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Should we just stop talking about bias frames?

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#1 kingjamez

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Posted 28 May 2020 - 09:26 PM

Background, this thread: https://www.cloudyni...onger-exposure/

 

The OP was asking if their 0.001" bias frame from an ASI183 CMOS camera was "good".

 

Bias frames are designed to be the constant electronic signature of the sensor, with no dark current. They are the part that should always be subtracted from all frames that come off the camera.

 

Dark frames contain both the bias and additional dark current that accumulates from the sensor being active.

 

When dark-scaling it's useful to differentiate between the electronic signature and the dark current produced noise from the sensor. However, I'm not aware of anyone dark-scaling with most of todays most used cooled cameras. 

 

So why do we even talk about bias frames anymore? Why add a level of confusion about what the difference between a dark, a bias, and a dark-flat, flat-dark is?

 

Why not just say: "Each and every light and flat frame that comes off the camera should be subtracted by a dark of reasonably same characteristics as the light"?

 

If you have a light of 300 seconds, you subtract a dark of 300 seconds.

 

If you have a flat of 0.1 seconds, you subtract a dark of 0.1 seconds.

 

No bias, no dark-flats, just darks.

 

-Jim


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#2 ChrisWhite

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Posted 28 May 2020 - 09:39 PM

Are you assuming that everyone uses cmos these days?
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#3 kingjamez

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Posted 28 May 2020 - 09:49 PM

Are you assuming that everyone uses cmos these days?

In explaining the concept to folks in the beginner forum. Yes.

 

However, good point.

 

-Jim



#4 fmeschia

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Posted 28 May 2020 - 09:55 PM

Indeed if you use flat darks, you don’t need bias frames. There are two equivalent ways to calibrate lights, one of them with darks matching the duration of lights + flats + bias, and the other one with darks matching the duration of lights + darks matching the duration of flats (flat darks) + flats.



#5 dswtan

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Posted 28 May 2020 - 10:14 PM

I like your simplification Jim, and it would've helped me when starting out. FWIW. But the boat has sailed on this terminology as-is, I think!



#6 bobzeq25

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Posted 28 May 2020 - 10:31 PM

Background, this thread: https://www.cloudyni...onger-exposure/

 

The OP was asking if their 0.001" bias frame from an ASI183 CMOS camera was "good".

 

Bias frames are designed to be the constant electronic signature of the sensor, with no dark current. They are the part that should always be subtracted from all frames that come off the camera.

 

Dark frames contain both the bias and additional dark current that accumulates from the sensor being active.

 

When dark-scaling it's useful to differentiate between the electronic signature and the dark current produced noise from the sensor. However, I'm not aware of anyone dark-scaling with most of todays most used cooled cameras. 

 

So why do we even talk about bias frames anymore? Why add a level of confusion about what the difference between a dark, a bias, and a dark-flat, flat-dark is?

 

Why not just say: "Each and every light and flat frame that comes off the camera should be subtracted by a dark of reasonably same characteristics as the light"?

 

If you have a light of 300 seconds, you subtract a dark of 300 seconds.

 

If you have a flat of 0.1 seconds, you subtract a dark of 0.1 seconds.

 

No bias, no dark-flats, just darks.

 

-Jim

A dark of 0.1 seconds (or whatever the flat exposure is) IS a dark flat.  Or a flat dark, if you prefer.

 

You need 3 things.  Flats.  Darks.  Either dark flats or bias.  3 things.

 

With my 183s (I have both) I find bias work perfectly.  No need for the complexity of dark flats.  I reuse one master bias for weeks or months.  I don't care if my flat exposures change.

 

That simplicity of one master bias for all flats regardless of flat exposure is why I use bias, instead of dark flats.  But either can work, provided you have 3 things.  Some cameras, notably the 1600 do require dark flats instead of bias.  So, no, I do not think we should talking about bias.  Many of us use them.  Some with CMOS cameras.

 

You're saying it's easier to have flats, and 2 different "darks".  I do not think that's easier.  If you have 2 different kinds of darks, you're going to need to keep them straight when you enter them into your stacking/calibration program.  Which will _not_ be using that terminology.

 

Neither do the large majority of imagers use that terminology.


Edited by bobzeq25, 28 May 2020 - 10:40 PM.

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#7 schmeah

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Posted 28 May 2020 - 10:50 PM

That simplicity of one master bias for all flats regardless of flat exposure is why I use bias, instead of dark flats.  

