Re: [APML] A beginners question

[Philip Perkins <pgp@astrocruise.com>]

Hi Ian,

By all means give it a try.  As you say one of the disadvantages of this 
type of lens is the speed, so you would be exposing four times longer than 
with a conventional telephoto (often used at f/4 even though some are 
faster).  There are some other things to consider with this type of lens:

- the lens itself will need firmly mounting (in addition to the camera) and 
I'm not sure how easy this is with mirror lenses. The usual method with 
telephotos is a front ring - presumably this is also possible with a mirror 
lens, provided that a large enough ring can be found.  You can see how a 
conventional telephoto is mounted in the following pictures:
http://www.astrocruise.com/for_sale/Nikon_400mm/Nikon_400mm.htm

- I have seen reports that the optical quality of these lenses is often 
somewhat below that of the best conventional telephotos

- there could be some shift of the optical components during the exposure 
(same problem as SCT mirror flop on a reduced scale) - bearing in mind that 
the lens is moving the whole time during the exposure.  This problem is not 
evident in conventional photos because of the shortness of the exposure, 
and the lens is not moving.

The first and second problems would be evident as trailed stars in the 
image.  Unfortunately these problems are more likely to appear because of 
the need to expose four times longer (on average). But I would certainly 
give it a try and (hopefully) you will be pleasantly surprised by the result.

--Philip
-------

At 21:02 17/07/2004 +0100, you wrote:
>Hi All
>
>Being a total newbie to astrophotography,and still in the process of
>getting the bits and pieces together I would appreciate some thoughts
>on something.
>
>Apart from their being on perhaps the "slow" side:on average f8,are
>there any other inherent problems involved in using a mirror telephoto
>lens for astrophotography when piggy backed on a telescope.I feel that
>there maybe because after a look through the archives it would seem
>that although a number of people achieve some stunning results by piggy
>backing a camera and telephoto lens,I see no mention (unless I am
>looking in the wrong place) of the use pf "mirror" lenses.
>
>My reason for asking is that my wife uses a 500mm f8 mirror lense for
>her wildlife photography,and I wonder if it would be practical for me
>to use it during the winter for astro-photography.
>
>Looking forward to any thoughts or suggestions
>
>Ian & Linda Jordan
>_______________________________________________
>Astro-Photo mailing list
>Astro-Photo@seds.org
>http://seds.org/mailman/listinfo/astro-photo

Philip Perkins
<pgp@astrocruise.com>
Wiltshire UK & Luberon France
www.astrocruise.com


_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo