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[Sticky] What is the bias pedestal and why does it matter?  

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(@mabula-admin)
Quasar Admin
Joined: 2 years ago
Posts: 2148
June 7, 2017 10:58  

The bias pedestal is an ADU (Analog Digital Unit) offset in your data. This pedestal is always there, in your light, flat, dark and bias frames. So the ADU values up until this offset value is always there and it actually is not real data. All ADU values above the bias pedestal are real data, but should be interpreted as the ADU value minus the Bias pedestal.

Let's look at a master bias frame of a zwo optical asi1600mm-cool camera and it's histogram:

app BiasPedestal
app BiasPedestal2

 The histogram shows you the value of the bias pedestal. It's almost exactly located at the peak of the histogram. Try to know this value for your camera's sensor with the ISO/gain and /or offset that you are shooting with.

  • For DSLR camera's this value normally is the same for all ISO settings.
  • For users of astronomical CCD/CMOS camera's, the bias pedestal corresponds to the offset and, usually, can be set manually depending on the camera's manufacturer.

In the FITS header of the master bias, created by APP, you'll find the ADU value of the bias pedestal, in this case we see:

HDU1 - CALFRAME= 'master bias'        / master bias frame                             
HDU1 - INSTRUME= 'ASI Camera (1)'     / instrument name                               
HDU1 - CFAIMAGE= 'no      '           / Color Filter Array pattern                    
HDU1 - MEAN    = ' 1,53E+02'          / mean of channel                               
HDU1 - MED     = ' 1,53E+02'          / median of channel                             
HDU1 - SIGMA   = ' 3,12E+00'          / standard deviation of channel                 
HDU1 - NOISE   = ' 2,93E+00'          / MRS gaussian noise estimate of channel    

The median/mean values indicate it: 1,53E+02, which is a scientific notation for 153 ADUs.

To come back to:

 "So the ADU values up until this offset value is always there and it actually is not real data."

This is the fundamental reason, why this bias pedestal is so important to understand.

In light frame calibration this bias pedestal must be subtracted by either

  • a Master Bias of the same ISO or gain + offset
  • A Master Darks of the same ISO or gain + offset.
  • Or a Master Dark of MasterBias subtracted darks + a Master Bias. both of the same ISO or gain + offset.

APP, currently, will not allow the use of a MasterBias shot with a different ISO or gain+offset than the ISO or gain+offset of your lights, since this will lead to suboptimal calibration for the read noise patterns of your sensor.

For flat frame calibration the same holds, flats need to be calibrated using:

  • a Master Bias of the same ISO or gain + offset
  • A Master Dark of the same ISO or gain + offset.
  • Or a Master Dark of MasterBias subtracted darks + a Master Bias, both of the same ISO or gain + offset.

Main developer of Astro Pixel Processor and owner of Aries Productions


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