Do you know what is wrong with my image capture?

astrob · Aug 15, 2025, 08:23 PM

I've been troubleshooting my imaging on the Lobster Claw Nebula over a few sessions now. I have an artifact that I'm stumped as the the problem.

1. Collimation has be verified to be good
2. Tilt has been verified to be little or none
3. Focus it good and consistent with U-curve autofocus procedure.

Attached is the worst image of the group that I kept - i.e. 80% star shape of the best image.

It shows an elongation somewhere between the DEC and RA axis - looks about 45 degrees or more. This elongation matches after meridian flip as well. I also see a little chromatic aberration which looks to be part of the elongation but I'm not sure. I think this artifact is worse at 5 min exposure - this image is at 3 min exposure.

Can someone analyze this image a tell me what the issue is. I'm thinking it might be guiding - flexure? maybe in the focuser tube? My friend says to buy a new focuser but I need positive confirmation that the focuser is the problem. Is the elongation just CA? where does the CA come from on quite a good APO scope?

I've tried using ASTAP and CCD Inspector on test images but inconclusively.

Am I picking at hairs? Its clearly visible to me - the elongation is about the same all over the image, more pronounced with longer exposure but some images are better than others, even taken close together.

Here is the link to the fits file in my Dropbox: https://www.dropbox.com/scl/fi/sxjgr6t9y8bwuar4c98yu/Light_120_30_180s_022.fits?rlkey=8dk5moc5fip40ghujb4f975q1&st=xlsfxjxi&dl=0

Thanks

Can’t look at the fits file on my phone, so can’t comment on it, but before buying a new focuser, you should verify there’s a problem by checking the focus with a bahtinov mask.

Cheers,

Scott

Ferran Bosch:
Hi, I checked your image and the elongation is obvious. If you’ve gone through everything and it all looks fine (collimation, tilt, backfocus, etc.), it could be that some part of your imaging train is a bit loose. Just last month a buddy of mine had the same issue and it turned out to be the angle rotator not tightened enough. Go over everything to rule things out — either that, or your guiding isn’t good enough for your resolution. By the way, I saw in the header that you’re shooting with an ASI 294 and optics at 560mm, which gives you a resolution of 1.71" per pixel. Even though that’s not super high, it already requires pretty tight guiding RMS — unless you’re running the 294 in unlocked BIN1 mode, which would mean you’re shooting at 0.80" per pixel, and that demands really precise guiding. If it’s not guiding, then check any element in your imaging train that might be loose or not properly secured.