Stacking without darks in DSS as I used dithering.

[Danjc]

Do I just go ahead and stack lights, flats and bias or is there something else I need to do to let DSS know I dithered ?

I have read that some use bias as darks but this did confuse a little. Is it a case of just renaming your bias frames and adding as darks so it doesn’t confuse DSS ?

Any advice is much appreciated. 

Dan. 

[Gerry Casa Christiana]

Yes choose median for the Lights bias and flats and stack them. If you have over 12 lights you can use sigma clipping otherwise use median. 

Let us see the result 

Gerry

[x6gas]

Bias frames and Dark frames are both subtracted from lights as part of the calibration process - the former to subtract read noise and the latter thermal noise.  If you don't have much thermal noise (because the exposure is short or the camera cooled) then you can just subtract Bias frames.  Can't remember if DSS allows you to add Bias frames.  If it does then add them to DSS as Bias (I suspect it will also then subtract the Bias from the Flat frames) and don't put anything in as Darks, if not then just put them in as Darks and DSS will sort it out.

HTH, Ian

[Gerry Casa Christiana]

Yes the bias can be added on DSS and the bias is subtracted automatically from the flats.  

[Knight of Clear Skies]

Just now, Gerry Casa Christiana said:

Yes the bias can be added on DSS and the bias is subtracted automatically from the flats.

Does anyone know what happens in DSS if you add dark bias but no flats please?

[Gerry Casa Christiana]

You will probably end up with vignetting in the picture. I wouldn't leave the out. 

[Knight of Clear Skies]

I try to take flats but I regularly have problems with them overcorrecting, so I often end up using GradEx to sort out vignetting. I'm more interested in whether DSS will remove fixed-noise patterns if only dark-bias frames are added.

[Danjc]

23 minutes ago, Gerry Casa Christiana said:

Yes choose median for the Lights bias and flats and stack them. If you have over 12 lights you can use sigma clipping otherwise use median. 

Let us see the result 

Gerry

I have 30 lights so would it be kappa sigma clipping or median kappa sigma clipping ?

and just for confirmation I would choose this for all lights, flats and bias. 

[x6gas]

Just now, Knight of Clear Skies said:

I try to take flats but I regularly have problems with them overcorrecting, so I often end up using GradEx to sort out vignetting. I'm more interested in whether DSS will remove fixed-noise patterns if only dark-bias frames are added.

When you say "dark-bias" do you mean bias corrected darks or darks and bias frames?  My understanding is that, as the bias signal is present in the dark frames as well, you just need to subtract the dark from the light frame as this contains both the read noise and the thermal noise.

There is no point in using bias corrected darks if you are not using flats.  To put it another way, if you are using darks but not flats then you don't need bias frames at all...

[Gerry Casa Christiana]

7 minutes ago, Danjc said:

I have 30 lights so would it be kappa sigma clipping or median kappa sigma clipping ?

and just for confirmation I would choose this for all lights, flats and bias. 

Either is fine and yes apply the same to all. Median will also work fine too. 

 

[Gerry Casa Christiana]

Can I ask what equipment are you using as that might help others to answer more deeply.  I just do what works!  

[Danjc]

44 minutes ago, Gerry Casa Christiana said:

Can I ask what equipment are you using as that might help others to answer more deeply.  I just do what works!  

It’s in my Signature if you are on your phone turn it to landscape and it will become visible on  posts. 

 

Optics: Skywatcher ED80 DS Pro
Guiding: Zwo ASI120 Mini + 9x50 Finderguider
Mount: Skywatcher HEQ5 Pro
Filters & Gear: SkyTech CLS, Pegasus Astro PPB, Startech USB Hub
Imaging: Canon 1100d Non Modified

[tooth_dr]

2 minutes ago, Danjc said:

It’s in my Signature if you are on your phone turn it to landscape and it will become visible on  posts. 

It doesn’t on my iPhone 

[Danjc]

7 minutes ago, tooth_dr said:

It doesn’t on my iPhone 

Not sure then as I also have an iPhone ?

[Knight of Clear Skies]

55 minutes ago, x6gas said:

When you say "dark-bias" do you mean bias corrected darks or darks and bias frames?  My understanding is that, as the bias signal is present in the dark frames as well, you just need to subtract the dark from the light frame as this contains both the read noise and the thermal noise.

There is no point in using bias corrected darks if you are not using flats.  To put it another way, if you are using darks but not flats then you don't need bias frames at all...

