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How to achieve perfect focus with the canon 200mm F2:8 lens


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Dear all,I recently purchased a canon 200mm F2.8L lens. I am used to focusing a Bhatinov mask with the telescope. I am very interested in finding the away to nail focusing with the 200mm lens. How do you achieve this? Focusing with a star?  using some landmark or landscape feature far away? Using APT or similar software package? Are there any bhatinov masks for these lenses? I would appreciate it that you could help me with this.

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Personally I use a home made Y mask and focus on a bright star. Live view can help immensely.

You could make a B mask but you may find it fiddly to construct with the correct spacing etc. you could combine a mask with software to absolutely nail your focus.

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Although i havent used mine since i bought my scope a few months back, i did use APTs FWHM function combined with a TS Microfocuser. I heavily recommend the microfocuser as it also locks the focus, great bit of kit. I also have some front aperture rings for my lens to stop they dreaded star spikes!

I love that lens, talking about it makes me want to start putting it to use again!

Callum

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Telescope Service;

IMG_1193-M.jpg

I used FWHM and followed Psychobilly's excellent advice to focus on a star placed at the one of the four intersections of the 1/3 lines.

Regular rechecking is essential. A front aperture mask also wrks well.

APERTURE%20MASK-M.jpg

Olly

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I made a Bahtinov mask for my Canon 200l and it works a treat but there are a couple of other 'secrets' too:-

1. I bought a 72mm to 52mm filter reducer which stops the lens down to F4 and has a 'knife edge' profile to ensure zero diffraction. This has a dramatically positive effect on star shapes.

2. Choose your focus position carefully, focus on a star at a 1/3rds position to help obviate field curvature (see below)

post-1029-0-27732900-1423339579.png

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Excellent bit of graphics from Steve, above, and point taken about the knife edge. I no longer have this lens since I don't have a suitable camera for it, but it really is worth setting up carefully.

Olly

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I'm sorry to hear that you no longer have the lens, Olly. Did you find the TS focus adjustment rings to be any good? The set shown on your lens look perhaps a little on the large size, did you choose that set for a particular reason?

Edited to say that I see Callum obviously likes them!

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I also use a stepping ring on my 200L but a 62mm which still gives you F3.2 and good star shapes. This lens really needs to be at ambient temperature when you focus it otherwise it will drift markedly as it cools.

Mel

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I'm sorry to hear that you no longer have the lens, Olly. Did you find the TS focus adjustment rings to be any good? The set shown on your lens look perhaps a little on the large size, did you choose that set for a particular reason?

Edited to say that I see Callum obviously likes them!

I'm sure Olly will agree that the TS microfocuser is an excellent pairing with the EF200L, allowing minuscule adjustments that frozen fingers are not capable of! My focuser is also a bit on the large size, i chose it because the next size down sounded like it was cutting it a bit close, and i didnt want to risk ordering it if it wasnt going to fit. But i have since seen someone using the smaller version which looks to be more of a perfect fit and works fine, probably a bit better.

Callum

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I'm sure Olly will agree that the TS microfocuser is an excellent pairing with the EF200L, allowing minuscule adjustments that frozen fingers are not capable of! My focuser is also a bit on the large size, i chose it because the next size down sounded like it was cutting it a bit close, and i didnt want to risk ordering it if it wasnt going to fit. But i have since seen someone using the smaller version which looks to be more of a perfect fit and works fine, probably a bit better.

Callum

My fear was exactly Callum's and informed my 'safe side' choice of bigger rings. I'm sure you could ask TS if the smaller ones will fir the 200L, though. They are likely to know. From memory I thought it might be a bit iffy. Smaller, if they fitted, would be better because the big ones are a bit spindley, as you suspect. Workable though.

Olly

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Well heres a link if you fancy letting somebody treat you Steve ;)

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p3286_Microfocuser-and-focus-lock-fuer-camera-lenses-up-to-D-135mm.html

Like i said above the smallest rings (75mm) fit the EF200L, just found this image confirming this:

FM7.jpg

Like Olly said the next size up is a bit spindly and it looks like the smaller size would fit and preform better. I may end up buying the 75mm version if i start using my 200mm again. 

My only complaint is the black screws are plastic and bend quite easily, one was even snapped when i received mine. But TS were good and sent me a shiny metal one which is much better quality than the default screws. The plastic screws bending is probably due to me being heavy handed though, plus the fact that the rings are too big. You can see here on my lens here how the screws are almost fully inserted allowing room to bend:

15176289579_08d29d7ee2_c.jpg

This of course would not be a problem with the 75mm version. Plus the rings wouldnt protrude as much and i wouldnt of had to make my own dovetail just to accommodate them!

Callum

Edited by iksose7
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Thanks for your comments, Callum and Olly - the 75mm version would suit me better because of the closer fit and the photograph shows that this would work. Apologies to Pixueto for the minor divergence from the original thrust of the question.

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Thanks for your comments, Callum and Olly - the 75mm version would suit me better because of the closer fit and the photograph shows that this would work. Apologies to Pixueto for the minor divergence from the original thrust of the question.

Not at all. I am considering buying the microfocuser too but I wasn't sure as to which size I would need. Thank you guys for all your comments.

One last thing: I've been using the 200mm F2:8L for a couple of nights now but I'm not having great results. The vignetting I am getting is horrendous (despite the fact that I use an Aurora flat panel (I also use a step down ring to avoid diffraction spikes). Also, even at ISO 400, I can't take any subs longer than 3 minutes without flooding the sensor with light. I am based in a light polluted area but did not experience these problems with the 200P or the ED80. I am also using a light pollution CLS CCD clip in  filter. I am a bit baffled, to be honest as I was expecting great things from this lens.

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I've been using the 200mm F2:8L for a couple of nights now but I'm not having great results. The vignetting I am getting is horrendous (despite the fact that I use an Aurora flat panel (I also use a step down ring to avoid diffraction spikes). Also, even at ISO 400, I can't take any subs longer than 3 minutes without flooding the sensor with light. I am based in a light polluted area but did not experience these problems with the 200P or the ED80. 

To be fair, the ED80 is F7.5 (F6.3 if you use the FF/FR). So your F2.8 lens is 3 full Stops faster than the ED80, so you should only be able to expose for 1/8th of the time before reaching the same level of saturation.

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