INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES
 

page 18

PHOTOGRAPHY BY OCULAR PROJECTION
Inserting the ocular in the camera connection, an enlarged image of the subject is obtained that can be projected onto the film surface. In this way the effective focal length of the telescope is increased. The ocular acts as a magnifying lens, or, by lessening the focal of the ocular being used, a progressive increase of the telescope's focal is obtained. This type of photograph is generally used to photograph the Moon or the planets, and the use of focal oculars of between 12 mm and 25 mm is advised.
To calculate the effective focal length and the ratio employed for a photograph taken using this method use the following formula

EFFECTIVE FOCAL length= Telescope focal length x M
EFFECTIVE FOCAL RATIO= Telescope focal ratio x M
The photograph will be taken with an objective of focal length 10200mm f/51.
With magnification factors of from 5 to 10 best results are usually obtained for photographs of the Moon and the planets..

THE GUIDE TELESCOPE
This is an indispensable instrument when the time of exposure exceeds 10 - 20 minutes. The inevitable imperfections of the timing device and positioning mean that the image does not remain fixed on the surface of the film. Prompt correction is necessary, either in declination or right ascension are establish the exact position.
For this reason, it will be necessary to fix a second telescope besides the main one, and lying parallel to it and this second one should be fitted with an illuminated grid. Before taking photographs, the following steps should be taken: look through the guide telescope with the grid (illuminated to facilitate viewing) and centre the star nearest to the object you wish to photograph. This operation should not cause the main telescope to move, and so the guide should be moved an its three mounting screws which keep it in place. It is important, after the centering of the star, that the screws are firmly in place as otherwise there is the danger of the guide telescope's shifting during exposure. At this point, the photograph can be taken, but it will be necessary to check throughout the exposure that the centering star remains at the centre of the grid, at the reference pointing chosen.
Should you notice any shift in position, correct it immediately. This system ensures stability of the image, which otherwise would be smeared (and the star would appear to be a comet).