Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

CANON EF200L PRIME LENS


ollypenrice
 Share

Recommended Posts

  • Replies 84
  • Created
  • Last Reply

Top Posters In This Topic

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...

Edited by ollypenrice
Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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).

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

  • 2 weeks later...

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!

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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

Edited by ollypenrice
Link to comment
Share on other sites

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

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.