Crescent nebula +/- HA

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

Crescent nebula +/- HA

#1

Post by timh »

SW200PDS Newtonian F5.0, Baader MkIII coma corrector, CEM70 mount, Baader steeltrack focuser, Pegasus Cube2 focus controller, ASI 120 mm guide camera, Startravel80, f= 400mm guidescope

1) 46 x 110s exposures at gain 124 -10C with an ASI 1294 MC camera and Astronomik UHC filter preceding late last quarter moonrise under Bortle 6 skies, 0.946 arcsec/pixel

2) 38 x 180s exposures at gain 151 -10C with an ASI 1294 MM camera and Optolong 7 nm HA filter with last ~ quarter low moonlight under Bortle 6 skies. 0.946 arcsec/ pixel

Opportunistic capture using Sharpcap and FWHM / brightness filter during brief cloud gaps over several nights. Guiding using PHD2 multistar, Preprocessing in PixInsight rejecting rogue frames and selecting frames with good brightness (Star number) and FWHM averaging about 2.6. Drizzle x1 integrations.

The HA and RGB images were separately processed (noise reduction, background subtraction etc) and then, after stretching to non-linear- the Luminance was extracted from the RGB, calibrated to scale consistent with the HA luminance and then the two luminances combined using the Max function in PixMath. The resulting combined luminance was then replaced into the RGB image which was then adjusted with PI curves and tidied up using Affinity Photo.

Bubble nebula of material ejected during a red giant phase of the central Wolf Rayet star now glowing as it collides with the fast moving stellar wind from the star. The bright red edge of NGC6888 largely comprises HA and NII. As shown in the side by side pictures adding in the HA filter luminance enhances these red parts of the nebula and adds in a good deal of detail as well as having the effect of brightening the nebula relative to the background stars. However this view of the nebula is somewhat unbalanced since much of the fainter white appearing on the darker edge of the nebula glows blue green from OIII emission. So the picture is a work in progress with the aim now to add in about 4h of OIII to try and get an overall more balanced RGBHOO image.

Tim
Attachments
CaptureCrescentpair.JPG
CaptureCrescentpair.JPG (89 KiB) Viewed 1521 times
CrescentHAcapture.JPG
CrescentHAcapture.JPG (111.85 KiB) Viewed 1521 times
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oopfan
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Re: Crescent nebula +/- HA

#2

Post by oopfan »

The blue/black is definitely OIII.

Brian
timh
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Re: Crescent nebula +/- HA

#3

Post by timh »

Hi Brian,

Yes I think that you are right ..see O3 pictures below also suggesting the same . Now started on the O3 data collection and it is interesting in a number of ways --

Firstly the subtle ways in which OIII is differently distributed than HA in the nebula. It is interesting to consider whether that really reflects a different spatial distribution of H and O atoms --- or - perhaps more likely ? - there is more OIII emission wherever the nebula is denser leading to more collisions because that is the process (rather than UV excitation) that double ionizes oxygen?

Secondly though - on a more technical imaging note - I am now thinking that there could well be a problem with my normal image processing flow which means I see less blue/ green than I should? When you look at the QE versus wavelength curve of the ASI294MC camera it is relatively weak on blue and the OIII line at ~ 500nm will mainly be detected by the G pixels in the RGGB Bayer matrix. That would be fine except for the fact that one of the steps quite routinely used in PixInsight post-processing is called 'SCNR' which is used to remove green colour from the image - basically any green stars are rendered white. But problematic when you want to keep the true teal colour of the OIII line ! So I will process differently now by - as well as luminance - also adding back in the O3 and HA colours at the end.

Still think I need more OIII data -- it is quite strong but still only 50% or so of the HA.


Tim
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crescentnarrowbandCapture.JPG
crescentnarrowbandCapture.JPG (280.3 KiB) Viewed 1465 times
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oopfan
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Re: Crescent nebula +/- HA

#4

Post by oopfan »

Eagerly awaiting your OIII stack. My photometric measurements tell me that Ha is 5x stronger than OIII. Some of the more impressive images on Astrobin clearly show the outer lobe of OIII at the 7 o'clock position that you are just starting to pick up.

Brian
timh
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Re: Crescent nebula +/- HA AND O3

#5

Post by timh »

You are probably right with that number Brian -- the O3 is certainly a lot weaker. .... but still a much stronger feature in this particular rather unusual emission nebula than in most.

I didn't go quite as far as a 5 fold 'leveling up' of the O3 signal but did about 3 fold as a compromise so that the O3 can be clearly seen but without giving too much of a misleading impression of its true relative strength versus the HA + NII signal.

I did this by..

