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Types Of Telescopes For Astronomy

Basic Telescope Designs

The job of a telescope is to collect light, not to magnify an image (the eyepiece does that job). The larger the objective (the part that collects the light) whether it be a lens, in refractors, or a mirror, in reflectors, the more light the telescope will collect. The more light you can collect, the more detail you will be able to capture, and also important for astrophotography, the shorter your exposures will need to be to capture this detail.

Refractor Telescopes

The type of telescope most people visualize when they hear the word telescope is the 'Refractor'. This is what Galileo used for his break-thru discoveries. A refractor has an objective lens at the front which passes the light straight through to the back of the tube, focusing this light at an eyepiece or for astrophotography a camera.

Advantages:

-No central obstruction (see more in the reflecting scopes), giving higher contrast.

-Due to the simple design they require little maintenance.

-Excellent for planetary and lunar viewing and photography.

-Excellent for wide field viewing and astrophotography especially in shorter focal lengths (more on this later).

-Because the objective is permanently mounted and aligned there is no need for collimation (again more on this in another article).

-Excellent color in apochromatic and ED (Extra Dispersion) designs.

Disadvantages:

-Costlier per inch of aperture (objective) than reflectors and catadioptric telescopes.

-Can become bulky and difficult to manage, especially in larger lens designs.

Newtonian Telescopes

This design was invented by Sir Isaac Newton (he of the apple on the head fame). Instead of a lens at the front of the tube this telescope design uses a concave, parabolic mirror to collect light reflecting it back towards the front of the tube to a flat diagonal mirror which reflects the light out the side of the telescope to the eyepiece or camera for astrophotography.

Advantages:

-Lowest cost per inch of all the telescope designs.

-More light gathering power per dollar because of the lower cost design.

-Absolutely perfect color rendition.

-More compact design compared to a refractor of similar light gathering ability.

-Excellent contrast for planetary and lunar astrophotography and viewing in longer focal lengths.

-Can get excellent wide-field astrophotos and short exposures in shorter focal lengths.

Disadvantages:

-Slight loss of contrast due to the central obstruction (the flat secondary mirror) as compared to a refractor.

-Requires more maintenance, such as collimation (discussed in another article) which is vital for great results in your astrophotography, although you will learn how to do this quickly with practice.

Schmidt-Cassegrain Telescopes

This is a very popular design, with a high tech look. Also known as a CAT (Catadrioptics). They use a combination of lenses and mirrors to collect and focus the light onto the eyepiece or camera. The light enters the telescope through a thin 'lens' called a schmidt corrector plate, goes to the back of the scope to a spherical primary mirror which reflects the light back towards the front. Here the light strikes another mirror, the secondary mirror which is mounted on the corrector plate. This secondary mirror then reflects the light back towards the back where it is focused onto a hole in the primary mirror where the light is collected by an eyepiece or your astrophotography camera.

Advantages:

-Compact and portable.

-Low maintenance although once again collimation is required for top performance.

-Many, many astrophotography accessories available.

-Cheaper per inch of aperture as compared with refractors.

-Excellent all-round telescope, good to very good for both visual and astrophography.

-Very good for planetary and lunar viewing and astrophotography.

-Very good to excellent for DSO (Deep Space Object) astrophotography with a caveat (see the disadvantages).

-Very good to excellent optics, both Meade and Celestron are putting out excellent optics on a consistant basis.

Disadvantages:

-Costlier per inch of aperture as compared with Newtonian telescopes.

-Loss of contrast due to the central obstruction which is even larger than that in the Newtonian scopes.

-Due to their longer focal lengths the field of view is smaller and longer exposures are required for astrophotography, although a lens known as a focal reducer is available which minimizes or removes this problem. The longer focal length is actually an advantage in planetary and lunar photography.

Maksutov-Cassegrain

The Maksutov-Cassegrain telescope design is basically the same as the Schmidt-Cassegrain design except it uses a meniscus lens at the front instead of a Schmidt corrector plate. The main advantage to the Mak telescope design is you will get sharper higher contrast planetary and lunar images when compared with the Schmidt design.

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Celestron T-adapter for all Schmidt-Cassegrains. Threads onto Rear Cell.

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The Cambridge Guide to Stargazing with your Telescope

The Cambridge Guide to Stargazing with your Telescope
The Cambridge Guide to Stargazing with Your Telescope is an indispensable guide to telescopes and how to choose the right one for your needs. It gives straighforward explanations of how they work, and how to progress from first-time user to skilled observer. It gives practical help for setting up and using any telescope, and provides lists of objects to look at with different sizes of telescope, from both town and country, including the Sun, Moon and planets, comets, asteroids, star clusters, variable and double stars, supernovae, nebulae and galaxies. Aimed principally at newcomers to astronomy, of all ages, this book describes the full range of telescopes that are internationally available, with examples of objects to observe taken from both hemispheres, appropriate for all observing conditions. It also gives advice about accessories, such as eye-pieces and filters, plus suggestions for photography through the telescope and choice of camera and film type.

Star Testing Astronomical Telescopes: A Manual for Optical Evaluation and Adjustment

Star Testing Astronomical Telescopes: A Manual for Optical Evaluation and Adjustment
Many observers harbor misgivings about their telescope. The manufacturer may have guaranteed accuracy to one-quarter wavelength or as diffraction-limited but most telescope users have, at best, only a hazy idea of how to personally verifying such claims. Sure, there are ways to check the accuracy of individual components but for many they are hard to understand or require costly reference optics and other test equipment. Besides, telescope users are interested in the performance of the entire optical train, not just the main optical element. What is really needed is a test that can be used at the observing site, so that all the problems that impact on a telescope's performance can be diagnosed. Isn't there a simpler and more complete way than the complicated shop tests? Yes, the star test is such a method. It uses the entire working telescope. It isnot a poor substitute or a work-around that uses bits and pieces of the optical system. It is the oldest and most sensitive of the optical tests an inspection of the diffraction image itself. Star-test results apply to the complete imaging performance of the telescope. The star test is lightning-fast and requires only a good high-power eyepiece. It tests the telescope for precisely what it was meant to do. Bad or poorly-aligned instruments fail the star test unambiguously. The star test often allows you to correct the optical difficulty immediately in the field, when you might be frantic t