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Small Telescopes - Choosing the right 'scope

 

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Refractor Telescopes

Refractors (also known as dioptrics) are what the average person identifies with the word "telescope", a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. Lately those focal lengths have got shorter as astrophotography matures and gets cheaper with the advent of Digital SLR Cameras with better low light performance.

Advantages

Disadvantages

In summary a small reflector is the best general purpose telescope for beginners as they are good on all types of objects and for starting astrophotography, if they are lower than f/10 focal ratio, f/5 being very fast. 90mm f/6 is an ideal portable telescope.

If you wish for the ultimate planetary telescope, and also a scope which is virtually maintenance free then look no further than a refractor.

Catadioptric Telescopes
Catadioptrics use a combination of mirrors and lenses to fold the optics and form the image. There are two popular designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain. In the Schmidt-Cassegrain the light enters through a thin aspheric Schmidt correcting lens, it then strikes the spherical primary mirror which reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece. Catadioptrics are the most popular type of instrument, with the most modern design, marketed throughout the world in 3.5" and larger apertures.

Schmidt-Cassegrain Advantages

Schmidt-Cassegrain Disadvantages

Maksutov-Cassegrain
The Maksutov-Cassegrain design is similar to the Schmidt with basically the same advantages and disadvantages but is not quite as good optically given the same focal ratios. It uses a thick meniscus correcting lens with a heavy curvature and a secondary mirror that is usually an aluminized spot on the corrector.
The Maksutov is heavier than the Schmidt and because of the thick correcting lens takes a long time to reach thermal stability at night in larger apertures (over 90mm). At 150mm it's not that portable.
The Maksutov optical design typically is easier to make and should be less expensive than the Schmidt-Cassegrain.

In summary a small reflector is the best general purpose telescope for beginners as they are good on all types of objects and for starting astrophotography.
 
If you wish for the ultimate planetary telescope, and also a scope which is virtually maintenance free then look no further than a refractor.

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