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While nearly all telescopes have them, few observers ever consider using finders
for anything other than aiming their telescopes. Yet finders can be very
effective tools for both visual and photographic astronomy. In his Deep-Sky
Wonders column, Walter Scott Houston has mentioned that finders are excellent
for viewing objects like the Rosette nebula in Monoceros. Indeed, some finders
can provide better wide-field views of certain objects than the main telescopes
themselves.
This is by no means a modern discovery. During the late 19th century
the skilled observer Thomas W. Webb in England and Lewis Swift in the United
States could see nebulosity around the Pleiades in 2-inch telescopes but not in
larger instruments. And the German astronomer Heinrich L. D'Arrest commented
that some nebulae were invisible or barely seen in great telescopes but were
easily perceived in their finders.
Today, many finders with apertures of 50-mm and larger have star
diagonals that allow comfortable right-angle viewing. Some even have
interchangeable eyepieces for greater flexibility. With a little modification a
finder becomes a highly portable, tripod-mountable, traveling telescope.
Given its advantages for visual use, we decided to investigate the
finder's potential for astrophotography. Its large relative aperture, moderate
focal length, and firm attachment to the main telescope tube make it seem well
suited for piggy-back photography, especially since the main instrument can be
conveniently used for guiding.
We experimented with an 8 x 50 finder that Celestron International
marketed a few years ago. Its 50-mm f/5.2 objective had a 260-mm focal length.
The finder had a helical focuser and adapters for either straight-through or
right-angle viewing. The image plane fell far enough behind the focuser to
reach the film plane or a camera without having to shorten the finder tube. The
finder's only drawback was that it used 0.965-inch rather than 1 1/4-inch
eyepieces, which limited our options for attaching a camera.
We found two solutions to this problem. The first was a standard
adapter that fit the 0.965-inch threads on the focuser but accepted 1 1/4-inch
accessories. Our camera was then fitted with a T-ring and an adapter that fit 1
1/4-inch draw tubes. We had to shorten the adapter on the focuser by about 3/4
inch so the camera would reach focus. While this was done on a lathe, careful
use of a hacksaw would also have done the job. We had to drill and tap a small
hole for a new locking screw. This modification also allowed us to use 1
1/4-inch eyepieces for visual work.
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For our second approach, we made a special adapter that coupled the
camera's T-ring directly to the threads of the focuser. While this required
more sophisticated machining, it provided a firm, stable attachment that could
not come loose and shift the camera's focus, or, worse yet, let the camera fall
off.
To make the adapter the right length, we aimed the finder at a distant
object (one that was at least a half mile away) and slid the camera, with a
T-ring in place, back and forth until the image was in sharp focus. We measured
the distance between the back of the focuser and the front of the T-ring and
then subtracted about 1/4 inch to allow enough clearance for the camera to move
through infinity focus.
As the pictures here show, our results were quite satisfactory. We
could record fainter stars with the 260-mm focal length f/5.2 finder objective
than we could with a 200-mm f/4 telephoto lens. The finder, of course, was not
as well color corrected as the lens. If the blue halo around bright objects
proves bothersome, it can be reduced by placing a Wratten 2a (weak-yellow or
so-called minus-violet) filter in front of the objective.
Many mounting brackets use rubber O-rings to support one end of the
finder. While this is fine for the lightweight finder alone, it can cause
problems with the added weight of a camera body. The rubber might compress,
causing flexure and trailed images. It's best to have a rigid connection
between the finder and main telescope.
Finder photography is a logical step up for amateurs who have mastered
piggy-back astrophotography with normal and wide-angle lenses. It's also an
inexpensive alternative to buying a telephoto lens.
END
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