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Does a Dark Flat replace a Bias frame?

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#1 Sean13

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Posted 08 October 2017 - 11:40 AM

Just wondering if there is any need for bias calibration if you are using dark flats, or does the dark flat take the place of bias and pretty much do the same thing?



#2 leveye

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Posted 08 October 2017 - 11:54 AM

I don't use bias frames if I use dark flats and get very good results in DSS using median/mosaic stacking method.


Edited by leveye, 08 October 2017 - 12:37 PM.


#3 pfile

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Posted 08 October 2017 - 12:01 PM

yes, any dark frame contains both the thermal signal and the bias signal.

 

rob



#4 Jon Rista

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Posted 08 October 2017 - 12:03 PM

Flat Darks (i.e. darks for flats), as in contrast to Dark Flats (which implies a flat that is dark...which would not make much sense. tongue2.gif)

They would indeed be used in place of a master bias to calibrate flat frames. The only time you would need flat darks is either if your flats are very long, or if your flats are shorter but your camera has amp glow and the flats are long enough for glow to show. Flat darks must match the flats on exposure, gain, offset and temp. 


Edited by Jon Rista, 08 October 2017 - 01:01 PM.


#5 Michael Covington

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Posted 08 October 2017 - 12:05 PM

The latter.  Dark flats *are* bias frames, actually, unless they are unusually long exposures.

Processing with lights, darks, flats, and dark flats gives you a theoretically perfect image.
Throwing bias frames into it actually complicates the situation.

There are 2 reasons for wanting bias frames other than dark flats:
(1) Your dark flats are unusually long exposures (maybe you are taking sky flats at night and the dark flats are several seconds to match them); or
(2) You want to scale the dark frames to a different exposure time.  That is an older practice that I think most people have decided was never very accurate.

More here:
http://www.covington...lr/#Calibration
http://www.covington...slr/#Arithmetic



#6 Jon Rista

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Posted 08 October 2017 - 12:11 PM

The latter.  Dark flats *are* bias frames, actually, unless they are unusually long exposures.


It's important to keep in mind, that what may have held true historically for CCDs may not hold true for CMOS cameras.

The above quote is not true if your camera has amp glow. Many modern CMOS cameras have enough amp glow in a fraction of a second that you would need flat darks instead of biases to avoid darkening the areas with the glows when calibrating the lights.

#7 Sean13

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Posted 08 October 2017 - 12:18 PM

Gotcha, my longest flats are 0.75 seconds, so not terribly long, but I figured with glow that Flat Darks were a better way to go regardless of exposure time.


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#8 bobzeq25

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Posted 08 October 2017 - 12:32 PM

Flat Darks (i.e. darks for flats), as in contrast to Dark Flats (which implies a flat that is dark...which would not make much sense. tongue2.gif)

They would indeed be used in place of a master bias to calibrate flat frames. The only time you would need flat darks is either if your flats are very long, or if your flats are shorter but your camera has amp glow and the flats are long enough for glow to show.

Since the thread has concluded.... <smile>

 

I laughed when I saw, in a military surplus store, "Insect, repellent"  An ugly bug?  <grin>



#9 Michael Covington

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Posted 08 October 2017 - 12:48 PM

Should I call them "dark flats" or "flat darks"?  I'm writing a book and need to standardize... I had settled on the former...



#10 Kendahl

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Posted 08 October 2017 - 12:58 PM

In principle, you should match a flat with a dark flat of the same ISO and exposure duration just like you do with lights and darks. In practice, flats are usually short enough that there is no significant difference between them and bias frames. I take flats and dark flats for each imaging session. It's easy and takes only a few minutes.


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#11 Jon Rista

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Posted 08 October 2017 - 01:01 PM

Flat Dark would be more correct, grammatically speaking, IMO. It's a dark for a flat, "dark" being the subject and noun, with "flat" being an adjective that modifies the noun. It's a "flat dark". In contrast, when you say "dark flat", it makes it sound like it's a flat frame that's dark (which may be where the all too frequent confusion comes from), as "flat" is the noun, and "dark" is the adjective in this case. A flat, by definition really, cannot be dark, it intrinsically depends on having light to be a flat. So a "dark flat" would seem to be improper, while "flat dark" would be proper. 

 

But, that's just me. I'm picky like that. :p


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#12 Michael Covington

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Posted 08 October 2017 - 01:13 PM

Flat Dark would be more correct, grammatically speaking, IMO. It's a dark for a flat, "dark" being the subject and noun, with "flat" being an adjective that modifies the noun. It's a "flat dark". In contrast, when you say "dark flat", it makes it sound like it's a flat frame that's dark (which may be where the all too frequent confusion comes from), as "flat" is the noun, and "dark" is the adjective in this case. A flat, by definition really, cannot be dark, it intrinsically depends on having light to be a flat. So a "dark flat" would seem to be improper, while "flat dark" would be proper. 

