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Technical card

Resolution: 2815x2010

Dates: Sept. 14, 2015

Frames: Hutech IDAS 2" LPS-D1: 18x300" ISO800

Integration: 1.5 hours

Darks: ~30

Flats: ~20

Bias: ~50

Avg. Moon age: 0.64 days

Avg. Moon phase: 0.46%

Astrometry.net job: 825243

RA center: 23.363 degrees

DEC center: 60.644 degrees

Pixel scale: 0.930 arcsec/pixel

Orientation: -4.615 degrees

Field radius: 0.447 degrees

Locations: Home base, Yorktown, VA, United States

Description

Here is my image of the open cluster M103 in Cassiopeia that I captured on September 14th, 2015 as a break from the week long streak of clear nights where I banged relentlessly away on the Pacman Nebula. This was the first time I tried out the IDAS-D1 light pollution filter on a star cluster and I am very pleased with the quality colors that are retained while noticeably suppressing background brightness.

I always prefer diffraction spikes on star cluster images, so I used braided fishing line taped to my dew shield to produce those seen here. I have experimented in the past with different thickness mono filament fishing line and I have yet to find one that I really like. In all cases, and including here, the diffraction spikes broaden with distance from the star and produce rainbow colors. It's kind of a neat effect, but I'm not sold on it. In the future I will try metal guitar string instead.

It took time to figure out the best framing for this cluster because a wide view makes it difficult to figure out what you are supposed to be looking at (i.e., the subject) and too close of a view makes it hard to recognize that this is a cluster rather than just a picture of a bunch of stars. I think I found a good balance here. Just for kicks I tried off-setting the cluster to the lower left or upper right following the rule of thirds, but no orientation had other bright stars that would lead the eye to the cluster and again the subject was lost. In the end, it just made sense to position M103 dead center. Sure, that's a lot of thought for a simple star cluster, but it is good practice for any image.

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Processing Workflow (PixInsight)
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Post-processing
1. Initial crop (Dynamic crop)
2. Reduce background gradient and neutralize background (DynamicBackgroundExtraction, subtraction)
3. Neutralize background (BackgroundNeutralization)
4. Set white balance (ColorCalibration; use entire image, disable structure detection)
5. Set luminance coefficients to 0.33333 (RGBWorkingSpace)
6. Reduce lightness noise (MultiscaleLinearTransform; 7 layers, noise reduction threshold 5, 1 and amount 0.5 ,0.5 to layers 1 and 2, respectively with inverted range mask, target: Lightness)
7. Non-linear stretch (MaskedStretch; target median background 0.125, clipping fraction 0.01)
8. Reduce background brightness; increase contrast (CurvesTransformation to RGB/K)
9. Reduce background luminance noise (ACDNR to luminance with luminance mask)
10. Reduce background chrominance noise (ACDNR to chrominance with luminance mask)
11. Increase color saturation (CurvesTransformation to Saturation)
12. Reduce green (SCNR at 92%)
13.Sharpen stars slightly (MultiScaleMedianTransform, bias +0.05 to layer 3 of 5 layers total, target Lightness)
14. Lower background brightness slightly (CurvesTransformation to RGB/K)
15. Cosmetic correction to reduce pink cores in a few over-exposed stars. Using a smooth range mask selecting just the offending star cores, first brighten cores close to white (InterchannelCurves, Target CIE c, Reference CIE L, reduce saturation).
16. Final Crop to 5 x 7 aspect ratio (DynamicCrop)
17. Set ICC profile to sRGB for web publishing (ICCProfileTransformation)

Comments

Author

Jason Tackett
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Open cluster M103 in Cassiopeia, Jason Tackett

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