SHO picture of the wizard nebula

[timh]

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 NB, both 4.63 uM pixels cooled to -10C

ZWO IR/UV cut filter, Astronomik UHC filter , Astronomik 6.0 nm OIII filter, Optolong HA 7 nm and SII 6 nm filters

81 x 110s RGB (UHC) frames at gain 124 and 72 x 3min gain 151 OIII under moonless Bortle 6 skies. 72 x 3 min gain 151 HA frames and 63 x 3 min gain 151 frames SII frames captured with the moon risen but opposite in the sky. All frames (0.95 AS/ pixel) pre-selected for quality using the FWHM and brightness filter within Sharpcap, darks and grayscale master flats (no bias) prepared using Sharpcap.

HA, SII and O3 images were background subtracted (DBE), noise-reduced (MLT) stretched (HisTRAN) and combined using Pixmath, made starless using StarExterminator 2.03, the green band hue shifted using curves and then further stretched using an Exponential stretch, slight LHE (1.3) and curves again. The RGB OSC image was background subtracted (DBE), colour corrected (PCC), noise-reduced (MLT) and stretched (HisTRAN). A high quality (unscreened) star mask was subtracted from the RGB image using StarExterminator, colour saturation increased using curves and then blended back into the starless NB image using pixmath. Finally MLT noise reduction was applied.

A couple of things that I learned here that may be useful to others?

1) I had not before appreciated that curves could used to apply hue shift to an SHO image in a way that stops the image from merely looking predominantly green and that really enhances the the discrimination between the HA, SII and OIII regions without compromising the integrity of the data -- i.e. checking back with my grayscale data - while HA underpins everywhere the yellow/orange is indeed coincident with the main SII signal, the cyan with mixed HA and OIII and the deeper blue where there is less HA

2) StarExterminator 2.05 works very well indeed - and star blending works far better than mere pixmath addition to re-add stars (prevents saturation and preserves more colour)

As expected for a Type II nebula - the SHO image shows HA throughout, yellow orange S+ forbidden transitions at the proximal ionisation edge , OIII nearer the presumptively hotter ionizing stars near the nebula centre with cyan where the O2+ forbidden transitions underly the HA and a deeper blue where the HA is less strong. I wonder though whether the very deep blue near the mid right bright star is an artifact relating to star removal?

Tim

[turfpit]

Tim

As always, a good image supplemented with detailed post-processing information.

Dave

[timh]

Many thanks Dave - actually slight errors in the processing description and image a bit overstretched -- which is now fixed.

I hadn't realized that the starnet version that is bundled into in PI is only version 1.0 and wasn't working very well (leaving artifacts after star removal). The StarExtractor 2.03 works a lot better -- Starnet 2 may also be OK? It is amazing how fast these tools are advancing. Also the combination of exponential stretch plus LHE seemed to work well on the starless image.

One interesting (to me anyway) small point about the astronomy that I hadn't previously appreciated is that the star, a typeO(5.5) responsible for ionizing the Wizard is known and visible. It is DH Cepheus which is the brightest star within the small star cluster and close to the wizard's hand

Tim