In the wise words of Fred - narrow field rules dude!
Hi All,
It’s been a while between posts, slowly working my way back into imaging after a break. So, I’m pleased to present a collaborative effort between Alvin Jeng from
Lightbuckets and I of the infamous planetary,
NGC7293 – The Helix Nebula in mapped colour
About the target;
The Helix Nebula (NGC 7293) resides in the constellation Aquarius and is dubbed the "Eye of God". It is one of the closest of all planetary nebula at a distance of approximately 450 light years and is about half the size of a full moon. Despite its size, the light is spread over a large area making it a difficult object for visual observation. The nebula displays different ionisation levels of ejected matter from the dying central star. The inner blue hue indicates the presence of excited oxygen atoms, while the vibrant outer structure consists of hydrogen and nitrogen atoms.
About the image;
The image is a SII:Ha:OIII palette mapped as RGB respectively. Alvin performed the data acquisition on Lightbucket’s monster 24” F/8 RC (4876mm FL). Not a huge amount of data for this target, but not really needed as it was acquired 2x2 bin on the sensitive Apogee U42 camera - SII: 90min, Ha: 60min, OIII: 60min (total 3.5hrs) – all 15min subs – overall good quality data. The binned U42 is only 1024x1024 pixels, so the image presented is actually upscaled by ~60% to show some pleasing details. If the Ha data was 1x1, I think the resolution would be remarkable, though I highly doubt seeing would get down to .57 arcsecs! The upscaling perhaps lost some resolution in the process. The SII data gave me the most grief during the processing, despite the quantity of data, it couldn’t be stretched hard. I will disappoint the NB purist by stating that I blended in some Ha data in to the SII to compensate. The blend however was low – 20%. This settled the somewhat noisy SII data when stretch hard. I constructed two data sets, the first was the SII:Ha:OIII image which was weighted to taste using MaximDL colour combine. I noted the weights and used pixel math to drop equalize the backgrounds between combined subs, and then raise them again based on the noted weights. Seems to deliver a reasonable result as opposed to using clipping masks – change is as good as a holiday right... I was expecting a greater transition of green Ha hue between the SII and OIII, but it simply is not as distinct as I would expect. The other data set being the Ha was ran through deconvolution and used as a luminance to bring some depth and resolution to the field. 2x2 bin still delivers 1.14 arcsec/pixel on the 24” RC so good sampling is present. All layers brought into PS where further colour balance tweaks and enhancement layers such as highlights and noise reduction were invoked. Needed to ensure those cometary globs were in full effect. Experimental, I desaturated the star colour as they typically turn a warm pink hue as the narrowband palettes mix. I think this provides a more aesthetic feel for this target. Talking stars, the diffraction spikes disappeared when I added the Ha data. Not sure what happened there. Perhaps I didn’t stretch the data hard enough. Interestingly, they weren’t very pronounced in the raw data to start with. I even thought I’d remove the stars all together, but thought that would look a little ridiculous considering the planetary nebula’s structure and surely such an activity wouldn’t secure an APOD would it…
Presented image is actually a crop of the upscaled frame to work in with website metrics. The full field also captures part of the notorious arm though it is faint with SII data.
Anyway, enough rambling, hope you enjoy it. All comments welcome.
PS. Anyone heading to
AIC 2008 this year? I’m there! Need to glean more processing info.