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Flat Calibration

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#1 ribuck

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Posted 17 March 2020 - 06:09 PM

Hi all, 

 

I'm a bit confused about flat calibration, so wanted to double check something.

 

So, i created my master Darks, which were all Bias Subtracted and i'm now creating my flats. I'm following the light vortex guide, which says flats should be calibrated with both Darks & Bias - Should this not just be darks as the bias was already subtracted in the master dark creation. It just sounds odd to have to do it again on the flats and not sure if it will actually introduce more noise.

 

Am i way off the mark, or is my thought process sound ?

 

By the way i'm using a Kaf830 based ccd chip

 

Cheers,

Rich.


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#2 zer0

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Posted 17 March 2020 - 06:30 PM

The flats need to be calibrated with their own "flat" darks - not the same darks you would have created for calibrating your lights.
The flat darks are dark frames to be taken at the same exposures as your flats will be.

Then, similar to lights calibration with darks, in order to not subtract the bias twice, you either don't bother with bias frames at all (keep the bias signal in the darks) or, if you subtract bias from the darks, you need to then subtract both biasless darks from the flats and the bias itself.

Master flat = flat - dark signal - bias signal.
If you don't bother with subtracting bias from the darks, your master dark will contain both the dark signal and the bias signal, so your master flat becomes just flat - master dark.

A nice graphical scheme is available here: http://deepskystacke...ibrationProcess
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#3 ribuck

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Posted 17 March 2020 - 07:12 PM

Ah I see, I thought the dark flat method was only for cmos cameras and didn't apply to ccd.

 

This is the problem with the internet, there are so many different guides and they all gives advice.

 

Ok I'll recalibrate my files with the dark flat method tomorrow.

 

Many thanks  for the quick response, 


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#4 bobzeq25

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Posted 17 March 2020 - 07:23 PM

Hi all, 

 

I'm a bit confused about flat calibration, so wanted to double check something.

 

So, i created my master Darks, which were all Bias Subtracted and i'm now creating my flats. I'm following the light vortex guide, which says flats should be calibrated with both Darks & Bias - Should this not just be darks as the bias was already subtracted in the master dark creation. It just sounds odd to have to do it again on the flats and not sure if it will actually introduce more noise.

 

Am i way off the mark, or is my thought process sound ?

 

By the way i'm using a Kaf830 based ccd chip

 

Cheers,

Rich.

You are way off the mark.

 

CCDs can use bias, they don't need flat darks, nor will flat darks improve things.  You need three things.  Flats, darks, and _either_ bias or flar darks depending on the camera.  Not both.

 

Darks should be stacked, they don't need bias.  In fact, subtracting bias on darks can easily lead to double subtracting bias, a _very_ common beginner mistake.

 

Flats _must_ be bias subtracted before stacking them, or the math for flat correction doesn't work.

 

Then lights are calibrated with a master dark and master flat before stacking.

 

DSS handles it all automatically, as does BPP in PixInsight.


Edited by bobzeq25, 17 March 2020 - 07:25 PM.

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#5 zer0

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Posted 17 March 2020 - 07:40 PM

_either_ bias or flar darks depending on the camera.  Not both.

 

I think I hear what you're saying, especially about the dangers of subtracting bias from the darks (why would anyone do this BTW, other than just for dark current measurement purposes?)

But, could you please elaborate a bit on not needing both bias and flat darks? If I understand correctly they are not quite mutually exclusive because flat darks CONTAIN bias. If the camera is really clean of dark current, then I see why only bias could be enough. But what if there's a plenty of dark current?

 

What so far sounds right to me is to not bother taking bias frames and use darks and flat darks (together with bias they contain) for subtracting from lights and flats respectively. Please correct me where I'm wrong.  


Edited by zer0, 17 March 2020 - 07:42 PM.

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#6 ribuck

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Posted 17 March 2020 - 07:40 PM

Maybe I'll start to use BPP from now on to save a lot of hassle.


