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NGC 2392: an experiment with short exposure imaging


iansmith

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Also known as the Eskimo Nebula, Clown Nebula, PNG197.8+17.3, and a great many other names, I used this nebula as an experiment in short exposure imaging. Instead of exposing for 5 or 10 minutes I thought I would try with 10s exposures in each of the narrow band channels to see if I could get any higher resolution. After calibration and stacking the nebula and stars have been processed separately.

For the nebula exposures were: Ha: 720 x 10s; O3: 720 x 10s; N2: 720 x 10s

The N2, Ha and O3 images for the nebular were assigned to the RGB channels as: red: 50% Ha and 50% N2; green: 50% Ha and 50% O3; blue: 50% O3 and 50% Ha which is kind of weird, but it produced a pleasing (to my eyes) colour combination.

For the stars they were: red: 15 x 120s; green: 15 x 120s; blue: 15 x 120s

Did the experiment work? In a word: no.

The 10s exposures were too long to freeze the seeing, but I had wondered if they would reduce the blurring caused by other errors, such as tracking. However, I don’t think this image is any better than I could have produced with 5- or 10-minute subs. The Mesu is a good mount and can be easily guided for 20 or 30 minutes if required. I suspect it could go for an hour, but I’ve never had the patience for that test. In fact, I suspect that the longer subs would have produced a better image as I would have gone for a much longer integration time: 8 to 10 hours per filter rather than 2. That would have increased the SNR and given a much nicer image. I didn’t go for so long with the 10s exposures as that would have left me with a huge number of subs to process and it was tedious enough as it was with 720 per narrow band channel.

As it was, processing the short exposure subs proved to be a lot of effort. PI complained that there weren't enough stars for it to register the subs. In the end I found and used SIRIL to stack them into a single sub which I then processed in PI. But even SIRIL could only register one on star.

In short, I think short exposure imaging would be of benefit for people with mounts that cannot handle 5 minute plus exposures (and have cameras with better gain control), but it offers little benefit to me and is definitely not worth the extra hassle involved. But at least now I know that!

Anyway, here is the image, I hope you like it:

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And do please do let me know if you have had any better success with this style of imaging.

Cheers, Ian

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I think if you don’t go short enough on the exposures to beat the seeing, then there would be little benefit on capturing lots of short (by CMOS standards) exposures, especially if your imaging rig is riding on a Mesu.

A nice result nonetheless, I would tag it “the Pool Ball Nebula”.

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6 hours ago, tomato said:

I think if you don’t go short enough on the exposures to beat the seeing, then there would be little benefit on capturing lots of short (by CMOS standards) exposures, especially if your imaging rig is riding on a Mesu.

A nice result nonetheless, I would tag it “the Pool Ball Nebula”.

Thanks. I would agree that the 10s exposures aren’t enough to affect the blurring due to seeing and on a mount like the Mesu they don’t really help with tracking errors either. But it’s nice to know that, rather than suspect it, which was the point of trying the experiment 😀
 

Cheers, Ian

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