Except if your flat exposures happen to be particularly long, depending on your cameras dark current, then you may have to take flat darks.



#8 Ken Sturrock

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Posted 28 May 2020 - 10:55 PM

I think that bias frames serve a useful purpose in certain cases but they have, somehow, become a fetish for many new imagers. As always, it helps to learn the theory and experiment; not just memorize "work flows". step.gif


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#9 bobzeq25

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Posted 28 May 2020 - 11:01 PM

Except if your flat exposures happen to be particularly long, depending on your cameras dark current, then you may have to take flat darks.

Sure.  But I use a Spike a Flat at maximum brightness, no (unnecessary) t-shirts, just adjust the histogram towards the middle with exposure.  Broadband that's a few hundredths of a second, it changes with filter, and OTA.  Narrowband, a few tenths.  None of my cooled cameras has much dark current when cooled.

 

One master bias (for each camera) works fine for it all.  Different filters, different OTAs, even camera lenses.  Dark flats would need to be constantly retaken for different filters, different OTAs.


Edited by bobzeq25, 28 May 2020 - 11:07 PM.

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#10 schmeah

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Posted 28 May 2020 - 11:13 PM

Sure.  But I use a Spike a Flat at maximum brightness, no (unnecessary) t-shirts, just adjust the histogram towards the middle with exposure.  Broadband that's a few hundredths of a second, it changes with filter, and OTA.  Narrowband, a few tenths.  None of my cooled cameras has much dark current when cooled.

 

One master bias (for each camera) works fine for it all.  Different filters, different OTAs, even camera lenses.  Dark flats would need to be constantly retaken for different filters, different OTAs.

Right, but I would say that most don’t use Spike a Flat and probably do sky flats. Ideally you just want to take very short flat exposures and only use master bias. Its rare that I have had to use flat darks.


Edited by schmeah, 29 May 2020 - 06:38 AM.


#11 kingjamez

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Posted 28 May 2020 - 11:16 PM

Sure.  But I use a Spike a Flat at maximum brightness, no (unnecessary) t-shirts, just adjust the histogram towards the middle with exposure.  Broadband that's a few hundredths of a second, it changes with filter, and OTA.  Narrowband, a few tenths.  None of my cooled cameras has much dark current when cooled.

 

One master bias (for each camera) works fine for it all.  Different filters, different OTAs, even camera lenses.  Dark flats would need to be constantly retaken for different filters, different OTAs.

I might suggest that you aren't using your bias as a bias. You are using it as a close enough dark... which can and does work quite well for CMOS cameras.  

 

All I'm suggesting is that for most new folks we just say that lights and flats are calibrated with appropriate darks. 

 

-Jim



#12 bobzeq25

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Posted 28 May 2020 - 11:34 PM

I might suggest that you aren't using your bias as a bias. You are using it as a close enough dark... which can and does work quite well for CMOS cameras.  

 

All I'm suggesting is that for most new folks we just say that lights and flats are calibrated with appropriate darks. 

 

-Jim

How so?  I do it at about 30 microseconds exposure, the minimum the camera will do.  That's hardly a flat dark.  It's a bias, measuring the irreducible minimum noise of the camera, exposure independent.  The exposure independent deal is important.

 

As I'm sure you know my specialty is advising beginners.  I truly think your suggestion is _more_ confusing.  How will we tell beginners that the nomenclature bears no resemblance to what they're seeing in their software, or reading in their books?  If someone wandered into CN, and people here are using a different system than elsewhere, they're going to be _very_ confused.

 

If you can get Bracken, Woodhouse, Stark, Glover, Hall, and Lodriguss (forgive my many omissions) on board....  I don't see that happening.

 

Bottom line.  My bias are bias, as the term is commonly defined.  One of your darks is a flat dark, as the term is commonly defined.  That's a big barrier to your suggestion.


Edited by bobzeq25, 28 May 2020 - 11:45 PM.


#13 bobzeq25

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Posted 28 May 2020 - 11:35 PM

Right, but I would say that most don’t use Spike a Flat and probably do sky darks. Ideally you just want to take very short flat exposures and only use master bias. Its rare that I have had to use flat darks.

I'd take an even money bet that more people use a Spike a Flat, or similar high end panel, than do sky flats.  I'd also bet that the most used solution is an inexpensive panel or a computer screen.