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#7 bobzeq25

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Posted 17 March 2020 - 11:31 PM

I think I hear what you're saying, especially about the dangers of subtracting bias from the darks (why would anyone do this BTW, other than just for dark current measurement purposes?)

But, could you please elaborate a bit on not needing both bias and flat darks? If I understand correctly they are not quite mutually exclusive because flat darks CONTAIN bias. If the camera is really clean of dark current, then I see why only bias could be enough. But what if there's a plenty of dark current?

 

What so far sounds right to me is to not bother taking bias frames and use darks and flat darks (together with bias they contain) for subtracting from lights and flats respectively. Please correct me where I'm wrong.  

It's not so much that you're wrong, it's that you're introducing complexity needlessly.  I'm going to be blunt, I hope no personal offense is taken.

 

It's that there is _no_ value with regard to image quality to do both.  And you could mess the processing up easily.  All risk, _no_ gain.  So what if dark current is large?  Dark current is handled by doing darks, not dark flats.

 

Some cameras don't like bias (the 1600 is the prime example), so you do flat darks.  But, if your camera is not one of those, dark flats, again, provide _no_ gain in image quality.

 

As is so often true with DSO AP, this may be unintuitive.  But the fact is that dark flats have no inherent value "just because".  The fact that some cameras require them (so you get posts saying, properly, with _that_ camera you must use dark flats) has led many people to overestimate their value.  It's a mirage.

 

The OP has an 8300.  People have been making superb images with that camera using bias, for a very long time.  Dark flats would not improve those images.

 

If someone wants to do dark flats, fine.  You can do DSO AP all kinds of ways.   But advice that overvalues dark flats is bad advice.

 

Minor point.  People bias correct darks all the time.  If their workflow doesn't then subtract bias a second time, no harm.  But, again, there's just no reason to do it.


Edited by bobzeq25, 17 March 2020 - 11:37 PM.


#8 rgsalinger

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Posted 17 March 2020 - 11:43 PM

Could you elaborate on why a camera would need dark flats? My flats are 1 second or less for LRGB and around 5 seconds for my NB filters. So, taken at -10 I'm not sure why I would need dark flats. What would the characteristic be of a camera that needs them. Now I had an ASI1600 and sadly I never experimented with dark flats for the same reason - I keep my flats very short.

 

Rgrds-Ross


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#9 Melgabean

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Posted 18 March 2020 - 12:54 AM

Bias frames are normally used when scaling your dark frames (Darks are taken at different exposure length than lights or Flats) and scaling doesn't work very well on CMOS sensors as it will not calibrate out things like AMP glow. hence the need for Flat Darks. Bias frames are typically taken as close to a 0 second exposure as possible. The ASI1600 does not do well with exposures < 0.1 second. because of this and the poor scaling you are better off with Flat Darks instead. Jon Rista explains how to use Flat Darks very well in this thread https://www.cloudyni...tion-correctly/ see post 4.

 

Light Frames will contain the Subject data, Dark Current, Bias, and Misc (Dust Motes, Smudges, Vignetting).

Flat Frames will contain Misc, Dark Current (but a different Dark Current due to exposure length), Bias

Dark Frames will contain Dark Current, Bias

Bias will contain Bias.

 

We stack all the Bias together to get the Master Bias.

Dark Frames - Master Bias = Calibrated Dark Frames. Calibrated Darks Stacked together gets us the Master Dark (no more bias)

Flat Frames - Master Bias - Scaled Master Dark (master dark gets scaled as the flat frames are a different exposure length than the darks) = Calibrated Flats. Stacked together gets Master Flat (just the misc data)

Light Frames - Master Bias - Master Dark - Master Flat = Calibrated Lights. Stacked together gets us a pretty image.

 

sorry this is kinda ugly to read but didn't know how else to explain.



#10 ks__observer

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Posted 18 March 2020 - 03:29 AM

You only need bias frames when doing flats.

