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Barnard's Loop 2-9-13

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#1 Erk1024

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Posted 13 February 2013 - 12:16 AM

I was having some technical difficulties, but I did manage to get some exposures of the constellation of Orion. I love how the entire, huge constellation is like one giant nebula. This photo shows the North Orion Bubble, Barnard's Loop, Running Man, Orion Nebula, Flame, Horsehead, and a teeny tiny M78.

Next time I'll go with 10 minute exposures. I didn't quite get all the signal I wanted.

25 x 5 min, unguided
Baader Modified Canon 450D + LPS filter
50mm f/1.8 Canon II lens @ f/4
Losmandy GM-8 mount, Backyard EOS

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#2 pfile

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Posted 13 February 2013 - 12:28 AM

nice one, i love that lens for orion. if you frame over just a tiny bit you'll pick up the witch head as well.

#3 Erk1024

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Posted 13 February 2013 - 01:22 AM

Hi pfile,

Good idea! Have you tried the lens with an Ha filter? I was thinking because the band-pass is so narrow, that I could try opening it up it up a bit. Maybe to f/2.8? Or do you think it will still lose the sharpness? I guess the question is: why does the lens have so much distortion at f/1.8? I've tried it and stars were streaks on the corners.

#4 pfile

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Posted 13 February 2013 - 02:09 AM

i have in fact tried with an Ha filter, but i think i stayed stopped to f/4. i'd have to go dig back thru my archives to find out.

based on what i've seen with the 200L and a 5nm Ha filter, it's possible that f/2.8 would work. the light is so narrow that chromatic aberrations just don't enter the picture.

one thing though is that with some narrowband filters the transmission goes way down at low f/ numbers, presumably due to the light reflecting off of the front of the filter instead of going thru it. you might be better off at the higher f/ ratio in that case. i think the astronomik filters have a transmission ratio table on the back of the package.

i'm not sure what causes the distortion but i assume it's something like field curvature.

#5 mmalik

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Posted 13 February 2013 - 03:33 AM

I didn't quite get all the signal I wanted.


True; good attempt after all.

#6 srosenfraz

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Posted 13 February 2013 - 04:37 PM

Great looking image, Eric - all the nebulosity shows extremely well.

FWIW, I imaged this with a different 50mm f/1.8 (an Olympus lens). I stopped down to f/4 for both the RGB and Ha (6nm Astronomik Clip filter), as the lens is extremely sharp at that point. I did 10 minute subs on the Ha and 3 minutes on the RGB. I put in more total time than you did on yours (about 7 hours of RGB plus 3 hours of Ha). My Ha data was much thinner than I would have preferred. So, if I had it to do over, I probably would try either f/2.8 or just a lot more time in Ha.

Anyway, great job on your image!

#7 Erk1024

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Posted 13 February 2013 - 06:10 PM

Thanks for the comments. ;)

srosenfranz: I do have the 6nm filter, but the I've been a little hesitant to use it because so little light gets through. I'd probably try the 12nm one first. Thanks for the exposure times. ;)

pfile: I know somebody who opens up the Tair-3c lens when they are doing Ha. Maybe the situation is not analogous though. The Tair is a weird beast. I don't know enough about lenses to know what the problem is with the 50mm wide open. I'd think field curvature would be the same regardless of f-stop? Basically the elongation of stars in the corners suggests that the extra aperture isn't getting the light in the right spot off-axis. But the elongated stars weren't rainbows, so not chromatic aberration. So if not chromatic aberration, then the narrow bandpass won't help. (If only I knew more about optics.)






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