NGC 2237, 2238, 2239, 2244, 2246, the Rosette Nebula in Monoceros
Gold Canyon, AZ
November 7 - 11, 2021

The Rosette Nebula was the third target of three imaged each night in Arizona.  Hydrogen-a, Oxygen-III and Sulfur-II 7nm narrowband filter data were combined using the Hubble pallet by assigning S-II to the red, Ha to the green, and O-III to the blue channel (SHO).  By doing three targets in a long night over several nights, I was able to collect many hours of subframes for each filter.  I used an evaluation copy of Russ Croman’s StarXTerminator tool in PixInsight to remove the stars so that the nebula could be processed separately from the star field.  StarXTerminator worked exceedingly well on all three linear monochrome images.  The stars image from the Ha and O-III integrated data were combined as an HOO image to create a RGB star field.  Starless and stars images were processed separately in PixInsight.  It turns out that the processing of starless images was much easier and faster than with conventional images.  A luminance image was extracted from the combined SHO color image.  The luminance was denoised, stretched and sharpened, while the RGB image was denoised, stretched and blurred.  The luminance was combined with the starless RGB image, and the saturation and color balance were adjusted.  The starless and stars images were combined using the PixelMath combine function with op_screen() to produce the final color image.  The first image uses the classical Hubble pallet with the green removed.  In the second image, some green was retained and tonal mapping was applied in Photoshop to give more of a rainbow appearance.

The Rosette Nebula is a large, circular emission region in the constellation Monoceros.  A small cluster of hot, young stars, NGC 2244, is in the center.  The nebula spans over 1o of sky and it has a diameter of 130 light years.  The nebula is estimated to contain around 10,000 solar masses.  It is about 5,200 light years from Earth.  Radiation from the open cluster in the center has pushed out the gas and dust.  The pressure from this has initiated new star formation (SkySafari Pro 7).


Imaging details:

Celestron 9.25" Edge HD SCT
Celestron 0.7x Focal Reducer (FL = 1645mm, f/7)
Celestron off-axis guider with a ZWO ASI 174MM mini guide camera
Losmandy G11 mount with Gemini 2
ZWO ASI 2600MM Pro cooled monochrome camera (-10oC)
ZWO 36mm Hydrogen-a, Oxygen-III and Sulfur-II filters
RA: 97.8074o, DEC: 5.003o, Equatorial camera rotation: 340o

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 1.8.8-11 with StarXTerminator,
                  Photoshop CC 2022


Hydrogen-a 10 min x 33 subframes (330 min), Gain 100, Offset 68, 1x1 binning
Oxygen-III    10 min x 33 subframes (330 min), Gain 100, Offset 68, 1x1 binning
Sulfur-II       10 min x 26 subframes (260 min), Gain 100, Offset 68, 1x1 binning