IC 4603 - A reflection Nebula in Ophiuchus
We finally had some clear skies on 5/11/21 and 5/12/21, and they happened to coincide with the new moon. Near ideal conditions for astrophotography. I did some searching for suitable targets to image, and found that the constellation Ophiuchus would be well positioned around 10:30pm when I expected to be able to begin imaging. Several years ago one of my last targets was IC 4603 / IC 4604, in this region. I’d gotten some decent data, but not enough for a final image as many of my shots were ruined by a reflection.
My previous shot of the region, Lum only.
Happy to finally have another chance at the region, I decided to focus on the reflection nebula IC 4603. It’s a fascinating bit of space, dark clouds of dust obscure the region. Several bright stars light up the dust blue in some areas, while in others the dust is so thick no light makes it through, and in others it’s a dusky brown. Reds from pockets of glowing hot gas giving off HA light add to the mix of color in the region.
Initially I was going to image from a new location, Rock Run Youth Park, which is East of town and looks to have very dark skies. Unforunately it also has very very tall floodlights for the softball diamonds that would be in any view of the sky I wanted - even once the lights were off for the night, they made the area a poor choice, so I headed back to the farm southwest of town where I typically image. The light pollution at the farm has gotten noticeably worse since I last imaged there, but it’s still far darker than in town. I found a new spot to set up which gave me far better views of the entire eastern sky, from North to South.
I completed setting up in the dark, and got to figuring out my workflow. Polar alignment with the CEM70 went well, but then I hit my first snag - I’d forgotten that my last time using this camera was on my SCT, not the short refractor. I was unable to get into focus, so plate solving wasn’t working. Once I realized that, I swapped out spacers so I could get into focus, and reset. Having solved that and getting things into focus, plate solving was then working. However, the next challenge was that my guiding camera, the iGuider built into the CEM70 mount, would not stay connected. It took some trial and error before I found it worked reliably on my laptop’s USB2 port. Once that was resolved getting it guiding was pretty straightforward.
Finally, around 11pm, I was able to slew to my target and work on framing. This is when the final surprise of my evening happened. The framing wizard view and my telescope’s view were off. My actual FOV was 1.51 degrees, where the framing wizard showed 1.85 degrees. I quickly ran the numbers through the framing wizard, correcting for the error, and chose a slightly different framing, centering entirely on IC 4603.
I took a sample 90 second sub, and examined the histogram. This target is very low on the southern horizon, meaning I was imaging through a lot of light pollution and atmosphere, but the histogram looked reasonable and the stars looked sharp, so I went for it. In total I took 83 subs that first night, wrapping up around 2am before taking flats and heading home around 3am.
I went back the next night, tired, but determined to get more data. I set up in the same spot, and things went much more smoothly. I always keep a checklist of my steps, even once it’s routine, and so following the previous night’s list setup and imaging went well. I was set up before my target was above the horizon, so I took the chance to gather 20 x 90s dark frames then. I was imaging a bit earlier, by 11:10pm. However I noticed this meant my subs had quite a bit more light pollution. I was pretty sure my early shots wouldn’t be usable, so adjusted my auto-focus settings to run a bit more often. I ended up imaging until 2:30am, gathering a total of 120 subs, and getting some of the best data right at the end of the night. I was home by 3:30 am.
Processing the data was a challenge - it was very easy to stack, and low in sensor noise. My guiding was good, but not perfect (I think I was chasing seeing from shooting through so much atmosphere). The color wasn’t as bright as I’d like, I think due to the same light pollution and atmosphere issues. Even with all of that, the image itself was great - well framed, and capturing the uniqueness of the region.
In my processing, I try to bring out what my camera caught, and not add what I wish it had. I did have to reject a handful of the shots from early on the second night, dropping from 203 total subs to 186. I tried for a subtle touch in processing in Pixinsight on this image. The synthetic Lum cleaned up very nicely with a denoise process, most of my challenge was in getting good color out of the RGB image - the region has so many unique colors, it’s a challenge to balance. Once combined it resulted in a very ghostly reflection nebula, which I like very much. This is one I may revisit later as my post-processing skills improve.
My only real disappointment with this image is that it didn’t have the original framing. I’m actually looking at selling my ASI533MC camera to move to a bigger sensor that pairs better with my scope - the FOV is more narrow that I’d anticipated.
You can see the full technical details for the image at my astrobin page.
