Calibration frame DARK

[cardconvict]

Could anyone please advise on potentially a issue I'm having. When I stack my dark files I find everything looks good as expected.

 MASTER DARK.xisf

 

But when I apply the bias to the dark file in image calibration the pixel value (I think) goes to zero and the dark is not completely black.

 

MASTER DARK_c.xisf

 

Can someone tell me if this is expected, the above dark is also auto stretched. The dark was taken at -20C five minute exposer on a SX TRIUS 694 CCD. I have also included the master bias

 

MASTER BIAS.xisf

 

Regards Ian]

 

[ollypenrice]

Why are you subtracting a bias from a dark? The bias signal is contained in the dark.   The bias signal is the faintest of the inherent camera noise. Brighter noise is thermal in origin. If, for some reason unknown to me, you subtract the bias from the dark then you'd get the thermal noise minus the faint bias signal - which is what you've got.

The question is, Why do you want it?

😁lly

[cardconvict]

I'm following guides on how to process images in Pixinsight and they are all going in the direction of calibrating the dark with a bias in image calibration. 

This is one guide I'm following: Light Vortex Astronomy - Tutorial (PixInsight): Pre-processing (Calibrating and Stacking) Images in PixInsight

But something does not feel right

Ian

[ollypenrice]

1 minute ago, cardconvict said:

I'm following guides on how to process images in Pixinsight and they are all going in the direction of calibrating the dark with a bias in image calibration. 

This is one guide I'm following: Light Vortex Astronomy - Tutorial (PixInsight): Pre-processing (Calibrating and Stacking) Images in PixInsight

But something does not feel right

Ian

I can't help you with calibration like this. I just keep it simple and use AstroArt. Calibrating a dark with a bias does not involve subtracting the bias from the dark, as I understand it. The bias is used to scale the dark, usually because the darks and lights have not been shot at the same temperature. Given the complexities of CMOS bias I've no idea whether this works for CMOS calibration or not.

I think you'll need Vlad for chapter and verse on this. 😁

Olly

[cardconvict]

Ok thank you for looking I'm also on CCD not CMOS

 

Ian

[Stuart1971]

1 hour ago, cardconvict said:

Could anyone please advise on potentially a issue I'm having. When I stack my dark files I find everything looks good as expected.

  MASTER DARK.xisf 23.24 MB · 2 downloads

 

But when I apply the bias to the dark file in image calibration the pixel value (I think) goes to zero and the dark is not completely black.

 

MASTER DARK_c.xisf 23.12 MB · 3 downloads

 

Can someone tell me if this is expected, the above dark is also auto stretched. The dark was taken at -20C five minute exposer on a SX TRIUS 694 CCD. I have also included the master bias

 

MASTER BIAS.xisf 23.48 MB · 1 download

 

Regards Ian]

 

The dark is not supposed to be pure black, and as has been said the bias is not subtracted from the dark, are your lights and darks taken at the same temp..? They should be.

Simply put, What you  are seeing above is correct, the first image shows the noise from the camera in the dark frame, when calibrated with the bias frames the noise has now gone, correct, but what you are now left with is the hot and cold pixels, (white dots on much darker background) which will also be in the light frames, when you calibrate the darks with these light frames they will then be removed from the light frames…also correct…so I don’t know what you are asking here….

Unless I am completely wrong, and it’s very possible….😂

[cardconvict]

I suppose what I'm trying to get at is the Dark frame with the bias frame subtracted should be as expected and from what you are saying it is ?. The light and dark were exposed at the same times and temperatures.

 

Ian

[scotty38]

There is a lot of information but have a look here:

https://pixinsight.com/forum/index.php?threads/for-beginners-guide-to-pis-imagecalibration.11547/

Also check out Adam Block's WBPP video series. These should give you all you need to know at least for now and probably for a long time....

Edited March 2, 2022 by scotty38

[cardconvict]

4 minutes ago, scotty38 said:

There is a lot of information but have a look here:

https://pixinsight.com/forum/index.php?threads/for-beginners-guide-to-pis-imagecalibration.11547/

Also check out Adam Block's WBPP video series. These should give you all you need to know at least for now and probably for a long time....

Thank you I will check these out