| Entire books have been dedicated to astrophotography
and the subject is quiet large. Almost any telescope can be configured
to accept a camera.
There are several aspects of astrophotography that should be addressed:
Some scope mounts are better suited to astrophotography than others.
For long exposure photography you will want an equatorial mount.
Vibration must be controlled. All scopes will have a small amount
of vibration to deal with, but with astrophotography vibration control
is critical. Thus, you should have a solid mount for your scope.
You should use a shutter cable as well as flip mirror systems and/or
"the hat trick" to reduce vibration. Anti-vibration pads for your
tripod legs are also useful.
Fast films will require short exposure times, but will have the
lowest resolution. Additionally, an accessory called a "Focal Reducer"
can change an f10 telescope into an f6.3 telescope creating smaller,
but brighter, images that also reduce exposure time by one third.
It is generally a good idea to keep a log of all your exposures.
So when you review your images after you develop your film, you
can see what worked well and what did not. It is also a good idea
for the first exposure on a roll to be some sort of standard photo
like a picture of a white board sign to serve as identifier for
the roll. This will allow the film processor to calibrate the frames
for printing the roll since the negatives of astrophotos tend to
look like unexposed film.
Many times the technician processing the film will not print pictures
that appear unexposed or may cut through the center of a frame.
I always put a label on the envelope for the film processor which
reads:
"Astronomical photographs. DO NOT cut the negatives.
Print all frames."
Wide Field Photography
The simplest form of astrophotography is wide field photography.
Simply place your camera on a steady tripod and take 30-second to
3-minute exposures. The closer your exposure is taken to Polaris
the slower the stars will drift and the longer you can expose you
film without significant star trails.
If you use this technique with long exposures you can create star
trail photographs. The sky moves at 15 degrees an hour so that a
3 hour exposure will produce 45 degree arcs of star lights.
Piggyback Mounts
Piggyback mounting your camera to an equatorial aligned telescope
will also allow wide field photography while allowing for much longer
exposures.
Prime Focus 
Prime focus photography uses your telescope is used as the lens
of the camera. Assuming you have an SLR camera, you would remove
the camera lens assembly and replace it with a T-Ring adapter, and
depending on the kind of telescope you have, you would need to put
an appropriate T-Ring where the eyepiece normally goes. These two
pieces, the T-Ring and the Camera's T-Ring adapter, thread together
using the industry-standard T-Thread.
This kind of photography requires a steady equatorial mount that
is precisely polar aligned plus precision guiding for the duration
of the exposure.
Eyepiece Projection
Eyepiece Projection is like Prime Focus except an eyepiece is place
into a tele-extender type T-Ring to increase the magnification.
This makes polar alignment of the mount, guiding accuracy and vibration
suppression become even more critical.
Afocal Photography
A rarely used technique in photography is a kind of eyepiece projection
called "Afocal Photography". Some cameras, like digital cameras,
cannot have their lens assemblies removed and must shoot through
the camera lens and through an eyepiece to get a satisfactory image.
The camera lens and the eyepiece need to be as close as possible.
The T-ring system will not work in these cases and vignetting is
usually a problem.
To solve the problem of vignetting, we recommend that the eyepiece
have a wide field of view, that the camera should have a physically
small size lens and that the camera lens and the eyepiece be connected
to be close as possible. Also if the camera has macro focus mode
and or zoom focus you may want to experiment to see what might give
an optimal image. |
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