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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’ll be easily 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 & lunar viewing & photography.

-Excellent for wide field viewing & astrophotography especially in shorter focal lengths (more

on this later).

-Because the objective is permanently mounted & aligned there is no need for collimation (again

more on this in another article).

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

Disadvantages:

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

-Can become bulky & hard 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 & lunar astrophotography & viewing in longer focal lengths.

-Can get excellent wide-field astrophotos & 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, even though you’ll learn how to do this rapidly 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 & mirrors to collect & 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’s focused onto a hole in the primary mirror where the light is collected by an eyepiece or your astrophotography camera.

Advantages:

-Compact & portable.

-Low maintenance even though 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 & astrophography.

-Very good for planetary & lunar viewing & astrophotography.

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

-Very good to excellent optics, both Meade & 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 & longer exposures are required for astrophotography, even though 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 & 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’ll get sharper higher contrast planetary & lunar images when compared with the Schmidt design.

Carl Gilbertson has been involved in amatuer astronomy for over thirty years & astrophotography for over twenty-five years. Learn more at his site: Telescopes For Astrophotography

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Carl Gilbertson

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