Sorry, I wasn't clear enough. I don't bother with DSLR darks as I suspect they can introduce noise when the temperature profile doesn't match the lights. I've stacked with just lights and bias frames, and wonder if that makes any sense (I've read contradictory advice). Perhaps I should load my master bias as a master dark in this case?

[tooth_dr]

23 minutes ago, Danjc said:

Not sure then as I also have an iPhone ?

I don’t have those tabs either though !

 

[Danjc]

15 minutes ago, tooth_dr said:

I don’t have those tabs either though !

 

It is a newish model maybe it’s that. 

[x6gas]

40 minutes ago, Knight of Clear Skies said:

Sorry, I wasn't clear enough. I don't bother with DSLR darks as I suspect they can introduce noise when the temperature profile doesn't match the lights. I've stacked with just lights and bias frames, and wonder if that makes any sense (I've read contradictory advice). Perhaps I should load my master bias as a master dark in this case?

Yes it does make sense to stack with lights and bias frames.  So long as you are not using flats (or darks) then I don't believe it matters whether you tell the stacking software that they are bias or darks since both are subtracted from the light frames.

Scrub that last bit.  I think you probably do want to tell the stacking software they are darks rather than bias frames - reason being the bias frames may only be subtracted from the flats since the software should expect the darks to correct the read noise as well as thermal noise.  If the software is smart enough to know that in the absence of darks, it needs to subtract bias from the lights then that's fine, but probably safer just to tell the software to treat them as darks then you can be sure that they'll be subtracted from the lights...

I have, by the way, seen workflows telling you to correct the dark frames with bias frames which seems odd because then you need to correct the lights with darks and bias (because you've taken the bias out of the darks, if you follow) - so you are adding two steps you don't need.  If there is an advantage to this then I'd be grateful if someone could explain!

[Knight of Clear Skies]

22 minutes ago, x6gas said:

Scrub that last bit.  I think you probably do want to tell the stacking software they are darks rather than bias frames - reason being the bias frames may only be subtracted from the flats since the software should expect the darks to correct the read noise as well as thermal noise.  If the software is smart enough to know that in the absence of darks, it needs to subtract bias from the lights then that's fine, but probably safer just to tell the software to treat them as darks then you can be sure that they'll be subtracted from the lights...

Thanks - that makes sense to me but it sounds like I need to run a quick test or two to confirm DSS behaves in the expected manner.

[dph1nm]

You do not need to use darks with DSS, or load bias or anything else to the darks tab.

NigelM

[Danjc]

On ‎12‎/‎04‎/‎2019 at 11:46, Gerry Casa Christiana said:

Yes choose median for the Lights bias and flats and stack them. If you have over 12 lights you can use sigma clipping otherwise use median. 

Let us see the result 

Gerry

Apologies for the delayed response. 

This is the image after stacking in DSS 30 x 300s lights, 50 x bias and 20 x flats using median kappa sigma clipping for all. Then taken into Startools with only a stretch and no other processing.

 

 

 

 

[Gerry Casa Christiana]

Looks fine now you just need to process it. Have fun! I use cs2 photoshop which is free. 

Let us know if you need any further help 

Gerry

[rickwayne]

BTW the original post confused me a little. It looks as if you've gotten everything straightened out, but in case someone reading doesn't get the relationship:

Darks and dithering don't interfere with each other at all.

In each dither-displaced frame, the stars and objects will apparently move w/r/t the edges of the frame, true. But the thermal noise is characteristic of the sensor alone, it doesn't depend on where the scope is pointed (or, indeed, if a scope was attached at all!). Which is why, if dark current is significant in your camera and you wish to eliminate it, you use a dark to calibrate each light frame as taken, before any registration and cropping that happens during the stacking sequence. If you registered all your frames first and then applied darks to cropped result somehow, yeah, the darks would at best fail to get rid of thermal noise and in fact would probably add noise.

Dithering defeats a completely different problem: patterns across the sensor of differential response to incident light. It's sorta-kinda like dark current in that the pattern is fixed for a given sensor, but the cause and solution is not the same. If pixel A is slightly more sensitive than adjacent pixel B, the stacking algorithm will interpret that as a difference in impinging light, it will be amplified in stacking and stretching, and will show up as noise. Displacing the scope a bit for each exposure randomizes that process so the patterns fail to self-reinforce in stacking.

Sorry if y'all knew that already, but I didn't want newbies (that is, folks with a week's less experience than myself) tossing darks out of their workflow because they saw "walking noise" in their stacked images and decided to start dithering.