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Author Topic: Workflow for astrophotography  (Read 3112 times)


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Workflow for astrophotography
« on: June 17, 2009, 12:22:34 PM »

Hello all,
I am writing as a landscape photographer trying to utilize the best Adobe LR and PS4 techniques for astrophotography.  So I am tapping into the expertise on this list for advice!  I have used dedicated astronomical software packages, but they all convert RAW to .fit formats early in the image calibration phase and I am not sure that is the best way to go about handling RAW files.  I am also not sure about the quality of RAW conversion by these vendors.  There is a lot that happens to the image through these programs that is "behind the scenes" and that bothers me as well.  The image calibration steps achieved by programs like Images Plus are dark frame subraction, bias frame subtraction, and flat field subtraction.  I recently was evaluating some modified (original filter removed in front of CMOS window and replaced with a filter that allows 656nm light to pass more readily) Canon 450D images in LR 2.0 and felt that the noise level was so low that I wanted to attempt to process the images as completely as possible in LR and then stack the images for signal improvement and finish in PS3.  For me, I am getting more out of the RAW images processed through LR and then stacking.  The color correction is coming out much better than a RAW converted and calibrated file through a dedicated astronomical processing package.  Now I will admit to a great amount of user error and lack of time for exploring all options with the dedicated software packages, but I have worked with them a fair amount.  I also see others who are using these programs and I don't believe are getting the optimal results for given exposure times.  Basically, I have more confidence in RAW processing with LR than what is happening with other dedicated astronomical programs.  I am also not sure if it would be of benefit to work on the .fit file in PS.  I have done this in the past, years ago, but haven't loaded the plugin for .fit in PS and don't hold out much hope for any improvement.  Perhaps I am wrong on that account.  

I would like to ask the group about whether I may be correct in assuming that working on a RAW image is inherently better than simply allowing a program to RAW convert to a .fit file with no adjustment applied?  The .fit file is then calibrated via the dark, bias, and flat fields and then aligned and stacked and then simply converted to a loss-less 16 bit tiff file for PS processing.  

I am also applying some noise reduction to the RAW file prior to converting in LR and then stacking.  This may not be optimal and I may pursue some options for RAW processing dark frames in LR and then applying the subtraction via PS4.  But I am skeptical that this may indeed be an unnecessary step-please see notes on signal to noise below.  

The other factor to consider is that we are stacking multiple exposures that are dithered.  Dithering can take place manually or automatically and is simply where the image is shifting across the frame so that noise hopefully can be overcome by signal by accumulating signal at random locations on the chip.  The images are then aligned and stacked based on the star pattern.  My guess is that this is the most powerful form of improving signal to noise ratio and I am not sure that dark frame subtraction adds a lot to improving that ratio with a given amount of stacked exposures.  

I hope I have been clear in explaining what I am attempting to do.  I guess I am looking at maintaining control over the entire process via the programs I am familiar with in LR and PS.  

Thanks for any insights into these questions:

Is it inherently better to process RAW files in a converter with adjustments applied vs. a simple conversion to a .fit file with no adjustments?  
Should I apply noise reduction in the RAW processing?  If it depends on whether dark frame subtraction is being made, then perhaps apply the same noise reduction to a stack of dark frames and then subtract in PS?
I believe the formula for improving signal to noise is exponential and for every half step improvement in signal/noise requires a doubling of the total exposure time?  

I am also careful to expose each frame so that the histogram is peaking at least 1/3 of the way from the left side with no pixels near black.  Some objects do require HDR treatment.

I am posting some of my experiments on my blog at

Thanks so much for considering the topic and I look forward to any insight you can provide.  
Take care.  
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