From: Pertti (pertti_at_antispamdomain.not)
Date: Fri Nov 28 2003 - 01:58:09 EST
--- In digital_astro_.at._yahoogroups.com, "Pepe Cuellar" <kueyar_.at._e...> wrote:
> Hi all,
> if I want to take some afocal pics with my DSLR camera, wich lens is the
best to do this?, do I need a macro lens or a zoom lens or a normal fixed
lens?, wich focal distance is the best? Thanks in advance!
> Pepe Cuéllar.
I think the important question here would be: "Which eyepiece to use?"
because the DSLR lenses are very large. If your eyepiece is smaller
than your lens - or to be more precise, if the exit pupil of
your eyepiece is smaller than the entry pupil of your lens - you
will get vignetting. Very severe, most probably, making a large
part of the image plain black ("looking in a tunnel" -effect).
The other important question is: "Why would you even want to?"
An afocal setup always seems to have a lower image quality
than a prime focus setup.
If you want more magnification, you can get that with:
1) Eyepiece projection - however, there are significant image quality
problems and the setup always seems to become very long,
which increases the shaking of the camera (I have tested this
personally, with 10D and several different types of eyepieces),
2) Barlow lenses - these seem to work quite OK, even with
inexpensive barlows (mine cost about 60 Euros, if I remember
correctly),
3) Teleconverters - I don't have any experience on this, but
others have tried it. If you already have one, or need one
for other purposes, this is definitely worth trying,
4) Televue PowerMates - the most expensive but the most
recommended setup. I haven't yet tested this, but most
probably some day will. They can also easily be stacked
and image quality is reputed to be very high.
At the moment I cannot much use magnification because I would
need guiding, as my mount tracks only for 60 seconds
with my scope's 990 mm focal length. So the only case is the Moon,
where I use my existing 2x barlow with rather good results
(see my Moon terminator image at this month's challenge).
Pertti
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