ASTRAL TELESCOPE

BASIC NOTES
 

page 13

IMAGE RECTIFICATION

IMAGE RECTIFICATION
The more modest telescopes, when used for observation of objects on our planet, becomes a powerful instrument despite furnishing an inverted image (something that is of no account in astronomy). In order to remedy this a suitable prism is inserted between the ocular holder and the ocular. This is called an image rectifier or corrector. There are two types: Porro prisms and lenticular. Porro prisms consist of two prisms in the form of a pyramid, mounted in the same way in a tube of between 24.5 mm and 31.8 mm. It is a compact device and gives better light transmission than the Porro prism does. Another tube of rectifier is the lens type. It is simpler and cheaper, and often increases the focal of an instrument.

THE SUN OBSERVATION
The observation and/or the photography of the sun can be a very satisfactory experience; the sunspots and others phenomenes of the Sun surface are easy to observe while they are happening on the Sun.
The Sun observation is also very dangerous if you do not use the right protections that means filters (please check with accuracy among the optional accessories the ones which can be used in sun observations). The unprotected observation may damage seriously your eyes. Please read carefully the handbook with all necessary information.
The most used observation systems are: all open filters and sun screens.

OPEN FILTER:
It is a Mylar pellicle which is mounted on a metallic support. It has to be put on the telescope's objective. With this system the light is filtered before to pass in the internal of the telescope, and so, moreover to guarantee the safety of the observer, also avoid the problems connected with the overheating that may exist with instruments at low focal rapport.

SUN SCREEN:
A cheaper method for the observation is the sun screen. Easy to use on refracting telescopes with small diameter. Is made of a support link to be inserted on the hole of the finder bracket.
It has two screens: one black and holed, which is used as a diaphragm and the other white on which is projected the image given by the eyepiece. The magnification hangs on the eyepiece and on the distance between the screen and the eyepiece. For example, with an eyepiece of 16 mm at a distance of 17 cm, we will have the same enlargement obtained with an eyepiece of 20 mm at a distance of 21 cm.
There are some practice limits on the projection distance, sometimes if you enlarge too much, the image will be less bright and is necessary to protect it from the external light with a box.

MOON FILTERS
These have the function of subduing or weakening lunar light when it is too strong in order to avoid risk to the eyes. The system most in use is that of screwing the filter with the tube of the ocular as in the case of the sun filter.