance to the components and a clear optical path without vignetting. The alloy beams/spars are easily cut to length with a hacksaw with a fine tooth blade. The bases of the pots can be cut out with a pad saw handle fitted with a fine tooth hacksaw blade. Or an electric stick saw with a fine metal cutting blade. Power tools are no longer as expensive as they once were thanks to [slave wage] Chinese manufacture.

A [possibly] easier but time consuming alternative is to mark the required circle. Then "chain drill" right around inside that circle with lots of small holes. A series of ever-larger drills is then run through the same holes. This will have the effect of narrowing the gaps between the holes until the pot base will eventually drop out. Leaving only a bit of filing necessary to smooth the edges of the large hole. Marking a smaller circle inside the required circle will allow a range of larger drills to be used without breaking through the desired, finished circle. The closer to each other and more accurately the first ring of small holes is drilled the better.

A centre punch [or substituted masonry nail] will ensure accuracy of drilling at the expense of rather more work. Some pans have a useful series of circles on the base from the spinning process. This will aid centring of the large hole for a pair of compasses. Taping two masonry nails together makes a handy and accurate double punch. The point of one nail rests in the first punched hole while the second marks the next with a light tap of a small hammer. A hacksaw blade can be run between the elongated, drilled holes if the cut disk should "hang by a thread" in only a few places.    

In my very simple OTA design the primary and secondary cells can be easily slid along the beams and re-clamped to match any major changes in eyepiece or camera position required at the focus. Meanwhile the entire OTA can be slid up and down, along the equatorial mounting saddle, for balance. This is simply achieved by loosening the two large wing nuts. Then re-tightening them when the desired balance point is achieved. This saves carrying the extra weight of a sliding OTA balance weight. Or having to fit one later after carrying the OTA out to the mounting.

Anything added permanently to the OTA may end up being carried for some distance. The whole point of an ultra-lightweight OTA is to avoid adding unnecessary weight! My entire OTA including the 10" primary mirror, collimation cell, focuser and spider is almost lighter than a bare, 12" diameter, cardboard tube of the same length! At 197cm/77.5" long the entire OTA presently weighs 12.7kg/28lbs. [sans Finder] This seems quite comfortable to lift by the new handle where I saved some weight by choosing a hollow stainless steel example. Rather than the usual, but heavier,  matt chrome plated brass in this common form. I have just checked back to an earlier post and the bare cardboard tube I made actually weighed 22lbs. I thought I remembered it being heavier because it was so difficult to handle! I haven't destroyed the cardboard tube because I may press it into service later.

The twin beams/spars I employed are builders [plasterers and flooring screed layers] straight edges. These nicely finished, rectangular, alloy profiles are readily available in huge range of lengths at remarkably low cost in all the Danish builders merchants I have visited so far. They are further improved from the bare, rectangular profile by rubbery/plastic end stops. Mine were black but red is an option on some makes. The end stops allow them to safely stand on end for vertical OTA storage. It also avoids anything taking up home inside.

I used some scrap 4" x 2" x 2"channel section alloy to join the two beams together. Though a number of alternative spacer options exist. Alloy tubing spacers with internal lengths of studding [screwed rod] could be easily made. Even by somebody who owns nothing more in the way of tools than a junior hacksaw.

I used furniture screws [actually hex-socket nuts] in an attractive "climate" gold finish. These have an internal thread [hence the term nut] which matches suitable electroplated zinc, galvanised or stainless steel studding. The zinc will eventually corrode so stainless steel is probably best for appearance if it is permanently exposed to dampness. Hiding the studding inside alloy pipes will protect them. Adding suitably large washers to the studding will keep them concentric within the chosen tubing. A nut on either side of each washer will ensure the studding remains perpendicular to the studding. All without so much as owning a picture of a lathe.

The length of the spacers obviously sets the distance between the beams. The further apart the beams are fixed [within reason] the stiffer the overall OTA structure. Anyone building an ultra-lightweight Dobsonian could space the spars to go on either side of matching cooking pots. They would have to be bolted directly to the cooking pots since solid spacers would obscure the aperture. Concave curved spacers cot on a rectangular cross section material fitted between spar and pot would ensure alignment and joint strength. The Dob's altitude bearings could then be attached directly to the outside of the spars. Making for a very stiff, but lightweight unit easily carried around.   