The lights are divided by the flats to get a "flat" field with no vignette.  

For flat division to work you have to start at zero point -- so you need to subtract bias from flat frames.



#11 bulrichl

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Posted 18 March 2020 - 04:30 AM

The topic (Should darks be calibrated with bias? Do I need darks and bias? Flat darks or bias for flat calibration?) is explained in detail in my guide: https://pixinsight.c...ibration.11547/

 

Bernd

 



#12 ribuck

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Posted 18 March 2020 - 05:46 AM

Apologies - I've been away form imaging for a few years so just trying to remember everything.  

 

So in this thread alone we have 2 different opinions on calibration in regards to the use of Bias. Melgabean, suggest that i should be calibrating the Darks with the Master Bias file but ks__observer is saying that i dont need to calibrate the darks with Bias, and only to use it when calibrating the flats

 

So which is the right method ?

 

Bernd,

 

I had a look at you link and read through, but the instructions as per section 3.1 assume master darks have been prepared as per seciton 1, but section 1 doesn't appear to say how to prepare them, e.g. in relation to the bias etc unless i missed it somehow.

 

3.1 MasterDark and MasterBias
It is assumed that MasterDark and MasterBias are prepared according to [1].

 


Edited by ribuck, 18 March 2020 - 05:48 AM.


#13 Melgabean

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Posted 18 March 2020 - 10:41 AM

I'm of the opinion that the Light Vortex Astronomy tutorial for PI is correct.

 

you could always try both ways yourself in PI and see how your images turn out.



#14 ks__observer

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Posted 18 March 2020 - 11:05 AM

We stack all the Bias together to get the Master Bias.

Dark Frames - Master Bias = Calibrated Dark Frames. Calibrated Darks Stacked together gets us the Master Dark (no more bias)

Flat Frames - Master Bias - Scaled Master Dark (master dark gets scaled as the flat frames are a different exposure length than the darks) = Calibrated Flats. Stacked together gets Master Flat (just the misc data)

Light Frames - Master Bias - Master Dark - Master Flat = Calibrated Lights. Stacked together gets us a pretty image.

 

sorry this is kinda ugly to read but didn't know how else to explain.

Dark Frames = Stacked together gets us the Master Dark (with bias)

Flat Frames - Master Bias = Calibrated Flats.

Light Frames (with bias) - Master Dark (with bias) divided by Master Flat = Calibrated Lights. Stacked together gets us a pretty image.

 

The above achieves the same thing: the Darks contain bias so when you subtract dark from light you are also subtracting the bias.

If you were not using flats -- some people skip flats -- you don't have to worry about the bias.



#15 spokeshave

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Posted 18 March 2020 - 11:19 AM

There are several different approaches to correct image calibration - and some that are incorrect.

 

It is helpful to understand the purpose of image calibration. We do image calibration to correct 4 primary image aberrations*:

 

1) Bias noise: When a sensor is read, there will be slight differences in gain for each row or column read and from one part of the sensor to another. This type of noise is "fixed pattern" meaning that it is essentially the same every time the sensor is read. As a result, it can be corrected by subtraction of a suitable calibration frame (bias).

2) Thermal fixed pattern noise: This is what we typically think of as hot/warm pixels. The brightness of these pixels is directly proportional to the exposure time and sensor temperature. Since this is a fixed pattern aberration, it can also be corrected by subtraction of a suitable calibration frame of the same duration and temperature as the light frames (dark).

3) Pixel response non-uniformity (PRNU): This aberration is often ignored when discussing image calibration. PRNU, as the name implies, describes the variation in which individuals vary in response when exposed to the same level of light. This effect is not additive - i.e. it does not increase as exposure increases - so it can be corrected by division by a suitable calibration frame (flat).

4) Vignetting: This is an optical falloff in field illumination that typically results in darkened corners of the image. Since it is also time-independent, it can also be corrected by division by a suitable calibration frame (flat).

