SH2-155 - The Cave Nebula
Date: November 15, 2020
Cosgrove’s Cosmos Catalog ➤#0062
SH2-155 - The Cave Nebula as captured by my William Optics 132mm Platform. (click image for full resolution via Astrobin.com)
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About the Target
SH2-155 - The Cave Nebula - also known as Caldwell 9, is a diffuse nebula found in the constellation Cepheus. It is actually a part of a much larger complex of nebulae n the region which includes the emission, reflection, and dark nebulae. The designation indicates that this part of the Sharpless Catalog of emission nebulae which are located north of the declinnation -27 degrees. Here is more background on this catalog in can be found HERE. Sh2-155 is located 2400 light-years away and is an ionized HII region with evidence of star formation activity.
About the Project
I have described this image as my problem child. This is a faint region and you really need a lot of integrated exposure to show it well.
I took 150 x 2.5-minute exposures over two nights, thinking that I had enough to accomplish what I wanted to do with this image. After I stacked the images and began to process them, I found what looked like a bullseye made of rainbows superimposed over my image! What the heck? What was going on? First I found a problem with my flat exposures and sorted that out. Then I looked at the data more analytically. It turns out that the subs I took the second night have 3X worse noise than what I took the first night. This target is located to the north of me - an area of light pollution. I think some high-level cirrus clouds came in and reflected the light from the polluted area and messed up my subs. The first night there was no evidence of cirrus clouds so they were much better. I ended up eliminating half of my exposures - which meant that I had to work like the devil to handle the noise on this image.
So not my best work, but at least I was able to salvage something from the exercise…
Thanks for looking and reading my tale of woe!
Updated Comments 7-24-21 (as I publish this into the website):
What I did not realize at the time was the fact that I was having dew forming inside the sensor window. It could have also been frosting - which would explain the rainbow defraction effect. This is the same problem I was having with my M74 image. The data was compromised and I had not really figured out what was going on at the time - all I could do is try to compensate, fix and bury the problem in the processing. I was not very pleased with the result I ended up with for M74 - this one I am more pleased with. Could it be a better image? Sure - but I don’t hate this one! :-)
The Annotated Image
An annotated image of SH2-155 created using Pixinsight’s ImageSolver and AnnotateImage Scripts.
The Location in the Sky
IAU/Sky & Telescope Constellation Map of Cepheus with SH2-155 marked by the yellow arrow.
Capture Details
Light frames
75 x 150 seconds, bin 1x1 @ -15C.
Total integration time: 3.12 hours
Cal Frames
45 Bias exposures
25 Dark exposures
45 Flats
Capture Hardware
Scope: William Optics 132mm FLT F/7 APO
Guide Scope: Apterna 60mm
Camera: ZWO ASI294MC-Pro
Guide Camera: ZWO ASI290Mini
Focus Motor: Pegasus Astro Focus Cube 2
Field Rotator: Pegasus Astro Falcon
Mount: Ioptron CEM60
Polar Alignment: Ioptron Ipolar integrated alignment cameras
Software
Capture Software: PHD2 Guider, Sequence Generator Pro controller
Image Processing: Deepsky Stacker, Pixinsight, Photoshop, Coffee, extensive processing indecision and second-guessing, and much swearing…..
During the Summer of 2020, whilst stuck at home because of Covid I became very in improving the automation of the platform and achieving a more precise framing of the shots I made…