INSTRUMENTS

ASTRAL TELESCOPE

BASIC NOTES

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Schmidt Telescope

The eyepiece is positioned behind the mirror as in the refractors. It is easy to handle and of convenient size, but it is expensive. It is suitable above all for the observation of planets and relatively bright objects.
It is suitable for photography only when the focus is on the inside. It is made on a spherical mirror and a corrector plate positioned at the centre of the curve of the mirror. The photographic plate has to be positioned at the focus inside and has to be curved insofar as the field of this instrument is curved. The diameter of the plate is smallest than the diameter of the mirror, this contributes to the elimination of aberrations of sphericity and coma.
There are two advantages to this type of telescope. HIGH DEGREE OF LUMINOSITY and BROAD FIELD. The RATIO F/D can be very small and therefore have very short focal.
The extension of the field can reach 25 ° in some cases, with only one photographic plate it is possible to take a shot of sky 25 ° in diameter and have good definition right to the edges.
When defining the characteristics of a Schmidt, it is necessary, first of all, to indicate the diameter of the plate, then that of the mirror and finally that of the focal.

CATADIOPTRIC
Telescopes that use a system of lenses and mirrors similar to those of photography objectives exist. These are called catadioptric.
Sehmidt-Cassegrain Telescopes This is the best-known and most widely used of the catadioptric telescopes, due to its being both easy to handle and compact. It is based on a principle which incorporates features of both Schmidt and Cassegrain systems.

The spherical mirror has a hole bored at its centre. The rays reflected by this mirror towards the corrector plate are collected by a small di- verging mirror that, besides lengthening the focal, beams them towards the hole in the mirror, behind which they are focused. The thin corrector lens, causes an aberration equal and opposite to that produced by the two mirrors and so there will be no final defects.
The big advantages offered by this system are that the weight and size are reduced while in no way compromising the quality of the result. (It should be noted that the optic scheme is similar to that of photography telescopes).
Thanks to the minimum focusing distance, this type of telescope is also suitable for observing terrestrial objects. Great magnified images are obtained. The only disadvantage to this type of telescope is its high cost relative to its aperture.
The optic system of Schmidt-Cassegrain
In the image, light enters from the right, passes across the thin lens with double correction plate and proceeds as far as the primary mirror, thus converging on the aspheric surface of the secondary mirror. The secondary mirror multiplies the effective focal length of the primary mirror and arrives at the focus on the focal plane with the light that passes through a central perforation of the primary mirror.

Schmidt-Newton Telescope

This is a combination of the Newton and Schmidt systems, where the ray of light, once it has passed through the corrector plate reaches the secondary mirror which sends it back to the ocular which is positioned at 90 ° to the optic axis of the telescope.