Help with flats

[bhwolf]

Hi all,

I decided the best place to get comfortable with processing was to work with image data I already processed previously, and go through the steps of creating masters, calibrating, registering, then combining manually.  I isolated my first problem to my flats, and I'm hoping it's something simple. 

The first image is a single-frame Rosette, after dark frame calibration and auto-stretch, showing some pretty heavy vignetting.   

The second image is the same RAW file, but with both dark frame and flat calibration after auto stretch.  It has what looks like a reverse or over-applied flat correction.

The last is just a sample of what one flat looks like. 

I was following the DSLR_RAW work flow, I think, but I must've goofed somewhere.  Any suggestions what I did wrong?

[Carlos Milovic]

Are you taking Bias?
I'd suggest to use the Batch-Preprocessing script. If you left the integration out, it will give you a very good result, with little effort.

[bhwolf]

Thank you Carlos--

I tried the batch script with the same (or similar) result.  I wanted to do it manually so I could get familiar with the process. 

I haven't tried with bias frames yet.

Any other suggestions?  Happy to post whatever info might help.

[chris.bailey]

Your flat seems a little bright to me, try again with a flat with about 1/2 of that exposure.

[Carlos Milovic]

You have to use bias frames to properly apply the flats (and to perform the dark scalation). Take them. Always.

[bhwolf]

Thank you Chris --

I looked over my flats and they varied quite a bit in brightness.  In looking at the histogram only, the darkest were in the 30% range and the brightest around the 75% range.  I was including all of them in my master flat. 

I re-processed everything only including the 3 darkest flats.  The results were much better -- see attached.  This was with Bias, Darks, and 3 flats, auto-stretch. 

Now, I also checked CFA in the batch process script and now I can't remember if I'm supposed to do that or not with DSLR_RAW files.   I think some of my issues revolve around understanding the correct color settings to use.   Under Format Explorer/DSLR_RAW prefs, I have it configured as in the screenshot below, but am uncertain of the true difference between "Create RAW Bayer Image" and "Create RAW Bayer CFA image" means, and whether or not we should/shouldn't use black point correction.

Also, one thing I noticed this time around -- when I opened the master flat, it opened as a greyscale image, as it should, I think.  I noticed when I opened it earlier, after creating it manually, that it was color.  I'm not sure if this has anything to do with it, but I thought it seemed strange my first image above had such a blue tint to it.

I might be wrong on this, too, but to clarify my own understanding: I didn't think the brightness of the flats mattered so much as long as the histogram was not clipped.  I would think it's the uniformity/luminosity that is measured, not so much the brightness, to create a model of correction?

Thanks much, this is a fun learning experience

[bhwolf]

You have to use bias frames to properly apply the flats (and to perform the dark scalation). Take them. Always.

Thank you.  (Honestly, I didn't realize the bias were so important.)  While my new results in my post above are much better, I made a mistake of changing 3 things at the same time.  I added Bias, I checked CFA in the Batch Processing (unsure if I did that before), and I limited it to only 3 of the darkest flats. 

I should probably run each to get a better understanding of the processing. 

[topboxman]

I find it best to expose the flats until the peak is at or near the center of histogram. I used to pay attention to max value but it can be mis-leading because the max value could come from hot pixel rather than useful pixel from each flat.

Peter

[chris.bailey]

Trickier with OSC flats as there will be three peaks in different places, all should ideally be below the half way mark on the histogram. As Carlos says you do need bias (or flat darks) to correctly calibrate flat frames. You will also need bias frames if you intend to optimise darks.