Bias as Dark Clarification Please

[richyrich_one]

Hi

Using a DSLR and dithering it's accepted that darks are not helpful. But I've seen it suggested in various places to use bias frames as darks.

Using PI I currently do the following when preprocessing:

1) Integrate bias to create master bias

2) Calibrate flats with master bias only, nothing selected for the dark option.

3) Integrate flats to create master flat

4) Calibrate lights with master flat, master bias and nothing selected for the dark option

Then debayer, register and integrate lights

Where should I be referencing the master bias to use as a dark?

Am I missing a trick here or is my current process the best I can get?

Many thanks

[wxsatuser]

Never quite understood that a bias on a dslr can substitute for darks, would be pleased to hear how this works.

A dark can substitute for a bias as the bias is in the dark, in fact, don't use any bias if you have darks.
Bias are useful and needed to calibrate flats, unless you have dark flats.
Bias are also useful in scaling dark frames that don't match the camera temperature.

If your dithering you most likely only want flats and dark flats, the bias for flats is in the dark flats.

[alacant]

 

1 hour ago, richyrich_one said:

master bias to use as a dark?

Hi. I think you should use a master bias as a master bias. In DSS it's light, flat, dark flat and bias.

0. a master bias is produced

1. a master dark flat is produced from the dark flat frames and master bias frame

2. this is mixed with the flat frames to produce a master flat

3. the master flat is now mixed with the light (and master bias?)

Stack the lot with median or the sigma clipper thing.

HTH

[richyrich_one]

So I am missing darks for my flats.

And if I use them then don't use bias frames as well on the flats because the bias signal is in the dark.

Think I've got it. Thanks.

[wxsatuser]

When the dark is subtracted it also does the bias as it's part of the dark.

[richyrich_one]

Are they temperature dependent? Providing I get the exposure time the same, could I make some now for previous flats?

[Stub Mandrel]

3 hours ago, wxsatuser said:

When the dark is subtracted it also does the bias as it's part of the dark.

DSS uses subtracts the bias from the lights and the other control frames so needs them to be separate from the darks.

If not included on its own the bias will be subtracted from the lights with the dark AND divided out with the master flat.

But as the Dark is subtracted from the light and the Dark Flat is subtracted from the flat they remove the need for bias frames:

Fully explained here:

http://deepskystacker.free.fr/english/index.html

 

[ollypenrice]

9 hours ago, wxsatuser said:

Never quite understood that a bias on a dslr can substitute for darks, would be pleased to hear how this works.
 

The bias is a component of the dark, as you know. It has the virtue of being highly consistent, unlike darks which are thermally unpredictable in cameras without set point cooling. The hot pixels and thermal noise seem, for many users, to be better dealt with by dithering than by darks. Many find that darks can do more harm than good. (Even with set point CCD I'm not persuaded by darks. On my low noise Sony chip they give exactly the same result as a bias used as a dark, so I don't bother with them. On my positively raucous Kodak chips I find darks inconsistent. Taking a few stacking sessions as a sample I do better with bias-as-dark than with dark-as-dark. Sometimes considerably better - and its much less trouble!

I've always found that a master bias makes a perfectly good flat-dark (ie dark for calibrating flats.) At one time I spent ages trying to resolve a flats problem on one camera and did endless testing, using dedicated darks for flats and master bias as dark for flats. I never found any difference whatever.

As for how to make the software do as you wish, the simplest solution is to use AstroArt! One of the things I like about it is that there is a separate box for each calibration file. (Lights, darks for lights, flats, darks for flats, defect map.) If I put a master bias in the box marked darks for flats I know it will be used as a dark for flats. Simple. No second guessing. If I put a photograph of my Christmas dinner into the box marked 'darks for lights' then I know that's how it will be used. It won't give a very good result but I know what's going on all the same!  What I actually do is put a master bias in the box marked darks for lights... 

Olly

[richyrich_one]

25 minutes ago, ollypenrice said:

The bias is a component of the dark, as you know. It has the virtue of being highly consistent, unlike darks which are thermally unpredictable in cameras without set point cooling. The hot pixels and thermal noise seem, for many users, to be better dealt with by dithering than by darks. Many find that darks can do more harm than good. (Even with set point CCD I'm not persuaded by darks. On my low noise Sony chip they give exactly the same result as a bias used as a dark, so I don't bother with them. On my positively raucous Kodak chips I find darks inconsistent. Taking a few stacking sessions as a sample I do better with bias-as-dark than with dark-as-dark. Sometimes considerably better - and its much less trouble!

I've always found that a master bias makes a perfectly good flat-dark (ie dark for calibrating flats.) At one time I spent ages trying to resolve a flats problem on one camera and did endless testing, using dedicated darks for flats and master bias as dark for flats. I never found any difference whatever.

