I seem to be having trouble getting a decent image lately (9.25 SCT. CGEM mount, Canon 550d). The attachment show the problem I am having with stars. (Don't worry about the quality - I haven't processed it much) I know SCTs don't give stars as sharp as a good refractor - but I rarely used to get stars like this now I get it every time.
I use a Bhatinov mask to focus - and I am sure I get this quite close.
It's not mirror flop. Collimation is quite good.
I suspect it has something to do with seeing conditions? It is cloudy most of the time with just a few clear "gaps" (1 night) - perhaps this problem is caused by disturbances?
The problem is that the stars are either a little circle or a star with a circle around it.
Does anyone know what causes this and how to fix it? I used to use a CG-5 mount now I have a CGEM which tracks better - I haven't made any other changes.
Hi Tony,
That's a strange look you have there, I recently had been trying to do some imaging and by the way I have a similar setup to you and while trying to focus my guide scope and camera was getting a similar sort of halo around the stars in the PHD window, not as defined a circle as you have there but it turned out it was just the humidity level was so high, I mean 100% humidity according to the weather website for our location.
So my stars looked to be very soft on focus but no ring like that.
Maybe someone else has seen something that more resembles it.
Good luck sorting it out
I now suspect it may be caused by dust/grime on the corrector plate - but I don't know if this causes this problem. Anyway I will clean it. It doesn't look that bad - until you look at it closely.
I have had issues with atmospheric moisture - but I think that usually causes the stars to be big and fuzzy?
I have never seen anything like this before. I know that this probably doesn't help much, but I've done just about everything wrong whilst taking images and I've never seen this before.
The brighter stars look a bit like the image I use for collimation, i.e. just out of focus, though I don't think I have ever gotten such a bright central spot.
What sort of camera are you using? Have you got another, even a guide camera to take an image?
First things first you have to isolate the cause, is it optical or is it an artefact of the imaging system? Chuck an eyepiece in, put the biggest magnification eyepiece in, with a barlow if you've got one and just defocus the image. Do the stars look like you've imaged? If so it's a simple focus problem. If not, then maybe it's an out of focus reflection off some part of the imaging system.
Don't trust focus masks for wide field imaging, there great for high magnification work, but for wide field, isolate a star, put the camera in focus mode and move your focuser back and forth until the star image is the smallest possible, or if you've got software that does FWHM calculations, use that.
When the weather clears next in Melbourne (I'm about to spend four weeks OS, that should help), report back on the results.
I've had a similar effect when there has been a tiny amount of condensation and dust on the corrector plate. Try cleaning it and use a long dewshield and see if the problem goes away. Check that the CCD window is nice and clean whilst you have the cleaning gear out.
Thanks for your help everyone - but I still am not sure how to fix it.
I cleaned the corrector plate and took some pictures tonight - same result.
Seeing conditions weren't that good - but I have taken shots through light clouds/moisture before and the result didn't look like this (stars were fuzzy).
I didn't have this problem same time last year - now I have it every time.
Maybe it is as simple as focus - but surely I would get the focus close enough occasionally? Also the stars with the dot in the middle and the circle around it isn't what an out of focus star looks like? The camera sensor is clean, the primary and secondary mirror look clean.
Maybe it is just the weather? - but I guess other people in Victoria with SCTs aren't having the same problem. If the secondary mirror isn't in the correct rotational position - could this cause this problem (I doubt it ... but I'm getting desperate)
for troubleshooting take everything out of the imaging path that isn't absolutely necessary. take test photos of stars only and put back stuff one by one.
if the "feature" reappears at any stage you know where to look closer.
if it doesn't reappear then yay!
Thanks once again for your replies - everything helps to learn more.
I only have one camera - the sensor looks very clean, no smudges.
The seeing conditions have been poor - I can hardly remember when there were 2 consecutive clear nights! Possibly was high level (virtually invisible) clouds. I have taken photos in moisture ridden skies - usually the stars get big and fuzzy? I haven't seen a deep blue sky for a long time - always a slight haze - but I do get some half decent images - but nothing like most of you guys.
