7" f/12 iStar folded refractor: Proof of the pudding.

The image shows the permanent, stubby dewshield on the objective bayonet plate with its protective saucepan lid in place. The short dewshield, with its firmly fitting lid in place, offers a safe housing for storing the objective in its cell. The short dewshield also provides some dew protection and blocking of stray light when the telescope is in use. The main dewshield is a much longer 10" diameter, thin aluminium tube which slides easily over the stubby dewshield when required. The cranked, solid, stainless steel handles provide safe handling of the objective when climbing ladders to slide the bayonet plates together. I am seriously considering making a concentric [rotational] bayonet fitting so that I can fit the objective to either the straight tube or folded OTAs at will. I only stopped using the straight tube due to weight issues. Had I thought of the bayonet principle back then I need not have become involved in the folded design and expense of optically flat, folding mirrors. It was the objectives own weight which pushed the OTA well over my comfort zone. Lifting the entire thing overhead into the open rings of the equatorial mounting hurt my back.

I thought it might be interesting to test the practice against theory regarding my optical folding. So, just briefly, I pointed the 7" at the sun while mounted on the MkIV.

The brilliant light cone passed safely through the baffle without restriction and lit up the dust on the 1st [125mm, 5"] mirror with about a 1cm unlit margin all around. The 2nd [100mm, 4"] mirror uses slightly more of its full aperture but still had a generous, unlit margin. Finally the light passed unhindered through the focuser baffle tube without lighting up the baffle. All this suggests that I could have baffled the folded OTA tighter but at least I am not limiting the full aperture.

When I held a piece of abrasive paper up at the focal plane it instantly produced smoke at 60mm from the fully extended focuser. I normally use a 2" star diagonal so that uses up all of the extra focal length. I did not risk sending the full heat of the sun through the star diagonal or eyepieces as well. Nor did I leave the telescope long on the sun as I had no desire to overheat the folding mirrors with the intense beam of concentrated heat. Nor to start a fire on the matt black paint anywhere! When mirrors are used to reflect the sun into a fixed solar telescope they are only experiencing normal sunshine. 

The 2m [6'6"] long straight tube, with its alloy baffles, would be safest if I really wanted to do serious solar observing with a Herschel wedge prism. When using the folded arrangement it is all completely safe thanks to the Baader Astro-Solar 5.0 film blocking all of the heat and almost all of the light. So the folding mirrors are never put at risk. I have left the 7" folded refractor out ready for this evening as the sky is still completely clear and the wind is dropping. Jupiter and the Moon will be rising in the east later.

Click on any image for an enlargement.

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Jupiter and the full moon. 20th April.

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Having left the 7" folded refractor out of doors for several hours I started observing Jupiter before it was even getting dark. Initial impressions were of greater contrast in the two main belts than before but without any real detail. In other words they appeared darker but offered no insight into their real complexity. The Galilean moons were more rounded and differed more obviously in size than previously. 

The folded 7" refractor on the MkIV mounting taken by flash and brightened several stops.

There were fleeting moments when there was a hint of twisting and edge roughness in the belts but the entire evening passed without any real improvement as Jupiter climbed steadily higher. I tried the Fringe Killer filter and preferred the view without. Then the Yellow No8 and ND but again without any increase in detail. The odd thing was that I was using the 10mm eyepiece [and the 20mm with the Orion 2x Barlow] without loss of image quality. The image did not 'break down' at 216x as it had done on previous occasions. 

Attempts to capture afocal snaps were a complete waste of time. Jupiter was overexposed and misshapen by glare. The moons were just distorted blobs. The weakness of the OTA supporting battens may be the problem here. Touching the OTA produced flexure and caused brief vibration in the image. The 18mm, 3/4" birch plywood battens were only a temporary idea to get the OTA mounted. The top bars have to be rotatable to allow the OTA to be easily lifted on and off the MkIV's 60cm, 2' long cradle. Though there is no reason why I shouldn't beef up the lower crossbars for more stiffness.

The moon was full and blindingly bright as it rose behind the hedge. I tried a range of eyepieces once it was clear of obstruction but there was no great detail to be had. Perhaps the seeing was simply soft?  Jupiter reached just over 40 degrees altitude by 10.45pm while the Moon was considerably lower and further east by the time I packed up. The drives were working well all evening. Making centering at 216x much easier than tugging at the telescope with the clutches tightened.

