About debias and darks

[magnusl]

Hi!

I think I am a bit confused on what the option "deBias" means and does. I am trying to make sense of it but I think I need to at least reality-test my understanding. This is my thinking:

1.
When building a new dark, AIJ subtracts the master bias (assuming it is enabled) from the master dark made, that is, averaging all dark subs, then subtracting the bias from these.

In the subsequent calibration, bias is thus subtracted once (the bias subtraction). Question: why then not just subtract the dark, un-deBiased (that is, including the bias), and not bother with a bias at all?

2.
When using an existing master dark (in my case build by Aij), deBias means that the enabled bias is subtracted. If it was subtracted in the build process (following 1) it will thus be subtracted twice from the master dark... or? Anyway, leaving the resulting calibrated light with an extra pedestal of a bias value, but no other issue (like zeroeing out on pixels). Right or wrong?

3. When using an existing dark, built through the process 1, and NOT enabling deBias - would result in the same result as 1.... righ?

When I experiment, I find that:
a. building a new master dark leaves basically the same background level with or without deBias enabled.
b. using an existing master dark and enabling deBias gives the same result as a.
c. using an existing master dark without enabling deBias makes the flat correction fail - I get very visible remaining vignetting - thus influencing the flat correction process.... why?

I very much appreciate if anyone can help me untangle this or point to a relevant resource to clarify my thinking. Sorry if I am just confused....

Magnus

[karenacollins]

Hi Magnus,

If your dark exposure times and flat exposure times are the same as the science exposure times, then there's no need to use a separate bias subtract. If the darks you are using have a different exposure time than the science images or flat images, then separating the bias and darks allows for scaling the darks according to the ratio of the dark to science and dark to flat exposure times. This approach is not perfect, but it's better than no scaling at all.

If you created a master bias and an associated debiased master dark, then of course you'd need to use the same master bias and debiased master dark. Again, if your dark, flat, and science exposure times are all the same, just avoid using the bias altogether. Or if you don't want to keep up with it, just always use a bias and a debiased dark and you should have no problem in any case (of course you need to keep the "scale" and "deBias" dark settings enabled in DP).

Karen

On 1/20/2021 12:37 PM, magnusl [via AstroImageJ] wrote:

Hi!

I think I am a bit confused on what the option "deBias" means and does. I am trying to make sense of it but I think I need to at least reality-test my understanding. This is my thinking:

1.
When building a new dark, AIJ subtracts the master bias (assuming it is enabled) from the master dark made, that is, averaging all dark subs, then subtracting the bias from these.

In the subsequent calibration, bias is thus subtracted once (the bias subtraction). Question: why then not just subtract the dark, un-deBiased (that is, including the bias), and not bother with a bias at all?

2.
When using an existing master dark (in my case build by Aij), deBias means that the enabled bias is subtracted. If it was subtracted in the build process (following 1) it will thus be subtracted twice from the master dark... or? Anyway, leaving the resulting calibrated light with an extra pedestal of a bias value, but no other issue (like zeroeing out on pixels). Right or wrong?

3. When using an existing dark, built through the process 1, and NOT enabling deBias - would result in the same result as 1.... righ?

When I experiment, I find that:
a. building a new master dark leaves basically the same background level with or without deBias enabled.
b. using an existing master dark and enabling deBias gives the same result as a.
c. using an existing master dark without enabling deBias makes the flat correction fail - I get very visible remaining vignetting - thus influencing the flat correction process.... why?

I very much appreciate if anyone can help me untangle this or point to a relevant resource to clarify my thinking. Sorry if I am just confused....

Magnus

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[magnusl]

Hi!

Thanks.

Yes, of course, if scaling there is a need for bias. Got it. My case is that I have a bunch of subs with different exposure times (from 30 to 300 secs) and darks with 300 secs. Taking them all in one batch is the smooth way, makes it far easier and quicker.

But then, what does the deBias do when I re-use an already produced dark (produced with deBias)?

I also wonder why I get the same background when having enabled bias, and building darks with or without deBias enabled. Could this really be the case? Or maybe it is a measurement error on my part - but I really found them to be identical.

The reason I am asking, is that I have found that at times I get strange results from the calibration process, but so far not been able to find out why. And with strange I mean either too much subtracted (I don't know the term, but with zero values in the background) or where flat calibration seems not to be effective - and I have no clue of why, but I suspect there can be something wrong with the way I set the deBias feature.

Best,

Magnus

Den 2021-01-21 kl. 11:14, skrev karenacollins [via AstroImageJ]:

Hi Magnus,

If your dark exposure times and flat exposure times are the same as the science exposure times, then there's no need to use a separate bias subtract. If the darks you are using have a different exposure time than the science images or flat images, then separating the bias and darks allows for scaling the darks according to the ratio of the dark to science and dark to flat exposure times. This approach is not perfect, but it's better than no scaling at all.

If you created a master bias and an associated debiased master dark, then of course you'd need to use the same master bias and debiased master dark. Again, if your dark, flat, and science exposure times are all the same, just avoid using the bias altogether. Or if you don't want to keep up with it, just always use a bias and a debiased dark and you should have no problem in any case (of course you need to keep the "scale" and "deBias" dark settings enabled in DP).

