[rcheshire (Rowland)]

I thought I would share this. Perhaps it may be of help to DSLR imagers. Following discussion and assistance on another forum, I am much happier with results, revisiting and reprocessing old data. More of a query, really? Thoughts, experience...?

Taking an astro processing software neutral position, having tried this on two prominent preprocessing packages, reading time and again about issues with another "free program," DSLR RAW data and particularly Canon, in Craig Stark's own words, "is mangled." I hesitate to say, that what you get after all the magic is not necessarily the best result, though it may look OK.

You may be aware that RAW data is subject to camera firmware manipulation, such that it is not strictly linear and not ideal for scaling and application of processes usually applied to linear data - which is a default for some preprocessing software, but changeable.

I have spent quite a bit of time in recent weeks working with RAW data from an uncooled 5DMKII and Fuji X Pro1 and a temperature regulated, cooled 1000D. The Fuji, from what I can tell, using RawDigger, is not so mangled. The Canons hot or cold are not as consistent.

Regulated cooled DSLR data is remarkably consistent, but this falls apart where it is assumed that the data is necessarily linear. The adage "the bias is in the dark" holds true for DSLR data, which is better processed from that perspective, cooled or uncooled, from what I can tell? Bias frames or dark flats are required for flat calibration - bias and flats are nearly linear, as opposed to darks and lights. In a nutshell, don't calibrate the darks.

The real issue behind not calibrating darks is, dark and light DSLR data linearity is unreliable, however, both share approximately the same bias and dark current (other things being equal - exposure time etc.), from what I can tell? Therefore, it is "safer" to calibrate light frames with an uncalibrated master dark and forget about scaling.

Of all the combinations, with 3 sets of data, particularly the Canons (exclude supercooled DSLRs, -25C, if you have one), calibration of light frames with an uncalibrated master dark frame produced the best results. Even at -5C, bias only calibration hurt my light frames - go figure? I conclude that the most reliable and complete calibration applied to DSLR data is had with uncalibrated darks, even though at -5C it's difficult to tell which is a bias and which is a dark. Flats however, can be safely calibrated with a master bias.

If this is useful or someone has a sure fire way of scaling DSLR data, while retaining most of it, following preprocessing, I would love to hear from you.