INSTRUMENTS
ASTRAL TELESCOPE
BASIC NOTES
page 17
BASIC METHODS OF
ASTROPHOTOGRAPHY
There are three basic ways of taking astrophotographs. The method used to photograph a
particular subject depends mainly of the nature of the subject; whether widespread, as in
the case of nebulas or surfaces such as those of the Moon or the planets, or spread out
over a wide area as in the case of star fields.
PARALLEL PHOTOGRAPHY
In this, the easiest type of astrophotography, the camera and its objective are mounted o
the telescope by means of a parallel support. The camera is pointed at the desired object
and the exposure is made. The telescope discharges many functions:
1) It gives the camera and its objective a stable support
2) It follows the object being photographed throughout the exposure (this may be
controlled manually of, if the telescope is appropriate equipped, it can be done by means
of an automatic timing device).
3) It operates as a guide telescope for the camera, with the observer following the object
keeping it at the centre of the ocular with the control grid lit, correcting any
imprecisions in the following process by means of small electronic or manual movements of
the telescope.
Almost all photography objectives can be used for parallel photography, but focal
objectives of between 50 mm and 300 mm are probably the most commonly used. The focusing
mechanism for the photography objective has to be positioned for infinity. Parallel
photography can be a source of great satisfaction in wide field photography of the Milky
Way, of comets and of constellations. Due to the small degree of error in following, it
is, without a doubt the best technique available to the astrophile. However, because of
the small scale of the image, parallel astrophotography is not suitable for photography of
the Moon, planets or of objects deep in the sky. The exposure time can vary from 30
seconds to 30 minutes depending on the type of object and on the degree of detail
required. The standard 50 mm f/18 objective can give optimum results with an exposure time
of 10 minutes.
DIRECT FOCUS PHOTOGRAPHY
It is by means of this method that all published photographs of
galaxies, nebulas and objects deep in the sky (also terrestrial objects) are taken by
amateurs and professionals. In direct focus photography, the image formed by the telescope
is focused directly on the film's surface.
No other optic element (such as oculars or objectives) is put between the telescope and
the body of the camera. The adaptor for the camera performs the simple function of
mechanically reminding the telescope of the camera. This method results in the most
luminous of photographs available as well as the widest field. Direct focus photography
needs: images that are perfectly in focus, telescope vibration reduced to a minimum and,
during the exposure time, the photographer has to ensure that the subject remains in
exactly the same spot with respect to the telescope (in the case of long exposure
photography). This last entails checking the exposure and effecting small changes of
position of the telescope either manually or electronically should the subject move from
the centre of the control grid. Auxiliary instruments, such as the guide telescope, carry
out the same function with different types of telescope.
 |
Diagram for direct focus photography.On the focus tube (A)
of the telescope the photography adaptor (B) is attached, then the T/2 ring (C) relative
to your reflex camera (D). |
| The results obtainable with direct focus photography have
this value: many astrophiles have obtained stupendous shots, as good as or better than
those obtained in the biggest observatories, the effective focal length with which a
direct focus photograph is taken is, simply, the focal length of the main telescope. The
dimensions of the object in the photogram depend on the focal and on the angular dimension
of the object according to the following formula: |
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