Derek Vasselin:
I'm having trouble with stacking photos lately and not sure what changed. I'm seeing dust mote artifacts in stacked & calibrated images that shouldn't be there.
Below is an image straight out of DSS. No further editing, STF applied. You can see dust motes in the top left and an overall red/green gradient from the filter. It's more prominent after DBE.
Extra details:
- Camera: ZWO ASI2600MC
- Lights are 300s
- Total lights: 408 frames
- Flats taken with a light panel
- Flats ADU average around 32,000
- Flat batches mostly in the range of 400-900ms (one outlier batch is 3.8s)
- Use 40 flats, 30 darks, 30 bias
- Everything is grouped by date in the stacking software
What I've tried based on other forum suggestions (none of them worked):
- Using dark flats in place of bias.
- Resetting DSS settings (Load > restore default settings)
- Changing the alignment settings to bilinear instead of automatic
There's several nights of data. About 1/3 of them have lights and flats with those dust motes. The other 2/3 don't, as I cleaned off the sensor. Posted the two different flats & lights below:
https://drive.google.com/drive/folders/1LJgDgo7KcYTBPHdGFOGUQuozpX2Ie5lM?usp=drive_link
Something you'll notice is the dust mote in the flats are very noticeable, but in the light frame, they are really difficult to spot. I'm thinking that may be why? Perhaps that flat is over-calibrating?
There's also a slight rotation between all the frames (no more than 1°), but I doubt that's the issue.
Last comment, I usually avoid stacking in PI because my computer doesn't have the memory, but tried in FBPP. It gave me an error on the debayering process with one image, but I looked at the raw and calibrated version of that image and couldn't find any issues there either. But I did do a stack early on with about 80 frames in FBPP and the image came out just fine. So it's almost like some of the later images are causing issues, but I have no idea how to tell or how to fix.
The issue seems to be in the raw flats. Here is what the fits flat looks like when I opened it in my fits viewer, with an autostretch.
Your flat should be flat in color. With an OSC camera you should see a uniform green, though the green hue can change in a vignette type pattern due to the difference in brightness with a strong stretch. This is where your color gradient is coming from the raw flat itself. This has nothing to do with any settings, bias, darks, just the flat itself.
There is either a light leak, remember this is a strong stretch it could be a small leak. Or your light panel is not uniform in color intensity.
I have a mask for my light panel, in my case it covers everything but the part of the panel over my dew shield. This also acts to sort of seal the edge of the panel against the dew shield to any light leak. I then do my flats in reasonably dark conditions (my garage), before I wheel my telescope out. So little opportunity for any stray light.
Did you try doing 10 flats, then rotated the panel 90 degrees, do 10 more, and then do the same at 180 and 270 degrees, then average them? Might not work perfectly, but you can see if it improved your flat color gradient.
You could also try the famous t-shirt flat, I have used that in my garage just with the ambient light, not pointing near the light source, and it works fine too. Or against the sky, but that can be tricky near sunset in particular, you should point straight overhead where color gradients are least.
Once you can get a raw flat that is just green everywhere, your problems should go away. I think the overcorrection (the donuts are bright, so it is overcorrecting) is because the green has faded in that corner, so you are getting an extra green boost in the corner--again green dominates the flat.
I can't think of any other reasons, but I am reasonably sure it is the raw flat you must fix.
Hope this helps
Rick
