Quote:
Originally Posted by glend
Way over my head Rowland,  but I enjoy reading about what your up to now. 
Is there a way to explain what your trying to achieve in more basic (layman) terms? |
I thought I was explaining it in laymans terms! That explains a thing or two!
This is the theory - DSLR RAW data is heavily modified by camera firmware, and there is no way of turning it off. The camera manufacturer needs to present an image to the user that looks like a photo. Consequently, the linear data from the sensor is modified to look good - this is particularly true of light and dark frames. To some extent bias and flats are more linear, or so I am informed.
The problem arises where the bias is subtracted from both the dark and light frames, sometimes with a view to scaling data. Unfortunately, this can lead to truncating/loosing data. A better method, certainly for beginners, is to leave the bias in the dark.
This is not new by any means - to which "H" will attest, as he has reminded me and others on numerous occasions that the bias is in the dark, and better left there. He is correct, and Jim Solomon nicely presents the formula in his Astrophotography Cookbook.
I think, in some cases DSLR RAW data reduction/calibration has been hijacked - well meant with unintended consequences - by linear 16 bit data reduction methods, obliterating the finer detail available in DSLR RAW data, properly processed.
The script sets out a folder structure and manipulates the images as follows;
1. RAW to 16bit tiff conversion with dcraw; then using ImageMagick
2. Average combine bias and dark frames to produce master bias and master dark frames.
3. Subtract the master bias from flats and median combine the flats to produce a master flat.
4. Subtract the master dark from the light frames and divide by the master flat.
EDIT: If you have dark flats substitute these for bias frames - as they are intended for flat reduction.
Cooled - sub-zero application.
There is also a no dark frame option for cooled DSLRs. Selected, the script will not prompt for, or process, dark frames. Instead it uses the bias, quite contrary to what I said above. But as you know there is little to differentiate darks and bias frames at sub-zero temperatures - so I think its OK.
If you wanted to take flats at a lower ISO, you will take separate sets of bias frames for flat and light frame subtraction. In that case load the bias frames intended for the lights as dark frames an those intended for the flats as bias.
If you have a dark library, (providing the dark is not bias subtracted) just copy your master frames to the image reduction directory and rename per the convention; e.g., MDARK_project_name.tiff - then 'Skip" all of the processes you don't need.
Interesting exercise and lots learned. I have reprocessed pretty much all archived stuff and seen first hand the improvement in image quality.
Caveat. Temperature consistency between lights and darks is a prerequisite. I will get around to scaling data.
