New software changes OOH NB image of IC 1805

[timh]

Skywatcher PDS200 with 0.9X SW coma corrector/ reducer (--> F4.5), CEM70G mount, ASI 294MC and ASI294MM cameras, ZWO UV/IR cut, Astronomik 6 nm O3 filter and Optolong 7nm nm HA filter .

Data captured using Sharpcap in 2021 and 2022, 66 x 70s at unity gain with the ASI 294MC for the RGB image and 137 x 3min and 110 x 3 min at gain 151 for the O3 and HA images.

I reprocessed the data a number of times over the past few years as new methods and software came available to obtain images that were gradually improved in various ways.

A long-standing problem though with any long-focus image of the detail within a large nebula like the heart nebula (as here) is that it has been thus far impossible to get the background subtraction right because nowhere within the frame is 'background'. The latest update to the MARS DR2 dataset in PixInsight now seems to largely solve this problem as it seems to cover the Northern hemisphere and in multiple wavelengths (NB and broadband). I thought it interesting to compare my previous 'best' (2025) image of IC1805 taken from the above dataset with one using the new 'correct' background subtractions. I think that this tool will find great use retrospectively 'mending' a lot of nebula images like mine ? The 'bluer' image is the 2025 version -- in the 2026 version I preferred --as a nod to more realism - to also more realistically express the more localised and weaker nature of the O3 versus the HA light emission.

At least -according to my no doubt partial understanding - the O3 light distribution now looks more as expected with most of it localized near the hotter new stars (i.e higher temperature needed to double ionize O atoms and electron temperature to then kick O2+ up into its forbidden spin state) and less smearing out into darker regions?

[admin]

Hi Tim,

thanks for an interesting post about the use of the MARS tool in PI - I had seen some of the announcements about it, but your post prompted me to dig in and actually read the various web pages that explain how it works. A clever idea! I presume that PI have been capturing lots of widefield images at decent dark sky sites to get their reference data to implement this.

cheers,

Robin

[oopfan]

Hi Tim,

Beautiful colors, great focus. I especially like the frame orientation.

Brian

[timh]

Thanks Robin and Brian.

The MARS tool promises to be a really useful tool for us Newt and large telescope owners and especially in light-polluted locations -- we should be grateful to all those anonymous contributors of the wide field dark sky images that have made good background subtraction possible. In a sense I suppose that it is no longer really solely 'my' image.

The orientation was really just luck but it did compose well at 900 mm focal length which was just the way it worked out with the coma corrector/ reducer used. I think that yet more software advances - the multiscale adaptive stretch and RC Blur Exterminator helped in balancing both the NB and the RGB star colours/ sizes. It is interesting to look back and see that the tools I originally used to process the data in 2022 were almost all completely different from those used in 2026. Seems that any book written on deepsky image processing will be out of date within a year!

[timh]

Really all the same as the original post on IC405 - same telescope and method - although this time F 5.0 but a different object. Here I think subtracting the MARS2 definition of the correct background makes an even bigger difference showing dark nebulosity otherwise missed entirely - obviously the impact will vary from object to object. Most images of the wizard are processed to amplify the blue of the O3 (as in my 2025 version) -- As a personal preference I am coming to prefer OOH images which set the proportions of H and O3 light more naturalistically.
Overall the colours - including those of stars seem more realistic when the correct background is subtracted.

Upper picture is 2026 with MARS2 background subtraction. Lower is 2025 picture processed without