I looked more closely at the individual subs - I can't see this ring affect on them. However I did notice that a lot of the images had a lot of elongation from drift. My mount has (or rather had) a lot of play in the RA gear - I have now tightened it. This possibly gave poor polar alignment (the celestron all star polar alignment is usually very good - I usually get less than 1 arcminute drift/hour. I know you probably get better - but I have to do a full set up every time.) Last night the drift was more like 1 arcminute/4 minutes which is hopeless without guiding.
Possibly (?)- deepskystacker caused these star rings by trying to stack/process poor images - probably not, but at least now I have removed this. I also uninstalled and re-downloaded DSS - just in case there was a bug, also unlikely but I have to eliminate as many things as I can think of.
Next time I will concentrate on manual focusing - maybe my bhatinov mask is giving an inaccurate diffraction pattern? Also, now I have tightened the gear I will get the autoguider back in action.
I'll keep posting until I resolve the problem - maybe what I learn will help others.
I looked more closely at the individual subs - I can't see this ring affect on them.
Quote:
However I did notice that a lot of the images had a lot of elongation from drift.
I had misunderstood that we were looking at a single frame in your first post.
what stacking methods in the Result tab and Light tab do you choose in DSS?
I produce "a lot of elongation" (which is quite a relative term), too, but with
- "Mosaic" in the 'Result' tab
- and "median" in the 'Light' tab
- and an "only the best 80%" setting, DSS doesn't produce those circle stars in my otherwise very humble results.
Yes it is a stack - about 90% of 80x 30 second subs. I used to get reasonable results with this set-up. I have a guidescope but I had trouble with it recently (probably due to excessive play in the RA gears). Also the attached picture is a crop.
I reloaded dss in case there was a bug. I also deleted the worst 40 subs and restacked @ 50% so only the best 20 subs were used. The stars still looked like rings.
Next clear night (which seem rare) I'll experiment with the position of the centre spike on the bhatinov pattern and see if the centre position is the best focus or not. I'll also try some visual focusing. Eventually I'll eliminate all controllable factors - can't do much about the seeing conditions though.
Hopefully I'm getting closer to a solution (and learning more). How do I attach bigger pictures - the limiting size is 200kb?
I don't think the settings have changed, these are the main ones:
Standard Mode (no drizzle)
Light: Kappa-Sigma clipping, Kappa 2.00, iterations 5.
Dark: Median, Dark Optimization
Flat: Median
Automatic alignment
Intermediate Files: FITS files
Cosmetic: Detect and clean Hot pixels: 3px, 50%
Cold Pixels: 2px,50%
I think these are the default settings, I actually don't know what the optimum settings are - but if someone can suggest better settings I'll try them.
when you inspected the single light frames, did you enhance exposure/luminance to see what is actually captured?
just thinking that, maybe, the single frames in fact do show the circle but you just didn't notice them...
I haven't solved the problem yet - still not sure if I'm even getting closer.
Took a photo last night - same result. I took a closer look at some of my past pictures - they actually do have the same problem but not as bad. The tracking (unguided) was near perfect - virtually 0 drift just PE. So at least tightening the RA gears has fixed the polar alignment issue.
I also took a closer look at last nights subs - I adjusted exposure etc. but I couldn't see any sign of the circles around the stars. If this is correct then it must be processing? (but I still haven't ruled out focusing or weather). If it is weather then other people in Melbourne with SCTs would have the same issue?
The green selections in DSS recommended settings are "Use median combination method" - for dark files and flat files (I don't use dark flats or offset bias). and "Use RGB background calibration".
I have the trial version of nebulosity - I'll do some stacking with that and see if it is better, I also have AIP but I haven't used the stacking in this - it seems more complicated.
I use an unmodded canon 550d - attached directly with an adapter/t-ring, no filters or reducer.