Background Pattern in DLSR Preprocessing

[Michael Blaylock]

Hello all,

This is my first post so please bear with me.
This link will show you the problem I'm having...

http://www.astrobin.com/180994/

As you see there is some blotchy chromience going on, but even worse, the diagonal lines going from about 10 to 5 oclock. The image is stacked and just STF applied.

Here is a single debayered sub frame,

http://www.astrobin.com/181022/

This image has the chromience motley look, but no diagonal lines.

With all this, I imagine that I must be doing something wrong with Image Intergration.
Any hints?

Thanks,
Michael

[gvanhau]

Hello Michael

As far as I know, this pattern comes out when there is a small drift between the images.  The diferences in pixel brithness due to different sensitivity of them, cause such a pattern.
The best way to avoid this pattern is to dither the images.

Regards
Geert

[IanL]

Yes, the classic DSLR "rainfall" pattern strikes again. You really need to dither by at least 15 pixels between frames to avoid this. You'll still have blotchy chrominance even so, but the dithering will eliminate the objectionable drift pattern that builds up otherwise.

[pfile]

on the other hand, since the pattern is caused by under corrected hot pixels, maybe it makes sense to try running CosmeticCorrection on the calibrated subs to try to eliminate any residual hot pixels. DSLR calibration is notoriously difficult to do perfectly so some post-calibration cleanup can be beneficial.

rob

[IanL]

on the other hand, since the pattern is caused by under corrected hot pixels, maybe it makes sense to try running CosmeticCorrection on the calibrated subs to try to eliminate any residual hot pixels. DSLR calibration is notoriously difficult to do perfectly so some post-calibration cleanup can be beneficial.

rob

Sorry to say that, whilst anything is worth a try, this probably won't cure the problem for anything other than fully hot or cold pixels. Hot pixels are a good diagnostic for this issue as you can see then forming trails that correspond with the overall pattern. They can be removed using CC but that isn't the issue here. Even using very aggressive CC settings, you will always be left with a pattern that builds during stacking.

The fact is that DSLR subs tend to have a lot of chrominence noise in the background that can't be calibrated away, and believe me I have tried everything. In dithered images the final stack is noisy but amenable to various noise reduction processes if used with care. In undithered stacks, it is much less successful as the pattern always breaks through.

Best bet is to get new data, but in the meantime experiment with the different techniques and then darken / clip the background hard. That isn't a generally recommended approach but you might rescue something from the data.

[pfile]

well it worked for me, that's all i know. i don't use a DSLR anymore due to these kinds of issues.

rob

[iksose7]

I had this issue once last year with my DSLR. I didnt change a thing, it just fixed itself and never happened again. You may be as lucky!

Callum

[Michael Blaylock]

Thanks, for ll the advice. I haven't been able to work on any file till now, so I'll be dinking with it this weekend.
I have been playing around with different techniques for preprocessing DSLRs. I think that somewhere among the rebuilding of calibration frames, use of darks as apposed to cosmetic correction, or both. Anyway, I think I screwed something up in the workflow.
I'm going back to ground zero. This time, i'll keep copious step by step notes and only change one thing at a time. Sometimes I will get in a hurry to change things and this is what happens...LOL

Thanks again,
Michael
astrobin.com/users/michael535/

[Cleon_Wells]

Michael, while you’re dinking, take some time and watch this UTUBE video on some of what PI can do, this could cause you to start drinking. LOL
I had similar problems when I started using PI, don’t give up.
https://www.youtube.com/watch?v=nakOHL02XFo&feature=youtu.be
Cleon
PS. PI is like a Formula One race car, a lot of us have a key and know how to put it in gear, but it takes practice and patience and help to learn how to drive it.