Compare RGB and NB imaging of the bubble and the wizard

A place to share images that you have taken with SharpCap.
Forum rules
Please upload large images to photo sharing sites (flickr, etc) rather than trying to upload them as forum attachments.

Please share the equipment used and if possible camera settings to help others.
Post Reply
timh
Posts: 515
Joined: Mon Aug 26, 2019 5:50 pm

Compare RGB and NB imaging of the bubble and the wizard

#1

Post by timh »

A quick explanation of what this study was about should it happen to be of any use to anyone doing similar things....

The imaging of HA emission regions can be enhanced in a number of ways. For example, in the first place, the RGB image from a OS colour camera can be enhanced simply by using a UHC or similar filter to suppress background and increase the relative contribution from the emission lines of HA, HB, NII and OIII etc. Use of a mono camera creates further possibilities. HA and OIII narrow band images can be combined into the luminance and or colour channels of an RGB image to improve detail and or colour differentiation. Since HA is usually by far the strongest signal adding HA to the luminance to produce an "RGB H" image is usually the single step that produces the most dramatic improvement in SNR/ detail. Other possibilities are to also co-add OIII to the luminance and/or colour channels to produce "RGB OOH" images or to drop the RGB altogether and simply produce dual narrowband "OOH" images.

The dilemma that all these possibilities create is around how to make best use of those - all too rare - dark, clear and moonless opportunities? Bearing in mind that HA data can be collected even under poor conditions, should the darker nights be best used for RGB(UHC) or OIII?

Thus far, the main conclusion for me is that - for the majority of HII emission regions - where the OIII is relatively faint and located coincident with the HA - it really isn't worth spending any of those precious dark hours on NB OIII and better to spend it on RGB (UHC) which will, in any case, provide adequate OIII coloration of the image. In fact adding in the OIII can be counterproductive. Echoing a comment from Brian (Oopfan) on this forum - unless hours and hours have been spent on the OIII - the SNR of the weaker OIII image is often poor and thus adding it in just a route to adding noise. BUT there are also of course many objects where NB OIII really is the best approach- like the Crescent, the veil and here, the bubble nebula - where the OIII is not purely co-localised with the HA and dual narrowband images look highly effective.
-------------------

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) 110s exposures at gain 124 -10C with an ASI 1294 MC camera and Astronomik UHC filter under moonless Bortle 6 skies, 0.946 arcsec/pixel

2) 180s exposures at gain 151 -10C with an ASI 1294 MM camera and Optolong 7 nm HA filter and Bortle 6 skies. 0.946 arcsec/ pixel

3) 180s exposures at gain 151 -10C with an ASI 1294 MM camera and Astronimik 6 nm OIII filter under moonless Bortle 6 skies. 0.946 arcsec/ pixel

Capture using Sharpcap and FWHM / brightness filter. FWHM of individual frames 2.5 to 3.0. Guiding using PHD2 multistar, RGB frames preprocessed and stacked in PixInsight.

NGC7380 (Wizard nebula) 81 x 110s (UHC), 39 x 3min (OIII) and 39 x 3 min (HA)

NGC7635 (Bubble nebula) 64 x 110s (UHC), 29 x 3min (OIII) and 22 x 3 min (HA)


The HA , OIII and RGB images were separately processed (noise reduction, background removal etc) and then, after stretching to non-linear- the Luminance was extracted from the RGB, calibrated to scale consistently 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 and PhotoscapeX. RGB OOH images were produced by adding HA and OIII into the R,G and B channels using the PI NBRGB combination script. Alternatively NB images were combined as purely narrowband images in the proportions R 100% HA, G 90% O3,10% HA, B 100% O3 using the PI LRGB combination tool.

The images show the original calibrated HA, OIII and RGB_luminance images in gray scale and then the "RGB Ha", "OOH NB" and "RGB OOH" images derived from these.

I think that the nearly starless OOH NB image of the bubble nebula looks quite effective as well as the RGB OOH image but that the best image of the wizard nebula is the RGBHa image whereas all my attempts to add in the extra OIII data into the colour channels accentuated the OIII region but at the expense of a noisier image and colour distortion that was difficult to adjust..

So for probably most of the HII objects I will be sticking to RGB Ha in future I think ..?

Tim
Attachments
aa_col_wizbubble.JPG
aa_col_wizbubble.JPG (257.39 KiB) Viewed 762 times
aa_bubwiz.JPG
aa_bubwiz.JPG (198.35 KiB) Viewed 784 times
aa_nb_wiz.JPG
aa_nb_wiz.JPG (190.86 KiB) Viewed 784 times
User avatar
turfpit
Posts: 1779
Joined: Mon Feb 13, 2017 8:13 pm
Location: UK
Contact:

Re: Compare RGB and NB imaging of the bubble and the wizard

#2

Post by turfpit »

Tim

Thanks for the detailed write-up. My personal preference is for the RGBHa combination for both images.

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

Re: Compare RGB and NB imaging of the bubble and the wizard

#3

Post by timh »

Thanks Dave,

Writing things up helps me to clarify where I have got to in this hobby of never ending learning - and especially around processing. Agree that O3 is a bit of a marginal call on the bubble.

Tim
Post Reply