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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