How Do I Choose the Best Cassegrain Telescope?

G. Wiesen

If you are interested in a Cassegrain telescope, there are a number of different aspects to look at and consider before making a purchase. One of the first things you should look at is the aperture size of the telescope, as well as its focal length. Though there are a few different types of Cassegrain telescopes, these differences are fairly minor and not likely to impact amateur usage. You should look for a Cassegrain telescope with the right combination of power and size for your needs, and consider a mounting system that is sturdy and includes a “GoTo” interface.

Inside Cassegrain telescopes are mirrors and lenses that allow close-up views of faraway things.
Inside Cassegrain telescopes are mirrors and lenses that allow close-up views of faraway things.

A Cassegrain telescope is a type of catadioptic telescope, a telescope that uses multiple mirrors or lenses, first developed in 17th century France by the man from whom it takes its name. The basic construction is quite simple, and allows these telescopes to have impressive focal lengths despite small tube lengths. These telescopes typically have two internal mirrors within the tube and an eyepiece at the far end of the telescope. Light enters the end of the telescope and reaches a concave reflector at the bottom of the tube; the light is then reflected back toward the open end but is focused onto a small mirror within the tube that reflects the light back toward an opening in the primary lens so the light can reach an eyepiece.

As you consider the best Cassegrain telescope for your needs, you should typically begin with the aperture size. This is usually the size of the primary reflector in the tube, which has a tremendous impact on how much light is gathered by the telescope and on its maximum magnification, so you should look for as large an aperture size as possible. The maximum magnification for a telescope is basically its aperture size in inches multiplied by 50, or the size in millimeters multiplied by two.

A Cassegrain telescope’s focal length also has an impact on the magnification and the quality of the images you can see through it. You should look for as large a focal length as possible, while still keeping the tube itself to a reasonable length. Though larger telescopes are often better than smaller ones, you should be sure to choose a telescope you can actually use in a reliable way. Smaller telescopes are typically easier to transport and set up, so unless you have a permanent observation point, you should choose a telescope that is small enough for you to easily set up and use.

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You should also consider the type of mounting or stand you can use with any Cassegrain telescope you choose. While some telescopes come with a stand, many are sold individually and you may end up spending as much on a good telescope tripod as you do on the telescope itself. You should consider a Cassegrain telescope with a stand that features a “GoTo” system. This is an automated computer system that allows you to quickly and easily aim your telescope toward a selected object in the night sky.

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