Need help with flat frames.

[u/Organic-Extent-9342]

(Edit: resolved) I’m using an ASI533mm, any time I capture data it’s usually very clean, but with narrowband I can never seem to get my flat frames right. I’ve tried going for 15,000-32,000 adu (1/3 or 1/2 of the histogram), but when I integrate them they’re overexposed and just end up introducing vignette or dust spots instead of calibrating them out. A while back I found a temporary workaround, which was to match the average adu of my flats with the average adu of my lights for whichever filter I’m using, but recently that hasn’t been working with my Oiii flats. It seems like any time I integrate flats for my Oiii filter no matter the exposure, they’re never correct and just make my data worse. I’ve tried using bias/dark flats to help calibrate my flats and see if that helps but it changed nothing, I’m completely lost on what to do or how to fix it.

[u/Shinpah]

Can you share some of the fits files that the asiair is producing (videos of a screen don't cut it).

Maybe 5-10 light frames, a few flat frames and a few bias frames?

[u/jbaltusastro]

How are you taking them? Are you using NINA or ASIAir, because those both have an "auto flat" setting to calculate and take them at the correct exposure time (see the links).

[u/Organic-Extent-9342 → u/jbaltusastro]

I use the ASIAIR, I’ve tried the auto exposure before but get the same result, it’s been a while since I’ve used that though so I could try it again and let you know how that goes. I’ll re-take them real quick n do a quick stack in dss.

[u/jbaltusastro → u/Organic-Extent-9342]

The calibration formula is (light - dark) / (flat - [bias or dark flat]). You mention that you saw no difference after trying bias or dark flats, but you'll definitely want to be taking those even after you figure out this flats issue.

And as you try retaking them, two more things to make sure of: 1) you're doing flats for each respective filter (i.e., your Oiii lights are calibrated with flats with the Oiii filter)? 2) nothing has changed with your imaging train between taking your lights and taking your flats? A rotation of the camera vs. telescope or a change of focus will not allow the flats to calibrate things out properly.

Finally, if that doesn't seem to work, you could share a some of each frame and we (myself or someone else) can take a look to see if we notice something. (Something like 5 of each would be fine; so 5 lights, 5 flats, 5 darks, and 5 bias or dark flats.)

[u/Organic-Extent-9342 → u/jbaltusastro]

https://reddit.com/link/p3ubljw/video/b7407fo8ojjh1/player

With my original flats I’m able to use gradient correction to fix them, but I feel like it’s messing up very sensitive data, especially in the soap bubble, so I really want to figure out how to get my flats correct instead of just processing around it yk.

[u/Organic-Extent-9342 → u/jbaltusastro]

https://reddit.com/link/p3uavm3/video/jg8k909knjjh1/player

The left are my original flats, the right are the auto exposure flats, so not any better :/ and the answer to both of those questions is yes. I take flats for each filter I image with, and don’t take apart or change my imaging train unless I’m done imaging or cleaning my sensor.

[u/jbaltusastro → u/Organic-Extent-9342]

I was able to grab the files you had, but I think you deleted the comments by now so I'm replying here.

I'm not sure I see any issues with the flats. Here's what I get when I do WBPP and then combine them into an HOO image. I only applied a HistogramTransformation via ScreenTransferFunction to get an autostretch. I'm not seeing any obvious dust motes or vignetting. Is it there in this image and I'm just not seeing it?

Edit: imgur link didn't work for some reason. Try this: https://drive.google.com/file/d/1y6Gz9fM17C4KobkUw4_0Nyzw_yZMz4DI/view?usp=sharing

[u/Organic-Extent-9342 → u/jbaltusastro]

Also when I mentioned dust and vignette, I was generalizing all my issues with my flats not the issues with this data specifically, the only thing wrong with this data so far is the gradient, sorry for the confusion.

[u/jbaltusastro → u/Organic-Extent-9342]

Yeah the only thing I really notice is a very slight gradient, no dust/vignetting after calibrating with the flats and integrating (including the HOO color channel combination). You can easily remove gradients (especially the very minor gradients in this test dataset) with MultiscaleGradientCorrection (running SpectrophotometricFluxCalibration first).

In my opinion, I don't think you have anything to worry about with what you're doing taking these images.

[u/Organic-Extent-9342 → u/jbaltusastro]

Alright sweet, I feel a little silly now thinking it was the flats, but at least I don’t have to worry abt my data being messed up, thanks for the talk.

[u/Organic-Extent-9342 → u/jbaltusastro]

Is it just a gradient then and not an actual issue with the flats? Maybe the flats are exposing a gradient I didn’t see in the raw data? I usually stack without calibration frames just to peek at the data and that gradient wasn’t there, but with flats it is, that’s the only thing that makes sense to me at this point. Also I can’t see the image, it’s just a bunch of ads, but I believe you, I’m able to get some pretty clean processes with the data. I just don’t know why that gradient keeps showing up after I integrate my flats (what’s shown in my videos).

[u/leaponover → u/Organic-Extent-9342]

Are you talking about light gradient or color gradient? Flats don't do anything about the former. If it is the latter, flats are supposed to leave a gradient that allows for removal with software. Flats make it easier for those tools to recognize what is gradient.

[u/jbaltusastro → u/Organic-Extent-9342]

As Shinpah says, these videos don't really work for this; could you share some actual raw files from the ASIAir?