As for how to make the software do as you wish, the simplest solution is to use AstroArt! One of the things I like about it is that there is a separate box for each calibration file. (Lights, darks for lights, flats, darks for flats, defect map.) If I put a master bias in the box marked darks for flats I know it will be used as a dark for flats. Simple. No second guessing. If I put a photograph of my Christmas dinner into the box marked 'darks for lights' then I know that's how it will be used. It won't give a very good result but I know what's going on all the same!  What I actually do is put a master bias in the box marked darks for lights... 

Olly

So..... What would be the ideal usage of the calibration files? In PI you can also specify what you want it to do with any particular file. (I don't use the batch script)

If I use a master bias for a dark for my lights, I then leave the bias box empty? And would it make any difference to using it in the bias box and leaving the dark box empty, as I currently do? And would the same apply to the flats calibration? 

(pretty sure I know the answer to that is to try it and see ) 

 

[ollypenrice]

1 minute ago, richyrich_one said:

So..... What would be the ideal usage of the calibration files? In PI you can also specify what you want it to do with any particular file. (I don't use the batch script)

If I use a master bias for a dark for my lights, I then leave the bias box empty? And would it make any difference to using it in the bias box and leaving the dark box empty, as I currently do? And would the same apply to the flats calibration? 

(pretty sure I know the answer to that is to try it and see ) 

 

We'd need a PI expert for this one! Where's Harry Page?

Olly

[steppenwolf]

20 minutes ago, richyrich_one said:

In PI you can also specify what you want it to do with any particular file. (I don't use the batch script)

Purely as a matter of interest, if you wanted to use the script - (it works exceedingly well by the way) you can use the Add Custom button to place any frame anywhere (Bias/Dark/Flat/Light).

[richyrich_one]

5 hours ago, steppenwolf said:

Purely as a matter of interest, if you wanted to use the script - (it works exceedingly well by the way) you can use the Add Custom button to place any frame anywhere (Bias/Dark/Flat/Light).

Thanks. I didn't know that. 

5 hours ago, ollypenrice said:

We'd need a PI expert for this one! Where's Harry Page?

Olly

Wouldn't the principles apply to whatever software you use? 

[ollypenrice]

4 minutes ago, richyrich_one said:

Thanks. I didn't know that. 

I'll check it out, it's been a while. Wouldn't the principles apply to whatever software you use? 

The principles, yes, but some software strikes me as rather obscure in that it tries to second guess your intentions.

Olly

[steppenwolf]

I have just tried calibrating my test flats in PI using a Bias Master as the 'Dark Master' and then again using just the normal Bias subtraction using the same Master Bias with the Darks box disabled. It is interesting to note that the two images are different in brightness when checked using Blink, however, this is assuming that ASTF has treated both samples equally - although there is no reason why it shouldn't. I will investigate further!

For other users, it may be worth bearing in mind that if you do use Dark frames to calibrate your Flats and you have a camera with set point cooling (I which I appreciate you haven't), PI will auto scale the Darks to match the Flats exposure and from what I understand, it does a good job of doing so. Unfortunately this last bit of information is for general info. only, not specific to your question.

[Stub Mandrel]

1 hour ago, ollypenrice said:

I've always found that a master bias makes a perfectly good flat-dark (ie dark for calibrating flats.) At one time I spent ages trying to resolve a flats problem on one camera and did endless testing, using dedicated darks for flats and master bias as dark for flats. I never found any difference whatever.

If (in DSS) you use a master bias as a dark master, it gets subtracted from itself (master dark -  master bias)  giving exactly the same result as not using a dark at all. See the flow diagram above.

That would explain why you couldn't see any difference!

With my cooled 450D DSLR darks seem to make very little difference, as even at 10 minutes they show barely any noise, but they did seem to help with the uncooled 10D and 450D. I used to temperature match to within about +/-5C using exif data but in the end segregating darks into 'freezing cold nights', 'cool nights' and 'warm nights' seemed to work well enough.

I suspect some people who see very little impact from using DSLR darks may be using either cooled or more recent and lower-noise models?

[steppenwolf]

48 minutes ago, steppenwolf said:

It is interesting to note that the two images are different in brightness when checked using Blink, however, this is assuming that ASTF has treated both samples equally - although there is no reason why it shouldn't. I will investigate further!

OK, I have completed my analysis.

Calibrating my test flats in PI using a Bias Master as the 'Dark Master' and then again using just the normal Bias subtraction using the same Master Bias with the Darks box disabled results in exactly the same calibrated Flat. My first test was erroneous as I had left optimisation 'ON'  - Doh!!

Soo, to answer your question " And would it make any difference to using it in the bias box and leaving the dark box empty, as I currently do? And would the same apply to the flats calibration?  " is NO, it makes no difference.

[ollypenrice]

1 hour ago, Stub Mandrel said:

If (in DSS) you use a master bias as a dark master, it gets subtracted from itself (master dark -  master bias)  giving exactly the same result as not using a dark at all. See the flow diagram above.

That would explain why you couldn't see any difference!

