Do I Need Darks When Dithering?

[Mikey0368]

I've had a wee bit of a problem with Walking Noise so I'm going to add Dithering to my APT plan. I read somewhere that you don't need to take Darks if Dithering, is that the case?

I would normally take half the number of Darks that I take of Lights and I'm quite happy to continue doing so if the results will be better.

My camera is a modified Canon 600D if that makes any difference and I'm tracking, not guiding. Exposures will be 60 seconds.

Thanks.

[wimvb]

Maybe, maybe not. You have to experiment. Darks don't always work for dslrs. 

Dslrs don't have temperature control, so it will be difficult to use darks for anything temperature related (faulty pixels, possible amp glow). There's only one answer: experiment. Adjust clipping parameters during stacking, apply cosmetic correction (deep sky stacker and pixinsight allow that), etc. 

[adoldesa]

Hi,

when I was using a Nikon D750 I noticed a huge difference when I started dithering. In my experience, however, darks are still very useful to get rid of bad pixels before the stacking process, especially with DSLRs, this helps the rejection filter (or simply the median if no rejection is used) to get rid of true outliers which may be skipped if stacking without darks. Dithering will help with walking noise and it will also allow use to use frames that contain satellites/meteors/airplanes, I was surprised when I staked 50 frames, 10 of which with airplanes trails and I could not detect any of it in the final image!

[Z3roCool]

I find that if I can get darks at a very similar temp to my lights it makes a difference, even when dithering. Hard to do though!

[MarsG76]

Personally I dither and never use darks.. I only use bias and flats.

I find that all thermal noise (and only the noise) gets eradicated by the "Dust and Scratches" filter in photoshop.. set to 2 pixels at threshold 30.

 

[StuartJPP]

55 minutes ago, MarsG76 said:

Personally I dither and never use darks.. I only use bias and flats.

I do the same.

Once I started to dither my subs I ditched darks, good riddance I say. I did experiment and found they did nothing for me other than waste time and disk space.

You have to make sure that your dithering is adequate though.

[MarsG76]

4 hours ago, StuartJPP said:

I do the same.

Once I started to dither my subs I ditched darks, good riddance I say. I did experiment and found they did nothing for me other than waste time and disk space.

You have to make sure that your dithering is adequate though.

And I found that it can actually introduce "reverse noise"... black single pixel dots..

[wimvb]

10 hours ago, MarsG76 said:

And I found that it can actually introduce "reverse noise"... black single pixel dots..

Telltale sign of darks not matching: light - dark < 0. However, if you do need darks for other reasons, you could add a small positive offset to the lights. Personally, I'd only do that when needing darks for other corrections. Otherwise I wouldn't bother. 

[Mikey0368]

Thanks everyone, that's really useful.