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Old 07-09-2007, 03:59 PM
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[1ponders] (Paul)
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500mm Mirror lens - Any comments on it's usefulness?

I know these have been mentioned on the site before but I think it was a while ago. So just to refresh my aging memory would someone like to comment on the suitability/usefulness of the 500mm Mirror lens like this one on ebay.

Not that I'm really looking to buy one, really, but if the price is right
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Old 07-09-2007, 04:11 PM
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Starkler (Geoff)
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Looks like a miniature sct
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Old 07-09-2007, 04:28 PM
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ballaratdragons (Ken)
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Paul,

it worries me when no brand name is mentioned.

But this does sound good: 'ALL items will carry a manufacturers warranty which can be serviced in Australia.'

I doubt it would have much light gathering capability though (for astro work). The central obstruction is huge!
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Old 07-09-2007, 04:32 PM
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it's similar to a prime focus telescope, apparently they give really weird looking bokeh, and in daylight, weird donut shaped light rings and whatnot...

i would do some research on teh wide world of the internets, then make a judgement....

if you do end up getting it, i owuld be most interested to see your results!!!!

btw, it was me who asked a while back
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Old 07-09-2007, 04:47 PM
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[1ponders] (Paul)
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Oh I'm not really serious about it, I'd rather use the ED80, though I'm sure someone sometime will be, which is why I was fairly specific in the thread title.

At f8 (500mm/100mm = f5 so that obstruction is pretty significant) though it might be ok for luna imaging without having to carry a telescope around. Or maybe you could adapt a visual back for it and create a cheap 500mm mak? Just ideas.
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Old 07-09-2007, 04:50 PM
Dennis
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Hi Paul

I have a Tokina 500mm F8 and it produces photos that are a little soft. It really requires the use of a tripod to minimise camera shake. These lenses are quite compact and lightweight; often fooling the unsuspecting photographer into thinking they can hand hold it!

I’ve had mine for over 15 years and only used it on a handful of occasions with average results.

Cheers

Dennis
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Old 07-09-2007, 04:54 PM
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Do you think the softness is from poor focusing mechanisms, poor mirror or because of the large CO? What's the contrast like with it Dennis? Normally that would be one thing I thought the CO would have caused problems with.
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Old 07-09-2007, 04:56 PM
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Actually I wonder if a visual back could be fitted to it to take a webcam. Wonder how it would go as a cheap guidescope?
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Old 07-09-2007, 05:18 PM
Dennis
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Quote:
Originally Posted by [1ponders] View Post
Do you think the softness is from poor focusing mechanisms, poor mirror or because of the large CO? What's the contrast like with it Dennis? Normally that would be one thing I thought the CO would have caused problems with.
Hi Paul

I have used the lens on a substantial tripod, using a cable release, at shutter speeds of 1/1000 sec in an attempt to constrain those factors, but the overall image still looks a little soft compared to my normal lenses, or the WO 80mm ED II Apo that I have.

Cheers

Dennis
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Old 07-09-2007, 05:31 PM
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Paul, I've been playing with an MTO 10.5/1100 mirror lens, and probably agree with Dennis that images end up a little soft. I used the lens for all my eclipse shots - lovely scale! My tripod is way too light & ricketty, and anything involving any longer exposure ends up fuzzy (eg totality!). Suspect this is not only movement through the field, but vibration from shutter opening as well. It is the same as shooting through a small scope, but control of focus is much more tricky. In fact you can buy (or make) visual backs to convert the lenses into scopes! Adds a fair bit of weight if you want to piggyback too.

That having been said, it's a lot of fun, with good terrestrial applications too. Anyone got a good heavy tripod?

Cheers -
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Old 07-09-2007, 05:33 PM
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Hi Paul

Here are a couple of 800 x 600 crops from the 3008 x 2008 jpg original. The 1st image is as is; the 2nd one has been smart sharpened in CS3.

The 3rd file is the original full frame 3008x2008 resampled down to 1024x681. No processing applied.

Subject to lens distance around 10 metres.

Cheers

Dennis
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Old 07-09-2007, 06:42 PM
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you shouldnt show me things like this
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  #13  
Old 07-09-2007, 06:50 PM
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Got Him!!!!!


I knew you'd stick your head in here
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Old 07-09-2007, 06:51 PM
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Looks like it could be a fun lens though.

Price is about right, 'eh h0ughy

What are your thoughts as a guidescope Dennis?
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Old 07-09-2007, 06:56 PM
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If it wasn't optically that good, I suspect it would still make a good guidescope with the addition of a mogg adapter.
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Old 07-09-2007, 06:56 PM
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BTW Stephan has a 4 inch mak in the icetrade section.
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Old 07-09-2007, 06:59 PM
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I wonder how these things go for mirror flop? Also notice the rear thread is only 30.5mm, less than 1.25". Wonder that the thread is and how much vignetting would occur?


Dennis? Rob?

I'm really liking the idea of a guidescope. If it worked then $200 for a guidescope wouldn't be too bad, really.
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Old 07-09-2007, 07:05 PM
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I'd imagine the focus would be some sort of helical system, go on give it a try, it's only money.....
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Old 07-09-2007, 07:06 PM
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24.6x18.5 arcmin FOV using a ToUcam/DMK. Not bad, almost full moon width.
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  #20  
Old 07-09-2007, 07:06 PM
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Yeah but it's money that could go towards a Tak


Besides I'd need some clear skies to give it a go.
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