 

No it wouldn't: I don't use DSS. I'm talking about using either a master bias or a master dark in AstroArt where what you subtract is what you subtract. Bias, dark, photograph of Christmas dinner, whatever. So I can compare with absolute reliability the difference between subtracting a dark and subtracting a bias from the stack.

Olly

[harry page]

2 hours ago, ollypenrice said:

We'd need a PI expert for this one! Where's Harry Page?

Olly

Hi

I use the script in pixinsight , do not see why you would not

I also do not use darks for most off the time using a master Bias for Darks as well ( using cosmetic correction to remove warm pixels) , So I just select the master bias in the bias box and that's it

Unless things have changed in the last year , if you do not input anything in the dark box the script will use the master bias for everthing  ( will check this later )

Also if you do put a bias in the dark box , uncheck the "optimise dark frame " as this will cause problems

Lastley you can not select the same file twice in the script ( I.e bias / dark ) so you will have to make a copy and use this

Regards

Harry

 

P.s

 

I also agree with Olly Astroart is a great place to start and very simple to use and give good results

[wimvb]

5 minutes ago, harry page said:

if you do not input anything in the dark box the script will use the master bias for everthing

Yes

Here's my (PI biased) € 0.02 worth:

Dark frames have the bias signal incorporated, so basically, you wouldn't need a master Bias. Using a Dark master and a Dark-Flat master is enough to calibrate the light frames and flat frames. However, if the stacking software (such as PI) can scale the Dark master, then the bias signal must first be subtracted. In PI this is called dark optimization. It allows using dark frames that are not ideally matched in exposure time, to the light frames. You need to subtract the bias because the darks contain a time dependent component (such as dark current) and a pedestal (the bias).

The scaling feature also allows you to use a master dark for both the light frames and the flat frames. The master dark replaces the dark-flat.

In practice this doesn't always work. One reason is that dark masters also contain components that don't necessarily scale linearly with time (or temperature).

PI works like this:

Master Bias is subtracted from dark frames.

Dark frames are combined into a master. This master ONLY contains scalable signal.

Master Bias is subtracted from flat frames.

Master Dark is subtracted from flat frames. Dark scaling is used.

Flat frames are combined into master Flat.

Master Bias and Master Dark are subtracted from light frames.

Light frames are divided by master Flat

https://www.pixinsight.com/tutorials/master-frames/

If you don't want to use dark frames, just leave those boxes empty. Don't use the same masters in the bias and dark fields of image-calibration.

For cooled CMOS cameras, such as ZWO cameras, you might need a different method, because these cameras have amp glow that can be removed by dark subtraction.

For my ASI174, I don't use bias frames. I use dark frames and dark-flat frames which I combine to make dark masters (with the bias signal still present). Then I don't use dark scaling (dark optimization) during flat frame and light frame calibration.

Here's a comparison: a section of an uncalibrated light frame, showing amp glow and vignetting. And a section of the calibrated light frame, using the recipe above.

[richyrich_one]

1 hour ago, harry page said:

I use the script in pixinsight , do not see why you would not

Like Olly, I like to know what's being used for what. Oh and I am a control freak!

[richyrich_one]

My head hurts...

[Stub Mandrel]

2 hours ago, ollypenrice said:

No it wouldn't: I don't use DSS. I'm talking about using either a master bias or a master dark in AstroArt where what you subtract is what you subtract. Bias, dark, photograph of Christmas dinner, whatever. So I can compare with absolute reliability the difference between subtracting a dark and subtracting a bias from the stack.

Olly

I'm a fairy, my name's Nuff.

Fairy 'nuff!

[Stub Mandrel]

19 minutes ago, richyrich_one said:

Like Olly, I like to know what's being used for what. Oh and I am a control freak!

 

To be fair, I think the much-maligned DSS is VERY clear about how it uses the control frames. Do any of the other programs provide clear diagrams explaining what the operations are depending on the supplied frames?

[richyrich_one]

7 minutes ago, Stub Mandrel said:

28 minutes ago, richyrich_one said:

Like Olly, I like to know what's being used for what. Oh and I am a control freak!

 

To be fair, I think the much-maligned DSS is VERY clear about how it uses the control frames. Do any of the other programs provide clear diagrams explaining what the operations are depending on the supplied frames?

To be fair, that was only in response to why I didn't use the preprocessing script in PI 

I agree DSS is very clear. 

[ollypenrice]

11 hours ago, Stub Mandrel said:

 

To be fair, I think the much-maligned DSS is VERY clear about how it uses the control frames. Do any of the other programs provide clear diagrams explaining what the operations are depending on the supplied frames?

The thing is, for me, that I don't really want 'clever,' I want trasparency and simplicity. That's what I get in AA. I also like the speed. Even on this ancient Pc with 4 gig of RAM, AA rattles through a stack of 30 full frame lights, applying flats, bias, defect map, hot pixel filtration and column repair in less than a minute. Given that we might have 24 hours of data to reduce on a winter morning, this speed really is a godsend. I'm surprised that AstroArt isn't more popular.

Olly