Link to inverted Ha image
StDr 4 was discovered in November 2019 by Xavier Strottner and Marcel Drechsler on an image collected as part of the INT Photometric Hydrogen Alpha Survey (IPHAS), which uses the 2.5-meter Isaac Newton Telescope on La Palma in the Canary Islands to collect 2-minute H-alpha images of the northern portion of the galactic plane. A matching survey (VPHAS+) is being made of the southern galactic plane.
The nebula was listed as a planetary nebula candidate in a table published in the March 2020 issue of L’Astronomie, but on planetarynebulae.net, the French site which maintains this list, it is now listed as a supernova remnant candidate. But at roughly 3 arcminutes in diameter it seems to me much too small to be an SNR.
The target appears to have Ha and NII signatures, but no OIII, according to a spectrum obtained by Pascal Le Du in December 2019. My single 20 minute OIII exposure showed no signal, so I took all my narrowband data with a 5nm Ha filter, which has a bandpass wide enough to transmit most of the NII signal as well.
The area is on the southern edge of the large ionized gas region IC 1396, and the region is suffused with faint H-alpha emitting clouds, as well as a few dark nebulae. This image uses Ha colorized to red for StDr 4 and the surrounding background nebulosity, and RGB for the stars.
Exposure: Total exposure time 22.67 hours, 68 x 20 minutes Ha, 60:60:60 x 2 minutes R:G:B. All bin 1x1. Data collected in September and October 2021.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Seeing: FWHM of integrated Ha around 2.15 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% scale).
Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.1 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.17140 (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 2.4.3.0 driver
Filters: Astrodon Type IIi LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB nStep motor with driver version 6.0.7 on stock Celestron focuser
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.1.0.457
Guide SW: PHD 2.6.7, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: PlateSolve 2, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2
Processing Software: Pixinisight, Affinity Photo, Photoshop CS2
Processing Workflow by Workspace in PixInsight 1.8.8:
1. Calibration
Calibration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Blink to preview and reject a few frames
Weighting and registration with WBPP
2. Stack and Mure Denoise
Image Integration on each channel
Mure Denoise on each channel
RGB Combination for RGB frames
Dynamic Background Extraction
No deconvolution was applied to these images
3. Ha Stretching
Histo Trans x 2
Curves Trans
StarNet to remove stars
TGVDenoise
Aggressive Multiscale Median Transform (with an inverted luminance mask, so that the effect was not applied to the nebula) to smooth out background
TGVDenoise using a mask to limit the effect to the nebula
Curves to darken the background and brighten the nebula
4. RGB Linear Processing
Photometric Color Calibration, using Average Spiral Galaxy white reference
Align histogram peaks with Histo Trans – PCC didn’t quite do the job
5. RGB Stretching
Histo Trans x 2
Curves
Histo Trans
TGVDenoise
6. Color Combination
From stretched Ha image, create an image for the Chrominance channel by stretching the highlights and clipping the blacks, so that bright areas will have strongest color and background will be neutral
Create a Hue image using pure black (Red has the value 0 in Hue space)
ChannelCombination to blend Ha:Chrominance:Hue to L:c:h
TGVDenoise
In PixelMath, add stars image to nebula image ($T+[stars-0.08])
7. Final
Final Histogram Transformation to darken background
ICC Profile Transform to sRGB
Resample at 50% scale
Save as JPG
