Refractor views of the Pelican and the Elephants trunk regions

[timh]

Revisited the Pelican and the elephants trunk. I have imaged these previously using my longer focus Newt but wanted to capture them at a wider image scale as well as test outa new dew controller kit (a Pegasus mini power box controlling dew straps on the main and guide scopes).

The two nebulae were imaged as a blend of UHC-filtered RGB from an OSC camera and OOHa narrowband using the mono camera.

Although we think of these objects as HA emission nebulae there must also be some baseline broadband component to the nebula light from scattered and reflected starlight? So I always find myself wondering to what extent the nebula-specific light can be faithfully represented as purely the sum of its NB components or whether the broader band RGB channels also add something rendering the image more realistic?

Here I tried to keep a consistent approach to colour in order to deliver something that is 'pseudo' realistic wrt human vision - although in reality I suspect that, from any vantage point- the light would be much too diffuse to stimulate the cones in the eye. It is probably quixotic but it feels uncomfortable just mixing the information arbitrarily.

I defined the SNR' of the separate HA, OIII, R, G, and B channels in terms of the difference signal between clear regions of nebula-only signal and the background signal in patches of dark nebula then divided by the standard deviation of this background signal (all within about 5000 pixel samples). The R , G and B channels used to form the final image were then built by adding, HA +R, OIII + G and OIII + B in proportion to these SNR numbers. In addition 12% of the HA signal was added into the G channel and 18% of the HA into the blue channel to account for H beta at 486 nm at about a third of the 656 nm HA intensity.

Mixing the channels as above basically delivers nebula images that are red with subtle pink coloration wherever the OIII is found - because in both of these nebulae- and as is usually the case - the HA signal far outstrips the OIII. From rather approximate photometry, in the brighter parts of the two nebulae HA is perhaps 7-10X the strength of the OIII signal in the Elephants trunk and maybe 15X the strength of the OIII signal in the Pelican. So in forming the final images- and after normalising for gain x exposure- the G and B channels are stretched up by 10X relative to the R channel in order to make the OIII regions more clearly visible- and also because that is close to realistic wrt the insensitivity of the human eye to red light at 656 nm relative to cyan light at 500 nm.

The star colours are close to true because they were rescreened into the final image from the colour corrected (SPCC) RGB image.

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The set up was an old Zenithstar WO 110mm apo refractor (F 7.0) taken down to F 5.6, f = 620 mm using a 0.8X WO MkIII flattener/ reducer, CEM 40 mount, ASI 294 MM and MC PRO cameras (4.63 u pixel image scale 1.53 arcsec/ pixel) and PHD2 guiding using a 400mm guide scope and ASI120 MM camera. Polar alignment and capture was in Sharpcap using livestacking with FWHM and brightness filters. Filters were Baader UV/ IR, Astronomik UHC, Optolong 7 nm HA, and Antila 3 nm OIII. Preprocessing and processing was in PixInsight and Affinity serif. BlurXt was used - it does not perceptibly add to sharpness at such low sampling -1.53 arcsec/ pixel - but it did correct star shapes at the periphery of the field where the field flattener was struggling.

The Pelican image was 2.3h of UHC-filtered OSC RGB, 6 h of HA and 4h of OIII.

The Pelican is just a small part of a massive region of Nebulosity. The Herbig Haro objects can be seen quite clearly and, relative to the Elephants trunk it is also notable how little OIII signal there is with the overall impression being red. The bright blue star was a challenge for processing -still not resolved.


The Elephants trunk region image was 3.3 h of UHC-filtered OSC RGB, 4h of UHC and 6.2h of OIII

This image scale frames the Elephants trunk region to show the sphere of OIII surrounding the central triplet of hot type O stars, HD206267 (of the star cluster IC1396) which irradiates the entire region. There is a a slightly 3D appearance with depth cues from the dark nebula and surrounding HA dominated outer regions. The image shows only a small part of the overall nebula. The elephants trunk - IC1396A - is a dark region of dust being eroded under radiation pressure from the type O stars and newly-formed stars forming within it. The whole region is about 2500 light years away - and for scale covers about 5 moon widths with the elephant's trunk feature being about 20 light years long.


Note that the OIII and HA subs for each were 3 minutes at gain ~ 0.6 electron/ ADU - the Sharpcap Brain suggested that 8 -6 min subs would have been better to suppress 90% of the read noise but conditions were such that longer subs would have meant that more failed so a compromise there.

[turfpit]

Tim

Great images and an interesting detailed description.

Dave

[timh]

Thanks Dave,

I always seem to get into a raging debate with myself around how to 'correctly' or at least to consistently colour NB images

Tim

[turfpit]

Tim

NB is false colour, you can have anything you want - artistic license.

Dave

[timh]

Well you are right of course --- but I do think that there is a case to be made for consistency in depicting the ratios of OIII to HA - otherwise it creates the impression that all HAII nebulae are similar when in reality OIII varies from being dominant to negligible -- made me think of working back from the electronic image to depict what an idealized eyepiece view might actually look like with a large enough telescope and ideal conditions. A retro approach -like going back to vinyl that could be applied to some of the new Unistellar type generation of electronic telescopes e.g. so perhaps something like this in the case of NGC7822 ?..