The nice thing about pots and pans is their wide availability in a huge range of sizes, finish, weights and appearance. [Not to mention price!] I sourced my primary mirror cell cooking pot from an organised camping outlet. Scouts use these pots for large quantity cooking at their camps. The material is very light, quite thin and has an attractive brushed appearance. The secondary cell pot came from a charity/thrift shop. I searched in a number of such inexpensive outlets until I found exactly what I wanted. It was of slightly heavier material than the primary cell but balanced the OTA perfectly. Had the primary cell been any heavier the OTA would have balanced too low on the twin spars. The addition of a finder is very likely to move the balance point even higher on the OTA. Possibly requiring a slightly taller pier but having no effect on the total height of the mounted telescope. The pier height is chosen for the OTA to clear the ground and no more.

Here is a YouTube video showing the remarkable skills of aluminium, metal  spinners making various utensils:

BTW: Just in case you didn't know: OTA = Optical Tube Assembly. This is the accepted term even when the tube doesn't look anything like a "normal" tube in shape. OTA is useful shorthand and a recognised term amongst astronomers for all forms of telescope. OTA is always understood to NOT include the telescope mounting. One often sees adverts for OTAs where the owner wishes to keep a desirable mounting but wants to sell only the optical assembly. An image alongside the advert will often show both the mounting and the telescope together. Hence the term "OTA only for sale" to save lots of extra, descriptive text. Conversely "Telescope for sale" is expected to mean both the OTA and the mounting are being offered together.

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10" f/8 Steady progress 2 Under the night sky.

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 Having fixed the primary mirror cell in place on the new rails I could collimate the optics with the Cheshire eyepiece. Venus and Mars were already lost to the intervening trees low in the south west. Jupiter was bright but just visible above the eastern hedge but very low. So I moved the mounting with the sack trucks to a point in the garden where I could see Jupiter above the hedge and began observing. 

As expected the planet was soft. Two strong plus one minor belt were continuously visible. With the Galilean moons expanded into tiny, spiky jewels.

I moved onto Orion, again rather low but towards the south east. The M42 nebula was well extended against a jet black sky. One of the advantages of living rurally, with largely unlit roads, is the very dark skies. The Milky Way is often etched clearly against the night sky when it is clear of cloud. Though clear skies are actually quite unusual where I live.

After looking at Orion I moved the telescope to point high overhead and browsed around  the Cassiopeia area. To reach the eyepiece I needed the full height of my builders folding stepladders! I should add an upward extension pole to the stepladder as a handle for extra safety in the dark. Or simply buy a taller and lighter, dedicated stepladder for observing. I find leaning forwards onto stepladders to be very relaxing. It is much less tiring than standing upright on the ground for hours on end. One's feet do not get so cold as when standing about on permafrost or snow.
 

Even with the drive clutches disengaged I found damping was excellent after touching the focuser. Less than a second to settle to perfectly still again. Not bad at all for such a long OTA. The curved secondary spider has a much higher natural frequency which I hope to remove. More details to follow. I still need to add some sights and a finder. So I am presently squinting along the upper edge of whichever spar is convenient. [Or inconvenient with anything more powerful than a long focus 26mm eyepiece! Even that is providing 77x with a 2000mm primary!  

I shall go out again after dinner to see if Jupiter is any sharper.  It wasn't. Not very. The planet was still very low. The secondary had misted over slightly by the time I returned. I managed to remove the dew with the warmth of my hand without actually touching the optical surface. By now the entire telescope was literally dripping with heavy dew. Though the primary was still fine and dry. With sky conditions worsening I packed up for the night. A glanced out of the window at bedtime showed Jupiter was now very high in the south east.

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10" f/8 Steady progress on the Planetary Newtonian.

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I have redesigned the primary cell support to make the alignment far more reliable. Still some small details to attend to. A strip of aluminium inside the cell will help to reduce distortion of the pot wall which forms the primary cell when the wing nut is tightened.