I fiddled with the collimation using a torch to light the folding components but achieved little or nothing. It made no difference to the view of Jupiter. I am tempted to try some different coloured filters but have never really invested. A green, a blue or a red might be worth trying. At least I am getting telescope time and my practicing staring at fuzzy planets must good experience. Had there been fleeting moments of clarity it would have made it seem rather more worthwhile. 

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Baader Astro-Solar filter for the 7" f/12 iStar for the Mercury transit.

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Having the 7" refractor available suggests that I use it for the coming Mercury transit on the 9th of May 2016.

I already have a Baader Astro-Solar film filter for the original Celestron 6" f/8 lens stop down cap for a 112mm f/11 approx. Plus the full aperture film filter for the 90mm which really ought to be replaced by  now. So it seems a waste to stop down the 7" when it could and should be used at full aperture for highest resolution. I have plans to do far more solar observing, photography and hopefully some imaging.

Above: Bare solar filter holding alloy rim trial fitted in place over the 180mm 7" iStar objective cell. 

Sorting through my collection of pots and pans from the earlier, 7" straight tube build I found a thin, parallel sided, perforated, inner steamer pan which would allow some clearance over the objective cell. Enough, in fact, to allow a lining of thin, closed cell foam. The 'Funky' foam lining will protect the objective cell's finish during fitting and removal of the filter and ensure a snug fit which won't fall off in a gust or the telescope is accidentally tilted 'nose down.' The alloy filter rim will hopefully provide long service without stressing the special filter foil it is designed to protect.

First I cut the base of the pan away to a depth of 40mm with an electric jigsaw. This measurement ensured the sides of the rim would not strike the collimation screws but would still reach down enough to ensure a safe grip on the objective cell.

In retrospect I should have cut the 185mm hole in the base first to allow saw blade clearance on the workbench. Instead of which I had to work in short arcs over the slot between the workbench jaws. 

The 185mm hole was chosen to give a slight clearance for the light passing through the filter before the objective cell did its usual job of setting the full aperture of 180mm. Any grazing surface will scatter incident light. So it probably best not to allow such surfaces to protrude into the light path.  

The images show the basic, alloy rim after smoothing the saw cuts and giving the alloy a rub over with abrasive paper to tidy things up. Followed by a thorough wiping with a clean rag to collect all the alloy dust and swarf.

The question now is how best to use the alloy filter rim I have made from the steamer pan. The Solarfilm has to be supported without the risk of snagging on sharp edges. The usual ATM construction is a sandwich of two rings of packaging cardboard and double sided tape. The tape is to hold the film smoothly and securely without stretching or wrinkling. I don't think the film wants to be in direct contact with the alloy ring. But, there just isn't enough room for a pair of useful cardboard rings inside the narrow, front rim.

If I made a cardboard strip into a short tube I could stretch the solar filter film over that.Then add a tube of foam made from a similar strip. Both then go inside the alloy rim for security and support. It is a [sort of] a plan but the proof is in the actual making. There isn't very much clearance between the cell and the rim.

Alternatively, I could cut a narrow ring of foam to line the filter rim and cover it with double sided tape. That would protect the film from ever touching the alloy rim. Then I'd attach the solar film, followed by another ring of foam with double sided tape already applied. Hey presto! Protective sandwich completed. Except it will be quite a fiddle working in the bottom of the 40mm deep filter rim. One mistake and the expensive filter foil is wasted.

It really needs a jig which can be built into the foam/foil/foam sandwich and then the rim gently lowed over it once complete. With the strip of cell gripping 'Funky' foam then applied to the inside of the rim in a long strip. Or [rather] several shorter strips with ventilation gaps between them to allow air to escape. If the filter was made too airtight the film will balloon on fitting and suck back in on removal. Best avoided to avoid a repeated chance of damage to the solar film!

I have just ordered some new Baader Astro-Solar foil and may even come up with new fixing ideas by the time it arrives. Some sort of stiffening ring would be really useful to avoid distortion of the foam rings. In fact the stiffening ring could come first followed by the foam rings and foil sandwich.

