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The Sunflower Galaxy - true colour


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About this observation
Dean Ashton
Time of observation
25/03/2018 - 20:21
Observing location
St Austell, Cornwall UK
Celestron 235mm EdgeHD
Celestron CGEM Mount
Celestron Nightscape 10100 RGB CCD
LoadstarX2 Guide Camera with Off-Axis Guiding
PrimaluceLab EagleS Telescope Control
Starlight Instruments Boss-II Focus Control
20x600second f/10
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M63, the Sunflower Galaxy, looking remarkably like a sunflower.

On the 25 March, 65% of the moons surface, as seen from the observation location, was illuminated and visible for the whole night.  Apart from this impediment the sky was unusually clear over Cornwall with seeing predicted at 1.4".  A total of 3 hours and 20 minutes of good light frames were capture of M63.  During the image capture process the altitude of M63 increased from 42 to 77 degrees.

Air temperature was around 6C with 91% humidity.

20x600 second light frames captured with camera TEC set at -10C

Images calibrated with master calibration frames created from: 100 bias frames; 40 600s dark frames; 40 flat frames.

Images calibrated, stacked and processed in PixInsight with final processing in Photoshop.

In an image such as this one, the issue of colour is bound to come up.  I used a one shot RGB colour camera for this image.  The process that I use to achieve correct colour balance is:

  1. The light frames are captured as monochrome RAW files and deBayered after calibration.  
  2. I then extract the R, G and B channels from each calibrated light frame and save the separated channels as if I had actually captured separate R, G and B filtered images.
  3. The R channel images are then registered against the highest quality red frame; the G channel images are also registered against the highest quality red frame; and the B channel images are also registered against the highest quality red frame.
  4. The registered R frame images are stacked to create a master R image; the registered G frames images are stacked to create a master G image; and finally the registered B frame images are stacked to create a master B image.  (Note that the stacked or integrated R, G and B images are all registered to each other already because all registrations were carried out against the highest quality red frame).
  5. Separating and registering the R, G and B channels of the original light frames in this way reduces the 'smearing' of the original images due to atmospheric chromatic aberration.  Some of the original RAW images were capture with M63 at an altitude of only 42 degrees.
  6. The R, G and B stacked images are now combined using PixInsights channel combination to generate the single stacked RGB image.
  7. Now for some PixInsight magic, applying the PI Photometric Colour Calibration (PCC) process will ensure that the background colour of the image is neutralised and that the colour balance is corrected. PCC matches the known stellar fluxes of the actual stars to the stars in the image.  This information is used to set the correct white balance to optimise the stellar fluxes in the image to the real and known stellar fluxes.  Current versions of the PCC tool use the AAVSO Photometric All-Sky Survey (APASS) star catalog as the reference to calculate white points.
  8. With colour balance restored while the image is in the linear state, processing can now continue in the usual manner.
  9. After stretching the linear image to create a non-linear image, the colour saturation is washed out by the non-linear stretching process.  PixInsight's Colour Saturation process can be applied to restore the colour saturation.  Applying colour saturation of course is a subjective judgement.  Achieving correct colour balance is a technical process.  Achieving aesthetic colour saturation is an artistic process.  No problem as long as we understand the difference.

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