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Thursday, May 1, 2014

Astrophotography DSLR RAW Data Calibration: preprocessing

This thread has its origins in Craig Stark’s article about the non-linearity of DLSR RAW data, as well as discussions on various forums about preprocessing of DSLR RAW frames.

The non-linearity of DSLR RAW data (compared to 16bit CCD linear data), is a product of camera firmware corrections, which are designed to create a RAW image that is easily processed, out of the box!

The issues discussed here were more evident when preprocessing cooled, temperature regulated, DSLR images, but does not rule out the same application to uncooled DSLR RAW data. However, if you have one of those supercooled DSLRs, perhaps dark frames are a thing of the past. A carefully constructed master bias and flat may be all that is required.

“The point of this discussion is, that problems arise with DSLR RAW data when dark frames are bias subtracted and then the master dark frame is subtracted from the light frames, as well as subtracting a master bias - as one would do if scaling darks. This is a process normally applied to linear data and is not suited to DSLR RAW data calibration. Linear processes applied to DSLR data may produce less than optimal and sometimes devastating results due to data truncation.”

Because DSLR RAW data is processed in the camera, it is “clean” to begin with, and applying pixel rejection algorithms to calibration frames is unnecessary. However light frames are exposed to unfriendly light sources and pixel rejection is necessary.

A note on bias frames - CMOS sensors do not have a bias and instead produce a pattern of fixed noise, which, to all tense and purpose, may be considered bias and applied in the same way.

The practice of not calibrating dark frames is common place, and has been for years; “the-bias-is-in-the-dark.” However, the advantages and reasons, applied to DSLR RAW data, may not be clearly understood - “it’s always been done that way.” Some software defaults to dark scaling and is not suitable for DSLR RAW data.

“A typical DSLR RAW calibration data set, comprises uncalibrated darks and bias subtracted flats. Flats and bias frames can be taken at the cameras lowest ISO setting.” Denoising the master flat frame is recommended by some imagers.

AstroArt preprocessing is a one time operation, and uncomplicated. In PixInsight, master bias, master dark and master flat frames are prepared separately to avoid bias subtraction of dark and light frames. Either way, pixel rejection is applied to light frames only.