LDN 1251 - Humpback Anglerfish Nebula

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oopfan
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LDN 1251 - Humpback Anglerfish Nebula

Post by oopfan » Mon Sep 21, 2020 6:15 pm

LDN-1251_Siril_StarTools_NEAT.jpg
LDN-1251_Siril_StarTools_NEAT.jpg (50.33 KiB) Viewed 331 times
William Optics ZenithStar 71
Atik 314E Cooled CCD
Luminance: 237x90s bin3
Total Integration Time: 5.9 hours
Bortle 5 (on my good days)
Processing: Siril, StarTools, NEAT

Each morning I make the acquaintance with the latest NASA APOD seen here:
https://apod.nasa.gov/apod/astropix.html

A couple months ago the following image was selected as Astronomy Picture of the Day:
https://www.astrobin.com/lg85ah/?nc=user

I've always loved Dark Nebulae and especially their "smokey" appearance. When I drilled down into the image details I was shocked to see that he rated his skies at Bortle 5. I'm Bortle 5 but I had no luck with the one dark nebula I tried last year. Now I understand why. The key to capturing these elusive nebulae are dark skies. That's pretty obvious given their low surface brightness. What I learned these past couple nights answered how it is possible:

#1. Allow the object to reach its highest altitude above your horizon. The sky is more transparent there. At my latitude LDN 1251 reaches a maximum altitude of 55 degrees above the horizon. On the east side of the meridian I have to wait for the object to clear trees but even then it only gives me 2 hours of imaging time. On the west side of the meridian I can follow it down quite low but I had to quit after 3 hours due to the decreased transparency when the altitude fell below 40 degrees.

#2. Your Bortle class is measured at or near the zenith (i.e. directly overhead). You can see this difference when you look overhead and see the Milky Way but as you scan down towards your horizon your sky turns gray where you can clearly see the outline of trees. So I repeat, wait for the object to climb in altitude. You can begin imaging sooner but just know that you will need to significantly increase your total integration time.

#3. Match your latitude to the declination of the object. Like I said LDN 1251 reached a maximum altitude of 55 degrees as it crossed the meridian. The gentleman who captured the APOD of LDN 1251 lives in the State of Maine which is 3 degrees farther north than me. Last year I tried imaging a dark nebula just north of Aldebaran in Taurus. In my backyard Taurus is blocked by trees as it nears the meridian so I was only able to image it when it was about 40 degrees above the horizon. That's not high enough.

Now for some comments about my image. First off, it is monochrome, not color like the APOD. Secondly, the image size is small. That is entirely due to my choice of using bin3 together with some image rotation after the meridian flip. I chose bin3 in order to turn my little refractor into a light bucket. I wasn't so much interested in creating a beautiful image as much as I wanted to prove that I could capture it. Tonight, I'll get another clear night so I might choose to switch to bin2. That will yield a better quality image but at the expense of longer integration time. Just off the top of my head it will double the time from 6 hours (at bin3) to approximately 12 hours (at bin2).

When you compare my image to his, please know that my field of view is narrower. That big bright red star in his image is just above the top of my frame. I wanted to keep it out because it is a real blow torch at 6th magnitude. In my image the bright star just below the top of the frame and left of center is the "eye of the fish" in his image.

My image is just starting to capture some of the filament structure in the smokey parts of the nebula but I would need more integration time to truly pull it out. Finally, I was hoping to pick up the "bow wave" as seen in his image but I just didn't have enough integration time.

I want to compare his scope and camera to mine. My guess is his refractor runs circles around mine!

Brian
Last edited by oopfan on Mon Sep 21, 2020 7:22 pm, edited 2 times in total.

