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PixInsight => General => Off-topic => Topic started by: Komet on 2012 January 15 03:24:19

Title: Combining Bias frames
Post by: Komet on 2012 January 15 03:24:19

Hello all ....

I am having problems combining bias frames in PI. Each sub frame has an average background of around 669 and yet when combined in PI (using Average/ No normalization) I get a master bias with a background of around 33000. This of course is useless.

Any pointers please about where I am going wrong?

Cheers

--
Mick

Title: Re: Combining Bias frames
Post by: vicent_peris on 2012 January 15 05:40:05

Hi Mick,

Be sure you don't have a flat frame inside your bias image set... This happened to me more than one time.


Regards,
Vicent.

Title: Re: Combining Bias frames
Post by: oldwexi on 2012 January 15 08:16:55

Hi Mick!
Could be a FITS format problem. Comes usually with Fits-files from Maxim...

Have a look here for one of the many solutions:
http://pixinsight.com/forum/index.php?topic=3597.msg24669#msg24669 (http://pixinsight.com/forum/index.php?topic=3597.msg24669#msg24669)



Aloha
Gerald

Title: Re: Combining Bias frames
Post by: Carlos Milovic on 2012 January 15 09:00:32

It seems that the data has been stored as signed integer, or something like that... 65536/2 + 669 = 33437. That explains  the values you are reading.

To fix that, make sure that you are saving the bias frames with the appropriate format.

Another workaround: Use PixelMath to correct the value of the master bias. Just subtract 0.5 to the target image (expression: "$T-0.5"), disabling the rescale option.