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jmX
super member
Reged: 04/23/09
Posts: 164
Loc: Orange County, CA
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The Rosette Nebula (NGC2237)
Skywatcher Equinox 80 + Televue 0.8x flattener
Unmodified Canon 50D
Astronomik Clip-in CLS filter
26 x 2 minutes (52 minutes total exposure time)
CGEM mount, guided via C6N + SSAG
This is uncropped...nice and flat all the way out to the edge. I just love the Televue0.8x+Equinox80 combo.
Larger size: http://jmx.ls1howto.com/pics/200910_ngc2237_1280.jpg
Largest size: http://jmx.ls1howto.com/pics/200910_ngc2237_1920.jpg
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Jon
C6N + CG5
Skywatcher Equinox 80 + CGEM
http://jmx.ls1howto.com
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Tapio
super member
Reged: 09/24/06
Posts: 157
Loc: Tampere, Finland
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Really nice image.
What version of the flattener do you have ?
I might someday buy one (I have a similar scope).
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Tapio
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templec
member
Reged: 08/01/09
Posts: 51
Loc: Hendersonville, TN USA
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Nice image, Jon. Looks like you have a real winning combination there!
- Craig
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Orion 10" f/4.7 Newtonian
Astro Tech AT8IN 8" f/4 Newtonian
Orion 80ED
William Optics ZS66
Atlas EQ-G w/EQMOD
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kiarash
super member
Reged: 05/11/08
Posts: 129
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nice image
what set of ISO did you use ?
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location : isfahan , iran
equipments :
meade LXD75 sn-10"
orion dob 4"
400D canon
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xatamec
member
Reged: 09/12/09
Posts: 49
Loc: Catalonia, Spain
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Very good image being a non-modded cam!!!
I see you are guiding your mount. Why are you making so short exposures?
SNR is optimised if you make exposures long enough that produce a histogram well separated from the low lights.
You would have got a much better result with 5x10' exposures than with 26x2'.
Sergi
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My telescopes: WO Megrez 88FD APO and SW 200/1000
My mount: SW EQ6
My cameras: Cooled Canon 350D, QHY5
...and very little time to play with these toys!
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jmX
super member
Reged: 04/23/09
Posts: 164
Loc: Orange County, CA
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Tapio, I used the 0.8x flattener for 400mm-600mm scopes. TRF-2008
kiarash, ISO1600.
xatamec, it was only until recently that I started guiding, so I'm still learning the ropes. I'm still new to the hobby. However, it does seem like I need a minimum of 20-30 exposures to average out noise, as there is usually quite a decent amount of it. The temps are around 70 degrees F when I'm shooting, and it gets noisy. On the shots where I've gotten <20 subs, I can't fully stretch the image without it getting ugly real fast.
It also seems like my few test 5 minute subs I've taken have been blurrier due to non-perfect guiding, even at this mild focal length.
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Jon
C6N + CG5
Skywatcher Equinox 80 + CGEM
http://jmx.ls1howto.com
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Nils_Lars
Post Laureate
Reged: 01/04/08
Posts: 3422
Loc: Santa Cruz Mountains , CA
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Thats very impressive for a Ha rich target on that camera , well done Jon.
I found when I had my CG-5 that 4-5 min was possible with guiding but everything else had to be perfect , balance , polar align , and you just get used to tossing out 30-40% of the subs.
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Erik
Orion Atlas Self Hypertuned (EQMOD)
Orion ED 80
Williams Optics VII reducer
Celestron 8" SCT
Orion Starshoot Autoguider
PHD guide
Canon 400D Hap Griffin Mod w/Baader filter
Astronomik clip-in LP filter and 12nm Ha
Stilleto CVF and Bahtinov mask
Tamron 75-300mm&28-80mm lenses
NexImage webcam
http://www.flickr.com/photos/31986095@N05/
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xatamec
member
Reged: 09/12/09
Posts: 49
Loc: Catalonia, Spain
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jmX, the more cumulative exposure time the better SNR we have, all of us agree with that. Many exposures are very useful if you do dithering between them to get rid of the hot/cold pixels and defects of the sensor efficiently, combined with a sigma clipping or min/max excluded average stacking. However, if you do not do dithering many exposures are not a clear advantage. The hot/cold pixels and the sensor defects fall always on the same pixels.
Regarding thermal noise... once the sensor has reached its working temperature, it increases linearly during the exposure, so, if you consider only this source of noise, the SNR doesn't change along the exposure. If you consider the read noise as well, obviously the longer the exposure the better the SNR. And finally, if you consider the main source of noise, the photon noise, the longer the exposure the better the SNR. The maths about noise are a bit complex and I will not go into detail, there are many web pages where you can find them, but basically you need to now that the photon noise is equal to the square root of the number of photons that your pixel has received. So, it does not increase linearly with the exposure time, it increases much more slowly, so it's convenient to make long exposures.
Coming back to thermal noise, the main problem in long exposures is that it reduces the dynamic range, so the solution is to lower the ISO setting as much as possible, to preserve the maximum dynamic range.
How long should our exposures be?... the minimum time should be that that makes the sky background photon noise 3 times bigger than the read noise... again a bit complicated to explain here... in practice you should try to make exposures that produce images with histograms that lay between 1/3 and 2/3rds of the dynamic range... be careful, the histogram must be measured on a raw file, not on a jpg one (it has been white balanced by the cam), and you need to look at all three RGB channels.
This is the optimum way to get very good astrophotos, as many as possible exposures as long as possible, with an ISO setting as low as possible. Obviously, if you have guiding or drift problems you will need to make shorter exposures.
Cheers,
Sergi
Edited by xatamec (10/29/09 05:27 PM)
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Skyshooter
member
Reged: 04/07/08
Posts: 47
Loc: S. Utah, U.S.A.
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Absolutely gorgeous! I have wanted to image the Rosette for a long time. You give me an incentive to try it with my stock Canon XS. Thank you for sharing and keep 'em coming!
Ciao
Ed
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C8 SCT, f/6.3 reducer, WO ZS66SD
Losmandy GM8, SSAG, Canon XS (stock)
Toshiba NB205 (field computer)
DSS, PS Elements 7, Bahtinov Mask
Plethora of eyepieces and lots of patience !
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jmX
super member
Reged: 04/23/09
Posts: 164
Loc: Orange County, CA
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thanks guys.
xatamec, I agree and have heard much of what you're saying, but I'm still very hesitant. I do plan on slowly working my exposure times upwards and seeing how it goes. I'm more than happy with my 2 minute data for now, but of course I'm totally new to the hobby.
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Jon
C6N + CG5
Skywatcher Equinox 80 + CGEM
http://jmx.ls1howto.com
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