M42 in SHO (Gabriel Cardona)

M42 in SHO

I used M42 as my first target for the ASI1600MM-Pro camera, mated to my Edge HD9.25 in Hyperstar form. Since I image mostly from my home under the Dallas, TX light dome, I invested in a set of Badder fast (F/2) narrowband filters, and these are what I used for this image.

Here are some observations regarding imaging in monochrome vs one-shot color:

Flats are much easier and less complicated. With my OSC camera, the flats for the blue channel were consistently under-exposed, especially when I used the LPS-V4 dual-band LP filter. This caused all kinds of gradients that had to be corrected in post-processing.

There is more freedom in post-processing to manage the individual channels before they are merged.

Light pollution gradients are easier to deal with on an individual channel basis.

One thing I noticed with the Badder filters, especially the OIII and to a lesser extent the SII, is they caused a reflection halo around the brightest stars that was difficult to remove. Tha Ha filter did not have this problem, and I used this to correct the other two master images. What I did was used PixInsight’s Clone Stamp tool to copy the halo-less stars from the Ha image to the OIII and SII images. It turns out the Clone Stamp tool has a dy-dx reading that allows copying from one image to the exact relative location in another image. This is similar to the Paste-in-place feature in Photoshop.

I thought this worked fine until I combined the images to get an RGB composite. All of the stars had a magenta tint to them (typical of narrowband), except the clone-stamped ones which were perfectly white, since these were copies of the same star. What I ended up doing was clone-stamping the original star (minus the halo) back into each image to get them to be similar in color to the other stars in the combined image.

Now I have my 3 masters, Ha, OIII, and SII. I used Pixelmath to add these together and generate a master Luminance channel. I then combined the 3 masters to generate and RGB copmposite using Pixelmath and the following ratios: R=0.7XSII+0.3*Ha, G=0.2*OIII+0.6*Ha+SII*0.2, B=OIII. THis gives stars a whiter tone that doing straight R=SII, G=Ha and B=OIII. I then did a masked stretch to the linear image and generated a StarMask using the Starnet process in PixInsight.

I used the inverse Starmask to reduce the saturation of the stars in the RGB image, then I used the SCNR process to reduce the green in the image and turn the yellow tones into orange. The last thing I did was used HDR Multi-scale Transform to reduce the dynamic range of the image and bring out the core details. The RGB image is now ready for LRGB combining.

To the luminance image I used deconvolution to sharpen things up a little and MLT to de-noise the dust areas. I then used two applications of ArcSinh stretch to de-linearize the image and bring out the interstellar dust which is prominent in Ha and SII. I then applied HDR Multi-scale Transform to bring out the core details.

The final process was to combine the RGB and Luminance using LRGB Combination, and the image is complete. I then did some adjusting to the curves and saturation, brought it into Photoshop to do final tweaking and conversion to JPG for publication. I’m including here all of the masters, including the original OIII master with the awful halos.

C

Description: OIII Master with awful halos.

Uploaded: ...

D

Description: OIII Master with the worst halos removed.

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E

Description: SII Master.

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F

Description: Luminance Master.

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G

Description: RGB Master.

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H

Description: Luminance combined with RGB.

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I

Description: Re-processed using new techniques.

Uploaded: ...

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Status	Advanced success
Started	...
PixInsight job	ZB1QF5TKJJI6O81NC8RZN2H3PVHM6NEO
PixInsight queue size	n/a
PixInsight stage	TASK_LOG

RA (center)	05h35m15s.57
RA (top/left)	05h32m34s.58
RA (top/right)	05h32m31s.94
RA (bottom/right)	05h37m57s.46
RA (bottom/left)	05h37m59s.60

Dec (center)	-05°21′52″.4
Dec (top/left)	-04°26′51″.1
Dec (top/right)	-06°16′29″.5
Dec (bottom/right)	-06°16′53″.3
Dec (bottom/left)	-04°27′12″.6

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