 

It is also helpful to understand which of these aberrations are contained (and thus need to be corrected) in calibration frames.

 

Bias frames are essentially zero-duration frames. Because they have essentially zero duration and the sensor is not exposed to light, they contain only the bias noise and no thermal fixed pattern noise, PRNU or vignetting.

 

Dark frames are exposures of the same duration and temperature as the lights. As a result they contain bias noise and thermal fixed pattern noise, but no PRNU or vignetting.

 

Flats are typically close to zero duration light frames. They contain bias noise and thermal fixed pattern noise as well as PRNU and vignetting. However, as the exposures are typically very short, the amount of thermal fixed pattern noise (which is time-dependent) is usually considered negligible. There is one caveat to that - in some circumstances, flat frames have longer durations. There are typically two reasons for this. The first is due to limitation of the sensor itself. Some CMOS sensors have response characteristics that are different at very short exposures as compared to longer exposures. For this reason, some manufacturers recommend that flats be exposed for a minimum time - typically no less than 0.3 - 1 second. While that does not sound like a long exposure, it is long enough for the thermal fixed pattern noise to begin to manifest. Additionally, taking flats with narrowband filters often requires longer exposures with the same issue of thermal fixed pattern noise accumulation. For this reason, we often take "dark flats", or as I prefer to call them "flat darks".

 

Flat darks are dark exposures of the same duration as the flats. As a result, they contain bias noise and thermal fixed pattern noise.

 

Now let's put everything together. The intent is to correct all 4 aberrations in our light frames. Bias and thermal fixed pattern noise are corrected by subtraction and PRNU and vignetting are corrected by division. Here's the catch - we want to correct each aberration only once. Let's start with bias noise. We can correct that two ways. Both bias frames and dark frames contain the bias noise. So subtracting either will subtract the bias noise from the light frame. If we subtract the bias from the light, that takes care of the bias calibration. But that leaves us in a quandary for the thermal fixed pattern noise. We can't just subtract the dark frame, because it also contains the bias noise - which has already been corrected. So, we would need to calibrate the dark frame first by subtracting the bias frame. So the calibration scheme for bias noise and thermal fixed pattern noise would be:

 

(Light - Bias) - (Dark - Bias)

 

A simpler approach is to just subtract the dark from the light since the dark contains both the thermal fixed pattern noise and the bias noise:

 

Light - Dark

 

So, why not always do this? The reason is that we sometimes want to scale or optimize the darks. The idea is to match the thermal fixed pattern noise as closely as possible, and since the thermal fixed pattern noise scales with time and temperature, we can use darks that don't perfectly match the lights either in duration or acquisition temperature by simply applying a scaling factor to the darks. When we do dark scaling or optimization, though, we don't want to apply the scaling to the bias noise in the dark. That would throw off the bias calibration. So if dark scaling is used, the darks must first be calibrated with bias frames. That, of course, means that the lights must also be calibrated with bias frames since the dark calibration frame will no longer contain the bias noise. 

 

It is worth nothing that things can be a little bit different in CMOS cameras. First, many of them have amp glow that does not behave like thermal fixed pattern noise. As a result, scaling or optimization of the darks can interfere with the amp glow correction. Additionally, as mentioned above, some CMOS cameras behave differently for short exposures versus long exposures. For this reason, the bias noise in a short duration bias frame may not be the same as the bias noise in a long duration dark or light frame. For these reasons, it of often recommended that bias frames not be used for light calibration for CMOS cameras. Instead, the lights should be calibrated with uncalibrated darks. Additionally, if is typically recommended that no dark scaling or optimization be used. So, in summary, calibration of bias noise and thermal fixed pattern noise is accomplished in one of two ways. If dark scaling or optimization is used, lights and darks are both calibrated with bias frames and the lights are then calibrated with the calibrated dark frames.

