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Small Telescopes - Choosing the right 'scope

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Newtonian Reflector Telescopes

Newtonians (also known as catoptrics) usually use a concave parabolic primary mirror to collect and focus incoming light onto a flat secondary (diagonal) mirror that in turn reflects the image out of an opening at the side of the main tube and into the eyepiece. A bit like the picture above.

Advantages

Disadvantages

Dobsonian Telescopes

Most Newtonian Telescopes have been supplied on equatorial mounts. The last few years has seen a new commercial telescope available on the market - the Dobsonian. A Dobsonian is a simple altazimuth mounted Newtonian telescope which is excellent for beginners and in large sizes is an economical "Light Bucket". Simple construction means they are easy to look after and a lot of aperture can be carted around, 200mm to 250mm Dob's can be slung across the back seat of a car and transported anywhere.

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. At the moment refractors are in vogue due to their portability, image quality and robustness.

Dobsonian's have their place if you know the sky, love deep sky object's and are just happy to roam the stars. The newest designs are even more portable, and even have adapters to fit small digital cameras.

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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