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[Solved] If the calibration frames are different from the principle.

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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
Posts: 86
Topic starter  

Hello everyone

I have read the "Data calibration principles/rules".
Of course, it is best to follow this principle, and I understand that it is essential to get the best image.

However, the reality is tough. If the calibration frames are different from the principle, are there some function that can be used to regain the disadvantages of calibration?

Thank you

This topic was modified 4 months ago by momo

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(@vincent-mod)
Quasar Admin
Joined: 5 years ago
Posts: 4727
 

Mm, what do you mean with "not in principle"? Calibration data is there so APP (and other programs) can do proper background calibration, which helps a lot in further processes of say, light pollution correction, etc. You can do without calibration, but the chances are much higher that you'll end up with other issues.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
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Topic starter  

@vincent-mod

Hello Vincent

For example, bias gain does not match flat. dark gain does not match light and so on.


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(@wvreeven)
Galaxy Admin
Joined: 4 years ago
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@mo-mo Basically you have two posts now where you ask the same question. I will answer here only.

What you should always do is

  1. Suppose that you shoot your lights at gain 100 and a certain offset, temperature and exposure time. Then make sure to have darks of the same gain (100), offset, temperature and exposure time. You can make use of darks of a different exposure time and in that case you need bias of the same gain, offset and temperature.
  2. If you shoot the flats at a certain gain, offset and temperature then you need dark flats of the same gain, offset and temperature. If the dark flats have the same exposure time as the flats then you're done. If they have a different exposure time then you'll need bias of the same gain, offset and temperature.
  3. Note that the gain, offset and temperature of the flats do not have to be the same as those of the lights. If that is the case and you need to use bias for both, then you will need to make sure that you shoot 2 sets of bias frames.

As long as you make sure that the darks have exactly the same gain, offset, temperature and exposure time as the lights, you will not need bias for the lights.

As long as you make sure that the dark flats have exactly the same gain, offset, temperature and exposure time as the flats, you will not need bias for the flats.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
Posts: 86
Topic starter  

@vincent-mod

@wvreeven

Hello

I got a comment from Vincent at the following URL. Do I understand the content correctly?

https://www.astropixelprocessor.com/community/main-forum/an-error-happened-version1-082/

I understand that:

I think that I need the following 1-5 conditions.

1.Bias should be the same gain as flat.
2.Flat does not have to be the same gain as light.
3.By lowering the gain of flat, the bias gain is also lowered (same as flat)to make the exposure time about 0.5 seconds.

4. I need to make dark and bias gain the same as light gain 100

5.if I change the gain on dark or bias, Another bias or darkflat (dark with the exposure lenght and gain of the flat) for the flats is needed.

By Conditions 1 to 3, will be the next gain, isn't it?
light gain: 100, flat gain: 50 (provisional), dark gain: 100, Bias gain: 50

If I apply Condition 4 and 5 to this, it will be the next gain to meet the condition 3, isn't it?
light gain: 100, flat gain: 50 (provisional), dark gain: 100, bias gain: 50 (for Flat) +100 (for Light), need 2bias

Is my understanding of the above correct?

This post was modified 4 months ago by momo

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(@vincent-mod)
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You're mostly right yes, it's not required to lower the gain on flats though, but if you do you should have darkflats or a bias with the same gain and offset as those flats.

So to summarize:

Lights: Gain 100, exposure A, offset X
Darks: Gain 100, exposure A, offset X
Bias: Gain 100, offset X

Flats: Gain 100 (or lower if you want), exposure B, offset X
Darkflats: Gain 100 (or lower if you lowered the gain of the flats), exposure B, offset X


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(@wvreeven)
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Joined: 4 years ago
Posts: 1647
 

Adding to what Vincent wrote, from the screenshots in your other post it looks like you are using an ASI camera. Which one is it exactly? Because for the newest cameras a gain of 50 wouldn't be correct for the flats but a gain of 0 would be what you should use.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
Posts: 86
Topic starter  

@vincent-mod

According to your comments,
The bias is shot at the fastest possible setting and I wonder if that might be an issue. Some sensors don't behave well in that range so we advice to go for 0.5 second per bias frame.

If it is faithful to that comment, I think that it can not be 0.5 seconds with the gain 100, so lower the BIAS gain. In conjunction, don't I need to lower the FLAT gain?


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 momo
(@mo-mo)
Main Sequence Star Customer
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Topic starter  

@wvreeven

Yes, ASI Camera. The model is 2600MC Pro.


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(@wvreeven)
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@mo-mo For an ASI2600MC you should use either gain 0 or gain 100. Don't use gain 50.

Bias frames can be shot at the shortest exposure time of 0.001 sec. I do this as well. If you don't trust that, you can use 0.5 seconds for bias.

And once more: if you shoot your lights at gain 100 and a certain temperature and exposure time, then you need darks of the same gain, temperature and exposure time. If you then shoot your flats at gain 100 and a certain temperature but a much shorter exposure time, then you need dark flats of the same gain, temperature and short exposure time as the flats.

If you decide to shoot the flats at gain 0 and a certain temperature and short exposure time, then you need dark flats of the same gain (in this case 0), temperature and short exposure time as the flats.

You don't need anything else. ONLY if your darks don't have the same exposure time as the lights OR if the dark flats don't have the same exposure time as the flats, only then you will need bias.

