Sh2-155 Cave Nebula
Sh2-155 (Cave Nebula) - Click here for full resolution
Sh2-155, also known as Cave Nebula (R.A.: 22h 48m 0.47s, Dec: +61º 24’ 52.0”) is a region of Hydrogen-alpha emission in the constellation Cepheus at a distance of 2,400 lightyear from Earth. The nebula is relatively small with a size of 70 lightyears and is embedded in a much larger area of emission, reflection and dark nebulosity. The nebula is also known as Caldwell 9 and LBN 110.
Sky Plot (click to enlarge)
5º FoV + scope display (click to enlarge)
Conditions
The Images were taken on October 30 and 31, 2019 from the backyard in Groningen, The Netherlands (53.18, 6.54). Moon was absent. This object is visible all year round, but visibility from this location is best during autumn and early wintertime. Sh2-155 was visible over the northern horizon, with high altitudes up to 81 degrees. The Northern horizon has quite some light-glow from the city of Groningen, but in narrow-band images that does not have too much effect.
Visibility charts showing 22:00h altitude throughout the year (left) and throughout the session on 30 October 2019 (right).
Weather was generally good, with low temperatures just above 0. Humidity was high in the 95% range. Visibility was average with SQM values of 19.5 mag/arcsec2. There were clear skies on two consecutive nights, so the equipment was left out during the day.
Capturing
The image was captured using the Takahashi TOA-130 in combination with the ASI1600MM-Pro camera.
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Takahashi TOA-130, 35 flattener, Sesto Senso
10Micron GM1000HPS, Berlebach Planet
ZWO ASI1600MM Pro, cooled to -25 ºC
Astrodon 1.25” mounted Ha(5nm), OIII (5nm), SII (5nm), RGB, ZWO EFW 8-position
Unguided
Fitlet2, Flip Flat
KStars/Ekos, INDI, Mountwizzard4, SkySafari
The image was captured using 5nm H-alpha, OIII and SII filters. For star colours, a small set of RGB images were also taken. Below are the frames listed taken in each session that made it to the final image.
Image
The images was taken in October 2019. That was about 1.5 years into the astrophotography hobby. Focusing routines, camera settings, etc were still a work in progress. The result is an image that might lack a bit of focus, has some more noise than desired and does not show pin-point stars throughout the field. Nevertheless, it represents more than 3 hours of exposures in each channel, with a total imaging time of 11.8h.
After a little crop to straighten the edges, and a further crop at the end to optimise framing, the final image has a resolution of 4168 x 3148 pixels, or 13.1 Megapixels. It covers a field of view of 0.92 degrees horizontally.
Annotated image showing other deep sky objects, stars brighter than mag. 9 and the image’s orientation.
Processing
All frames were calibrated with Bias (100), Dark (50) and Flat (25) frames, registered and integrated using the BatchPreprocessing script.
After cropping away some rough edges from the alignment process, a modest background gradient in the HSO images was removed using DBE with a grid of only 5 points per row. On all images, including the RGB channels, noise reduction was applied using the MMT-method. The R, G and B channels were combined into an RGB image which was stretched to the non-linear stage with a HistogramTransformation. This images was intended to only supply the stars for the final image, so the brightness was not stretched a lot, to prevent bloating of the stars too much.
Before combining the narrowband channels, overall image signal had to be matched. Mean signal strength of H-alpha was 86 ADU, while OIII and SII were 134 and 66 respectively. Using LinearFit, both OIII and SII were brought to the same level as H-alpha. As expected, the H-alpha image has most of the detail in it. But the SII channel was surprisingly detailed as well, with a lot of detail at the heart of the nebula. OIII signal was more uniformly spread, with quite some glow above the nebula. The narrowband channels were combined according to the so-called Hubble palette, with SII mapping to Red, H-alpha mapping to Green and OIII mapping to Blue.
By means of experiment, the grading of the final colours was done on a starless image. Since the stars would be added later from the RGB image, this made good sense. The starless SHO image was easily created using StarNet. The ColorMask script (blur-factor 3) was used to create a cyan, magenta, green and yellow mask. The masks were subsequently applied and CurvesTransformation was used to modify individual colour components, roughly in the same way it was described for NGC2237, the Rosette Nebula. The final colours here are purely an aesthetic choice. The H-alpha signal came out more green-like, and the OIII less blue-like than expected. But the overall image seems well in balance. Contrast in the final image was enhance slightly by applying some mild DarkStructureEnhance. Then it was time to add back the stars.
The stretched RGB image also held quite a bit of nebulosity, so the stars were isolated here using StarNet. Then they were reduced in size a bit using Morphological Transformation. The background was protected by applying a star mask.
The stars from the RGB image were then combined with the nebulosity from the SHO image by simply adding both images together using PixelMath. This resulted almost in the final image. To focus a bit more on the nebula itself, a slightly further crop was applied which gave the image a bit more impact.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
