Casitas de Gila, Gila, New Mexico, October 28-29, 2022

During my trip to New Mexico, I imaged the Lobster Claw nebula as the first of two targets each night.  SH2-157 is a large, faint emission nebula with newly formed, bright stars and several bright nebulae in the field of view.  I captured narrowband Hydrogen-alpha, Oxygen-III and Sulfur-II filter data for the nebula and Red, Green and Blue broadband subframes for the stars.  With two rainy/cloudy nights, imaging time was limited for this target.

I processed the data using a similar workflow in PixInsight for the Lion and California nebulae.  A stars only image was created with Red, Green and Blue integrated masters.  StarXTerminator was used to remove all nebulosity.  The stars were stretched and saturated.  A modified Hubble pallet was used to create a near blue and gold image from narrowband calibrated master frames.  The rest of the image processing workflow was straight forward, made easy with starless images.  In this case, a starless RGB false color image was created with StarXTerminator.  A luminance image was extracted from the RGB color image.  The luminance was denoised with NoiseXTerminator, stretched, sharpened and then the contrast was enhanced with the HistogramEqualization tool.  The luminance was then combined with the stretched RGB color image.  Stretching was done with combinations of the GeneralizedHyperbolicStretch script and the ArcsinStretch tool.  In addition to providing good detail, the colors also give a sense of depth to the nebula.  In this case, I pushed the color combination and saturation using color masks.  I pushed further than I did with the Lion nebula, and the data itself guided me to the final color combinations.  Two versions of the stretched, starless nebula were created, one with a more muted blue and red color pallet, and one with more orange in the Ha regions and increased brightness in the star forming regions.  Neither were satisfying, so I blended them in Photoshop using the layer transparency slider.  The final starless nebula was combined with the stars using the PixelMath combine function with the opscreen() parameter.

SH2-157, also known as the Lobster Claw Nebula, is an emission nebula located about 11,000 light-years away in the constellation of Cassiopeia.  The red/orange portions of the nebula are made up of strong HII regions predominantly yellow part of the nebula is a very large H II region.  The blue-colored regions are predominately from the emission of light from molecular oxygen.  In the lower left corner of the image, NGC 7635, the Bubble Nebula, is visible.  The open star cluster NGC 7510 is seen at the top right of the image.  The bright nebula, NGC 7538 is in the upper left of the frame.
Imaging details:

Stellarvue SVX102T with SFR0.74 focal reducer (FL = 528mm, f/5.2)
ZWO off-axis guider (OAG-L) with a ZWO ASI 174MM mini guide camera
Losmandy G11 mount with Gemini 2
ZWO ASI 2600MM Pro cooled monochrome camera (-10C)
Chroma 36mm filters:  5nm Hydrogen-alpha, 3nm Oxygen-III, 3nm Sulfur-II, Luminance, Red, Green, and Blue
Equatorial camera rotation: 270 deg

Software:    Sequence Generator Pro, ASTAP plate solving, PHD2 guiding, 
    Losmandy Gemini ASCOM mount control and web client interface,
    SharpCap Pro for polar alignment with the Polemaster camera,
    PixInsight 1.8.9 with StarXTerminator (AI version 10) and NoiseXTerminator,
    Photoshop CC 2022

Hydrogen-a    10 min x 15 subframes (150 min), Gain 100, Offset 68, 1x1 binning
Oxygen-III      10 min x 14 subframes (140 min), Gain 100, Offset 68, 1x1 binning
Sulfur-II          10 min x 14 subframes (140 min), Gain 100, Offset 68, 1x1 binning
Red        4 min x 15 subframes (60 min), Gain 100, Offset 68, 1x1 binning
Green    4 min x 15 subframes (60 min), Gain 100, Offset 68, 1x1 binning
Blue       4 min x 15 subframes (60 min), Gain 100, Offset 68, 1x1 binning