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by oopfan » Mon Sep 21, 2020 7:20 pm

Here is another NASA APOD of the same object but at a different angle. Actually I like this better. I've rotated my image so as facilitate a comparison. At this point I think that adding color would be a terrific improvement. Everything in luminance is kind of "blah".

https://apod.nasa.gov/apod/ap190531.html

Brian

PS: I forgot to mention in my original post that you can see the difference between a single sub taken at 40 degrees altitude and another at 55 degrees. The 55 degree image shows a subtle difference between the luminance of the smokey nebula and the darkness of space. At 40 degrees there is hardly any difference at all. I've always believed that if you can't see what you're aiming for in a single sub then you must prepare yourself for very long integration times.
LDN-1251_Siril_StarTools_NEAT_vertical.jpg
LDN-1251_Siril_StarTools_NEAT_vertical.jpg (59.12 KiB) Viewed 329 times

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by Menno555 » Mon Sep 21, 2020 9:51 pm

Nice shots and great info!
This without any filter?

Menno

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by oopfan » Mon Sep 21, 2020 10:12 pm

Menno,

No filter except for your basic clear luminance. (It does cut UV and IR but that's it.)

So I've decided to give this object a try in LRGB. I'm going to switch to bin2 for all four filters. It'll increase my total integration time but I'll gain a larger image size, plus my CCD resolution will go from 7 arc-seconds per pixel to 4.5 arc-seconds.

In retrospect I realize now that I ran my bin3 subs a little "hot" at 90 seconds. I measured the ADU level of dark space and best case it was 6,000 ADU; worst case 10,000 ADU. It caused a lot of stars to saturate.

For the new LRGB at bin2 I figure I'll use these exposures:
L: 30s
R: 90s
G: 90s
B: 90s

Furthermore I am going to center the nebula so now I'll have to deal with that bright 6th magnitude star. It will saturate but it won't be as bad as if I used a 90-second exposure.

Tonight will be clear as will tomorrow and the day after. I'm going to take a break tonight. I've done three nights in a row already, and last night I didn't wrap up until 4am.

Thanks for your interest, Menno.

Brian

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by turfpit » Mon Sep 21, 2020 10:24 pm

Great work Brian - Beverly Lynds would enjoy this.

Dave

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by Menno555 » Mon Sep 21, 2020 10:27 pm

These kind of structures are just awesome, so I get all the info I can get :)
This particular one is almost in zenith for me, so that's good. The bad news is Bortle 7/8 but by now I have learned that that will not stop me from trying it ;)

Menno

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by umasscrew39 » Mon Sep 21, 2020 10:35 pm

Beautiful image, Brian- Congratulations

Bruce

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by oopfan » Tue Sep 22, 2020 2:56 am

Thank you, gentlemen.

Brian

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by oopfan » Tue Sep 22, 2020 2:59 am

Menno,

I say, definitely give it a try. This is what my stack looked like after the first night with only 2 hours of data:
LDN-1251_80x90s_bin3.png
LDN-1251_80x90s_bin3.png (339.52 KiB) Viewed 303 times
Brian

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Re: LDN 1251 - Humpback Anglerfish Nebula

Post by oopfan » Tue Sep 22, 2020 11:49 am

Well guys, I hate to disappoint you but I just ran the numbers and it doesn't look good for bin2. I would need to increase total integration time from 6 hours to 9 hours but settle for half the signal-to-noise ratio that I currently enjoy with my monochrome image at bin3. It doesn't sound like a good trade-off.

The only remaining option is to stay at bin3. These are the numbers: 11.3 hours total integration time in LRGB with the resulting signal-to-ratio ratio equal to what I enjoy with my monochrome image. I'd still have to re-capture luminance at a lower exposure in order to tame the saturated stars. So, an 11 hour project to capture a small sized image with poor CCD resolution of 7 arc-seconds per pixel but it's in color!

Is it really worth it? It sounds like it's better to rent some time at a remote site. What is interesting is that my equipment running at bin3 is equivalent to their fastest equipment but the difference is that they are running at bin1. Again, it comes down to the difference between my 7 arc-seconds per pixel and their 1.7 arc-seconds per pixel. So the total time at the remote host is still about 11.3 hours but I gain higher resolution, a larger field of view, and a camera with a much deeper full well. (My camera is 13,000 electrons and theirs is 100,000 electrons.)

I think I'll go with Option B :D

Brian

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