In the past few months my luminance flats have deteriorated so that I cannot calibrate my raw images any more. Attached is a YouTube video link and I'm hoping someone can help identify what is going on. One colleague suggested that my SBIG self-guiding filter wheel is not returning to the exact home position for the luminance filter. (I'm using an STXL-10002 camera). This prevents my lum raws from "flatting" out dust motes and vignetting. In fact, when I mean combine my lums the most offensive dust mote looks like a 3-D torus. My RGB, Ha, SII and OIII do not do this. Any notion whats happening at the top of the frame with the moving horizontal line? Also included in the video is what the Red filter flats look like which do not exhibit the problem at the top of the Lum frames. Is there some way to re-calibrate the filter wheel or is this a camera problem? Thanks, Ron
It would be more informative if you could post a couple of different luminance flat-field frames in FITS format.
Thanks, Doug. Let me know what else you need. Had to link to Dropbox as their sizes are too large. https://www.dropbox.com/s/75qn02050vlb8xo/flats_compare.zip?dl=0#
So the flats appear basically normal, except for the top. The dust doughnuts line up perfectly. I don't think there's anything going on with the filter wheel. That line across the top appears to be light contamination. It's not coming from the camera because a camera artifact would line up with the pixels, and this does not. I'm thinking you have a light leak somewhere. Light is getting into the optical system during your flat field calibration, in some route other than coming in directly through the objective.
Thanks for getting back to me Doug. Wouldn't it make sense that at least one of my six other filters would show the same artifact? I have done a high intensity light test of my image train and cannot introduce any streaks or reflections. I use the same light panel for all my flats so the workflow is consistent for all. Any idea why the line at the top is angled and not straight as one would expect? I am happy that it's not a camera or filtercwheel problem. I'm still stumped why the dust motes won't flat out if the doughnuts are aligned perfectly - any thoughts on that? I really appreciate your help. Thanks, Ron
If there's an offset in level then they won't cancel properly. So any change to the illumination levels will mess up the flat. If the stray light is IR it will go through some filters and not others, so that could possibly explain some of the difference. The only other thing I could suggest is that you take the camera off and look inside to make sure the filter wheel and shutter are both operating reliably 100% of the time. If the chip isn't being evenly illuminated due to some mechanical shutter/filter wheel issue that might possibly help explain things. As for sources of stray light, I don't know what your telescope setup looks like so it's hard to comment on that.
Hi Doug - just to close the loop on this, the moving line at the top of the flats video is shutter shadow caused by using extremely short duration flat exposures. When increased to greater than 1 second, the line disappears. Thanks for your suggestions above. Ron
Ron, if the short exposure was calculated by ACP's auto-panel-flat, you can control the minimum duration and the panel brightness specifically to avoid shutter vignetting. Post in the Comm Center if you need specifics. It is covered in ACP Help.
Hi Bob - thanks very much for checking in. I wasn't using ACP at the time and these were manual flats. Is there nothing ACP doesn't do!?!?!? Thanks again and CS, Ron
