Mis à jour : 9 oct. 2020
The Cave Nebula (or Caldwell 9) is a diffuse emission nebula in the constellation Cepheus. It is one of the prettiest objects in the Sharpless catalog, and seems to "sit" in front of a field of blue Oxygen gas. Lot of dust can be seen in the object and it is also surrounded by some dark nebulae and interstellar dust lanes.
Object Designation: Sh2-155
Also known as: The Cave Nebula
Object Type: Emission Nebula
Distance: 2,400 light-years away
Discovered in: 1959
This is most likely a work in progress as I am not 100% happy with the final result, despite the 37+ hours spent on the integration time. Who to blame? The California wildfires!
The main star forming region in the object (as seen on the left) is made up of double ionized hydrogen, that came from Cepheus B, a large molecular cloud home to all types of nebulae.
The Cave Nebula is faint, and is a challenge for beginner astrophotographers. We spent over 37 hours on this object to get a good result. It is best captured in narrowband with a high power telescope. You will also need to spend a considerable amount of time on it, especially if image from a light polluted place like us.
This is our first image with a new telescope we are trying this year thanks to our awesome friend Mark! Note that we plan on re-shooting this image, specifically the Sii and Oiii channels as those were taken during the California fires "season" and thin layers of smoke were present in the sky during that time.
The telescope we used to capture the Cave Nebula is the Stellarvue SVX130, an upgrade from our Meade 115mm. With the reducer, it has an almost identical field of view in terms of focal length (655mm vs 644mm) and the Cave Nebula fits really nice!
Something absolutely awesome about this telescope is that it came with a Moonlite Nightcrawler focuser/rotator, and it has been working perfectly so far! Unlike our ZWO focuser, this one has zero backlash!
You will find our full image below. Notice how much dust is present all over the image, and how bright the star forming region in the center is compared to the darker clouds of gas almost everywhere else in the frame!
The Cave Nebula from our Bortle 9 backyard in narrowband
Camera: ZWO ASI 1600MM
Telescope: Stellarvue SVX130
Mount: Paramount MyT
Guiding: ZWO ASI 290MM Mini
Accessories: Moonlite Nightcrawler focuser
Total Exposure Time: 37 hours and 15 minutes
Exposure Time per frame: 10 minutes
Filters: Chroma 3nm Ha/Sii/Oiii
How to find the Cave Nebula?
The Cave Nebula is too dim to be seen with the naked eye, and is also just not bright enough to be spotted with binoculars. You can observe it with a telescope, but do not expect this object to look incredible.
To find Sh2-155 in the night sky, first locate the constellation Cepheus, very close to the North Star Polaris. Once done, find the "triangle" shape on the North part of the constellation. The star you are looking for is the left base of the triangle. The Cave Nebula can be found just South of that bright star.
Processing of the Cave Nebula
I did not plan to be done with this image of the Cave Nebula so early. The original goal was to spend at least 50 hours of total exposure time, but the California fires have been raging for so long, and our sky hasn't been clear for weeks that I decided to finally be done with it. It is sad because I know that the OIII data was somewhat blurry, the stars would fluctuate so bad when going through the frames on Blink, but i decided to integrate them all anyway.
To process the Cave Nebula, I went through the processing workflow I always use for nebulae. If you'd like to learn to process your images exactly the way I do it, you can download a full PDF "follow along" file that contains 75 pages, a full 1 hour and 45 minutes walkthrough tutorial video and even raw data HERE.
Just like you will see in the PDF file and video, I used StarNet to remove all the stars in the image prior to bringing out all the details and colors in the object. I then re-added the stars at the very end. You can see the starless version of this image below. All the weird brown dots you see in the image are artifacts from bright stars, which, this time, I wasn't willing to spend hours removing individually. I do of course go through how to remove those in the tutorial, where I process the Pelican Nebula.
What did each narrowband channel look like?
Below you can see what each stacked frames looks like for the three narrowband channels. We used the following filters to capture Sh2-155:
Hydrogen Alpha (left) - Taken during the full moon
Sulfur II (center) - Taken at the beginning of the California fires
Oxygen III (right) - Taken through thin layers of smoke from the fires
As you can see, the OIII data is really bad, and the SII isn't that great either besides the center area. We shot with both of these filters when we knew there was a very thin layer of smoke in the sky.
The Cave Nebula was fun to shoot! The Hydrogen Alpha channel was really impressive, and we hope to soon re-shoot both SII and OIII on a very clear night and perhaps even from the desert, especially for OIII which lets some light pollution reach the sensor.
We are overall happy with this image, make sure to revisit this page when we upload our second version of it!
If you'd like a print, make sure to contact us! We also have prints for other images HERE!
Have you captured the Cave Nebula? Attach your image in the comments and let us know your acquisition details!
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