 

Some cameras, like the 1600, require flat darks (and low level flat light sources).  They do funny things at very short exposures.


Edited by bobzeq25, 28 May 2020 - 11:38 PM.

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#14 Stelios

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Posted 29 May 2020 - 01:33 AM

I'd take an even money bet that more people use a Spike a Flat, or similar high end panel, than do sky flats.  I'd also bet that the most used solution is an inexpensive panel or a computer screen.

 

Some cameras, like the 1600, require flat darks (and low level flat light sources).  They do funny things at very short exposures.

I would say it depends on the level of proficiency and experience.

 

I started with sky flats, then went to t-shirt flats, then got an EL-panel, then got a Spike-A-Flat. 

 

I don't think many beginners start with any kind of flat panel. As a matter of fact they tend to go through an even earlier stage than the ones I went through--no flats at all. Every beginner hates flats. Every.Single.One.

 

That said, by the time you get to Pixinsight, I think you may well be past the sky flats stage. Or should be.



#15 ChrisWhite

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Posted 29 May 2020 - 04:53 AM


 

That said, by the time you get to Pixinsight, I think you may well be past the sky flats stage. Or should be.

Completely disagree.  By the time you figure out how to do all this you find that sky flats are the absolute BEST matched flats you can take.  This requires accurate capturing though ideally with dithering. The set one exposure length and go wont work as the sky brightness changes over time.  You need to adjust exposure as you take the flats.  I've gotten really good at taking flats over the years with panels, etc... (I even owned and used a spike-a-flat for years) but the absolute best flats are properly exposed sky-flats, especially for those who experience the "ring" type of vignetting from filter mountings.  


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#16 fewayne

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Posted 29 May 2020 - 08:49 AM

??? Light goes into telescope. Light hits things in telescope. Some light gets to sensor.

 

How does the scope know where the photons are coming from? I mean, if it had stars or clouds or a bug or something, sure. But if you succeed in getting a perfect uniform sky, that will be...as good as a high-quality panel?

 

Inquiring minds want to know.


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#17 ChrisWhite

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Posted 29 May 2020 - 08:51 AM

??? Light goes into telescope. Light hits things in telescope. Some light gets to sensor.

How does the scope know where the photons are coming from? I mean, if it had stars or clouds or a bug or something, sure. But if you succeed in getting a perfect uniform sky, that will be...as good as a high-quality panel?

Inquiring minds want to know.


Well, I would argue that it is even better than a high quality panel.

 

I will also add that even little clouds or some weak stars at dawn or dusk won't make any trouble.  That's why you would want to dither. 


Edited by ChrisWhite, 29 May 2020 - 10:03 AM.

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#18 kingjamez

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Posted 29 May 2020 - 10:08 AM

 

 

If you can get Bracken, Woodhouse, Stark, Glover, Hall, and Lodriguss (forgive my many omissions) on board....  I don't see that happening.

 

 

All of those folks are certainly smarter than I am. They also grew up on CCD cameras where the concept of a bias frame is very useful. 

 

I'm not a huge fan of "it's always been done this way, everybody does it this way, thus it should continue to be done this way" type of a justification. I'm totally cool with that if it makes sense, but today in a largely CCD-less world for new imagers, why? 

 

If new folks move up to a CCD one day, they will not be as easily confused as one is when just starting out. 

 

For a CMOS camera there is no useful reason to differentiate between a bias and a dark frame, so why do it? 

 

-Jim



#19 bobzeq25

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Posted 29 May 2020 - 10:19 AM

All of those folks are certainly smarter than I am. They also grew up on CCD cameras where the concept of a bias frame is very useful. 

 

I'm not a huge fan of "it's always been done this way, everybody does it this way, thus it should continue to be done this way" type of a justification. I'm totally cool with that if it makes sense, but today in a largely CCD-less world for new imagers, why? 

 

If new folks move up to a CCD one day, they will not be as easily confused as one is when just starting out. 

 

For a CMOS camera there is no useful reason to differentiate between a bias and a dark frame, so why do it? 

 

-Jim

Here's why.

 

If you use a dark flat exposure that's the same as the flat exposure (which defines a "dark flat"), and you change the flat exposure, you naturally will change the dark flat exposure and reshoot.

 

If you use a bias exposure that's (properly) the shortest exposure the camera will do, and you change the flat exposure, you can still use the same bias, so no need to reshoot.  I use one bias for weeks, despite changes in setup.  It's completely camera specific, nothing else.