1) initially taking 1.5 x as many O3 exposures (I added a few more) as HA , then normalising the two integrated images so that they were scaled equivalently and then creating a combined luminance 'master' image by combining them using the 'Max' function in pixinsight so that- at every pixel- the combined image took the numerical value of either the maximum of the HA image or the value of 1.5 x the O3 image. This boosted the O3 image quite a lot -- so - at least visually - the resulting image seemed to about equally combine the main features of the HA and O3 images.

2) After appropriate rescaling I then used this combined HA/1.5XO3 master image to add in - again using the max function - and combine into the luminance of the RGB image.

3) After 2) you end up with a colour image that has all of the O3 and HA detail in it but that now looks rather pale. So at this point I then added carefully balanced HA and O3 signal into the R, G and B channels of the RGB. This bit is something of an art rather than science I think --but I tried to end up with reasonable-looking star colours. Then finally tidy up in Affinity.

The above electronic manipulation is a bit of a cheat and - if opportunity allows - I may do another 3h of O3 but I have been taking serious risks to get the data I have (using clear gaps between heavy rain clouds ---having to dash out to stop the camera etc getting wet)

So here it is as it is now -- I am very pleased with it and think that it is a really beautiful and interesting nebula now that I can see most of its halo of oxygen --- and there is also HA and a sea of faint O3 all around (maybe from even older bursts of shell losses?)

Tim
Attachments
NGC6888_small_Newt_Baader_UHC_040821_46x110s_gain124_pixint_CROP_ABE_PCCcol_MLT_strmask_TRAN_max_plus33percent2hHALUM_66pc3hO3LUM_NBRGB_curves_PX.jpg
NGC6888_small_Newt_Baader_UHC_040821_46x110s_gain124_pixint_CROP_ABE_PCCcol_MLT_strmask_TRAN_max_plus33percent2hHALUM_66pc3hO3LUM_NBRGB_curves_PX.jpg (831.98 KiB) Viewed 1453 times
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turfpit
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Re: Crescent nebula +/- HA

#6

Post by turfpit »

Tim

That is a good effort with this tricky object. My experiences, captured without astro-dark, are not worth posting.

Dave
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oopfan
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Re: Crescent nebula +/- HA

#7

Post by oopfan »

Very nice, Tim!

Yes, 5x is not huge. It is equivalent to two stars having a 1.75 magnitudes difference. The challenge is then a matter of sky brightness, read noise, and aperture. You've got all three tackled especially now that the UK has once again achieved astronomical darkness.

Congratulations!

Brian
timh
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Joined: Mon Aug 26, 2019 5:50 pm

Re: Crescent nebula +/- HA

#8

Post by timh »

Thanks Brian,

Yes I am certainly fortunate with equipment and aperture ..the revised guiding setup is working at less than 0.5 arc sec RMS and the new focuser on the Newt has been transformative.. but the biggest limitation for me is actually the 'darkness' bit. My location was labelled Bortle 6 but I suspect that in recent years it has gone up to 7 with surrounding development. I tried the experiment of adding yet more exposures to see just how much difference it might make. So the same as previously but now with 5.5h of O3, 2.9h of UHC RGB and still 2 h of HA all at F 5.0. The O3 hood is now better defined with a better colour balance but the overall improvement is beginning to approach a limit ---best strategy probably to make more trips to dark sky site (but weather very hit and miss at the moment).

Just for the sake of completeness I was finally able to process the background area properly- background subtraction of some of the O3 signal was tricky and so I have also added the wider field shot. Most of the Wolf Rayet UV light breaches the Crescent nebula and some excites surrounding clouds of H to give off HA light.

best wishes
Tim
Attachments
little_NGC6888_Newt_Baader_UHC_120821_94x110s_gain151_11Mb_pixint_CROP_DBE_PCC_SCNR_MLT_strmsk_TRAN_LIN_toO3_pxmathMAXlumfrom2hDBE_HA_DBE_DBE_5,5hO3_NBRGB_curves_PX.jpg
little_NGC6888_Newt_Baader_UHC_120821_94x110s_gain151_11Mb_pixint_CROP_DBE_PCC_SCNR_MLT_strmsk_TRAN_LIN_toO3_pxmathMAXlumfrom2hDBE_HA_DBE_DBE_5,5hO3_NBRGB_curves_PX.jpg (337.32 KiB) Viewed 1286 times
NGC6888_sm_Newt_Baader_UHC_120821_94x110s_gain151_11Mb_pixint_CROP_ABE_PCC_SCNR_MLT_strmsk_TRAN_lumfrom0.9x2hHA_5hO3_NBRGB_curves_mlt_PX.jpg
NGC6888_sm_Newt_Baader_UHC_120821_94x110s_gain151_11Mb_pixint_CROP_ABE_PCC_SCNR_MLT_strmsk_TRAN_lumfrom0.9x2hHA_5hO3_NBRGB_curves_mlt_PX.jpg (619.88 KiB) Viewed 1382 times
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