In the end I made two identical rings out of some 3mm, 1/8" plastic sheeting I had forgotten I owned. The rings should be stiff enough to allow safe handling while applying the double sided tape. The sticky tape can then be trimmed without distorting the rings. It is important that the rings fit easily inside the alloy rim. Or the solar film filter will be pulled out of shape when it is fitted. A relaxed smoothness without tension is desired for best optical performance. This is the advice of Baader themselves, in their printed instructions. Where it is recommend the prepared [sticky] rings are dropped gently onto the film while it is lying on a flat surface. Adding the second ring, to ensure it is concentric with the first, might be best with the first ring already seated in the alloy rim. Fingerprints on the solar filter film must also be avoided. I note that Baader now include thin paper protection for the film. They advise that the film be cut with scissors only while protected by paper on both sides. I must admit to being disappointed by my first sample of Baader film because it had large creases and actual folds caused during manufacturing. This was purchased some years ago. Whether these affected my views of the sun remain unknown.

I used sharp scissors with an angled bias to trim the outside of the double sided tape once firmly applied to the plastic rings. [With the protective top surface tape left in place to allow easier handling.] My careful trimming was to ensure the rings dropped easily to the bottom of the alloy filter rim without any protruding tape foam.

Cutting the inside circle was far more difficult and I used a pair of curved scissors with a final [rather rough] trim with a small bladed, craft knife. The foam backing of the tape does not respond well even to a gentle sawing action with a brand new blade. The foam is soft and sticky but the slightly untidy finish should be be invisible once in place inside the alloy rim. Rubbing around the rings with finger and thumb helped to smooth the tape foam and I carefully removed any straggling pieces which might stick to the active area of the solar foil. The width of the plastic rings is much narrower than the usual filters so even and firm adhesion of the solar film to the double sided tape is vital to the finished solar filter's longevity.

I managed to drop the first ring safely onto the solar foil but it still wrinkled. Adding the second ring to achieve concentricity with the first was an absolute nightmare! I had to carefully peel the 2nd ring off again, trim the foil closely around the 1st ring and then use the alloy rim to get one ring to overlap the other. The result is not remotely pretty but it seems to have worked. I added a few strips of double sided tape to get the rim to fit snugly over the objective cell and to hold the rings safely in place. Of course the weather is not going to cooperate. Gales, overcast and rain forecast for all afternoon! Peering through the filter it was only just possible to see the sun as a dim blob behind the uniform layer of clouds.

Click on any image for an enlargement.

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90mm astro telescope for birdwatching?

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A 90mm f/11 refractor makes a decent spotting scope at the expense of greater length and weight than the usual specialist instrument. At 7lbs with Vixen, closed rings and dovetail it is not ideal for carrying far but many 'birders' arrive in their car and just set up their tripod mounted telescope nearby. 

A 45 degree Amici prism 'diagonal' corrects the inverted and L-R reversed image. An astro star diagonal can only invert the normally inverted image but cannot manage the left to right reversal.  

This particular Amici "terrestial" prism is from Baader with a larger claimed clear aperture. It has a bronze compression sleeve to avoid marring a standard 1.25" eyepiece barrel. It is an attractive and solidly made piece of equipment which seems not to damage the image up to at least 50x. Which is the highest magnification I have tried so far with my familiar no-name 20mm Plossl and plastic, detergent bottle top, camera adapter. See below for visual magnifications. 

Here is the Vixen 90M 90/1000mm on the Bogen video tripod with original, rise and fall, pan and tilt head. The tripod was bought secondhand decades ago and is quite firm provided all the head adjustments are well tightened. 

I suppose a purist birder would paint the main tube and dewshield green or black or even camouflage. Or even drape a camouflage cloth over the entire telescope. Padded camouflage case, anybody?

Though all the birds will ever see is a distant and very dark lens hidden deep within a blackened dewshield. Not that one should ever underestimate the remarkable visual acuity of birds. The person standing behind the telescope is likely to present are far larger target for nervous creatures of all kinds.

A 32mm EP in the Vixen produces a 31x90mm with options to push the magnification to ridiculous levels if needed. This is quite a decent telescope for birding on lakes, wetlands and bigger ponds. The image is bright and razor sharp with incredible contrast and detail. At 45 meters I could see every detail of a Greenfinch's plumage as he swayed back and forth on some nearby larches in a gale.