 

Bias and dark flats are two different things.  Some CMOS cameras, like the 1600, cannot use bias, must use dark flats instead.

 

Some CMOS cameras, like the 183, _can_ use bias and there is no need to take dark flats.  Just like a CCD.

 

The confusion over this has mostly come from people who insist you must use dark flats with any CMOS camera.  That's just not true.


Edited by bobzeq25, 29 May 2020 - 10:28 AM.

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#20 Ken Sturrock

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Posted 29 May 2020 - 10:37 AM

How so?  I do it at about 30 microseconds exposure, the minimum the camera will do.  That's hardly a flat dark.  It's a bias, measuring the irreducible minimum noise of the camera, exposure independent. 

I'd agree with the King that you're not using a bias frame the way that it is traditionally intended to be used. What I mean is this: You are not using a bias frame to extract the "irreducible minimum noise of the camera" from a mismatched dark frame and then scaling (aka optimizing in PI speak) the remaining thermal noise from that mismatched dark frame in order to try to simulate the thermal noise pattern expected in a light frame shot at a different exposure (or with different settings). Rather, you are using a bias frame as a close-but-not-exact duration dark frame and just subtracting it from the light with no other dark frame in play. Right?

 

The purpose of the bias frame is to be paired with a mismatched dark in order to synthesize the equivalent of a matched dark frame. You are simply using a bias frame as a really short duration dark frame because it represents the vast majority of the noise found in a really short duration light frame.

 

They also grew up on CCD cameras where the concept of a bias frame is very useful.

 

<snip>

 

For a CMOS camera there is no useful reason to differentiate between a bias and a dark frame, so why do it?

Maybe I'm missing the point of the thread. I thought that you were advocating for us to dispense with the entire concept of bias frames versus claiming that bias frames are the same thing as dark frames. Regardless of if you advocate for their use, dark and bias frames should be differentiated because they are meant to serve different purposes.

 

I'm also not sure that the architecture of the sensor matters from a theoretical standpoint. I'd say that the role of a bias frame is the same: It's a pragmatic short cut that, when it works*, will reduce the number calibration frames that need to be dealt with.

 

For what it's worth, a long time ago, nobody I knew used bias frames, although I'm sure that they existed and were in use in other places or populations. Bias frames seemed to gain in popularity with the use of dSLRs because darks were inherently mismatched to lights due to lack of temperature control. Using a bias frame allowed one to scale the dark to better match the noise in the light. After that, their convenience caught on with more and more people.

 

 

 

 

 

* = Which it can work for all kinds of cameras regardless of sensor - the main issue is if the camera has non-linear noise. Some CMOS cameras don't experience non-linear glows. Some CCD cameras have them. I guess that I'm still not a huge believer in the whole "CMOS cameras are so unbelievably different that everything you ever knew before is useless" school of thought. Each camera is different and yet they all have similarities.


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#21 OldManSky

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Posted 29 May 2020 - 10:43 AM

Right, but I would say that most don’t use Spike a Flat and probably do sky flats. Ideally you just want to take very short flat exposures and only use master bias. Its rare that I have had to use flat darks.

To avoid refresh issues, and to use my system, my flats are typically about 3-5 seconds.

So I do flat-darks.  Because at 3 seconds, there's a tiny bit of amp glow on the 183 that shows up, and a bias wouldn't remove it.

Lots of ways to skin this particular cat... :)


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#22 bobzeq25

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Posted 29 May 2020 - 10:51 AM

I'd agree with the King that you're not using a bias frame the way that it is traditionally intended to be used. What I mean is this: You are not using a bias frame to extract the "irreducible minimum noise of the camera" from a mismatched dark frame and then scaling (aka optimizing in PI speak) the remaining thermal noise from that mismatched dark frame in order to try to simulate the thermal noise pattern expected in a light frame shot at a different exposure (or with different settings). Rather, you are using a bias frame as a close-but-not-exact duration dark frame and just subtracting it from the light with no other dark frame in play. Right?l.

 

 

No.  That may be how kingjamez misunderstood, also.  As I said I shoot _three_ types of calibration frames.  Count 'em, 3.  <smile>  Darks, carefully matched to the lights, as is good practice.  Never optimized or scaled, even with my CCD.  Flats.  Bias.

 

As a result the amp glow in my 183s vanishes.  That's not going to happen if I do no other darks than the bias. 

