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PostPosted: Mon Jan 05, 2009 12:45 pm 
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Hi CL astro-photographers! :D

My question about mirror
how mirror lenses focus infinity and how it applies to astro photography?

I'm thinking on a budget mirror lens between 600 and 1000mm fixed f/6.3 or f/8.
The application will be when make the new DMC-G1 acquisition, so I guess x factor will be 2 = 1200 or 2000mm

As I know the mirror lenses have a so small d.o.f.
Also, they manage well focus on infinity?
May or need also to think on a 2xteleconverter? this will bring focal distance of 2400mm or 4000mm
And about donuts? I'm thinking right it will not occur on this type of photography?

thank you all in advance,

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PostPosted: Mon Jan 05, 2009 6:50 pm 
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A practical distinction here, but are we talking about telescopes or actual mirror lenses?

On the manual focus mirror lenses I've seen, you rotate the part of the body of the lens. On a telescope, there will generally be a knob moving something depending on the type/design.

For astrophotography, DoF doesn't really apply since everything is close enough to infinity from our perspective. The focus adjustment can be very fine though.

You can use teleconverters in theory. I couldn't use mine so far as the camera gives an error if the TC is connected without lens. I haven't tried insulating or otherwise disconnecting it which should work. If using an telescope through the eyepiece port, a barlow lens added will give the magnification. Any magnification will reduce the brightness overall. So you may need to experiment to find how much you can usefully use.

At very long focal lengths, even the smallest wobble will cause the image to move a LOT. You will need a very stable mount and cable release or timer on camera to get shake free shots.

The donut bokeh doesn't apply to astrophotography since everything is effectively at infinity. It might only be an issue if you try to put something in the foreground.

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PostPosted: Mon Jan 05, 2009 8:17 pm 
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Thanks for your reply Popo!

In fact I'm talking about actual mirror lenses to be coupled on G1 mfthirds.
I don't have the camera yet, but when I was looking for some primes (and I got some good names between 50 and 170mm) then I realize the 2x crop factor on a sigma 600 (as an example) gives me 1200mm. And is quite cheap.

As many of these come on T mount, and some of them with a 2x T converter, thus gives 2400mm.

I saw many brands between 400 and 600... do you know if are out there some one plus longer, about 1000mm?

In terms of reputation, I think all of sigma, tokina, tamrom, nikon are side by side in performance.

Could you recommend such solution or there are others best by the same price range?

(it's only for beginner purposes and a bit of fun)

During this writing also find this

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PostPosted: Mon Jan 05, 2009 8:57 pm 
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I don't know about the various brands but I can only suggest you don't go cheap and get a Centon like I did in the past.

One thing I forgot to mention. For focus, live view with zoom is very handy and if I remember correctly the G1 can do this.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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PostPosted: Mon Jan 05, 2009 9:28 pm 
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popo wrote:
One thing I forgot to mention. For focus, live view with zoom is very handy and if I remember correctly the G1 can do this.


Thank you Popo!
In fact G1 only can do this :)

As I say you, I'm only able to buy brands well known. Here I think the 2x mfthirds factor is a plus. Although there's much to lead with noise.

What make me think on this sulotion was the lightness of the setup. Maybe will be good on a 055 or a 190 tripod.

The link I posted before clarified me about the possibilities.

Anyway, and since one adapter will be needed, what kind of mount do you recommend?
T mount preferable, with an adapter for mfthirds? (maybe on the market some lens to Oly OM or M42 mounts that I plan to buy on other lenses)

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