At 80 meters I could easily see the detail in the threads of seeds hanging from a neighbour's tree. At 135 meters I could easily see the texture on birch catkins and the detail on the bark. A fly would have been just as easily seen if it was daft enough to venture out in a gale in falling rain in the early evening. I pushed the power up to 100x, with the 10mm EP, but this was the limit under such a heavy overcast. I shall try again in bright sunshine to see how far the Vixen holds up for terrestrial use.

I took a couple of afocal 'snaps' through the 20mm Plossl to give an idea of image quality with both the Amici prism and "straight through." 

I have posted the un-retouched images here. [With the inverted one re-inverted in PhotoFiltre.] It might have helped if both images were of the same tree but I assure you they were at exactly the same distance.

Which is the laterally correct and upright and which was inverted and reversed should be obvious from the date and time stamp.

Remember that these images have a nominal magnification of 1000/20mm  = 50x. The view seen directly through the eyepiece is of much better quality. Camera shake may have something to do with this but the eye is very forgiving compared with a camera.

Perhaps I should try lower powers more in keeping with typical spotting scope powers? Though I would need a different [bottle top] adapter to manage the slightly larger body of the Meade 4000 32mm Plossl for ~31x. The rest of the 4000 range actually seem to be slightly smaller in body diameter than the no-name 20mm Plossl EP.

The conditions were heavily overcast and very windy but dry. The trees imaged are just over 600 meters/yards away and visibly moving in the wind. Shutter speeds on the Canon Ixus 117 HS, short zoom, compact digital camera lay in the range of 1/300-500th which is likely to freeze any movement.

Don't ask me to work out the true f/ratio or equivalent focal length of an afocal telescope/ digital camera system. These will both will depend on the camera's imaging sensor size. Picasa tells me that I used no zoom so the nominal "film" lens focal length is shown as 30mm. The human eye has an equivalent focal length [in camera terms only] of 50mm. So, perhaps I should reduce the 50x nominal telescope magnification by 3/5? Or increase the zoom to 50mm equivalent?  The former reduces magnification to a [very nominal] 30x. Or 50x with modest zoom. You do the maths. ;-)

I can't see much [any] difference in image quality from this simple image capture test.The light may even have changed slightly between taking images. Terrestrial seeing conditions are often inferior to astronomical observation due to thermal currents rising from the sun-heated ground over the entire length of the view.

Click on any image for an enlargement.

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17th April 2016 Spring cleaning.

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While trying to make room in the shed for the folded refractor and 10" f/8 rebuild I decided to check my assorted achromat refractors. These all stand on their dewshields in storage which means they collect dust inside from leakage around the focuser drawtubes. The no-name 60mm x 700mm, Bresser Skylux 70mm x 700mm and Vixen 90M 90mm x 1000mm all badly needed the backs of their objectives cleaning.

The DIY 5" f/15, in the spiral steel tube, was in rather a poorly state with thick fungus growing on the plywood baffles. So I removed the objective and focuser back plate and slipped the baffle tree out. I now have a plain steel tube for the 5" of exactly the same size to replace the spiral tube. It needs a new objective cell backplate because the original plywood ring has delaminated. The lens also needed cleaning.

This image shows the relative scale of objectives and dewshields. Photography and bright sunshine makes things look worse than they are!

Next up is the Celestron 6" f/8. The lens needs cleaning on the back. Fortunately I can unscrew the lens cell to avoid removing the glass elements. The same with the Vixen which required Prussic loops and opposed battens to loosen the [very firmly] screwed cell from the main tube. Trying to drop the glass elements out of a fixed cell is really asking for trouble. Or chips at least.

All of these refractors were bought secondhand except for the Bresser which was incredibly cheap on special offer, with mounting and tripod, from Lidl supermarket. A check through the empty focuser of the Bresser showed the black plastic drawtube had never been blackened inside. So out came an old toothbrush and I quickly painted the inside with blackboard paint. The toothbrush reaches in easily to such small [1.25"] tubes and leaves concentric rings when the tube is spun against the toothbrush bristles. These act as tiny screw-like baffles and help to kill glancing reflections.

Leaving the bare focuser out in the wind and sunshine soon dried the paint to a flat, matt black. The drawtube is baffled but it was easy to see the high shine surrounding the baffle before it was painted. Shine kills contrast by reflecting stray light into the eyepiece. The difference is very obvious with a 'milky' view in daylight.

