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badgers

Canon 200mm Lens Widefield - poor performance at f/2.8

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Hello Everyone,

I've finally got my widefield imaging setup working, many thanks to jnc71106 and his thread for help.

10178510936_59b4a4f511_z.jpg

I finally got some proper test images running with the lens wide-open at f/2.8 and aperture masked to about f/4.

I've been adopting the wisdom from psychobilly to focus on a star at a point near the 1/3 intersections on the sensor.

I did a series of tests last night on the pelican and north america nebula for 10 minute shots through a H-alpha filter.

click to enlarge to full size

10308978374_b7c926d035_b.jpg

I have seen some people getting fairly acceptable stars at f/2.8. 

However, I'm not sure the performance I'm seeing above on the left is really very good at f/2.8. The stars throughout the image all seem to be badly affected by coma.

At f/4 the results seem excellent, although you can see a lot more noise as expected there seems to be better detail and mostly absent coma.

For both images I focused using minute adjustments to a minimum FHWM on a moderately bright star roughly near the 1/3 intersections, I'm wondering how sensitive the results might be to this.

jnc's stars in his post looked pretty fantastic, although his was a smaller sensor (Atik 314L+). But I've seen some APS-C sensor DSLR images that seemed to be pretty crisp, although they may have done some processing to improve things.

I can't identify a point anywhere in the left image where star shapes aren't affected by coma.

Is it possible that I'm doing something wrong with the focusing or perhaps my imaging train isn't square ?

I guess it's possible that this individual lens isn't great at f/2.8 while other peoples might perform better ?

It's also possible that I'm being overly fussy and my f/2.8 images are ok.

Any suggestions appreciated, I'm thrilled at the f/4 performance, but It would be fantastic to be able to run this wide open on the Atik 460.

Thanks,

Anton

Edited by badgers
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I find it's pretty sensitive to focus... 

But seeing as I am using the lens on a DSLR I can use the focus motor in the lens to make minute adjustments and even let APT carry out FWHM based AF. I can even tape up the Focus ring on USM(Canon) or HSM (Sigma)  lenses to make sure  I don't tweak it by mistake... All I have to do is remember to switch the lens to MF or it will try and AF using the cameras focus sensor... Although some canon lenses will remote AF with the lens set to MF the Sigma need to be in AF for remote AF and then switched to MF...

Peter

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Yes, it's a shame there's no way to hack into the autofocus motors in this kind of CCD setup.

The metal focusing adapter though is good, I can make adjustments that change my FHWM by 0.2 or 0.3 so I think I'm doing a reasonable job nailing the focus.

I'm thinking its a combination of focus and that the imaging train might not be square. So the star I'm focusing on in the 1/3 intersection is blowing out the focus everywhere else as the optical centre isn't where I think it is. The gerd neumann adapter is nice, but it doesn't lock down the lens as well as I'd like, also the filter trays are very tight when they are inserted and might be pushing things off-square.

I think a night of experimentation with focusing in different parts of the image and rotating the CCD etc might be in order.

(and whoops, sorry I credited the wrong person for your 1/3 focusing strategy, fixed now.)

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it would be interesting to see how bad it would be using a central star....

I had a set of Stainless steel front aperture masks laser cut in IIRC 0.2 of a stop increments to fit into an old filter holder when I started playing about with the 200/2.8L ...

Peter...

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sorry i can't help regards the coma but i know there is a member on here that uses a atik 383 with this lens wide open and i believe the 383 has a larger chip maybe its worth sending him a PM to ask if he has any issues http://stargazerslounge.com/user/14823-rac/

my first night out with my setup i spent quite a bit of time practising with the focus rings trying to get the FWHM as low as i could but now i spend about 5 mins getting focus with my setup, i use a 10 sec loop and the best FWHM i've had in Artemis was 0.73 never been able to get it any lower. So now i know if i can get as close to that as possible i'm good to go

i have managed to find a way lock the filter drawer to the lens so my setup is rock solid now

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Interesting Anton I look forward to your results. Can I ask how you are connecting the lens to the imaging train.

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Thanks, 

Yes, I think I'll want to look at tilt using CCD inspector on a nice flat star-field and adjust the Gerd Neumann adapter to be more solid.

That and focusing on different reference points around the image.

The aperture mask I'm using is a piece of black PVC that's been perfectly cut to size by Monsieur Penrice the original owner of the lens. It may be that I should try and construct some incrementally smaller masks as you suggest Peter.

Anton

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Hi Sara,

The imaging train is:

Lens -> Eos side: Filter Drawer :T2 side -> T2 Spacer (5mm) -> Atik 460.

If I can get this running a bit faster, I can make a lot of use of the few nights that aren't cloudy here.

Thanks,

Anton

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Just a thought but was wondering if you just connected the lens to a canon camera to see if you can do an image test.  (should be orthogonal to the sensor)
That way you are only testing the lens and not the imaging train as well

When that is ok then you know that you have one of the variables out of the mix.

John
 

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