Sunday, September 27, 2020

First Light: M51

We chose M51 as the first image of our new scope:
(click on image for full resolution)

M51 is a spiral galaxy in a distance of 23 million light years and has a diameter of 76,000 light years - which means it's slightly smaller than our own galaxy (105,700 light years). It was discovered by Charless Messier on October 13, 1775 - though he only discovered the main galaxy. The smaller galaxy (NGC 5159) was discovered 6 years later by Pierre M├ęchain.
The most prominent feature of this galaxy is the encounter with the smaller galaxy (at the bottom in the image). Although it looks like a frontal encounter, the smaller galaxy (NGC 5159) is actually passing behind the larger galaxy. This process has been going on for hundreds of millions of years.
Another interesting aspect is the larger number of supernovae in M51. There have been supernovae in 1994, 2005 and 2011 - three supernovae in 17 years is much more than what we see in other galaxies. It's not clear what causes this - and if the encounter with NGC 5159 has something to do with it.

It took me a long time to figure out how to process images from it:
  1. Normal Calibration of the individual Frames
    We realized that the shutter on the ML50100 camera isn't completely shut and let some light in. In order to take bias and dark frames, we had to cover the scope (I used a Telegizmo cover for a Dobsonian that we could pull over the entire scope, finder scope and mount:
  2. Using the DefectMap process to correct dark (cold) and white (hot) columns.
  3. Equalizing all images.
  4. Using the SubframeSelector process to create weights for all images and mark the best images.
    And just so that I don't forget the parameters:
    Scale: 1x1: 0.48 2x2: 0.96
    Gain: 0.632
    Weighting: (15*(1-(FWHM-FWHMMin)/(FWHMMax-FWHMMin)) + 15*(1-(Eccentricity-EccentricityMin)/(EccentricityMax-EccentricityMin)) + 20*(SNRWeight-SNRWeightMin)/(SNRWeightMax-SNRWeightMin))+50
  5. Stack the images (I stacked all images against the best Ha image)
  6. Use the LocalNormalization process to improve normalization of all frames
  7. Use ImageIntegration to stack the images
  8. Use DrizzleIntegration to improve the stacked images
  9. Use LinearFit on all images
  10. Use DynamicBackgroundExtraction to remove any remaining gradients
  11. Use ColorCombine on the Red, Green and Luminance images to create a color image
  12. Use StarNet to remove the stars from the Ha image
  13. Use the NRGBCombination script to add the Ha data to the color image
  14. Use PhotometricColorCalibration to create the right color balance
  15. Use BackgroundNeutralization to create an even background
  16. Use SCNR to remove any green residues
  17. Stretch and process the image (no more noise reduction - the advantage of dark skies!!!)
  18. Enhancing the Feature Contrast (the Pixinsight tutorials from LightVortexAstronomy are awesome!!!)
  19. I then use the Convolution process on the RGB data to remove any processing noise in the colors.
  20. Process the Luminance image the same way
  21. Sharpen the image just so slightly
  22. Use LRGBCombination to apply the luminance image to the RGB image
  23. Do some final processing (usually just CurvesTransformation to drop the background a little and maybe adjust Lightness and Saturation to bring out the object better).
  24. Done!!!
Interestingly M51 was also the first images I took with my Celestron EDGE scope 7(!!) years ago: What a different darker skies, better equipment and 7 years experience make :-)

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