Paul Shings wrote:
The telescope adapters I've seen are made up of three pieces. One of
these is a T-adapter. There is a large extension tube with T-threads on
each end which allows you to insert an eyepiece in the light path for
increased magnification. Then there is the third piece which has a
T-thread at one end and the other end inserts into the eyepiece tube on
the telescope. The second piece is only used for eyepiece projection
photography.
Most amateur telescopes today accept 1-1/4" diamater eyepieces. Other
popular sizes are 0.965" (typically found on cheaper, low end models)
and 2". You need to get an adapter which matches the telescope's
eyepiece size. A good source for these is Orion. I think they are at
ww.telescope.com
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Paul is correct and I will amplify a little. I own a Celestron 8 and
have built a couple of smaller telescopes over the last 30 years. The
larger catadioptric scopes such as those from Celestron and Meade are
built with photography in mind. The way you attach a camera depends on
what you want to do. For "prime focus" photography the telescope is
basically just a telephoto lens. To maximize the width of the light
path to the camera you first remove the visual back. As Paul notes,
these are typically for 1-1/4" OD eyepieces and the size of the tube is
restrictive. In its place you attach a larger diameter extension tube
which is threaded for a T-mount. To that you attach the camera.
To increase the magnification you can use an ordinary teleconverter. To
decrease the magnfication you can attach a "telecompressor". A
telecompressor on the Celestron 8 turns it from a 2000mm f/10 to a
1260mm f/6.3.
If you want extremely high magnification you use "eyepiece projection"
with the multi-part adapters that Paul mentioned. A normal visual
eyepiece is held inside the camera/telescope adapter and magnifies the
image and projects it onto the film. Shorter focal length eyepieces
will give higher magnification. There are lots of variations on this
theme.
Now some caveats. Even with a scope built with photography in mind it
is still very difficult to do. The magnification is very high.
Vibration, air currents and the rotation of the earth are your constant
enemies. If you're going to use it on the sky at night you MUST have a
clock drive which drives the scope and counteracts the earth's rotation
during long exposures. The only exception to that is pictures of the
moon using fast film. The moon is so bright you can do short exposures.
Now even more caveats. Most telescopes aren't really built with
photography in mind and can be difficult to adapt. The easiest
photography will be at prime focus since this is the minimum
magnfication. However, you may well find that the focusing mechanism
(which is built for visual observation) doesn't allow the camera to move
in close enough to allow the image to reach the film. Even if it does
reach you may find that the visual focusing mechanism is of limited
diameter and is causing serious vignetting. If it's a Newtonian
reflector you may also find that the diagonal mirror is a cause of
serious vignetting. Fixing these problems means physical and/or optical
modification of the scope. One fix might be to move to a 2" focusing
mechanism from the typical 1-1/4"... spell that... expensive. To avoid
these problems you can use some sort of projection but that gets you
back to higher magnification and even more trouble with vibration, air
currents, etc., etc.
I would recommend ordering Orion's very nice color catalog. You can
learn a lot from looking through what's available. They sell good stuff
but that nice color catalog has to be paid for somehow.
I would be happy to give some more specific advice off-list if you want
to tell me more about the telescope. What size, type, brand, etc.
ps: Sorry for the late post but the original got lost in the bit bucket
somewhere along the way---
Chuck Norcutt
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