Sh2-188 Planetary Nebula


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October 31, 2014

This is the faint but interesting planetary nebula Sh2-188, sometimes called the Dolphin Nebula.  It lies in Cassiopeia a little more than 700 light years away.  It is estimated to be around 7,500 years old.  Although it is circular, it is much brighter to the southwest because the star that is energizing the nebula is moving rapidly in that direction.   One of the things that makes this planetary nebula a little unusual is how red it is, due to glowing hydrogen and not a lot else that’s visible with my setup.  Many other planetary nebulae have teal or blue in them due to oxygen atoms (for example M27 and the Helix Nebula).

For those who, like me, are interested in faint and faraway galaxy, this image has several to offer.  The brightest are a little above the pair of bright white stars at bottom centre.  The elongated galaxy on the left is 17th magnitude PGC137850.  To its right and a little closer to the bright stars is elliptical PGC137845, which is a little fainter still.


10″ ASA @ f/6.8 on a Paramount MX.  SBIG STL-11000 with Baader LRGB and Ha filters.  Guiding with 80 mm f/6 Stellar-Vue refractor and camera’s Remote Guide Head.  Acquisition with TheSkyX.  Focusing with FocusMax4.  Scripting using CCDCommander.   All pre-processing and processing in PixInsight.  No moon for L or RGB, gibbous moon for H-alpha.  Transparency and seeing  average for R, G and B and Ha and average for L.  Acquired from my SkyShed in Guelph, ON.   

13x15m R, 10x15m G, 8x15mB; 16x20m Ha, 32x10m L (total = 18hr25m)

The batch pre-processing script was used to calibrate, register and combine the frames.  R, G, B, L and Ha channels were cropped.

R, G and B were combined, and RGB, L and Ha were background corrected with DBE.  The NB-RGB Combine script was used to combine the Ha and RGB data (Ha scale of 8).  The resulting HaRGB file was processed with colour calibration, HistogramTransformation, ACDNR (small scale), another HistogramTransformation, and a curve to boost contrast and saturation.

The L image, which was background-corrected earlier, was processed using Deconvolution (PSF made from 40 stars using DynamicPSF), HistogramTransformation, TGV Noise reduction, and another HistogramTransformation.  Morphological transformation was applied to shrink the brighter stars somewhat, using a star mask to protect everything else.

The luminance of the HaRGB image was matched to the L image using LinearFit, and then combined back into the HaRGB image using ChannelCombine in the Lab colour space.  Then LRGBCombine was used to overlay the L onto the HaRGB.  A slight unsharp mask was applied to the centre of the nebula, and colour balance, brightness and saturation were adjusted.  Then MultiscaleLinearTransform was used to separate the large scale (>4px) from small scale structures.  Large and small scale structures were separately stretched, noise reduced and saturation adjusted before recombining using PixelMath.  Brightness and colour saturation were adjusted to complete the processing.  The Image scale is 1.1 arcsec/pixel for this camera/telescope combination.