And please note that the temperature of the darks NEED to be the same as the temperature of the lights. Similarly, the temperature of the dark flats NEEDS to be the same as the temperature of the flats. If you require bias frames for the lights then the temperature of the bias frames NEEDS to be the same as the temperature of the lights. If you require bias frames for the flats then the temperature of the bias frames NEEDS to be the same as the temperature of the flats.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
Posts: 86
Topic starter  

@wvreeven

Thank you for your polite answer to my question.

I've never used dark flats, how do I use it in an APP?

Then, according to the

[Sticky] Data calibration principles / rules --must read!

, I can use master bias.

Also, it is attractive to use master bias because there is no statement to the effect that the same temperature is required.

Do I need to adjust the temperature as Wouter says?

This post was modified 4 months ago by momo

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(@wvreeven)
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@mo-mo

Posted by: @mo-mo

I've never used dark flats, how do I use it in an APP?

You simply take darks that have the same gain, offset, temperature and exposure time as the flats. Then in APP instead of loading them as darks you load them as dark flats and you're done.

Posted by: @mo-mo

Also, it is attractive to use master bias because there is no statement to the effect that the same temperature is required.

I'm checking this with Mabula and will be back about this as soon as I can.


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(@wvreeven)
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Posted by: @mo-mo

Also, it is attractive to use master bias because there is no statement to the effect that the same temperature is required.

I checked with Mabula and temperature is VERY important. This sticky thread was posted a long time ago when APP had just been released and APP worked differently then than it does now. So my statements about the temperature were correct. I'll discuss with Mabula the option of writing a new Data Calibration Principles post.


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 momo
(@mo-mo)
Main Sequence Star Customer
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Topic starter  

@wvreeven

I am looking forward to the principle of calibration to be clarified by you.
Because I understand the principle of calibration very important.

By the way, it returns to the purpose of this thread.

My flat, dark and bias temperature is different from light.

The result is an attached image, but is there any way to correct and repair this using APP?

Rosette 010s RGB session 1 St

 


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
Posts: 86
Topic starter  

@wvreeven

Thank you for the explanation of Dark Flats.
I will try it.


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(@wvreeven)
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Posted by: @mo-mo

My flat, dark and bias temperature is different from light.

To repeat what I have written before: the dark and bias frames need to have the same temperature as the lights. If you use flats at the same gain and offset and the lights then they need to have the same temperature as well so you can apply the same dark and bias frames to them. If you use a different gain or offset then you will need at least dark flats of the same temperature, gain, offset and exposure time and the flats. If the exposure time is different then you will need also bias frames of the same gain, offset and temperature.

Posted by: @mo-mo

The result is an attached image, but is there any way to correct and repair this using APP?

You can try Remove Light Pollution in the Tools tab but it will be hard to correct this. It may be easier to shoot new darks and bias such that they have the same temperature, gain and offset as the lights plus new darks and bias such that those have the same temperature, gain and offset ans the flats.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
Posts: 86
Topic starter  

@wvreeven

I will try Remove Light Pollution. first.

Again, because I use the telescope hourly rental service, the service provider gives me a generic dark, flat, bias. I can't shoot them new.

When that happens, I think of a realistic option.
・ Use another service provider. iTelescope etc.
・ Negotiate with the current service provider to provide dark, flat, bias according to the principle
And so on

I think I will continue to use APP, but The APP change added a temperature condition to the bias, making it harder to use for me.

My suggestion is that APP will be able to provide the next best solutions for dark, flat and bias that deviate from the principle.

I hope that this will be a consideration.


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(@wvreeven)
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Joined: 4 years ago
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Posted by: @mo-mo

I think I will continue to use APP, but The APP change added a temperature condition to the bias, making it harder to use for me.

My suggestion is that APP will be able to provide the next best solutions for dark, flat and bias that deviate from the principle.

Please understand that this is not a requirement of APP but a requirement in general. Sensors of cameras introduce unwanted noise in the images and the amount of noise depends on gain, offset, temperature and exposure time. Any other image processing application will provide equally incorrect results if the calibration data differ in temperature, gain or offset. Please talk to your telescope provider to discuss this matter.

Having said that, APP does not stop processing if the calibration data differ from the lights. It continues processing and tries to make the best of the situation. Just be aware that the result would be better with better calibration data.


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 momo
(@mo-mo)
Main Sequence Star Customer
Joined: 6 months ago
Posts: 86
Topic starter  

@wvreeven

Yes, I understand the principles of calibration.

I am accepting what you have told me over and over.
The gist of my proposal is a remedy if unfortunately it deviates from that principle of calibration. If there is a remedy, I think more people and more images will benefit. Also, I don't make a reason not to use APP even if there are few remedies.

This post was modified 4 months ago by momo

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(@wvreeven)
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Posted by: @mo-mo

The gist of my proposal is a remedy if unfortunately it deviates from that principle of calibration. If there is a remedy, I think more people and more images will benefit. Also, I don't make a reason not to use APP even if there are few remedies.

Yes I understood that. Like I wrote, APP will continue processing and make the best of it. So your proposed remedy already exists.

Coming back to the image that you posted, it looks like the flats overcorrect the vignetting in the lights. I am not sure if this is because of the darks and bias used or if the flats are problematic themselves. Could you upload a small set of, say, 10 lights, flats, darks and bias to

https://upload.astropixelprocessor.com/

and use for both username and password: upload Note that you cannot use your AstroPixelProcessor username and password! Please create a directory called mo-mo_flats_overcorrect and put the files there. Let me know here when the upload is done and I'll have a look at the data. Thanks!


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