Yerkes McDonald 16 in Serpens Cauda
YM 16 (PK 38+02.1, PNG 038.7+01.9) is a fairly large (6 arcminutes) but quite faint object (mag 14.4, according to the one estimate I found) just north of the equator in Serpens. It was discovered during a 1953-1954 survey sponsored by the two observatories, and reported in 1955. I found one distance estimate of 2900 light years, and a diameter of 6 light years, but I haven’t found the source of those estimates.
This image is a colorized H-alpha for the nebula, and RGB for stars. I shot 4 hours of OIII, but it showed only the faintest hint of a signal for the nebula – just a very vague brightening toward the middle of the frame.
Exposure: Total exposure time about 37 hours, 93 x 20 minutes Ha and 61:61:66 x 2 minutes R:G:B. All bin 1x1. Data collected May to August 2021.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated Ha 2.5 arcsec
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% scale).
Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.1 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.17140 (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 2.4.3.0 driver
Filters: Astrodon Type IIi LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB nStep motor with driver version 6.0.7 on stock Celestron focuser
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.1.0.457
Guide SW: PHD 2.6.7, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: PlateSolve 2, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2
Processing Software: Pixinisight, Affinity Photo, Photoshop CS2
Processing Workflow by Workspace in PixInsight 1.8.9:
1. Calibration
Calibration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Blink to preview and reject a few frames
Weighting and registration with WBPP
2. Stack and Mure Denoise
Image Integration on each channel
Mure Denoise on each channel
RGB Combination for RGB frames
Dynamic Background Extraction
3. Narrowband Stretching
Histo Trans x 2
Curves Trans
StarNet v2 to remove stars
Mild Localized Histogram Equalization
TGVDenoise
Aggressive Multiscale Median Transform (with an inverted luminance mask, so that the effect was not applied to the nebula) to smooth out background.
4. RGB Linear Processing
Photometric Color Calibration, with average spiral galaxy as white reference
5. RGB Stretching
Histo Trans
Curves for color saturation, using a luminance mask so that color in background wasn’t also boosted
Histo Trans
Curves Trans to brighten
TGV Denoise on chrominance to eliminate a host of blue pixels in the background
6. Background Subtraction on RGB Stars Image
a. Create an image of the background:
1. StarNet++ to create a starless image. I used the spot healing brush in Photoshop to remove the nebula and a couple leftover star halos.
2. In PS, apply a dust & scratches filter at very low strength (5 pixels) to smooth image so that noise is not removed during the subtraction process
b. Subtract the background image from the original image in Photoshop to remove remaining gradients and any messy clumps in the background, applying an offset so that the background is not pure black
7. Color Combination
Ha was colorized to red using a curve, pulling up the red highlights and pulling down the green and blue, while the keeping the shadows anchored.
In Pixinsight, I added the stars to the starless nebula image
In Photoshop, I used the nebula + stars image from PI for the nebula, but used only the RGB stars image for the starfield
8. Final
ICC Profile Transform to sRGB
Resample at 50% scale
Save as JPG