 

To correct PRNU and vignetting, we use flats. However, the flats must first be calibrated at least for bias noise and if they are of any significant duration, they must also be calibrated for thermal fixed pattern noise. If the flats are of short duration, they can be calibrated with bias frames, but if they are longer in duration, they should be calibrated with flat darks. 

 

The resulting calibrated flat frame is used to calibrate the light frame by division. First the flat frame is normalized, and then divided into the light frame. In summary, the common approaches to image calibration are:

 

(Light - Dark)/(Flat - Bias)     Used when dark scaling is not used and when flats are very short duration. Also used for most CMOS cameras.

 

or

 

 (Light - Dark)/(Flat - Flat Dark)       Used when dark scaling is not used and when flats are longer duration. Also used for most CMOS cameras.

 

 

or

 

[(Light - Bias) - (Dark - Bias)]/(Flat - Bias)       Used when dark scaling is used and when flats are very short duration. Not used for most CMOS cameras.

 

or

 

 [(Light - Bias) - (Dark - Bias)]/(Flat - Flat Dark)      Used when dark scaling is used and when flats are longer duration. Not used for most CMOS cameras.

 

Tim

 

*For the purists, this discussion ignores Poisson noise. It is present in all light and calibration frames but is not discussed.


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#16 ks__observer

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Posted 18 March 2020 - 11:31 AM

*For the purists, this discussion ignores Poisson noise. It is present in all light and calibration frames but is not discussed.

Lately wondering if it is best to skip darks and do aggressive dithering.  Curious what people think.



#17 rgsalinger

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Posted 18 March 2020 - 11:32 AM

Thanks for that tutorial on calibration. I come from CCD imaging where there's no weird stuff typically like amp glow that needs to be considered when calibrating. So, my surmise is correct for my cameras which do not have amp glow. Keep the flat exposures short and match the dark exposures length to the lights and take a lot of bias frames should work for me. I use sky flats and have a wide horizon. As a result I can get very short narrow band exposures and then later take the LRGB. Again, thanks for the tutorial.

Rgrds-Ross


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#18 Melgabean

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Posted 18 March 2020 - 11:33 AM

Spokeshave thank you for jumping in on this. your explanation is fantastic. my take away is that we were all sort of correct but for different reasons.



#19 spokeshave

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Posted 18 March 2020 - 11:36 AM

Lately wondering if it is best to skip darks and do aggressive dithering.  Curious what people think.

I don't recommend it. Not all thermal fixed pattern noise is in the form of individual pixels. Some is larger scale and would not be corrected by dithering. Additionally while dithering and sigma rejection works well to correct hot and cold pixels, it does so at the expense of SNR. Any time a pixel is rejected, signal from that pixel is lost. Calibration with darks preserves the signal in pixels that would otherwise be rejected with dithering and sigma rejection.

 

Tim


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#20 bulrichl

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Posted 18 March 2020 - 11:53 AM


Bernd,

 

I had a look at you link and read through, but the instructions as per section 3.1 assume master darks have been prepared as per seciton 1, but section 1 doesn't appear to say how to prepare them, e.g. in relation to the bias etc unless i missed it somehow.

Of course I referenced the document that describes the preparation of the master calibration files. The numbers in squared brackets are the references, and these are compiled at the bottom of the tutorial, together with the corresponding link, e.g.:

 

[1] Tutorial by Vicent Peris: "Master Calibration Frames: Acquisition and Processing"

 

Bernd


Edited by bulrichl, 18 March 2020 - 11:53 AM.


#21 bobzeq25

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Posted 18 March 2020 - 01:09 PM

Of course I referenced the document that describes the preparation of the master calibration files. The numbers in squared brackets are the references, and these are compiled at the bottom of the tutorial, together with the corresponding link, e.g.:

 

[1] Tutorial by Vicent Peris: "Master Calibration Frames: Acquisition and Processing"

 

Bernd

Vicent Peris is fabulous, I had the good fortune to take a weeklong class from him.

