With Your Manual Calibration Tutorial Aren't We Subtracting The Bias Twice WRT Integrated Images??

Tony Benjamin

Hi,

This has probably been asked before.

If you are calibrating the Flats with Bias frames (subtracting the Bias) and Darks and then go ahead and calibrate the Lights with the Master Flat and Master Bias and Master Dark then don't you end up subtracting the Bias twice (once for the Flats and then again for the Lights)? Wouldn't that cause an over subtraction of the Bias?

BlockHead

Hi Tony,

I would have to look back... but I think the Master Flat is calibrated (meaning the bias is subtracted).

The Master Dark still has the bias.  Please look at approximately 14:19 of Section 2. 

https://www.adamblockstudios.com/articles/ManualCalibration_02

Do you see how "Calibrate Dark" is checked?

*This will subtract the bias from the master dark and leave what is called the thermal frame.

So in ImageCalibration the Light frame is first bias subtracted and then it is thermal frame subtracted and finally flat fielded (by calibrated flats, this isn't a subtraction but a division). 

So the Light frame is bias subtracted only once *if I am doing this correctly.

-the Blockhead