 

A possibly interesting question is whether I could get away with just using darks and flats.   I personally have no interest.  I don't believe in getting away with things that I can do right with so little time and effort.  I shoot a master bias of 100 frames, use it for weeks with everything.  Takes almost no time.  Why not just do it right? 

 

I always advise beginners to do it right, not take shortcuts.  I've noted people having problems with eliminating amp glow, when they don't.

 

Also, I've watched a fair amount of Mike Holmes.  <smile>

 

https://makeitright.ca/


Edited by bobzeq25, 29 May 2020 - 10:59 AM.

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#23 kingjamez

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Posted 29 May 2020 - 10:51 AM

I'd agree with the King that you're not using a bias frame the way that it is traditionally intended to be used. What I mean is this: You are not using a bias frame to extract the "irreducible minimum noise of the camera" from a mismatched dark frame and then scaling (aka optimizing in PI speak) the remaining thermal noise from that mismatched dark frame in order to try to simulate the thermal noise pattern expected in a light frame shot at a different exposure (or with different settings). Rather, you are using a bias frame as a close-but-not-exact duration dark frame and just subtracting it from the light with no other dark frame in play. Right?

 

The purpose of the bias frame is to be paired with a mismatched dark in order to synthesize the equivalent of a matched dark frame. You are simply using a bias frame as a really short duration dark frame because it represents the vast majority of the noise found in a really short duration light frame.

 

Maybe I'm missing the point of the thread. I thought that you were advocating for us to dispense with the entire concept of bias frames versus claiming that bias frames are the same thing as dark frames. Regardless of if you advocate for their use, dark and bias frames should be differentiated because they are meant to serve different purposes.

 

I'm also not sure that the architecture of the sensor matters from a theoretical standpoint. I'd say that the role of a bias frame is the same: It's a pragmatic short cut that, when it works*, will reduce the number calibration frames that need to be dealt with.

 

For what it's worth, a long time ago, nobody I knew used bias frames, although I'm sure that they existed and were in use in other places or populations. Bias frames seemed to gain in popularity with the use of dSLRs because darks were inherently mismatched to lights due to lack of temperature control. Using a bias frame allowed one to scale the dark to better match the noise in the light. After that, their convenience caught on with more and more people.

 

 

 

 

 

* = Which it can work for all kinds of cameras regardless of sensor - the main issue is if the camera has non-linear noise. Some CMOS cameras don't experience non-linear glows. Some CCD cameras have them. I guess that I'm still not a huge believer in the whole "CMOS cameras are so unbelievably different that everything you ever knew before is useless" school of thought. Each camera is different and yet they all have similarities.

I'm suggesting that we don't use bias frames as they are intended to be used with CMOS cameras. Today, when most folks talk about calibrating flats with bias frames, what they are really saying is that they are using short duration darks that are reasonably close in approximating a perfect dark frame. 

 

Ken, you just say things more clearly, and more accurately than I do. Thank you!

 

-Jim



#24 bobzeq25

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Posted 29 May 2020 - 11:05 AM

Today, when most folks talk about calibrating flats with bias frames, what they are really saying is that they are using short duration darks that are reasonably close in approximating a perfect dark frame. 

 

That's not what I do.  I approximate nothing, everything (darks, flats, bias) is done as precisely as possible.  Darks are carefully matched to lights.  I use a Spike a Flat for flats.

 

We work so hard, spend so much time and money to get good data, why not calibrate it correctly?  It just doesn't take that much time and effort.  Not calibrating lights correctly is throwing away effort and money spent on good equipment.

 

I also think it's not what "most" people do.  Maybe some beginners, surely not most experienced imagers.  Or even a significant minority.

 

The thing that frustrates me is some beginners not understanding the importance of proper calibration.  Not so much because of the effects on their images, but because it affects their ability to learn processing properly.
 


Edited by bobzeq25, 29 May 2020 - 11:14 AM.


#25 nimitz69

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Posted 29 May 2020 - 11:07 AM

Right, but I would say that most don’t use Spike a Flat and probably do sky flats. Ideally you just want to take very short flat exposures and only use master bias. Its rare that I have had to use flat darks.


It’s camera dependent. I can not take a classic bias frame with my 1600 because you get horrible banding at The very short exposure Times of a bias frame. Dark frames work because they are ‘longer’ since they are at teh same exposure as your flats which are never under .1 secs like bias frames can be ....
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