INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES

page 16

ASTRONOMICAL PHOTOGRAPHY
Today more than ever before astrophotography has become a must for the demanding astronomer. Firstly because it allows the astronomer to see much more that by looking alone, since the long exposure time results in much more detail being retained on film than it would be possible to see with the human eye.
Secondly, the photograph is something to keep. It is not the easiest of roads to follow but, with enthusiasm and perseverance, important results will follow. In order to obtain results, a telescope with solid equatorial mounting is necessary, well positioned according to the Polar Star and a timing mechanism to follow the motion of the star (in reality it is the Earth that moves). If the exposure exceeds five minutes, it will be necessary to guide the telescope using a guide telescope, possibly with a lighted ocular. The following things are necessary:
- The careful and stable positioning of the instrument, if necessary with extra weights.
- A timing device
- A guide telescope Some small alterations in declination may be necessary during the exposure time.
Obviously, a reflex camera with interchangeable objective is also necessary. Only the body of the camera will be used, on which the ocular holder of the telescope will be attached by means of suitable attachments.

SCALE OF IMAGE
This is a fact of prime importance in the choosing of the best method to photograph a particular subject (a planet, a galaxy, the Moon, the Milky Way etc) The level of magnification offered by a telescope is in direct proportion to its focal length. A telescope with a focal length of 2000 mm gives an image twice the size of that given by a telescope with a focal length of 1000 mm. For certain objects, (for example the Moon or the planets) the photograph will want to increase the effective focal length of the telescope, by projecting the ocular described, in order to achieve an image on a grand scale.
The surface details of the Moon and planets will be photographed. For other objects (such as galaxies and nebulas) use of the basic focal, without magnification, with the direct focus method described later in this manual is the best. The objects deep in the sky are, generally, of weak luminosity and the increase in the scale of the image will, of necessity, reduce the luminosity of the image.