Doughnut shaped stars

[philming]

Hi all !

I'm shooting with a GSO RC8 with a Canon DSLR (Canon) and OAG + Lacerta Mgen.

Two times in a row, I started shooting M63 that was pretty high in the sky. And both times, I got doughnut shaped stars.

I checked colimation both times with a takahashi microscope.

Focus was made with a bahtinov mask, so it should be ok as well.

Anyone knows how this can happen ? I'll post a picture tomorrow to make things clear.

Thanx for your input !

[oldpink]

A picture will be of great help
its hard to get an idea of whats wrong without seeing it
also how you took the shots (software processing etc) will help

[philming]

Hi

As promised, here's a picture.

A full frame, and a crop.

[Steve Ward]

Looks like a focus issue as much as anything ... ?

Double spikes would suggest this ... ?

[Steve Ward]

More possibilities here .... http://stargazerslounge.com/topic/147599-double-diffraction-spikes/

[Scott]

Is it possible that your focuser is slipping at all after you have reached focus with the baht mask?

[steppenwolf]

This looks very much like a classic focus issue to me too - it isn't a tracking problem as the double diffraction spikes from the spider vane are present in both axes.

[Xplode]

Spikes not being aligned and focus movement/temperature change i would guess. You also have coma in the cropped image.

[laser_jock99]

Are you using a focal reducer or field flattener? Get the CCD spacing wrong and this won't help. Some of the more critical ones need to be sub-millimetre perfect.

[philming]

OK Some news.

1 - It might have been a focus problem. I realized my Bahtinov mask wasn't straight when I put it on the opening of my RC8. It sort of felt inside ny about 10 or 15 mm.

Since my tests were made on M63, very much up on the zenith I might have had a distortion in the spikes due to that ?

2 - Comparing my GSO RC8 with the one of a friend's I realized the secondary mirror cage was two far towards the primary. I must have mounted it wrong when I cleaned my mirrors a while back.

I didnt have a chance to do any more tests yet, I'm waiting for a clear sky and some free time for this...

Regardles, thanx all for your input !

[philming]

Sorry can't edit yet.; The bahtinov was falling inside by 10-15mm on one side, not generally.

[KevinReid]

Have you tried focusing using FWHM (full width half maximum) on a suitable star? I have a Bahtinov and it is very good but recently I have used the FWHM method more. Perhaps I just prefer looking at the numbers rather than trying to judge when the horizontal line cuts the middle of the X. At least that might eliminate the possibility of your mask being slightly misaligned.