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Rho Ophiuchi - Canon 200mm USM L F/2.8

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#1 rob77

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Posted 12 June 2012 - 10:04 AM

Hi,
I would like to present you the first test I did with my new Canon 200mm F2.8 USM L lens.

  • 12 Lights @ F3.2, 400 ISO, 300s
  • 10 Darks
  • 20 Flats

Well, first of all, I have to say you that my location is really critical in terms of LP (I am in the centre of Belo Horizonte). I was barely able to see Al Niyat at naked eye.
Moreover, the weather this year is quite bad for astro imaging; there are lots of clouds every night.
This is the main reason why I just integrated one hour.

I used IRIS for processing, aligning and stacking.
Fits Liberator to stretch.
PS and NeatImage for refining.

Rho Ophiuchi

Regards

#2 jgibson1@emich

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Posted 12 June 2012 - 11:39 AM

Very nice image Rob77! LP really makes this target tough to image.

I'd just like to point out that the object is upside down as we perceive it in the US! ;)

Clear Skies,
Jason

#3 rob77

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Posted 12 June 2012 - 11:43 AM

Hi Jason,
you are right. I have noticed right now that I made the rotation in the wrong direction :)

The output RAW images were at an ADU equal to 900.
The dynamic of the Canon is 0-4096 if I am not wrong.

Should I increase the exposure time and/or go to F2.8?
I have been quite conservative since I was scared to got to much noise and burn the star colors.

Regards,

#4 rob77

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Posted 12 June 2012 - 11:46 AM

This is the JPG at the original size.

Rho Ophiuchi Full Size

Regards,

#5 jgibson1@emich

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Posted 14 June 2012 - 05:47 AM

Rob77,

I would not open the lens anymore than what you used to capture this image. This lens is great but at f/2.8 it does not produce tight nice stars over the entire frame. IMO I would go with longer exposures & possibly even stop the lens down a bit more. This assumes that LP is not overwhelming the image. For dark nebulae and reflection nebulae there is no substitute for dark skies.

When I imaged this target with the same lens while in Arizona this past April I took 4 min subs at f/5.6. The results are great. My imaging/observing location exhibited nearly ZERO LP!!! Based on your description of the LP in your location your image looks great.

Clear Skies,
Jason

#6 Nils_Lars

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Posted 14 June 2012 - 03:08 PM

You got a good result here with some good dusty details.

#7 Samir Kharusi

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Posted 15 June 2012 - 12:51 AM

Rob77,

I would not open the lens anymore than what you used to capture this image. This lens is great but at f/2.8 it does not produce tight nice stars over the entire frame.


I beg to differ. When I purchased this lens I did not expect satisfactory astro performance at f2.8 from its MTF graphs, but with a bit of sharpening it does deliver. The OP has done very well at tracking 5-minute subs, a challenge even at 200mm focal length. But his major challenge is the light pollution, meaning that he needs a huge lot of integration time. While an hour at a dark site with a modded camera is fairly reasonable on this object, I'd expect he needs several hours at his light-polluted site. Cutting down to f3.2 rather than shooting at f2.8 implies that he is sacrificing around 30% of his integration time; (3.2/2.8)^2; IMHO not worth the slightly better stars. Three hundred one-minute subs at f2.8 ought to deliver quite nice nebulosity, especially with a modded camera. Modding makes a huge difference because there is a lot of subtle Ha nebulosity around Antares. Here are my modded- and unmodded-camera versions at dark sites. Both at f2.8, but an hour or less integration time. I suspect that the OP used an unmodded camera and if that is the case then his result is very good for only an hour integration at f3.2 and light-polluted site. Lacking a dark site one has to be resigned to multi-hour integrations at the widest possible aperture. I use all my Canon primes of 100mm or longer focal length wide open. Below 100mm focal length one has to throttle that aperture. What we need is a simple-design, cheap and cheerful, 50mm f2.8 lens that can deliver astro wide open. All the manufacturers insist on giving us faster 50mm lenses that end up having to be closed down to f4 :bawling:

#8 Maverick199

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Posted 15 June 2012 - 10:01 AM

Wonderful image! My location is even more light polluted judging from the images I take and seeing images posted by others. Even a minute's worth seems washed out though I now get by with 2 minutes with a clip in filter.

#9 oldstargazer

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Posted 15 June 2012 - 07:32 PM

Hey Samir do you know what globular that is in the frame of that modded shot? That is a beautiful shot by the way!

Rob77 did you have to use 400iso due to the light pollution? It would be nice if you could use 800 or 1600 for that same length of exposure.

If I ever get a clear night I am going to try this with my 300mm canon lens and see what that looks. I have a dove tail that lets me mount just the camera and lens on my CG5-GTa mount. When I use that setup I have to use the bar and counter weight from my old super polaris mount to have the right balance. Luckily it screws right into the CG5 mount.

#10 Samir Kharusi

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Posted 16 June 2012 - 02:44 AM

The globular is M4, as pointed out by youngest kid, here.

Just as a general note that bears repeating: Under light-pollution, perhaps contrary to expectations, you can get away with quite short subs, shorter than at dark sites. Typically, one-minute subs at f2.8 will stack very nicely, dark site or light-polluted suburb, BUT(!) you need an enormous number of them to build up the integration time. You can never have "enough" integration time. Aim for at least 5 hours in exurbia to get a semi-decent image (300x1min subs) that might then be similar to a one-hour effort at a dark site. Often it's not the length of the subs where we fall short at a polluted site, it's simply that we hardly ever shoot enough of them. Inner suburbs may require 20 to 40 hours to match that one hour at a dark site. At that high level of light pollution one is consequently forced to go narrowband.

#11 rob77

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Posted 16 June 2012 - 05:13 AM

I will try tonight 18 lights, 200s, 800ISO, F2.8

#12 oldstargazer

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Posted 16 June 2012 - 07:24 AM

Wow Samir you have some really nice posters there! And thanks for the info on the globular.






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