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CANON EF200L PRIME LENS

ollypenrice

By ollypenrice
in Member Equipment Reviews

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ollypenrice

ollypenrice

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I've been considering a small electric motor mounted on the dovetail using a pulley with an elastic band round the focuser to make fine adjustments. It is very hard otherwise to make adjustments to the focus finely enough.

Telescope Service do a bolt on gadget that looks like a medeival dentist's brace and allows you to use a screw thread to adjust the lens. Trouble is, too slow to change lenses and I can't face more complications around here!!

Olly

Sorry Peter, we doubled as well there. See if your local NHS orthodontist wants to buy a few...

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Photosbykev

Photosbykev

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Very interesting. I'm seriously thinking about the F2 135 L and getting a canon spacer to give the option of 200mm.

Canon spacer? do you mean the Canon x1.4TC? I rate the x1.4 highly but the Canon x2TC is awful lol, Low contrast and a very expensive paper weight

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MG1

MG1

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Very interesting. I'm seriously thinking about the F2 135 L and getting a canon spacer to give the option of 200mm.

How does the spacer work to change the focal length...or am I being a little thick here...(answer:probably!)

I've used extension tubes for Macro before, but they prevent focus at infinity I seem to recall.

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JCJC

JCJC

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What is a Bahtinov Grabber??

Hi Olly,

Bahtinov Grabber is programme which can look at an image taken with a bahtinov and adjust focus by controlling a motorized focuser or you can manually adjust until the error is lowest possible.

I'm not sure if this is more accurate than the FWHM method you use or not.

Bit of a sidetrack, I've started using FWHM for DSLR now as APT has it integrated, but find a bahtinov mask much easier with Ha? Do you do the same?

Cheers,

Jordan

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ollypenrice

ollypenrice

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I thought FWHM was more to do with exposure quality rather than focus?

I think I need to do some reading again....anyone got links lol.

Thanks Jordan, I rely on someone young to keep me up to speed - or as near to it as I can get!! I do use Bahtinovs now but also use FWHM still. I find they don't always agree exactly, though they are very close. Then I dither for a bit as to which to believe. For Ha I try to remember not to spend half an hour composing a picture before doing critical focus because sometimes you can't get a result on any star in the field. Grrr...

Full Width Half Max is a focus aid. The software measures the full width of a stellar image half way up its brightness range. The faintest part is round the edge, the brightest more or less the middle, so half way between is what is measured. You try to get it a small and/or as bright as you can. Since the width is a small, intellectually digestible number like 1.4 that's the one I look at myself.

Olly

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robertm

robertm

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Thanks for explaining Bhatinov grabber Jordan. As a slight aside I always focus on a very bright star for NB with the 200L. Expect some corner softness when working with the tele extenders and also if you don't get precise focus.

The new MkIII tele extenders are probably better than their MkII predecessors but I'm not sure how well they would work for astro imaging. If they work well then they would add a lot of flexibility. Another thing to consider is the weight of the lens and the fractional displacement due to play in the bayonet. Both lens and extender would need to be a tight fit.

Robert

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brianb

brianb

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It is very hard otherwise to make adjustments to the focus finely enough.

Hmmm. There are mechanical devices which can help: see for instance here (other sizes available; don't know of a UK distributor but importing from Germany is not an issue).

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JCJC

JCJC

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I rely on someone young to keep me up to speed - or as near to it as I can get!!

I try Olly... I try :)

It was a shock first time I used my Ha filter. I thought I'd left the end cap on or something.

Thanks for clearing that up, it's a good point about focussing on a bright star then moving to the target, especially with narrowband.

Jordan

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Starblazer

Starblazer

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Full Width Half Max is a focus aid. The software measures the full width of a stellar image half way up its brightness range. The faintest part is round the edge, the brightest more or less the middle, so half way between is what is measured. You try to get it a small and/or as bright as you can. Since the width is a small, intellectually digestible number like 1.4 that's the one I look at myself.

Olly

Cheers for clearing that up for me Olly :)

Chris.

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  • 2 weeks later...
AndyUK

AndyUK

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I've only just stumbled across this thread (whilst google searching Olly's EF200L)... Over the last few days I've been racking my brain on how best to focus a DSLR/Lens combination... and hey presto - Here it is! Thanks all!

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chiltonstar

chiltonstar

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iinteresting thread. I have a 300mm Nikon prime lens (f4 AIS) which I use mainly for wildlife - works well on my D300 despite being older generation. Is it likely to give reasonable astro images (I've been attempting to try for some time, but the only clear nights since God knows when have been full Moon!)

Chris

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opticalpath

opticalpath

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Excellent review, Olly. The results speak for themselves.

<<Perhaps the 85mm 1.8 but then that has no weather sealing - always something to consider in a damp climate.>>

Am I right in thinking that the 200mm is not weather-sealed either, and if so would that be a consideration for use in the damper UK climate?

Adrian.

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ollypenrice

ollypenrice

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I don't know anything about weather sealing so I had better not leave it out in what passes for damp in my neck of the woods!

I bet the Nikon would be great, too, but you have to consider these lenses as having telescopic guiding requirements, albeit tolerant ones.

What strikes me is that using this kind of FL and scorching f ratio is the incredible productivity. Fast with easy guiding. The lens quality strikes me as being in the Tak FSQ league but at a fraction of the cost. Naturally the lens is smaller but what it does it does to top standards. What I had to learn was brutal star reduction, but there seems to be no aesthetic penalty for this. This is a crop from my Ha layer on the Rosette shown at full size. Just two hours in an aggressive 7 nanometer Ha filter in imperfect but still good transparency. Remember the full field goes almost to the Cone and would get there alltogether on a DSLR chip.

Olly

1178028992_Dzdtx-X3.jpg

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melsky

melsky

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Nice review Olly

I use the 200L @ F3.2 practically all the time on the Astrotrac and it's superb, I even managed to get a shot of Supernova 2011b with it this week (look in the deep sky imaging section). For you non DSLR users I do wonder what difference not having the lens set on the infinity mark makes because as Steve pointed out it has internal focusing, so the relative spacing of the elements is changing with focus setting. Having looked at Olly's images though I doubt there's any improvement to be had.

I have used the TS rings with this lens and they're almost to fine/slow to use even with x10 enlargment on the camera screen.

The 85mm F1.8 is also a great lens stopped down to F2.8 and I've had no problems using it with our British weather (apart from the clouds that is).

Mel

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