The plan was to mount these various refractors as guide-scopes or finders on the bigger instruments but they added weight and vulnerability during carriage. Each produces decent enough images so they really ought to get more use. The problem is that having a larger instrument available spoils the desire to look though anything smaller.

I now have an Amici prism 45 degree, terrestrial diagonal which should mean the refractors getting more use for distant bird watching. At which they excel but at the cost of greater length and weight [7lbs with Vixen rings and dovetail] than the very expensive, but ultra-compact, short focus, lightweight, prismatic, spotting scopes.

Sitting on my sturdy, Bogen, video tripod I have used the Vixen 90 for both eclipses and transits. I plan to use the Vixen 90 on the MkIII Fullerscopes mounting for the Mercury transit on the 9th of May 2016 [in Europe] using a DIY, Baader Solarfilm filter. Or I could mount the 5" Solarfilm filter on the folded 7" refractor somehow. Or use it, as intended, on the 6" f/8 objective stop. Or, I could just sit indoors watching the rain pouring down and feeling very sorry for myself.

Click on any image for an enlargement.

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10" f/8 Planetary Newtonian: OTA Rebuild 2.

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Serious, overnight doubts about spars and mirror torque around the beams has suggested I examine the cardboard tubes, plywood rings and well separated, alloy beams design. The present, single spar does not lend itself readily to Dobsonian mounting. The center of gravity is in mid air above the beam making altitude bearing attachment a difficult design exercise. Whatever I choose to add to the beam would add weight which has to be lifted and carried out to the mounting. I can no longer underestimate the importance of reduced OTA weight after suffering an agonizing back injury just from lifting my telescopes!

Separating the two or four beams adds enormously to the rigidity of the OTA. Altitude bearings are readily attached with the ability [via clamping] to slide up and down for precise OTA balance and alignment. The cardboard tubing is thermally neutral compared with alloy pots. The beams are not close to the light path in this design and have low thermal mass and excellent exposure to the air.

Though bulkier to mount, the OTA should be much easier to carry than the single spar type. Which always wanted to invert itself so that the cells were pointing straight downwards! Carrying the spar OTA was a major chore and very risky when negotiating doors. It was not only very long but had seriously overhanging pots at each end.

Beams surrounding tubes and rings will have a better defined overall dimension and be balanced in all planes. The downside is adding the weight of two more beams. Though weight savings will occur with removal of all the heavy, channel sections.

The image shows a mock-up to determine OTA stiffness using only two side rails. Rather than waste birch plywood I used the drilled rings from the earlier [failed] dowel OTA experiment. I simply cut diametrically opposed  slots for the beams in the widths of the four rings. The interesting point about this arrangement is the opposed leverage against the two rings ensures rigidity. The rings cannot twist on the beams provided the cells are firmly constructed [glued] as one unit.

Using just two side rails for a Dobsonian mounting would probably suffice. The altitude bearings could easily be clamped to the side rails to allow longitudinal balancing of the OTA. Even relying only on friction between the beams and slots and rings/tubes the structure already seems very stiff compared with the ease of twisting the pots around the original duplex spar.

If I proceeded with this design and cut new 12mm (1/2") birch plywood rings I would prefer not to drill holes in the alloy beams. Given my habit of constant updates and improvements some means of clamping would probably be best. The alloy beams are a commercial product and readily available but rather too expensive to spoil them cosmetically by drilling and screwing before the design is finalized. Opposed wedges could be inserted behind the beams with a length of studding pulling the wedges together to jam the beam against the cardboard tubes. The plywood rings would be glued to the cardboard tubes by then to avoid the whole thing falling apart during transport.

I am still trying to decide whether a central 'cell' similar to the other two, to support the altitude bearings would be beneficial [for stiffness] or merely add more weight. The balance point will almost certainly have changed relative to the single spar/pots design.

Attempts to distort the simple arrangement of two beams and two cells showed that it was immensely stiff in the vertical plane but could be slightly "lozenged" laterally. Whether this would alter the optical path once lateral pressure has been removed [for pointing at an object] is an unknown. The leverage applied for pointing, or nudging the tube for tracking, would be roughly halved by the midway position of the altitude bearings. Slightly increased stiffness is likely to be gained by the need for lateral location from the rocker box. 

Adding further beams, at 90 degrees to the other two, would provide an incredibly stiff OTA in all planes, but it would obviously be heavier. In both weight and appearance.  There is something rather special about an interesting and/or slender design. Though all four beams could be fitted with altitude bearings to allow the OTA to be rotated to bring the eyepiece to the most comfortable position.

