Narrowband of NGC2174/ 5. A systematic approach

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timh
Posts: 515
Joined: Mon Aug 26, 2019 5:50 pm

Narrowband of NGC2174/ 5. A systematic approach

#1

Post by timh »

NB images - be they SHO or OOH - can be difficult to interpret without knowing exactly how much each contributing channel has been stretched.

Here a go at imaging and then processing NGC2174/ 5 - the monkeyhead nebula and its core star cluster - in a systematic way. The recent addition of the 'starnet' tool to PixInsight made it a lot easier to do this quantitatively. The two contrasting ways were as follows...

1) The S2, HA and OIII channels were normalised for total exposure time and then each stretched an equal amount before being combined. The resulting NB image then reflects not only something of the positional distributions within the nebula but also the 'true' relative strengths of the contributions from HA, S2 and O3. Alternatively..

2) The S2, HA and OIII channels were each stretched by different amounts but to an equivalent defined endpoint so that at least one part of each image ended up equally bright. In this case for example the 3 starless nebula images were each stretched to a point where the pixels comprising their brightest parts were at ca 12K out of a maximum 16K electrons (14 bit ADU) before being combined. In this case the resulting NB image contains no information about the relative strengths of the HA, S2 and O3 emissions but provides a clearer and better picture of their positional distribution.
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SW200PDS Newtonian (f = 1000mm, F5.0) Baader MkIII coma corrector, CEM70 Ioptron mount, Baader steeltrack focuser, Pegasus Cube2 focus controller, PHD2 guiding using an ASI 120 mm guide camera and 80 mm SW startravel refractor at f = 400 mm.

ZWO AS1294 MC camera for RGB (UHC) captures or ZWO ASI294MM mono camera for HA and OIII , both 4.63 uM pixels cooled to -10C

ZWO IR/UV cut filter, Astronomik UHC filter, Astronomik 6.0 nm OIII filter, Optolong 7 nm HA filter, Optolong 6.5nm SII filter

24 x 70s RGB (UHC) frames at gain 124 and 44 x 3min gain 151 OIII under moonless Bortle 6 skies. 57 x 3 min HA frames and 42 x 3 min SII frames captured with the moon risen but low in the SW sky

All frames (0.95 AS/ pixel) pre-selected for quality using the FWHM and brightness filter within Sharpcap, darks and grayscale master flats (no bias) prepared using Sharpcap. Preprocessing in PixInsight.

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Following standard processing of the 4 aligned images - Crop, DBE background removal, Photometric colour calibration etc. the images were initially stretched the same amount. Then Starnet was used to create starless images of the three NB images as well as a star mask of the stars from the RGB image. The three channel images were then further stretched (or not) according to method 1) or 2) and as guided by the image statistics.

Starless NB images were built using Pix Math in both a naturalistic (RED HA plus S2; GREEN O3; BLUE 0.15*HA + 0,85*O3 ) and in the standard SHO palette. These starless images were further adjusted with curves, sharpening and noise reduction. Then the RGB stars from the starnet starmask were re-added.

The top two images produced according to the 'equal stretch' method 1 show what are - in a sense - the most honest images in the two palettes. Both pictures reflect the fact that in reality the visible light from the nebula is vastly dominated by HA - nevertheless the S2 contribution around the nebula periphery and O3 at the core is clearly visible.

The lower two images made according to the 'equal intensity' method 2 are akin to most on Astrobin - the O3 and S2 are much more easily located.

One nice thing about the NGC2174 (5) nebula image is that it does seem to exemplify theoretical expectation for HII regions. The OIII light does indeed appear to only come from near the NGC2175 star cluster at core reflecting that this locus best fulfills the requirements of - i) enough energy to ionize O+ to O2+, ii) collisional energy - temperature- to raise O2+ to the excited quantum spin state, iii) high enough 'metallicity' that O is sufficiently abundant and iv) sufficient density of plasma for OIII to be visible and yet low enough that the OIII excited state is not prematurely quenched by collision before it can emit a photon via the forbidden transition. The SII on the other hand is clearly brightest at the ionization edges reflecting the longer lifetime of the excited S+ ion spin state such that it can only survive long enough to emit a photon in relatively low density plasma environments.

https://www.astrobin.com/g53qco/


Tim
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ChrisMobley
Posts: 2
Joined: Sun Jan 23, 2022 11:16 pm

Re: Narrowband of NGC2174/ 5. A systematic approach

#2

Post by ChrisMobley »

Incredible, great explanation with points I had not considered.
timh
Posts: 515
Joined: Mon Aug 26, 2019 5:50 pm

Re: Narrowband of NGC2174/ 5. A systematic approach

#3

Post by timh »

Thanks indeed Chris! It's all just as new to me too and I really am no expert - it is just that writing stuff up as I learn helps to lock in my own understanding. I have certainly learned to like SHO images more now that I can interpret them better :). For example - in the above I now imagine the reddy/ yellow S2 features as nearest edges that are more likely to be coming out of the screen towards the viewer whereas the bluer regions would be recessed back into the core near the star cluster - so in my imagination at least - more 3D.
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