Karen

On 1/20/2021 12:37 PM, magnusl [via AstroImageJ] wrote:

Hi!

I think I am a bit confused on what the option "deBias" means and does. I am trying to make sense of it but I think I need to at least reality-test my understanding. This is my thinking:

1.
When building a new dark, AIJ subtracts the master bias (assuming it is enabled) from the master dark made, that is, averaging all dark subs, then subtracting the bias from these.

In the subsequent calibration, bias is thus subtracted once (the bias subtraction). Question: why then not just subtract the dark, un-deBiased (that is, including the bias), and not bother with a bias at all?

2.
When using an existing master dark (in my case build by Aij), deBias means that the enabled bias is subtracted. If it was subtracted in the build process (following 1) it will thus be subtracted twice from the master dark... or? Anyway, leaving the resulting calibrated light with an extra pedestal of a bias value, but no other issue (like zeroeing out on pixels). Right or wrong?

3. When using an existing dark, built through the process 1, and NOT enabling deBias - would result in the same result as 1.... righ?

When I experiment, I find that:
a. building a new master dark leaves basically the same background level with or without deBias enabled.
b. using an existing master dark and enabling deBias gives the same result as a.
c. using an existing master dark without enabling deBias makes the flat correction fail - I get very visible remaining vignetting - thus influencing the flat correction process.... why?

I very much appreciate if anyone can help me untangle this or point to a relevant resource to clarify my thinking. Sorry if I am just confused....

Magnus

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[karenacollins]

The simplest method is to always use a master bias and a master dark and keep deBias enabled. They there's nothing to keep up with whether dark scaling is required or not.

If you have a specific case of over-subtraction of (likely) bias (i.e. the bias has been subtracted twice), then it sounds like something has gone wrong. I'd need a specific case to help troubleshoot though, so send specific case details the next time you see the problem. I'd likely need to have access to a raw bias and dark file, as well as the master bias and dark files that were produced.

Karen

On 1/21/2021 11:07 AM, magnusl [via AstroImageJ] wrote:

Hi!

Thanks.

Yes, of course, if scaling there is a need for bias. Got it. My case is that I have a bunch of subs with different exposure times (from 30 to 300 secs) and darks with 300 secs. Taking them all in one batch is the smooth way, makes it far easier and quicker.

But then, what does the deBias do when I re-use an already produced dark (produced with deBias)?

I also wonder why I get the same background when having enabled bias, and building darks with or without deBias enabled. Could this really be the case? Or maybe it is a measurement error on my part - but I really found them to be identical.

The reason I am asking, is that I have found that at times I get strange results from the calibration process, but so far not been able to find out why. And with strange I mean either too much subtracted (I don't know the term, but with zero values in the background) or where flat calibration seems not to be effective - and I have no clue of why, but I suspect there can be something wrong with the way I set the deBias feature.

Best,

Magnus

Den 2021-01-21 kl. 11:14, skrev karenacollins [via AstroImageJ]:

Hi Magnus,

If your dark exposure times and flat exposure times are the same as the science exposure times, then there's no need to use a separate bias subtract. If the darks you are using have a different exposure time than the science images or flat images, then separating the bias and darks allows for scaling the darks according to the ratio of the dark to science and dark to flat exposure times. This approach is not perfect, but it's better than no scaling at all.

If you created a master bias and an associated debiased master dark, then of course you'd need to use the same master bias and debiased master dark. Again, if your dark, flat, and science exposure times are all the same, just avoid using the bias altogether. Or if you don't want to keep up with it, just always use a bias and a debiased dark and you should have no problem in any case (of course you need to keep the "scale" and "deBias" dark settings enabled in DP).

Karen

On 1/20/2021 12:37 PM, magnusl [via AstroImageJ] wrote:

Hi!

I think I am a bit confused on what the option "deBias" means and does. I am trying to make sense of it but I think I need to at least reality-test my understanding. This is my thinking:

1.
When building a new dark, AIJ subtracts the master bias (assuming it is enabled) from the master dark made, that is, averaging all dark subs, then subtracting the bias from these.

In the subsequent calibration, bias is thus subtracted once (the bias subtraction). Question: why then not just subtract the dark, un-deBiased (that is, including the bias), and not bother with a bias at all?

2.
When using an existing master dark (in my case build by Aij), deBias means that the enabled bias is subtracted. If it was subtracted in the build process (following 1) it will thus be subtracted twice from the master dark... or? Anyway, leaving the resulting calibrated light with an extra pedestal of a bias value, but no other issue (like zeroeing out on pixels). Right or wrong?

3. When using an existing dark, built through the process 1, and NOT enabling deBias - would result in the same result as 1.... righ?

When I experiment, I find that:
a. building a new master dark leaves basically the same background level with or without deBias enabled.
b. using an existing master dark and enabling deBias gives the same result as a.
c. using an existing master dark without enabling deBias makes the flat correction fail - I get very visible remaining vignetting - thus influencing the flat correction process.... why?

I very much appreciate if anyone can help me untangle this or point to a relevant resource to clarify my thinking. Sorry if I am just confused....

Magnus

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To unsubscribe from AstroImageJ, click here.
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