Image Processing

Part III - Image Processing

Unlike the previous sections, I am not going to try and cover all of the different methods for processing images, but stick with what I know and also what I believe to be your best option, namely using PixInsight. It is a fabulous piece of software dedicated to astro-imaging that offers a veritable giant toy box of tools to get the most out of your data. It also makes sense. One of my biggest compliants with PhotoShop and the like is that it feels like painting rather than working with your data. PixInsight, on the other hand, gives you real insight into the workings of your image and pushes you to expand and explore to constanstly improve your skill set. I can't say enough about how good this package is and how well the development team supports their work. And at its price, for what you get, its a steal. I should note, I have no affiliation with PixInsight other than as an extremely satisfied customer and a regular on the PixInsight forum board.

My cribsheet for PixInsight is now in its 28th revision (see what I mean about constantly learning) and is over 40 pages long. As such I am only going to include my workflow and some philosophical considerations below. If you are interested in the full version, please drop me a line at jkmorse57@gmail.com and I will be happy to send you a copy and add you to my distribution list for future updates.

 

PixInsight Cribsheet Excerpts

Image Processing Steps:

• Build Bias (including Superbias), Dark & Flat Master Calibration Frames
• Select Light Frames using SubFrameSelector
• Calibrate Image Stacks by Channel
• Run Cosmetic Correction to further clean images
• Separately Star Align Channel Stacks
• Dynamic Crop each Stack
• Integrate and, if undersampled, Drizzle combine each Aligned Channel Set
• Dynamic Background Extraction for each Channel
• Star Align RGB/NB Channel Stacks
• Dynamic Crop R, G, & B/NB) Stacks
• Apply Linear Fit to equalize R, G, & B/NB stack histograms
• If shooting RGB only, create a synthetic Lum (if NB, use Ha image as Lum)
• Create RGB/NB using ChannelCombination
• If binned, Resample RGB Image
• Neutralize Background in RGB Image (may not be needed if Step 10 used)
• RGB Color Calibration (may not be needed if Step 10 used)
• RGB/NB Linear Color Saturation with Curves Transformation*
• If necessary, run second Dynamic Background Extraction on RGB image*
• Deconvolve Lum to increase detail and tighten stars*
• MLT noise reduction of Lum and RGB/NB by removing first wavelet layer only*
• Consider very mild Unsharp mask to subtly sharpen features*
• If needed, Linear TGVD and/or MLT Noise reduction of RGB/NB and Lum*
• If separate Lum, use Channel Extraction to extract RGB L, a, and b channels
• If separate Lum, ChannelCombine a and b channels with separate Lum
• RGB and Ha and/or OIII Combine (if needed) w/NBRGBCombination or PixelMath
• Histogram Transform Stretch RGB/NB & Lum Images (my preferred method)
• (Optional: MaskedStretch for initial stretch (~20 iterations, 0.20 target median))
• Increase Color Saturation (Saturation or Curves)*
• SCNR
• Non-Linear Noise Reduction of RGB/NB and Lum (MLT, MMT and/or TGVD)*
• HT Touchup of RGB/NB and Lum (reset black point w/o clipping)
• Star Align RGB/NB and Lum Images
• LRGB/LNB Combine
• TVGD and/or MLT noise reduction*
• HT Touch-up (black point reset)
• If needed, for HDR image use HT to brighten target, w/o saturating**
• HDRMT to increase detail*
• Morphological Transformation to tighten stars*
• HT Touch-up (black point reset)
• MLT and/or MMT for sharpening*
• Dark Structure Enhancement*
• LHE application*
• Saturation Curve
• If necessary, Crop & Print
• = mask recommended

NB      = NarrowBand imaging with Ha, OIII, and SII filters

Philosophical Considerations (What PixInsight has taught me, I think)

• When shooting RGB images, based on discussions threads in the PI Forum with PI developer Juan and others, I now only image RGB frames (i.e., I shoot no luminance frames) using 1x1 binning and use the R, G, & B master integrated images to create a synthetic Luminance frame as per step 9, above.  If you are interested in the discussion, see the attached: http://pixinsight.com/forum/index.php?topic=6042.15
•  PixInsight has taught me that SNR quality definitely trumps target quantity.  PI rewards taking the time to maximize SNR for a single target versus padding your target list.  When I was just starting, I would shoot up to 6 targets a night.  Now I shoot one target over two nights if possible.  That typically gives me over 25 ten minute subframes with each of my R, G, and B filters (or my NB filters).
• Take the time to do the little things that add real value both in the field and when building images.  At the top of that list is building high quality bias (and superbias), dark and flat master frames.  I have a number of notes below on how I build each so I won’t repeat that here, but it is worth mentioning that I try to shoot as many calibration subframes as possible.  That means a minimum of 64 (currently using 100) dark frames for each dark master iteration (I shoot my darks to match the exact time of my lights), 25 or 36 frames for each flat in the field, and 400 frames to build my bias.  And, if you have a cooled CCD, you can shoot those darks and bias frames at home during full moons or cloudy weather so it’s not like you are wasting imaging time to build quality sets.  And remember to reshoot those darks and bias frames periodically (e.g., at least every 6 months)
•  I know several of the noise reduction tools can be used in the linear stage, but be gentle to avoid stretching artifacts that are hardly noticeable in the linear stage that become real processing problems in the non-linear stage.  I have learned that unless you have really noisy images (and that probably means you are not doing enough to maximize SNR; see point 2), there is no reason to do heavy noise reduction before you permanently stretch your images.  A light touch here goes a long way.  That said, I have been really impressed with how well MLT works for noise reduction in the linear stage so if you are using that tool, you are likely going to be fine.
• Experiment, experiment, experiment!  Anything in this cribsheet re settings are only intended to be starting points or settings that work for my rig.  You owe it to yourself to make tweaks.  The beauty of PixInsight is the combination of its logic with an infinite ability to tweak the tools to do what you need.  It really is amazing software and by playing with it you start to get a sense of what all those settings do.  I have yet to process an image where I don’t learn something new or discover a better way of doing it, often due to the great suggestions from the pros in the Forum. 

Please let me know if you find this helpful, but also where I can make it better.  This is especially true if I just have something flat out wrong.  It does no one any good for me to keep repeating the same error in revision after revision.  It’s also why this cribsheet is up to Rev38, and I am certain there will be at least another 20 to replace this one.  You can find me on the Forum on a regular basis, but also feel free to drop me an email at jkmorse57@gmail.com.