RCW 91: Beautiful but Shy

OTA: CDK 24
Camera: QHY 600Pro
Observatory: Telescope Live
Date of Capture: Jan 23
Date of Processing: Apr 24

Exposures Used:
H: 11 x 1200 sec
O: 13 x   "
S: 14 x   "
Total Exposure time: 6.3 hours
Image Width: 31 arc-minutes

Processing Tools:
1.    Commercial: PixInsight, Topaz, PhotoDirector
2.    Pixinsight Addons: NoiseXTerminator, BlurXTerminator, StarXTerminator
3.    My Scripts: NB_Assistant, AC_Restar, Subframe Weighting Tool (Excel w/ J. Hunt), ColorTweaker

Target Description:
This little-known nebula is sometimes caught in wide-field images of this nebula-rich region. But it is seldom the featured object of an image; my search of Astrobin failed to reveal a single image homed in upon RCW 91. Those images must be there…this data set is for sale to everyone.

Interestingly, the Ha images had a revealing signal level, and the SII had a readily discernable level. However, OIII was almost absent. The blue seen in the OIII image (and this image) is suspicious. I suspect the blue signal is not OIII emission but the blue signal from a reflection nebula surrounding the bright blue central star. If data the capture had included RGB data, the reflection and emission components (if any) could have been separated with a bit of high school mathematics.

Processing Description:
Perhaps one reason RCW 91 has not received more attention is that it is faint and, at first glance, bland. But sound processing uncovers a beautiful detail reminiscent of a garden painting. One challenge is pushing both the color and structural detail in the nebula while not fragmenting the subtly of the background variations. Usually, I avoid masking, which often causes unnatural boundaries among brightness and color domains. But here, it worked rather well.

Version B shows a starless rendering. I prefer the starred version for several reason, including the central star "explains" some of the feature of the nebula. However, some details certainly become more evident in the starless version.

Target Statistics:
Distance: 13 ly
Apparent Magnitude: ?
Apparent DSO Span: >fov (>100 trillion km)
Pixel Resolution: 0.39"
Pixel Span at Target: 23 billion km

Alex Woronow
(95)