Having fun with NGC 188

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oopfan
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Having fun with NGC 188

Post by oopfan » Wed Oct 21, 2020 3:00 am

Normally I look down upon those who take artistic license with their images but I couldn't help myself. Here is a work-in-progress of NGC 188, an open cluster in extreme northern Cepheus. It is a particularly difficult target with my camera having a full well depth of only 13,000 electrons. The bright 8th magnitude stars saturate easily at only 20 seconds exposure in luminance. As a result I've had to capture a literal ton of frames in order to push back the noise. At present I am 300-some frames totaling 3.7 hours but still the noise persists. I am willing to go a full 10 hours but the Moon has put a halt to progress now.

For fun I input the image into StarTools and used its "Synth" tool to simulate diffraction spikes from a reflecting telescope's secondary mirror. The tool does some other interesting things like performing an extreme stretch to bring out the brilliance of the 15th magnitude stars in the center of the cluster. Unfortunately as a result the diameter of the bright stars is exaggerated. The original image is much more normal looking but not as interesting as this one...
NGC-188_Bin1-LRGB-HDR-L125x7s-L30x20s-R60x80s-G60x45s-B60x72s-Balanced-SNR-3.7h-APP-NEAT-newtonian.jpg
NGC-188_Bin1-LRGB-HDR-L125x7s-L30x20s-R60x80s-G60x45s-B60x72s-Balanced-SNR-3.7h-APP-NEAT-newtonian.jpg (263.34 KiB) Viewed 269 times
Brian

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Menno555
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Re: Having fun with NGC 188

Post by Menno555 » Wed Oct 21, 2020 8:39 am

Look cool though.
It has a kind of depth effect now :)

Menno

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turfpit
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Re: Having fun with NGC 188

Post by turfpit » Wed Oct 21, 2020 8:59 am

I like the diffraction spikes Brian. Good job handling the wide range of star brightness and colours in the image.

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Re: Having fun with NGC 188

Post by umasscrew39 » Wed Oct 21, 2020 10:16 am

Hey Brian - I did not realize StarTools can even do that...that is incredible what all of these software programs can do. Pretty neat with the different colors of the diffraction spikes. Hopefully, your computer won't melt processing 300+ frames!

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oopfan
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Re: Having fun with NGC 188

Post by oopfan » Wed Oct 21, 2020 10:34 am

Hi Bruce, yeah I've toyed around with the idea of taking two lengths of rigid steel wire and hot gluing them to the objective lens cell, a few millimeters above the lens surface. I've known people who have taped dental floss over the end of the dew shield although I am not keen on that trick since I have a DIY flat field device that would interfere. If I go with the hot glue solution I'd have to test how difficult it would be to remove.

The advantage of this solution is that I'd be able to reduce my overall frame count by taking longer exposures. I find that saturated stars are not as unsightly in a reflector. I think that the diffraction spikes is nice eye candy.

Brian

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oopfan
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Re: Having fun with NGC 188

Post by oopfan » Wed Oct 21, 2020 11:18 am

Bruce, another possible solution is to 3D print a profile of the secondary mirror and spider vanes together with "clips" and "screws" that create an interesting diffraction pattern. StarTools allows you to modify those parameters so I figure that I can 3D print that too.

Something like the following. Make it about 5mm thick to give it some rigidity and then hold it down with Blu Tack although I don't know how it holds up in frigid temperatures. I guess I can test it in my freezer.
3D-print-secondary-mirror.jpg
3D-print-secondary-mirror.jpg (17.25 KiB) Viewed 247 times
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Re: Having fun with NGC 188

Post by oopfan » Wed Oct 21, 2020 11:57 am

Bruce, I reprocessed in StarTools using the following:

Aperture: 71mm
Focal length: 418mm
Central obstruction: 14mm (20% of aperture)
12 screws, no clips
Rotate OTA 45 degrees

It's pretty decent. I might give it a try.
NGC-188_Bin1-LRGB-HDR-L125x7s-L30x20s-R60x80s-G60x45s-B60x72s-Balanced-SNR-3.7h-APP-NEAT-newtonian-ver2.jpg
NGC-188_Bin1-LRGB-HDR-L125x7s-L30x20s-R60x80s-G60x45s-B60x72s-Balanced-SNR-3.7h-APP-NEAT-newtonian-ver2.jpg (261.52 KiB) Viewed 246 times
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