INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES

page 19

WHEN?
Atmospheric conditions greatly influence astrophotography. A sky slightly clouded with fog diffuses the light, thus darkening the film.
These conditions are good for photography. A limpid sky slightly clouded with fog diffuses the light, thus darkening the film. These conditions are good for photography. A limpid sky, with a transparent atmosphere, but with obvious fluttering among the stars (poor seeing) indicates a rather turbulent atmosphere.
It is therefore, not advisable to photograph in these conditions if you are anxious to obtain good details (of the Moon, planets, double stars). A limpid sky together with a transparent atmosphere and contained fluttering (good seeing) of the stars constitute the best conditions for photographing any object whatever. When the Moon is in the sky it is useless to try to photograph other objects. Whoever tries it will discover this. If the evening is good photograph the Moon, and, if it is possible, you will have to wait until it arrives at the meridian or in its immediate closely, at which time the light, crossing a small layer of atmosphere, will offer the best possible image. (this last pointing goes for any object whatsoever). If you are interested in the smallest details of the Moon, it is necessary to take photographs during the first quarter or the last quarter, as it is at these times that the Sun drawing near to the terminator (the boundary dividing the illuminated portions of the surface from those in shadow), projects the shadow of obstacles in its path, thereby throwing even the smallest details of the moon surface into relief.

REFRACTORS AND REFLECTORS
Normally, a refractor is constructed with a very small aperture ratio (f 1/8 - f 1/15). Given the poor luminosity, best results are obtained when photographing the Moon or the brighter planets. Of course the projection system will be the one used. In the case of reflectors, on the other hand, it is necessary to make some distinctions.
The Newton telescope is one which is normally constructed with a wide aperture ratio. For this reason it is excellent for direct focus photography and is quite good for the Moon and the planets. The Cassegrain, like the refractors, is normally constructed with a small aperture ratio and is therefore suitable for photographing the Moon and the planets. The Schmidt-Cassegrain catadioptric is a very versatile instrument and is available with both small and large aperture ratios. It is an excellent instrument for photography.

FOCUS
The focusing mechanism is operated by means of the reflex viewfinder or sight. Once the Moon is in focus, the correct position can be marked. This mark will be the reference point for the correct focus when dim objects are being photographed (some stars, nebulas etc.).

WHAT FILM SHOULD BE USED?
For the Moon and the planets a medium sensitive film should be sufficient (100 ASA) in black and white or in colour. The colour film does not give as good results as the black and white one, however. For other objects, films of higher sensitivity are necessary (400 ASA or higher). Black and white films such as the Ilford and the TRIX PAN Kodak are available in most shops and are colour films such as the 400 Kodak Ektachrome. It is possible to increase the of a film to twice its original sensitivity when it is being developed. The astrophile will need to ask for this when necessary.

EXPOSURE TIMES
The exposure times is influenced by many factors.
- The characteristics of the telescope
- Use of filters
- Type of film
- Lights from the city
- Atmospheric conditions
- The subject of the photograph
- The system used Here are approximate times, which do not, for obvious reasons, take all of the above factors into account. You will, however, learn by experience.
Moon less than a second Planets more than a second Star masses up to 15 minutes Galaxies up to 30 minutes Nebulas up to an hour