 

But, that's just me. I'm picky like that. tongue2.gif

Actually, being a linguist by profession, I'm picky too, and I'm surprised I didn't think of this already.  "Flat dark" it is.

A "flat dark" is, as you say, a dark for a flat (like "truck tire").

A "dark flat" would be a flat-field image which is dark -- which, as you say, makes us wonder in what sense it is supposed to be flat.


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#13 leveye

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Posted 08 October 2017 - 01:13 PM

This page is always helpful when in doubt. The term "dark flats" is proper.

 

http://deepskystacke...lish/index.html



#14 Michael Covington

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Posted 08 October 2017 - 01:13 PM

Dissenting arguments, anyone?



#15 Michael Covington

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Posted 08 October 2017 - 01:14 PM

This page is always helpful when in doubt. The term "dark flats" is proper.

 

http://deepskystacke...lish/index.html

I wouldn't take them as authoritative -- as I understand it, the author is not a native speaker of English.

 

Granted that both usages ("flat dark" and "dark flat") are common, we're trying to figure out which one makes better sense.


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#16 Jon Rista

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Posted 08 October 2017 - 01:22 PM

This page is always helpful when in doubt. The term "dark flats" is proper.

 

http://deepskystacke...lish/index.html

I completely disagree. "Dark flats" is even worse! That sounds even more like you are talking about flat frames that are dark, which is completely wrong and confusing. Probably the main reason why people seem to be so confused about the term "dark flat" or "dark flats" when it comes up. 

 

That's why I've been rather pointed lately about using the term "flat darks", as it is a far clearer phrase. 


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#17 pfile

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Posted 08 October 2017 - 01:32 PM

fwiw i think "flat dark" is correct as well, and i agree that the author of DSS probably does not speak english as a native language.

 

rob



#18 Sean13

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Posted 08 October 2017 - 01:42 PM

Dark flats just rolls off my tongue easier then flat darks and sounds better, to me. Flat darks should be the correct term I would think, I just don't like the way it sounds, and since an actual Dark flat would be completely pointless I figured everyone would know what I meant by it.



#19 Michael Covington

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Posted 08 October 2017 - 02:00 PM

Both terms are definitely widely used.

 

By analogy of "truck tire" vs. "tire truck," I think "flat dark" makes slightly more sense, but I'm not going to criticize people who go the other way.


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#20 leveye

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Posted 08 October 2017 - 02:30 PM

 

This page is always helpful when in doubt. The term "dark flats" is proper.

 

http://deepskystacke...lish/index.html

I completely disagree. "Dark flats" is even worse! That sounds even more like you are talking about flat frames that are dark, which is completely wrong and confusing. Probably the main reason why people seem to be so confused about the term "dark flat" or "dark flats" when it comes up. 

 

That's why I've been rather pointed lately about using the term "flat darks", as it is a far clearer phrase. 

 

It's right from the source and as intended. You kids go right ahead..



#21 Michael Covington

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Posted 08 October 2017 - 02:40 PM

 

 

This page is always helpful when in doubt. The term "dark flats" is proper.

 

http://deepskystacke...lish/index.html

I completely disagree. "Dark flats" is even worse! That sounds even more like you are talking about flat frames that are dark, which is completely wrong and confusing. Probably the main reason why people seem to be so confused about the term "dark flat" or "dark flats" when it comes up. 

 

That's why I've been rather pointed lately about using the term "flat darks", as it is a far clearer phrase. 

 

It's right from the source and as intended. You kids go right ahead..

 

Why is DeepSkyStacker the authority rather than somebody else?


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#22 leveye

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Posted 08 October 2017 - 02:54 PM

 

 

 

This page is always helpful when in doubt. The term "dark flats" is proper.

 

http://deepskystacke...lish/index.html

I completely disagree. "Dark flats" is even worse! That sounds even more like you are talking about flat frames that are dark, which is completely wrong and confusing. Probably the main reason why people seem to be so confused about the term "dark flat" or "dark flats" when it comes up. 

 

That's why I've been rather pointed lately about using the term "flat darks", as it is a far clearer phrase. 

 

It's right from the source and as intended. You kids go right ahead..

 

Why is DeepSkyStacker the authority rather than somebody else?

 

See what I mean? Lets see. Because it's right from the author and as intended? What's next "dark masters" instead of "master darks"? Enough bickering already move on people.


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#23 Michael Covington

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Posted 08 October 2017 - 03:13 PM

I am an author myself and am trying to choose between two rival terms for use in something I'm writing.  I know that both terms are in wide use.  Merely finding one of them used somewhere doesn't settle the issue.


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#24 Chucke

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Posted 08 October 2017 - 04:02 PM

I just call them "Darks for Flats" although I now use bias frames instead.


Edited by Chucke, 08 October 2017 - 04:02 PM.

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#25 Sean13

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Posted 08 October 2017 - 04:04 PM

I think to avoid any further confusion I will just call them long bias frames now lol


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