Dengel Hartl 2 in Ophiuchus

Inverted Ha and OIII Images

DeHt 2 (PN G047.1-04.2) is a highly evolved (age estimate 13,000 to 18,000 years), faint (mag 14.9), 1.9 x 1.5 arc minute located 6000 to 10,000 light years away in Ophiuchus. It was discovered in 1979 in a survey of Palomar Observatory Sky Survey (POSS) plates and described in the 1980 Astron. Astrophys. Research Note “A Search for Planetary Nebula on the “POSS”, vol. 85, pp 356-358 by J. Dengel, H. Hartl and R. Weinberger. A recent study (Aller et. al., Mon. Not. R. Astron. Soc., 446, 317-329 (2015/January-1) concluded that it formed from at least two different ejection processes. The first, between 13,000 and 19,000 years ago, formed a spheroid with a bright, ring-like region on its equatorial plane. A subsequent bipolar ejection, collimated along a different axis, interacted with and deformed the original spheroid. In PNe, multiple ejection events like this are correlated with binary central stars. Spectroscopy found no indication of [NII] emission in DeHt 2, which I think is fairly unusual in PNe.

The mag 13.5 spiral galaxy at upper left is UGC10943, located about 115 million light years away. LEDA 214626, near the spiral, has no distance information. The two faint objects near DeHt 2 aren't cataloged as far as I can tell, but the object nearer the PN does show in the images captured by the 2.5 meter Isaac Newton Telescope for the MNRAS study. It's stronger in Ha than OIII. It would be interesting if it could be determined whether it's associated with the nebula or whether it's a background galaxy.

Luminance for this image is a 50/50 Ha/OIII blend. Color for the nebula is Ha:OIII:(15% Ha + 85% OIII) mapped to RGB. Star colors are RGB.

Exposure: Total exposure time about 24.7 hours. 33:33 x 20 minutes Ha:OIII, 26:23:23 x 2 minutes RGB. All bin 1x1. Captured May to July 2020.
Light pollution: SQM ~18.38 (Bortle 7-8, NELM at zenith about 4.5, Red/white zone border.)
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 0.9 arcsecs/pixel (2/3 original size)

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 IIe 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 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
Dynamic Crop

3. Narrowband Linear Processing
(No deconvolution on these images)
STF applied and inverted images created

4. Narrowband Stretching
Sum Ha and OIII images in PixelMath to create 50/50 Ha/OIII for luminance
Histo Trans x 2
Curves Trans
Additional stretching for the nebula was done with a RangeMask
Local Histogram Equalization, using the RangeMask, to enhance contrast in the nebula
TGV Denoise
(similar processing on separate Ha and OIII images for color)

5. RGB Linear Processing
Photometric Color Calibration, using Average Spiral Galaxy white reference

6. RGB Stretching
Histo Trans
Curves
Boost color saturation with additional Curves

7. Color Combination
Color for nebula: combine Ha and OIII in PixelMath using HO(85% O, 15% Ha) palette to create HOO nebula image
Shift teal green to blue with Curves Trans
LRGB Combination, using 50/50 Ha:OIII as luminance and HOO for color

8. Star Reduction
I followed Adam Block’s star reduction technique to reduce the brightness of the stars embedded in the nebula:
StarNet to create “Starless Image”
Extract two copies of luminance from main image, then apply MLT to one to create a rough star mask
Binarize to select only the stars
MorphTrans to enlarge stars
Convolution to blur star edges
Pixel Math: subtract luminance image from blurred star mask so that cores are excluded from mask, and on ly halos are represented in the mask = “Halo Mask”
Apply Halo Mask to main image, then run PixelMath to use Starless Image where halos otherwise would be

9. Photoshop
Layer stretched nebula image onto starfield image with a mask
Repair blue comatic halos with Radial Spin Blur on a color layer
Slight additional noise reduction on nebula and background galaxies
Save as TIFF and move back into PI

10. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample to 67% of scale
Save as JPG