NGC 7023 - the Iris Nebula

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NGC 7023 - the Iris Nebula

Unread post by Pete »

220625-26 log NGC7023
Saturday, 25 June 2022
Tonight’s forecast is for perfect conditions and the forecast proves accurate. It’s a clear still warm night. I’d attempted the Iris Nebula on the 24th and got clouded out. So I’m setup for it already. It’s 76°F at 20:45 so cooling can’t reach last night’s -10C so I’ll be running at -5C.

I’ve done NGC 7023 in Nov 2020 and in June 2021. Here's the June '21 image:
Previous NGC 7023 .jpg
Previous NGC 7023 .jpg (1.02 MiB) Viewed 80 times
36 X 5 min, unbinned, 200 gain at prime focus.

This image was far better than the Nov 2020 image but in viewing images on the web I can see that there's huge room for improvement. And that’s what tonight’s about.

A stop of light is not a fundamental unit, but a method of measuring the increase or decrease of light in a photographic exposure. 1 light stop is either double (+ 1-stop) or half (- 1-stop). So, irrespective of your initial exposure, increasing it by 1-stop means twice as much light, and decreasing it by 1-stop means half the amount of light. By my dropping gain from 200 to 120 I lose the equivalent of 1.2 stops according to the ASI manual. However the focal reducer has increased the f/ ratio from 10 to 6.3, which is approximately 1 stop. The Iris has some subtle detail and to optimally stretch the image in post processing I need the improved dynamic range associated with 120 gain. So as far as sensitivity the focal reducer pretty much compensates for dropping the gain.
NGC 7023 s.jpg
NGC 7023 s.jpg (497.15 KiB) Viewed 80 times
70 X 3 min, 2X bin, 120 gain, -5C, 1 sec guiding
14” Meade LX200GPS @ f/6.3 with ASI2600MC-P camera and Optolong L-Pro filter

Comet C/2017 K2 subsequently imaged 12 X 1 min, 2X binning, 120 Gain, -5C, 1 sec guiding.

It’s getting late and this is just a quick capture to check in on comet C/2017 K2 PANSTARRS AND see if the comet’s grown a tail.
C2017 K2.jpg
C2017 K2.jpg (372.83 KiB) Viewed 80 times
Yup, it's got a tail. But I'm not up to using fancy software to keep the comet still. And even with only a 12 minute total exposure there’s a small linear track at the center of the nucleus. This one would have been better at 200 gain or higher but I didn’t have an appropriate dark.

In at 02:04. It’s still 67°F but pretty wet out here. Fortunately, dew's not an issue in a dome.

Conclusions & lessons learned:

Between the increased dynamic range and my increased post processing proficiency (since last year) the Iris image is now a keeper.

Before closing up the camera was set to capture 1 min -5C 2X 200 gain dark frames so that next time I go for the comet….. In fact, perhaps I should experiment with a gain of 300! Or even 400 as my lazy capture approach could stand a significant increase in sensitivity and the comet won't suffer for lack of dynamic range.

Left the camera running to capture the desired 1 min -5C 200 Gain 2X dark frame. Next time I'll be better prepared.
Pete P.
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Re: NGC 7023 - the Iris Nebula

Unread post by menardre »


I would say it is much better than just a 'keeper'.

Great color, lots of details, and nice stars. I would be thrilled with this image. It actually has a 3 dimensional quality to it.

Roger M.
Celestron CPC1100 EDGE, Stellarvue 130T refractor dual mounted on iOptron CEM120 on permanent pier mounted in Observatory. Imaging camera ZWO ASI2600 OSC, guide camera Lodestar or ZWO ASI290MM.
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