The mirror cell springs have also been upgraded with much stiffer examples. The mirror collimation screws now have no slack movement. The OTA now rests on three rubber door stops instead of its collimation screws. A small detail  but it keeps the screws clean and the OTA is more stable in vertical  storage.

I dragged it all outside to the mounting and took some pictures in the bright, winter sunshine. It's a shame the hedge in the background is so transparent and scruffy at this time of year. Unfortunately I had no other suitable background which didn't involve a lot more work.

The measuring pole leaning against the OTA is 230cm high to the red tape. About 7'6" in old money. This is as high as it gets but only when pointing at the Pole Star. Southerly objects will always be much more accessible from the ground.

The OTA can now be quickly dismantled via the wing nuts holding each cell to the twin spars or beams. The same goes for the OTA fixings to the 2' long [60cm] cradle on the Fullerscopes MkIV mounting. I just loosen the wing nuts, rotate the thick Tufnol clamping plates and lift the OTA away. The OTA can be very easily slid up and down in these clamps to balance the OTA longitudinally. Handy if a heavy eyepiece is inserted with a Barlow.

The new handle fitted to the primary cell is very useful. Previously the OTA tried to turn itself "upside down" whenever it was lifted. The handle makes carrying the OTA almost effortless thanks to the low balance point with the mirror in place.

The cooling fan is missing in this picture. It normally sits on the back of the cell [pot] to speed up the mirror blank temperature equalization with the night air. Though the OTA is stored in unheated accommodation to avoid large temperature differentials. That said, the trend is usually towards falling temperatures at night.

The large fan cooling holes in the cell are ideal for lifting the mirror in and out while it is resting on a heavy brass tube. The tube stands vertically as the entire cell is lowered over it. The mirror is then placed face up on the top of the tube. The complete cell can then be lifted carefully until the mirror enters the cell and rests on its three support pads. Finally, the cell can be lifted free of the brass tube and set aside with the mirror safely in place. There is not sufficient room for fingers on each side of the mirror blank. So the mirror cannot be lowered into the cell by hand.

I am sticking to a 3-point mirror support for the moment having looked at Plop and other websites for advice. I used adhesive furniture protector pads for the support points and the lightly sprung edge supports. These pads have a felt-like finish offering low friction. So the mirror can settle slightly and slide if need be. Note there are no clips over the front of the mirror to avoid unwanted diffraction effects.        
               

The upper cell shows the curved vane spider to the secondary mirror support. The secondary cell clamping wing nut is seen between the spars. Thanks to the angle profile "rails" between which it rests, there is no chance of movement relative to the spars.

The OTA innards still need to be sprayed matt black to kill all reflective surfaces near the optical path. Presently there is only sheet of thin, black foam opposite the focuser.

The overall, aluminium construction ensures lightness and [hopefully] a very long life without obvious corrosion. So many telescopes deteriorate badly over time that I was determined to avoid such problems if I could.

The vintage, Fullerscopes MkIV mounting rests on a temporary, slotted angle iron stand. This low pier is the minimum height possible without the OTA striking the ground when pointing vertically overhead east or west of the mounting. Here, the telescope is pointing south west. Where Venus and Mars are presently situated in the evening sky. They would be visible if it were not overcast and raining. 

I have yet to fit a strong plate to the top of the pier to support the MkIV base more firmly. More procrastination over a more suitable pier material has put a damper on progress. I could cast a solid concrete base with cast-in, anchor screws to hold the mounting. Because of the desperate need for mobility around the garden I would need to be able to push a sack truck underneath the cast block.

I have been rather repeating myself in discussing my long focus, 10" ultra-lightweight, planetary Newtonian. Most visitors here will not read every single post from start to finish. So they might miss the thinking behind some of the important details. 

I am now seriously considering a Hargreaves strut. This will join the tip of the declination shaft and the upper end of the OTA. This will hopefully help to kill any flexibility between the declination shaft and the very long OTA. Since the two points remain fixed, relative to each other, there is no need for the universal joints seen in some designs. The only real requirement is that there is no slop or flexibility and that the strut can be readily removed for storage of the instrument after each use.

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