7" f/12 iStar folded refractor 32: More testing, painting and fine tuning.

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A second night of Lunar observation and occasional views of Jupiter were far more successful from a mechanical point of view. The OTA went onto the MkIII mounting smoothly enough. I wore sturdy boots for the stepladder climb with the objective assembly dangling in my hand. This aided stability without the flexure of the soft shoes I wore yesterday.

All the images in this post show the matt black, blackboard paint drying. The front plate paint smudges will wipe off with steel wool. This surface is normally hidden by the bayonet plate. It is also the surface on which the OTA rests in storage. The smudges around the focuser hole will need to be removed to allow easy focuser rotation. Which is why I left that area largely unpainted.

I started the fitting of the objective with the OTA well down on the mounting rails and pointing at the Pole Star. After the clamping battens were gently tightened, for security, I could still push the OTA upwards to find its balance point.

I don't think the Moon's sharpness was quite as crisp as last night but was still acceptable up to 150x. Last night it held up to over 200x. Jupiter was sharper and much higher though still only 20 degrees up. One Jovian moon disappeared into the limb while a small jet black shadow appeared on the opposite side. The GRS was occasionally visible but the 1st mirror was well dewed over at this point.

A quick squint through the Cheshire eyepiece, with an LED torch lighting the target, showed a series of bright spots well spread out in a line. Tomorrow I shall try proper Cheshire alignment in daylight with the OTA easily accessible on the bench.

Next morning and the Cheshire EP was very helpful in aligning the mirrors and objective to concentricity. The Cheshire's small aperture is a great aid to centering the eye in the focuser. I used bright sunlight to improve the brightness of the spots reflected back from the objectives multi-coated surfaces. The 2nd mirror collimation screws are only just reachable from the EP. [With star diagonal in place] Gently springing the collimation bars made judging the likely effect of adjusting the wing nuts very easy.

I applied some matt black, blackboard paint to all the shiny interior surfaces, collimation bars and tilt strips. I shall have to remove the optical flats from their shells to paint the shells too. My piece of cloth from the charity shop worked well at excluding even bright sunlight. A sheen on the visible side and a rougher side inwards was just what I needed.



Later I added a thin aluminium, 150mm [6"] aperture baffle at 14" from the objective. The full sized paper drawing of the light cone was again very useful for judging where and what size of baffle was possible or desirable. The thin metal sheet drops down from the inside of the framework shoulder. I used small, curved metal shears to make the hole [starting from a drilled hole] and then filed the circular hole smooth to a knife edge. No point in adding grazing incidence surfaces to the optical path just to scatter light into the eyepiece. A coat of matt black paint applied both sides of the baffle plate helped the cause. Two coats seems to be essential for an even finish. I shall use some small pieces of alloy angle to anchor the lower end of the baffle sheet to the bottom bars of the Porsa framework.

It is still possible to see a small arc of the objective through the open focuser. Many folded refractors use an extended pipe projecting forwards from the focuser. A carefully sized, aperture baffle at the open end will exclude all direct light from the objective. Which should help to increase contrast on dim objects in the presence of extraneous light or bright moonlight. Some designs use multiple baffles with oddly shaped overlapping holes. I presume these are necessary when the objective and focuser are too closely situated to exclude direct light. I deliberately separated the parallel beams to avoid this problem. Minimum OTA size was not an issue since I was not trying to squeeze the optical components into an available size of tube.

I am considering adding a collimation plate for the focuser base as it seems not to point perfectly down the optical axis. Perhaps the mirrors are not set to precisely the correct height along their tilt strips. I shall have to look at this problem when I refit the mirrors after painting their retaining shells. It should be remembered that height problems can be dialed out by tilting the mirrors but then the final, reflected beam to the focuser, from the 2nd mirror, is no longer parallel with the objective axis.

I thought up a way to hold the larger mirror blank gently but safely. I could hardly keep the gaffer tape on there forever. Particularly since I wanted to paint the shell matt black. First I slightly over-sized the plywood backing disk with an insulating tape wrap. Then I could clamp the split shell securely, low down on the shell, without putting any pressure on the blank itself. The cut of the slot remained parallel as I tightened the simple clamp made up from a strip of pallet binding. Both shells have a narrow lip on their upper rim to retain the glass blank.

The smaller 4" flat mirror is a perfect fit in its shell with a slight amount of slop. I was advised on a forum that I wrap the mirror blank edges in Teflon tape to protect them from thermal stressing by their shells. Aluminium has a very much higher coefficient of thermal expansion compared to the ULE of Zerodur blanks. The shell for the 5" mirror is unlikely to suffer from serious contraction as it is open at the slit and the smaller blank has enough freedom in its shell not to be squashed at low temperatures. Unwanted mechanical pressure could alter the incredible precision of the optically flat surfaces.

The matt black paint was still rather soft and fragile so I will need to touch up the few small marks from fitting the clamp. I should add that the backing disks on both shells are each fixed with three short wood screws. A captive, stainless steel coach screw projects form each backing disk for holding the mirrors and their retaining shells securely to the slotted, tilt strips.

I decided the focuser must already be fairly straight relative to the OTA framework. So I dismantled the 2nd mirror collimation structure to lengthen the slot upwards in its tilt strip. I also enlarged the screw holes to allow more flexibility in the collimation screw seating as the angle of the collimation triangle changed.

Another hour under the stars with the moon not as high and with thin, high cloud. Image quality had fallen again with high frequency thermal effects visible and no sign of the craterlets in Plato. Jupiter was still struggling for height and similarly afflicted. The 2nd mirror collimation screws proved impossible to reach while viewing through the focuser with the OTA mounted. So I may reverse them to put the wingnuts to the inside of the OTA. Being able to see the results of turning the 'thumbscrews' is vital to rapid alignment progress.

This was a worthwhile change. See image above. The plastic pot in the 2nd mirror's reflection is the 1st mirror cover resting on the bench. I have yet to find a pot which fits the 2nd mirror shell.

I am still unhappy with the MkIII's ability to carry the heavy OTA but am considering how best to use the MkIV. 'Plant' a permanent pier with all its limitations or modify the existing, wheeled one?

Click on any image for an enlargement.

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