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The Cave Nebula, 


            Jonathan W MacCollum
The Cave Nebula

The Cave Nebula

Technical card

Resolution: 3289x3484

Dates:Sept. 24, 2019Sept. 25, 2019Oct. 1, 2019Oct. 2, 2019

Astrodon 1.25" 5nm Ha: 45x240" (gain: 53.00) -15C
Astrodon 1.25" 5nm Ha: 19x600" (gain: 53.00) -15C
Astrodon O3 1.25" 3nm: 46x300" (gain: 53.00) -15C
Astrodon O3 1.25" 3nm: 39x600" (gain: 53.00) -15C
Astrodon SII 1.25 5nm: 44x300" (gain: 53.00) -15C
Astrodon SII 1.25 5nm: 37x600" (gain: 53.00) -15C

Integration: 26.3 hours

Darks: ~60

Flats: ~20

Flat darks: ~20

Avg. Moon age: 14.43 days

Avg. Moon phase: 17.27%

Bortle Dark-Sky Scale: 4.00 job: 2975387

RA center: 344.359 degrees

DEC center: 62.649 degrees

Pixel scale: 0.498 arcsec/pixel

Orientation: 93.755 degrees

Field radius: 0.331 degrees

Data source: Backyard


The Cave Nebula - Sh2-155

This target has become one of my favorite targets in the northern sky... I first started
imaging it with my 135mm widefield setup next to the Bubble and the Lobster Claw and have
been mesmerized by it ever sense. Getting a chance to see it close up with the telescope
has been a real pleasure. I pushed myself to get the best data I could and as much as I
could reasonablly take given the weather but after the first few nights, a significant
amount of banding resulted in all of my integrations. I'm dithering quite a fair bit but
since I've been struggling with backlash in declination have been only dithering in RA.
Given the orientation of my framing has been at 0* when I started the dithering just wasn't
cutting it. So after about 11 hours of data in, I rotated the camera 90*, limiting my use
of the full field the camera has to offer but what a significant different it made. To still
make use of the data already acquired this image consists of the best data from all four
of the night's I spent on imaging this target. The remaining banding left is in the direction
of the origional orientation. Perhaps next year I'll give it a try in RGB.


* 8in f4.9 Newtonian Reflector w/ Sesto Senso Auto Focuser

* Skywatcher Quattro Coma Corrector

* ZWO Electronic Filter Wheel with 5nm Ha, 3nm Oiii and 5nm Sii Astrodon Narrowband Filters

* ZWO ASI 183mm Pro - Cooled to -15.0C

* Celestron CGEM Mount

* Dew Strips around the Primary mirror and Focuser draw tube

Integration Time per Channel: (Combination of 4 minute, 5 minute and 10 minute exposures)

* Ha: 6 Hours 10 minutes

* O3: 10 Hours 20 minutes

* S2: 9 Hours 50 minutes

Gain: 53 (Half Unity)

Offset: 10

Data Taken across 4 nights: September 24th, September 25th, October 1st, October 2nd

Guiding averaged ~0.8"/px RMS across all nights; Image Scale: 0.5"/px

Flats: 20 per filter; calibrated with matching Dark Flats (Thanks NINA flats wizard!)

Darks: 60 per exposure duration (4 minute, 5 minute and 10 minute)

Bias: None

Ha Processing:
let ha =
crop edges
|> linear fit to Sii
let decon=
|> Sharpen
|> Deconvolution iterations:60 deringing:none regularization:5 layers of decreasing amounts/thresholds
|> replace stars using a star mask and a copy of ha
|> with the star mask above MMT Sharpening to bringing out features
let nr =
|> TGVDenoise mask: inverted low contrast copy of ha
|> MMT mask:very high protection luminance mask
let blendMask =
|> stretch using screen transfer -> histogram tool
|> stretch more clipping white point and black point

let l =
// adjust the blend mask using the midpoint slider on the histogram tool
// until the noise reduction targets a blend of the low-signal areas and
// the sharpening is applied to the high-signal areas.
|> Stretch with Histogram Transformation
|> Motion Blur Deconvolution to round-off stars (Guiding in RA is considerablly worse than in Dec)
|> Morphological Transform to shrink stars

Oiii and Sii were processed with a similar yet simplified workflow:
let result =
|> TGVDenoise mask: inverted low contrast copy of the image
|> MMT mask:very high protection luminance mask
|> Stretch with Histogram Transformation
|> ACDNR to remove the smaller 'flakes' of noise throughout
|> Motion Blur Deconvolution to round-off stars (Guiding in RA is considerablly worse than in Dec)
|> Morphological Transform using Morphological Selection to shrink stars
|> Canon Banding Reduction script

The three channels were combined to produce the final images using the workflow below:
let result =
|> LRGB Combination
L = Ha
|> HDR Multiscale Transformation
Layers: 7
Target: Lightness
Lightness Mask: on
Additional Mask: Same starmask from previous processes to protect the star cores/halos
|> Curves to boost contrast and saturation
|> Exponential Transformation
Function: Power of Inverted Pixels
Order: 1.0
Smoothing: 3.5
Lightness Mask: on
|> Additional round of Curves and ACDNR
|> Dark Structure Enhancement script

A starless version was created with the processed Ha, Oiii and Sii image but was not
used in the making of the final image. All of the same processes applied in the final
image were re-run with the same values on the starless versions of these images.



Jonathan W MacCollum
License: Attribution-NoDerivs Creative Commons


  • Final
    The Cave Nebula, 


            Jonathan W MacCollum
  • The Cave Nebula, 


            Jonathan W MacCollum


Starless of each Processed Stack + Combined using same steps

Sky plot

Sky plot


The Cave Nebula, 


            Jonathan W MacCollum