M81 and M82 using very old PixelMath (Andy Wray)

Celestial hemisphere: Northern · Constellation: Ursa Major (UMa) · Contains: Bode's Galaxy · Cigar Galaxy · HD85458 · M 81 · M 82 · NGC 3031 · NGC 3034 · PGC 2726822 · PGC 2730379 · PGC 2730409 · PGC 2730709 · PGC 2731294 · PGC 2732102 · PGC 28505 · PGC 28529 · PGC 28757 · PGC 28848 · PGC 3086325 · PGC 3097961

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M81 and M82 using very old PixelMath

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M81 and M82 using very old PixelMath

I've noticed that a lot of sites use the following PixelMath equations to combine Ha with RGB:

RGB/K: ((HA * R_bandwidth) - (R * HA_bandwidth)) / (R_bandwidth - HA_bandwidth)
Symbols: R_bandwidth=100, HA_bandwidth=7

The big problem with this is that it assumes that you have taken your Ha and RGB exposures at the same exposure time (which you probably shouldn't be doing). In fact the PixInsight NRGBCombination script suffers from the same issue which is why many recommendations are to up the RGB bandwidth to 200 to somehow compensate for it.

I am now using the following, which is not my work, but old pixelmath equations going back to 2015:

Ha-((Ha_Bandwidth*Ha_Exposure)/(Red_Bandwidth*Red_Exposure))*(Red-med(Red))

This produces an Ha additive image that you can then add back into your red channel using:

$T+Boost_Factor*(Ha_Clean-med(Ha_Clean))

Where Ha_clean is the image that was created by the first equation.

N.B. the (Red-med(Red)) part of the equations are trying to make sure that you don't affect the overall colour balance of your image as they centralise the adjustments around the median value of that channel (or at least that's what I think that does)

The Boost_Factor thing is just if you want to emphasise the Ha component ... technically a Boost_Factor of 1.0 would be correct, but many like to up it

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Unknown

The “AstroBin Image of the Day and Top Picks”, or IOTD/TP, is a long-running system to promote beautiful, interesting, peculiar, or otherwise amazing astrophotographs, with a focus on technical excellence. Learn more.

Submitted The date and time when you submitted this image for IOTD/TP consideration.	...
Views by Submitters (available since September 19th, 2023) Every image is assigned to 50% of available Submitters. In the event that at least 80% of them don't view the image before its time in the IOTD/TP process expires, it's assigned to the other 50% of Submitters and the process begins anew.
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Promotions by Reviewers When 3 distinct Reviewers promote the image, it moves on to the next stage of the process: evaluation for IOTD status.
Early dismissal Staff members have a lot of images to inspect on a daily basis, and they can dismiss images if they believe they don't meet the requirements for IOTD/TP selection. If an image is dismissed 5 times, it's removed from the process. This streamlines the process and ensures that any bias present in promotions could be overruled by other staff members.

Status	Advanced success
Started	...
Astrometry.net job	5709965
PixInsight job	EEDVB99WTUJYDVI1X71GL2E2ETWXIXAZ
PixInsight queue size	n/a
PixInsight stage	TASK_LOG

RA (center)	09h56m05s.791
RA (top/left)	10h01m07s.353
RA (top/right)	09h58m33s.638
RA (bottom/right)	09h50m52s.728
RA (bottom/left)	09h53m45s.336

Dec (center)	+69°18′52″.32
Dec (top/left)	+68°55′01″.18
Dec (top/right)	+69°52′26″.88
Dec (bottom/right)	+69°42′11″.23
Dec (bottom/left)	+68°45′12″.80

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M81 and M82 using very old PixelMath

M81 and M82 using very old PixelMath

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M81 and M82 using very old PixelMath

M81 and M82 using very old PixelMath

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