I'm not sure I would try to mount the two beam OTA equatorially because of the degree of overhang. Certain orientations might also throw diagonal loads onto the parallel beam geometry. No stiffening can be applied which would intrude into the light path. Otherwise I could just add some diagonal wires for triangulation. The four beam design would obviously be far superior in this respect for equatorial mounting. Having an OTA which could be instantly mounted either equatorially or as an altazimuth Dobsonian would/might be an attractive combination.

 

Click on any image for an enlargement.

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10" f/8 Planetary Newtonian: OTA rebuild.

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I decided that being close to completion the folded refractor should have some direct competition. I wanted another [useful] telescope to confirm the seeing conditions and to make comparisons between the two instrument types.

The problem with the 10" f/8 reflector was flexure between the duplex spar/beam and the lower pot which contained the primary mirror in its collimation cell. I had used heavy sections of alloy channel to space the twin beams but only thin angle to support and align the cell 'pots.'

The spar design on a slightly taller pier is shown here. While it was simple to fix to the equatorial mounting it was unsuitable for Dobsonian mounting. For scale, the spar/beam is 2m or 6'6" long.

Today I took down the OTA from the shed roof space where it had been stored away from potential damage. I quickly removed the channel sections by removing the screws and socket head furniture nuts. Then I fitted another 2meter [6'6"] length of builders straight edge section between the two original beam sections. Trapping the new section with short lengths of studding and furniture socket head nuts produced an even stiffer beam than before. I have the equivalent of a simple 'plank' OTA but immensely stiff without having added a lot of weight.

The problem still exists that I had used rather flimsy channel sections to support the primary and secondary pots. These could be replaced with heavier angle sections. Which will not flex so easily when the pots are firmly clamped to the beam.

The alternative is to cut some curved strips to match and support the pots directly. These strips could be made of 18mm [3/4"] plywood on the bandsaw or using a circle cutting jig on the router.. Several strips will help to support the alloy pots without distortion. The relatively flimsy, angle sections would then become more cosmetic that structural. It is important that the lower and heavier pot [with primary mirror] does not flex relative to the beam.

Previously, this flexure tended to throw the optical alignment out as the OTA was moved to the east or west of the pier on the MkIV equatorial mounting. Having to re-collimate the OTA every time it was moved to a new celestial object was an exercise in futility and frustration! Which is why I became interested in building a refractor instead while I pondered the reflector design.

I think it would be best to have the lower pot easily removable to save my having to carry its 13+lbs weight [with mirror] out to the mounting. The pot provides excellent protection for the primary mirror and has a safety lid/plug lined with genuine chamois. The mirror could then be housed safely in unheated accommodation to avoid long waits for acclimation to the outside temperature. The plywood curved strips should provide a repeatable, location bed for the pot requiring very little collimation after fitting.

It is a long walk and/or climb from the Cheshire eyepiece to the mirror collimation screws on a 10" f/8! Hence my sudden desire to build something more user-friendly. Only to find a 7" f/12 refractor is a huge and heavy beast used in a straight line. Folding only added considerable expense and complexity. While the finished weight actually increased! Though the easily removable objective is a huge help in reducing the weight needed to be carried it is an added chore. If I had my time over again I would simply have made a circular bayonet system for the straight tube! Hindsight is always so perfect. 

 

Click on any image for an enlargement.

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7" f/12 Istar folded refractor 35: Focuser light baffle extension:

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I am seeing rather a  lot of stray light. So, perhaps, a baffled, focuser extender tube ought to be seriously considered as the next important step. There followed the usual optimistic but fruitless search of all the metal and plastic tubing I had saved for a rainy day. Various pots and containers were tried without success.

Finally I found a piece of cardboard poster tube with push-in plastic end caps. The image above shows the extension baffle for the focuser. It fits snugly inside the Vixen focuser base plate and through the clamping ring for focuser rotation on the OTA back plate. A few layers of electricians insulating tape ensured a nice tight fit to avoid it falling out.

