Something unpleasant has happened to my C11.....

[ngwillym]

It spent most of last year stored in a large plastic case in my dome (which has a frost protection heater and dehumdifier) as the planets were too low at my site to do any meaningful observing.

I decided to re-mount it and do some lunar work. BUT on trying to check the collimation - things didn't look right.

I've attached 3 pics - one from inside focus, one  at best focus and one outside focus. (images taken on 18th Feb - seeing wasn't brilliant but even so I would have hoped for a much better star profile that what I got)

 

anyone have any ideas what's causing this - any attempt to adjust only moves the star image around the field (mirror flop) but does not improve the star profile. .

The C11 has an aftermarket electric focuser attached, (Pegasus Focuscube) but otherwise is unmodded.

 

Help 🙂

 

 

 

Edited February 25, 2021 by ngwillym
typos

[dweller25]

Check from the front of the scope very carefully for any screws touching the mirror

[ngwillym]

Thanks @dweller - I'd thought of that too  - used a flashlight to check all around the perimeter of the main mirror- but can't see anything untoward.

 

Ahhhhh, but something else does just occur to me - it might be a protruding fixing screw from the electric focuser - I'll check tomorrow

[Peter Drew]

I suspect something is amiss with the secondary alignment as the image seems to show gross collimation error.  A screw protruding into the light path would only show up as an indentation to the defocused star image.  Check that the secondary collimation screws are all positively attached to the mirror holder.  If adjusting them provides no difference it could be that the secondary has detached partially from its holder.  Do these checks with the tube horizontal, just in case!     😱 

[ngwillym]

Thanks Peter - I wasn't sure if it was just collimation error or something preventing the main mirror racking in and out. I did a daylight collimation using thus approach - Daylight Collimation (robincasady.com) and it looked pretty good - hence me suspecting something more that just a dodgy secondary. As it the Hyperstar version, its not too difficult to remove the secondary for a check-out.

Neil

[ngwillym]

All,

 

thanks for the advice.. Secondary was removed and re-seated on its pivot and the collimation screws levelled up. A quick daytime collimation, followed by a star collimation has it looking much better - not perfect, the seeing wasn't really up to it - but I now have a doughnut to work on. There's a forecast for mist/fog over next couple of days, so hopefully the seeing will be much steadier

Happy days

Neil