Rosette nebula. Making a silk purse from a sow's ear

[timh]

"You can't make a silk purse out of a sow's ear" - seventeenth century probably originally Scottish proverb

Here the sow's ear was about two hours of RGB images taken through a UHC filter of the rosette nebula on 01/12 and 06/12 during brief dark moonless opportunities within unsettled passing weather systems and with poor seeing and the jetstream overhead. A backyard with almost no southern view because of blocking trees and houses meant a lot of experimentation with Stellarium and a protractor to find telescope positions that would allow tracking the rosette between about 25 and 35 deg elevation rising from E to SSE. Set up and collimation of the Newtonian that almost redefined astrophotography as an extreme sport because of the constant vigilance and need to keep dashing out to quickly throw a rain cover over everything. Normally I wouldn't try this hard under such marginal conditions and with an object so low in the sky but the rosette is special imo.

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 , both 4.63 uM pixels cooled to -10C

ZWO IR/UV cut filter, Astronomik UHC filter. Optolong HA 7 nm filter

All frames captured between 01'st and 10'th December.

RGB UHC frames captured under under moonless Bortle 6 skies, HA frames captured with the moon risen but low in the SW sky and far from NGC2244 in the E/SE

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

101 x 40s and 23 x 70s RGB (UHC) frames at gain 124 under moonless Bortle 6 skies - v. poor seeing and guiding - FWHM threshold about 3.7 up to 4.5!
28 x 3min HA at gain 151 - low first quarter crescent moon in SW - Bortle 6 skies poor-average seeing - FWHM 3 to 3.5

PixInsight pre-processing, integration and processing. Alignment of HA and RGB images, cropping, DBE background correction, Photometric colour calibration, star and noise reduction, histogram stretching and curves intensity transformation. Affinity used for final curves and contrast adjustment.

IMAGE 1. Here the luminance of the stretched HA image was calibrated versus that of the stretched RGB image and then the PIXMATH max function used to generate a luminance image combining the maxima of the RGB and HA. The combined luminance was then applied to the RGB image. This approach preserves the brightness of the NGC2244 star cluster while adding some HA detail to the nebulosity.

IMAGE 2. Here the luminance of the stretched HA image was simply applied directly to the RGB image. This approach maximises the addition of HA detail to the nebulosity but by minimising the stars reduces the 'jewel box' impact of the core star cluster

IMAGE 3. Here I tried to combine the best of image 1 and image 2 by taking just the starry NGC2244 core of image 1, combining it with the luminance of the core of image 2 using the max function, aligning with image 2 and then creating a new combined luminance to add back into image 2. Thus preserving both the nebula detail and the bright central stars.

Overall I was pleasantly surprised to find out just how much it was possible to get out of some initial poor - almost discardable - RGB captures by adding in some sharper HA and the use of some smart processing tools. But the rosette is the real star here - such an attractive object with the bright O and B stars in NGC2244 hot enough to carve out the central hole and give a lot of OIII as well as HA colour - lots of fine dust detail in the foreground - and overall lots of varied colour where the UHC filter will likely have subsumed some contributions from HB and HG into the bluish green as well as SII and NII into the reds - subtly modified everywhere by reflection and the scattering effects of dust ?

Tim

[Menno555]

Great captures again Tim!!
The 3rd one indeed has the best of the 2 worlds
Over here using Stellarium too to check if an object will be visible for me and how an object will fit in a FOV.

Menno

[turfpit]

Tim

I would say these are 'silk purse' given the low elevation of the object when captured.

I will use your NGC2244 results as a reference as I move my processing to PixInsight (if the weather ever lets me image again). I am using the 'Mastering PixInsight' book which you mention in the 'Riding the SNR Curve' post.

Dave