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Calibration Frames

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#1 svtskys

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Posted 25 May 2024 - 03:43 AM

Hi all,

 

I'm just wondering, I've been learning about calibration frames and watching lots of youtube videos and am wondering how important temperature is and how people go about it?

All the videos i watch say that temperature should be the same across all images, but if you're imaging all night, the temperature drops and changes all the time. So how does one watch the temperature for calibration frames?

 

Thanks in advance.

 

 



#2 maxsid

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Posted 25 May 2024 - 03:59 AM

For dedicated cooled astro cameras it's easy - the temperature is constant.

 

When I use my DSLR... Plus-minus several degrees is not that critical.

So I have a library of darks and biases for like 45, 55, 65, 75F.

I collect flats every session and then use darks/biases with the temperature closest to flats.


Edited by maxsid, 25 May 2024 - 04:01 AM.


#3 svtskys

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Posted 25 May 2024 - 04:28 AM

For dedicated cooled astro cameras it's easy - the temperature is constant.

 

When I use my DSLR... Plus-minus several degrees is not that critical.

So I have a library of darks and biases for like 45, 55, 65, 75F.

I collect flats every session and then use darks/biases with the temperature closest to flats.

I'm using DSLR, I live in a tropical part of Australia where our night starts off at 18 degrees and can drop to as low as 6 in one night so it makes it a bit hard!



#4 maxsid

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Posted 25 May 2024 - 04:38 AM

Like I said, it's not very critical.

If it starts at 18C and ends at 6C. Use 15C or 10C or so for darks/biases.


Edited by maxsid, 25 May 2024 - 04:40 AM.

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#5 DeepSky Di

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Posted 25 May 2024 - 08:07 AM

Welcome to Cloudy Nights!

 

Topic moved to this forum in accordance with forum rules (watch out for "no astrophotography" forums).



#6 PIEJr

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Posted 25 May 2024 - 08:23 AM

Welcome to Cloudy Nights Mate.

I use a dedicated astro camera and always have. So put me in the cooled camera side of the room.

 

I've done both cooled and ambient, and even used my DSLR some, especially for comets.

One big difference I've seen was the difference's with hot pixels, which look like little stars at ambient but shrink as the sensor is cooled.

 

My biggest reason for using a dedicated Astro Camera was because I felt I was wearing out my camera enough with my soccer pictures of the Grandkids.

My DSLR has a shutter projected lifespan of ~150,000 operations. I'm at ~ 87,000 +. So my thing is to try and save my camera a bit.

And a dedicated astro camera made my life easier.



#7 idclimber

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Posted 25 May 2024 - 10:36 AM

Be aware that there are multiple pathways for calibration. As such you can find a lot of conflicting information, which is just info that is out of date with the newer CMOS sensors. 

 

The currently common pathway is to only use a Master Dark to calibrate the Light via direct subtraction. (Light - Master Dark). This is done pixel by pixel. 

 

Either a Master Bias or a Master Dark-Flat are used to calibrate the Flats also via direct subtraction. 

 

The purpose is simple we are attempting to remove by subtraction the fixed pattern noise that is found in every raw digital frame. One component of the Bias and Darks is that they also have a pedestal in them. Sometimes this is adjustable. If I look at a typical Bias or Dark frame from the ASI2600 with an offset at 50 the ADU values displayed in 16 bit mode will measure a mean of around 502. 500 of that is the offset. 

 

Proper math is important because it is super easy to wind up double subtracting the bias. One alternative calibration pathway subtracts a bias from the dark. This can work but also requires careful subtraction elsewhere. It is not needed unless we are attempting to scale the dark, which is generally no longer done with CMOS sensors. 

 

Worse is that improper math or the failure to remove the offset can and does affect how the calibrated Master Flat works. This last step is one of division but it also multiplies by the mean. This is a ratio. If pixel in our flat frame has the same value as the mean that ratio is one. If we don't proper subtract the offsets found in the Light or Flat it changes the mean and these rations. The result is either over or under correction.

 

If we stick with that current common calibration pathway, we want to temperature match the frames as follows:

 

Light - Darks 

 

Flats - Bias. 

 

One common alternative with cameras like the ASI2600 is to substitute the Master Bias for the Master Dark. This only works if those frame are also matched. 



#8 BQ Octantis

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Posted 25 May 2024 - 11:48 AM

G'day svtskys,

 

What camera?

 

Canons have the sensor temperature in the EXIF metadata header. So you can bucket the captures into like temperatures and use temperature-matched darks to calibrate.

 

Cheers,

 

BQ


Edited by BQ Octantis, 25 May 2024 - 11:51 AM.



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