NGC 6791

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oopfan
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NGC 6791

Post by oopfan » Mon Jun 18, 2018 2:53 pm

NGC 6791 - Open Star Cluster in Lyra

NGC 6791 is an enigma. The stars are twice as old as our Sun but have an Iron-to-Hydrogen abundance ratio (metallicity) that is more than twice that of the Sun. This flies in the face of the rule of thumb that "older means metal-poor". NGC 6791 is one of the most studied star clusters.

(see following post for technical details and comments)

Brian
Attachments
NGC_6791_LRGB_258m_G100_BL20_Bin75.jpg
NGC_6791_LRGB_258m_G100_BL20_Bin75.jpg (167.75 KiB) Viewed 883 times

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oopfan
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Re: NGC 6791

Post by oopfan » Mon Jun 18, 2018 2:56 pm

Technical Details:

William Optics 71mm f/5.9
Altair 290M camera (uncooled)
Optolong LRGB filters
Unitron Model 142 GEM
Passive tracking with PEC
No active guiding

Gain 100 (FWD: 15ke-, 3.66 e-/ADU)
Offset: 20 ADU

Captured over 3 nights:
L: 45 x 60s
R: 80 x 50s
G: 80 x 50s
B: 80 x 60s

Total integration time: 4.3 hours

PIPP 2.5.9
Deep Sky Stacker 3.3.2
StarTools 1.3.5.289

Comments:

Low gain was required to capture wide dynamic range from brightest stars (8th magnitude) to faintest stars (16th magnitude) without saturation.

Luminance stack was acquired two weeks earlier with a gibbous Moon. Unfortunately I did not take Flats. No dust motes but noticeable vignetting.

First version had total integration time of 3.2 hours. I was unhappy with noise level so I captured more RGB in this second version. Only a very slight improvement in SNR, less than I expected. Since then I've come to learn that 50 darks, instead of 30, yields significantly lower noise.

I think it is time for a cooled camera.

Brian

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turfpit
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Re: NGC 6791

Post by turfpit » Mon Jun 18, 2018 8:54 pm

Brian

A good image backed up with technical details - should help others when trying to imaging this type of object.

Dave

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oopfan
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Re: NGC 6791

Post by oopfan » Sat Jun 23, 2018 2:10 am

I dug down into the StarTools documentation to discover how to create Synthetic Luminance from the RGB data. Using this technique cuts down my at-the-telescope time by 25% since I no longer need to capture with my Luminance filter. I've read that variations of this technique can be used to add Ha to the image of an emission nebula, and for processing SHO data. Much more to learn.

Brian

Here is my NGC 6791 using Synthetic Luminance:
Attachments
NGC_6791_SynLRGB_138m_G100_BL20_Bin75_20180618_BMorgan.jpg
NGC_6791_SynLRGB_138m_G100_BL20_Bin75_20180618_BMorgan.jpg (190.16 KiB) Viewed 811 times

mAnKiNd
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Re: NGC 6791

Post by mAnKiNd » Sat Jun 23, 2018 10:55 pm

Brian, nice image! Since you have captured a Luminance channel, would love to see your comparison of Luminance vs synthetic L on the RGB data.

Cheers
Minos

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oopfan
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Re: NGC 6791

Post by oopfan » Sun Jun 24, 2018 1:05 am

Minos,

Yeah, that's the tricky part. If you scroll all the way to the top of this topic you will see an image that used actual Luminance, not Synthetic. The problem with it is that it was taken a few weeks earlier with my Orion luminance filter, not my Optolong luminance filter. Processing in StarTools was quite challenging with a heavy blue cast on the initial stretch -- that is a clear indicator to me that something ain't right. (Ah, now I remember, the Gibbous Moon was bright and I did not take Flats.)

What I'd like to do is this: on the next moonless night I'd like to capture the target with my Optolong luminance filter. That way I can be assured that LRGB is balanced from the same manufacturer.

I've attached my latest Synthetic LRGB of NGC 6791. I learned from the StarTools docs that I was denoising the final image too soon.The colors are a little less dramatic but I think it agrees nicely with the image of NGC 6791 taken by Dark Horse Observatory with his 10-inch Takahashi Baker-Ritchey-Chretien !

http://darkhorseobservatory.org/images/ ... 1_full.jpg
http://www.takahashi-europe.com/en/BRC-250.php

Brian
Attachments
NGC_6791_SynLRGB_138m_G100_BL20_Bin75_20180618_BMorgan.jpg
NGC_6791_SynLRGB_138m_G100_BL20_Bin75_20180618_BMorgan.jpg (161.94 KiB) Viewed 781 times

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