I've been using my 20" skywatcher dob sparingly over the last few months due to the rather crappy Brisbane weather, but I've noticed some odd and fairly pronounced coma becoming evident when the scope is pointing close to zenith. I normally collimate it at around 45 degrees, and it holds collimation well all night, apart from the coma, which disappears again when the scope moves out of the vertical. Before I start ginning about with it, does anybody know what might be causing this? I'm assuming it'll have something to do with the primary seating, but any other theories would be appreciated.
Thanks Nick, you're probably right - the stars looked like the attached sketch. Odd (to me at least) that it's being asymmetrically stressed when lying virtually flat - something must be amiss somewhere. Thanks again.
What kind of material "glides" are the contact points the mirror sits on?
Mine originally had felt glides but they had a tendency to grip too much, I am currently trialling UHMWPE ones though plan to make some POM ones in the future.
Have you checked that the cell pivots are working and that none have hit the limit of their movement.
Patrick at the zenith any thin profile astro mirror larger than 6” will sag under its own weight if the forces supporting it are unequal or unequally distributed (the whole point of a multipoint mirror cell), causing the image to be deformed.
For a thin 20” solving this adequately is not a trivial matter and as Kunama suggested I’d be taking a very close look at the mirror supports.
Thanks guys - problem solved I hope. Apart from the three spring-loaded adjusting bolts and three locking bolts, the main mirror cell support has a type of central screw-on collar which was loose, basically allowing about 5mm of tilt and free rotation of the whole mirror. I appreciate the responses - cheers.