 

But, that page is old, and has one obsolete statement.

 

"Don't waste time acquiring dark-flat frames: IC will rescale the master dark to match the dark noise in your flat frames."

 

This does apply in most cases, as I said above.  It does not apply to some CMOS cameras, like the 1600.  And _any_ camera with sinificant amp glow should not use rescaling (sometimes called optimizing) for darks.  It won't work for amp glow.

 

But my 183 CMOS cameras (I have both) do just fine with flats, darks, and bias.  They have amp glow, so, no optimizing.


Edited by bobzeq25, 18 March 2020 - 01:10 PM.


#22 bulrichl

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Posted 18 March 2020 - 06:28 PM

Vicent Peris is fabulous, I had the good fortune to take a weeklong class from him.

 

But, that page is old, and has one obsolete statement.

 

"Don't waste time acquiring dark-flat frames: IC will rescale the master dark to match the dark noise in your flat frames."

 

This does apply in most cases, as I said above.  It does not apply to some CMOS cameras, like the 1600.  And _any_ camera with sinificant amp glow should not use rescaling (sometimes called optimizing) for darks.  It won't work for amp glow.

 

But my 183 CMOS cameras (I have both) do just fine with flats, darks, and bias.  They have amp glow, so, no optimizing.

Yes. Obviously you are talking about my guide without having read it correctly.

 

Section 3 of my guide reads as follows:

 

3 Generation of the Master Calibration Frames
It is advisable to prepare (according to [1]) the master calibration frames and check them before the light frame calibration is executed.

 

3.1 MasterDark and MasterBias
It is assumed that MasterDark and MasterBias are prepared according to [1].

 

3.2 MasterFlat
It is assumed, that the MasterFlat is prepared by calibrating the flat frames and integrating the calibrated flat frames according to [1].

 

Depending on equipment and the used method of flat frame acquisition, some people consider it favorable not to use the option of 'Dark frame optimization' in the calibration of the flat frames. Instead of the MasterDark with dark frame optimization they will use either
- a MasterFlat-Dark without dark frame optimization or
- a MasterBias.
In case of long exposure time of the flat frames (e. g. with narrow band filters) or a camera with high noise or with "amplifier glow", a MasterFlat-Dark may be favorable. In case of short exposure time of the flat frames and a camera with low noise, without "amplifier glow", a MasterBias will do well. Just try, which approach is best in your case.

 

If you use a monochrome camera, separate flat frames (and MasterFlats) have to be generated for each filter.

 

 

Bernd


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#23 zer0

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Posted 18 March 2020 - 11:27 PM

There are several different approaches to correct image calibration - and some that are incorrect.

...

Thank you for this great summary. At the top of this thread I was talking about the case with no dark scaling and longer flat exposures: with a temperature controlled CCD I do not see a problem creating darks perfectly matching the lights (at home while sleeping), and my 1x1 RGB flats are all over 1.5s long.

I also like the fact that the "(Light - Dark)/(Flat - Flat Dark)" approach works regardless of the flats duration, as long as the darks aren't scaled.



#24 Der_Pit

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Posted 19 March 2020 - 06:52 AM

I also like the fact that the "(Light - Dark)/(Flat - Flat Dark)" approach works regardless of the flats duration, as long as the darks aren't scaled.

Not only that - it works regardless of the camera type.  CCD or CMOS? Doesn't matter, that approach works.  Which is why I'm using it exclusively since 30 years....



#25 Hawkdl2

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Posted 19 March 2020 - 08:24 PM

I have a ASI1600mm-C and I was about to post pretty much the same question as the OP, but this has cleared up a lot of the confusion - for me.  However,  with the 1600, if the flat exposures are short (less than 1sec) is there still a problem with using a bias instead of dark flats?   From what I can glean from the discussion, the bias problem with the 1600, and perhaps similar cameras, is when flats are greater than 1 sec.




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