Michael A. Phillips' Astro Blog: Akule's Astigmatism

Sunday, October 30, 2022

Akule's Astigmatism

As the planets began to sink lower in declination and altitude the opportunities to observe and get high resolution photographs became more challenging. In hind sight it was easy to blame poor seeing and weather on increasingly poor results. A few key people in my life pointed out that things didn't look as sharp as before and I had no reason to blame anything other than the weather. When someone with similar equipment really spent time with me, comparing our results it was only then did I think to dig deeply to my issue.

Turn to Cloudy Nights and this long post - https://www.cloudynights.com/topic/842366-defocused-star-test-questions/

After back and forth I had a good guess it was astigmatism based on some star tests showing a cross shape.

I formed a game plan to isolate and break down to the lowest common denominator. I researched the links that the great folks on CloudyNights shared. I took the cooling unit out almost immediately for two reasons. First reason being I only have 2 of 3 working TEC Peltiers, meaning the cooling is already imbalanced. Secondly I had automated the whole thing - https://astromaphilli14.blogspot.com/2022/09/automated-telescope-cooling-with.html so that was disappointing. After these fixes didn't help I became worried that my mirror cell(s) were in need of replacing but I planned the following:

After reading this article in particular - http://www.loptics.com/articles/starshape/starshape.html, "Fifth condition: For this article, we are assuming that your mirror cell is properly designed and set up, including the edge support of the mirror. If your optics are glued to the cell, then you cannot easily rotate them to help rule out some possible causes of non-round stars. Gluing a mirror (other than a small secondary) to a cell made of something other than glass will often cause it to warp as the cell shrinks by a different amount and pulls on the mirror, creating optical problems. I do not recommend gluing large mirrors to their cells/holders, as it will often cause non-round stars. If your primary mirror and or large secondary mirror is glued to something, I recommend finding a way to mount it without glue, and then coming back to this article if problems remain."

Guess what I did? I glued it.

Still I planed to break down my issue into the following steps.

Hand from bottom, straight up from ground towards zenith, passing towards center of OTA / Secondary
Hand from EAST, moving west on compass rose

Defocus and use hand to determine angle of OTA relative to ground, in which I accidentally made a Schlieren experiment at home! https://imgur.com/gallery/LDNZpj5

Star test at night, STRAIGHT UP!! This will help isolate the primary and cell as sources of gravity stresses
Near Jupiter DEC/ALT near zeinth; note angle with star tests
Near Jupiter DEC/ALT near tree tops; note angle with star tests

At step three I noticed that my star was very round which to me meant some stress ON the mirror, not the mirror itself. I was convinced it was the mirror cell or the glue holding it to the cell.

I started the process of discovering how to fix the cell and what options I had. It does NOT seem that the ATM (Amateur Telescope Making) market for mirror cells is very well populated. I started to think about how I could make my own modifications to the existing cell I had. I took the whole assembly out to take some measurements. Upon disassembly I noticed some metal on metal rubbing of parts such as here:

This is similar to a pinching problem I had long ago and essentially the design of the whole area pictured above, from the length of the screws pressing against the mirror back to the white plastic spacers needed improvements. It just so happens that in my searching for some washers or spacers did I find that I already had the perfect tool for this job, hard Rubber screw bumpers!