DSS - Dark Multiplication Factor

[Xiga]

Okeydokey so, 

I was recently testing a very short stack, only 4 dithered lights taken with a DSLR, so not enough to take advantage of the dithering. Under normal circumstances, i would have taken a lot more (stating the obvious there!) and also taking into account the fact that my camera is not setpoint cooled, then i normally wouldn't bother with Darks (as they'll probably just add to the noise more than anything). However, what if dithering is not an option, then what? You either have to choose between adding extra noise from the darks, or do a lot of heavy lifting in processing to clean the bad pixels. DSS does have a cosmetic tab for supposedly cleaning some of the bad pixels, but i've found it often does more damage than good tbh.

In my example above, which admittedly is not exactly your standard situation, i was struggling to find a way to effectively clean a lot of these bad pixels (and believe me, there was A LOT), so i started experimenting with some of the setting in DSS and came across the following.

I did two stacks in DSS. Both were identical, except one had darks and one didn't. The darks had the same ISO and Exposure length, but the temperatures were no doubt miles apart from the lights (i took them indoors the following night). 

For the stack with the darks, on the 'Dark' stacking tab in DSS i ticked the 'Dark Multiplication Factor' box and set the amount to 0.05. I was purely guessing here, but my logic was 'I want as little dark subtraction as possible, but i do want the bad pixels fixed', so i just went with a small number. 

I then gave both stacks an identical and aggressive stretch in Maxim, then layered them one over the other in PS, to test the difference. I've tried to create a couple of GIF's below to show the results, hopefully they show up ok:

Max Zoom in PS:

As you can see, it's done a very good job indeed of fixing the bad pixels. Now, the thing is, i was also expecting to see at least some extra noise coming through in the deep space due to the darks, but is it just me or is there virtually no extra noise there? Does anyone know anything about this DSS setting, and does my logic of what it does seem to hold up?

I should also say, i did try another stack setting the factor to 0, but it came out much much worse. 

Anyways, just thought i'd post this up to see what others thought and in case someone might find it useful.

Off to bed now, it's late! 

[Astroboffin]

The whole point of using dark frames is to get rid of the noise and hot pixels, so by using them it will make the picture less noisy that is normal and the pint, you should have the setting higher in DSS to get the full benefit of using dark frames, the image will be a lot more noisy and dirty without dark frames......it sounds like you have understood it the wrong way round....or is it me ?.

AB

[Xiga]

Hi

Yes it's true that Darks are supposed to clean up an image. But i'm reading more and more that for those people who are using camera's that aren't cooled (such as DSLR's) that using Darks can actually be detrimental, in that they increase the background noise (so a separate issue to cleaning bad pixels) as the temperatures tend not to match the light frames.

So i surprised to see in my DSLR example above that specifically the backgound noise didn't seem to be any worse when using Darks and the DMF of 0.05.

Later tonight i might try a couple more examples with higher values than 0.05 and see what they throw up.