I used a jewellers saw to cut a 60mm aperture in one of the plastic end caps for the open end of the light baffle extension. Its required diameter was checked on the full sized, paper, light cone drawing. A quick and thick coat of blackboard paint has been applied to the extension tube and its baffle to finish the job. As soon as the paint is dry I can push it into the re-mounted focuser. The advantage of cardboard tubing is its thermal neutrality and relatively light weight. Hopefully the extension tube and its baffle will now reduce stray light from reaching the focuser.

The baffled extension tube should not be made too long [nor too large] in case it intercepts the light travelling between the folding mirrors. Not that this is very likely with my deliberately deep design to avoid off-axis, stray light from travelling straight from the objective into the focuser. Extraneous light falling on the folding mirrors could also light up the field of view. The black, cloth shroud has not yet been tried after dark. Jupiter's glow could be seen well outside the field of view. Hence the attempt to baffle the focuser more thoroughly. A smaller tube fitting the focuser draw tube would not have allowed a baffle without cutting off some valuable light.

The extension tube seemed to work. It became more difficult to locate an off-axis object by its stray light. The flash shows up all the dust on the 1st folding mirror but has no effect on image quality. It is interesting how the flash lights up the mat black paint. It is obviously not completely matt nor completely light absorbing.

After leaving the entire [already cool] OTA set up on the MkIV for an hour to temperature adapt I spent a couple of hours looking at the Moon and Jupiter. It steadily clouded over from the south.

Thanks to Stellarium I had a chance to see the ISS pass over at ~21.33pm [CET] The space station gradually brightened as it neared the south but the patchy cloud obscured it at times. On its flypast the ISS passed midway between Orion's belt and the two bright upper stars. [Betelgeuse and Bellatrix] ISS altitude was only about 20 degrees according to Stellarium.

Then it was back to the Moon and Jupiter using the 15mm and 12.4mm EPs for 144x and 174x. Again the 10mm for 216x was too much for the thermally agitated seeing conditions. There were rapid thermal movements [almost like occasional shaking] on the Moon's image. Jupiter appeared to be boiling at first. Seeing on the Moon was better earlier on but became steadily worse as it sank towards the local trees and through increasing cloud. It started out at only 32 degrees high.

Jupiter improved very slowly but was never sharp even at 40 degrees altitude. The two main belts were joined by two fleetingly seen belts nearer the poles. What looked like a small star hovered on the planet's shoulder and gradually drew away from it. This proved to be Callisto from checking Stellarium again. Since I was not getting very sharp images I inserted the Cheshire eyepiece but there was very reasonable optical alignment and objective collimation. My snaps of the crescent moon with the 20mm eyepiece were hardly worth the effort. As the sky became progressively cloudier I finally packed up at 22.40pm. Only for the cloud to clear again later. The incoming weather front had obviously upset the seeing conditions.

I decided that being close to completion the folded refractor should have some direct competition. The problem with the 10" f/8 reflector was flexure between the duplex spar/beam and the lower pot which contained the primary mirror in its collimation cell. I had used heavy section of alloy channel to space the twin beams.

Today I took down the OTA from the shed roof space and removed the channel sections. Then I fitted another 2meter 6'6"] length of builders straight edge section between the two original beams. Trapping the new section with short lengths of studding and furniture socket head nuts produced an even stiffer beam than before. I have the equivalent of a simple 'plank' OTA but immensely stiff without great weight.

I had used rather flimsy channel sections to support the primary and secondary pots. These will be removed, to be replaced with heavier sections, which will not flex so easily when the pots are firmly clamped to the beam. The alternative is to cut some curved strips to match and support the pots directly. These strips could be made of 18mm [3/4"] plywood on the bandsaw.

The relatively flimsy, angle sections would then become more cosmetic that structural. It is important that the lower and heavy pot [with primary mirror] does not flex relative to the beam. This tended to throw the optical alignment out as the OTA was moved to the east or west of the pier on the MkIV equatorial mounting. Having to re-collimate the OTA every time it was moved to a new object was an exercise in futility! Which is why I became interested in building a refractor.

I think it would be best to have the lower pot easily removable to save my having to carry its 13+lbs weight [with mirror] out to the mounting. The pot provided excellent protection for the primary mirror and has a lid/plug lined with genuine chamois. The mirror could then be housed safely in unheated accommodation to avoid long weights for acclimation to the outside temperature. The plywood curved strips should provide a repeatable bed for the pot requiring very little collimation.

Images of these changes to follow.

Click on any image for an enlargement. 
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