DSLR issue - help

[astroman001]

I ran a test using my DSLR on M101. This is a crop of a wider field view taken with a Nikon 5200 DSLR (unmodded) 30 x 2 min subs at 400 ISO with an ED80. Stacked with a dark frame only in deepsky stacker and tweaked in Photoshop.

What is causing the horrible mottled background and all the small black defects everywhere? It's all over the full frame.

Any DSLR users out there seen this before? I don't think it's dust.

Peter

[geordie85]

If you're only using 1 dark frame then it's going to do more damage than good to your images. Also try dithering, that should help smooth out your background noise and mottle

[PeterCPC]

You should have at least 20 darks and flats to stack.

Peter

[stash_old]

I do not use any darks or flats with my Canon 100d and I still stack images in live mode using Astrotoaster (which uses DSS in background) and I have never had that sort of effect. 

Yes if you are trying to achieve perfect images then add Darks etc .

What does the image look like BEFORE Photoshop - i.e. output of DSS ?

Try stacking DSS without Darks/Flats and see what you image looks like.

I guess you are zooming and croping the image too ?

Also now and again I find deleteing DSS temp folder help cure problems.

 

Attached is an example of DSS stacking(Using Astrotoaster) with no Darks or Flats applied 

 

Edited April 24, 2020 by stash_old

[happy-kat]

Bias are so easy to take should add them as well and if you go to the rl trouble of taking flats then do dark flats as well

[astroman001]

Here is a close crop of M101 after DSS stack with no darks at all. Just levels in PS to highlight whats going on. There is good correlation between the coloured hot pixels in this image and the black marks in the previous image. 

Can't explain why they end up black after processing. I did use Gradient Exterminator on the previous image as well.

Peter

 

Edited April 24, 2020 by astroman001

[happy-kat]

What settings have you selected on the Cosmetics tab?

[astroman001]

What is the cosmetics tab?

[happy-kat]

In DSS when you select stack. Don't register and stack together, better to register and then look at registering results before stacking. One of the cosmic options might cause black spots through usually in star centres

[astroman001]

I do register and stack separately. The black areas are not there without darks, see second image above. The coloured hot pixel areas coincide with the black defects in the first image. 

I'm trying to find what processing steps are causing problems. I noted that HLVG plug in makes dark areas in the background when it removes green.

Thanks anyway for the advice

 

[happy-kat]

Check your cosmetic tab and check what you have set regarding dead pixels

[almcl]

I think you have walking noise and when this is removed you end up with the black spots. (Not a PS user so can't comment on how to avoid this effect.)

To avoid the walking noise in future data sets, try dithering between frames?

[FaDG]

8 hours ago, astroman001 said:

There is good correlation between the coloured hot pixels in this image and the black marks in the previous image. 

Can't explain why they end up black after processing

They ended up dark because you only used one Dark frame in the calibration process, which is exactly the reason why you should average darks: if you don't, when the dark is subtracted from the lights during calibration, values could drop below 0 and leave you with those black holes; also, with only one dark you're actually adding noise into your Stack, mainly in the background. 

And, while you're there, also add bias and flats too (and dither, as was suggested): this will give you the best data to start with, and due to the low signal reaching us from those far far away galaxies, you'll really need that! 

 

[astroman001]

Sorry so many questions.

Thank you, FaDG. When I said one dark, that was in fact a stack/average of about 20 dark frames. It was about 6 months old, so I am wondering if doing a fresh average dark will be more accurate??

Thank you Almcl,  I will try dithering, it is not something I have used before. Any idea how much to use, how many pixels etc? I use PHD for guiding.

Thank you Happy-kat, how should DSS be set up for dead pixels?

Best regards

Peter

 

 

 

[happy-kat]

Thought I had just read don't use a master dark instead stack using the individual dark files.

[FaDG]

4 hours ago, astroman001 said:

 When I said one dark, that was in fact a stack/average of about 20 dark frames. It was about 6 months old, so I am wondering if doing a fresh average dark will be more accurate??

Then I assume that the stacked dark you used was acquired in a much warmer condition, which fully explains the issue: a dark is only usable for a small temperature range, and ideally darks should be acquired each session.

The thermal noise in your dark Stack is much higher than during the lights, so you're removing more signal than needed, henche the holes. 

Reshooting the darks (at night!!) will solve the problem

[almcl]

11 hours ago, astroman001 said:

Thank you Almcl,  I will try dithering, it is not something I have used before. Any idea how much to use, how many pixels etc? I use PHD for guiding.

Something that gives you half a dozen pixels might be a good starting point, but you will probably need to experiment?