This was one of the most challenging images to process I’ve encountered yet. I’m not 100% sure of what the reason is but I have a few possible theories. Actually, all of them could be true, but I have a plan to determine if what is happening. If none is true then it appears that I may have a hardware issue in the camera or perhaps an incomplete understanding of the problem.
And, speaking of problems, there are plenty of them to go around. My guiding woes continue so I had to image unguided. I ended up doing one minute exposures since it seemed that was about as long as I could go without the stars elongating too much. I also shot at gain 175 on the ASI294MC Pro. My hope was this combination would be enough to swamp the read noise on the sensor with sky glow. As it turns out, it may still have not been enough. I may need either more exposure time or more gain.
On the good news front, I managed about 7.5 hours worth of usable data, one minute at a time. Most of the frames were usable. However, Sequence Generator Pro’s direct mount guider seems to keep the mount moving through entire exposures every once in a while. It ends up skewing the intended framing of the object and makes for some really noisy transitions in the area where as much data wasn’t captured. It’s not clear whether it’s an SGP problem or a mount firmware problem but if I had to put money on it, I’d say it’s an SGP problem.
Ultimately I ended up with a lot of frames to integrate but they didn’t really line up all that well. However, if I wanted to use only the properly framed subexposures I’d have had to throw out around 75% of the subs.
The “magic” of averting lots of frames continues to amaze me. In the individual subs I can’t even see the nebula. I had to take it on faith that it was there, but once 450+ frames were averaged together it popped out and was clear, though still fairly faint.
I was fortunate to be able to do this without a moon in the sky. I can’t recall the last. time I had a clear night on a new moon. I had hoped it would make getting the gradients out a bit easier but that proved not to be the case. Since the integration time was spread over several hours, the motion of the object across the sky caused it to move through different light pollution gradients. The Dynamic Background Extraction tool relies on having clear background to reference and clear background was in short supply in this image. I managed to do a decent job clearing up the gradients but the image still looked muddy. However, I couldn’t seem to do anything about it so I did color calibration and a bit of modest noise reduction and hoped for the best
When I tried to stretch the image the background was just horrible. I couldn’t find a way to stretch the image without the background turning into a complete mess. After several tries of doing different things I was about to give up. But, an idea occurred to me that ultimately saved the day. At some point in one-shot color images it’s common to run the SCNR tool in PixInsight to remove any green cast in the image. I’ve always done it toward the end of processing but I tried it early this time while the image was still linear. To my amazement, a lot of the “mud” went away. There was still some blue in the background so I tried something unconventional and removed about 50% of the blue with SCNR and suddenly the image cleared up! I was then able to proceed and stretch the image without any issues.
This was not without consequences. The star colors pretty much washed out, but since the target was an emission nebula and basically red, the subject was not significantly damaged.
I think there are three possibilities for what was going on and hopefully my next time out I’ll get to confirm my ideas or eliminate some. One possibility is that the gradients were just so complex that DBE (or at least my limited skill with the tool) couldn’t handle the gradient. In favor of this was that removing green and blue basically removed yellow from the image and most light pollution is yellowish.
The second possibility is that the read noise was just really high and when averaged together it makes this mottling in the image that makes it hard to deal with. Some people think that the thermoelectric cooler on the sensor doesn’t do an even job in the ASI294MC Pro and this could be causing the mottling. Perhaps that interacts with the read noise in some strange way so as to cause the mottling. The night was fairly cold at around 3-4C and I was cooling to -10C so I wasn’t pushing the cooler hard.
The third possibility is that I have all the devices riding on the telescope (pc, camera and dew heaters) being powered via a device called a pocket powerbox. It’s a really convenient way to handle things but perhaps the dew heaters or PC is sending noise into the camera and that is showing up as mottled background.
I don’t see any mottling in the individual subs but definitely do in the final integration. Next time out I’m going to really work on guiding so I can reliably get longer exposures. Hopefully that clears everything up. If it doesn’t then the next time after that I’ll look at disabling the dew heaters. If that doesn’t help then I’ll need to find a separate batter or power supply to run the camera. I’ll also try targets that afford easier sampling places for DBE so I can ensure incompletely removed gradients aren’t an issue. If problems remain after that then it’s probably the camera cooler and I can try uncooled images to see if the noise remains. As cold as it’s been the noise shouldn’t be too bad. That should definitively answer the question.
Even with all the “learning experiences” I’m having, I found this image very satisfying! It’s a nebula I’ve never seen visually. It’s too faint to see in our light polluted skies. Yet, it’s quite pretty and it’s one of the few objects where you don’t need much imagination to see how it got its name.
I’ve worked on cable management and really have that cleaned up. Only one cable comes off the mount now and it’s fairly thin. I’ve locked down my guidescope in a way it should never be possible for it to move and I’ve found and fixed a problem in my guider settings. I’m hopeful that next time out I’ll be able to get good guiding. Now all I need is some clear sky to give this a try. And perhaps some weather that is above freezing. The next forecast clear night is expected to go down to 17F. That might be pushing how extreme an environment I want to put the equipment in, though I’ve read Losmandy says the mount should be fine.
Here is the image on astrobin.