NGC 6823 (and SH2-86) - HaRGB Processing in PixInsight

NGC 6823 is an open cluster surrounded by a nebula complex Sh2-86 in the constellation of Vulpecula.  The open star cluster is about 50 light years across and lies about 6000 light years away. The center of the cluster formed about two million years ago and is dominated by bright young blue stars (source: wikipedia and Astronomy).

I captured this object last year (image) using a UHC filter (29x180s) and Canon T3i and processed it using my former method.  This new image HaRGB was captured using the ZWO ASI1600MM-Pro and processed primarily using PixInsight. 

I wrote up a comparison between two methods for producing HaRGB images below.  The two methods are as follows: method 1 - using the LRGB combination and method 2 - using PixelMath.  If you are not familiar with PixelMath I recommend watching the Harry's Astro Shed video Adding Ha to RGB which is the procedure I followed.

Creating the HaRGB Image

1) Registering, aligning, and stacking was done in PixInsight using the Batch Preprocessing function.  I followed Chuck's Astrophotography (Stack and Combine Filters in PixInsight) method to do this.

2) Automatic Background Extraction was run on the Ha channel to make the Ha image.

Ha

Method 1 - Using LRGB Function to create the HaRGB image 

For this method I did not use anyone's specific directions, it was described on numerous websites and forums.

3) An RGB Image was created using the RGB combination function.  Processing included: ABE, Color Calibration, SNCR noise reduction, HistogramTransformation,  Extract CIE Component, Reduce star size - Star Mask - Morphological Transformation, Sharpness and Saturation - Mask - Convolution.  This was only for show as the image was not used for anything.

RGB

4) The simplest HaRGB Image was created by combining the separate channel using the LRGB process and the Ha was put into the Luminosity channel at 0.5 strength.  Further processing included: ABE, Color Calibration, HistogramTransformation,  Extract CIE Component, Reduce star size - Star Mask - Morphological Transformation, Sharpness and Saturation - Mask - Convolution.

HaRGB - Simple

In my opinion this image was better than my image from last year and the star colors came out very well.  However I was not satisfied with the detail or the look so I did some more research on combining HaRGB.  Most experienced PI users do not use the LRGB process to combine Narrowband data but rather a process using the PixelMath function.  

Method 2 - Using PixelMath to create the HaRGB image 

For this method I chose Harry's Astro Shed video Adding Ha to RGB to complete the combination (http://harrysastroshed.com/pixinsight/pixinsight%20video%20html/pixinsighthomeinter.html).

3)  The original was Ha image which ABE was already run was further enhanced using the using the following script: ((Ha*100)-(R*7)/(100-7) added to RGB/K channel.  That is the Ha image (Ha) and the Red Channel (R) are merged and the unwanted Ha is subtracted out of the image.  The '7' is the filter size and the '100' is I believe the intensity which can be adjusted. I ended up using 140 for my image.

Enhanced Ha

4)  Next up was adding the Enhanced Ha image to the Red channel again using PixelMath.  I used the following script for this:  $T+(Enhanced Ha-Med(Enhanced Ha))*4 and then applying this to the Red Channel.  What you get is an Enhanced Red Channel image.  The number '4' is the scale and can be adjusted to give the optimal look.

Red Channel

Enhanced Red Channel

5)  Finally you can combine your RGB channels using the Enhanced Red Channel to make the HaRGB image.  Further processing included: HistogramTransformation,  Extract CIE Component, Reduce star size - Star Mask - Morphological Transformation, Sharpness and Saturation - Mask - Convolution.

HaRGB-v1

Finally

In my opinion this method produced a far superior image than the simple LRGB method (method 1).  The detail and sharpness are greatly enhanced using the PixelMath process (method 2).  However, I do like the star colors of the LRGB method and will attempt to add them to the image using a star mask. I did no processing in Photoshop except for file saving as I wanted  to show a comparison between HaRGB images using the LRGB method and the PixelMath method.  I will add updated versions of the final image when they become available.

Version2
For this next addition I added the Enhanced Ha the LRGB function.  Enhanced Ha was added to the luminosity channel at 0.65 intensity.

HaRGB-v2

Version 3

Toned down the red channel, minor sharpening and noise reduction in Photoshop

HaRGB-v3

Version 4
Toned down the red channel, minor sharpening and noise reduction in Photoshop

HaRGB-v4

Version 5
Added the RGB star colors using photoshop and then back to PI - PixelMath, StarMask, and Morphological Transformation.

HaRGB-v5

Comparison

NGC 6823 (and SH2-86) - HaRGB Processing in PixInsight

Location: Home Monroe, CT
Date: 8-28-18 (Ha), 8-29-18 (RGB), 8-30-18 (RGB), 9-2-18 (RGB), 9-3-18 (Ha) 

Camera: ZWO ASI1600MM-Pro
Telescope: Orion ED80 80mm f/7.5 Apochromatic Refractor Telescope
Barlow: None
Focal Length: 600mm
f/7.5
Focal Reducer: Orion 0.8x Focal Reducer for Refractor Telescopes
Mount: Orion Sirius EQ
Filter Wheel: ZWO EFW 8x 1.25"
Filter: ZWO Ha, OIII, SII
Autoguiding: QHY-5L-II-M attached to an Agena 50mm Guide Scope with Helical Focuser
Exposure: (Ha 8/28) 44 x 180s, (Ha 9/3) 29 x 180, (R) 63 x 60, (G) 52 x 60, (B) 47 x 60

Gain: 139
Offset 21
Temp: -10 C
Processing: APT, PixInsight and Photoshop
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