Astronomical Refractor

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.

Carl Gilbertson has been involved in amatuer astronomy for over 30 years and astrophotography for over twenty-five years. Learn more at his site: Telescopes For Astrophotography

Question about telescopes?

I'm looking into buying a telescope.
Of course, budget is a concern,
so I'm wondering...

does anyone know what the minimum decent size is?
I see them ranging from 50mm x 450mm
to 1100mm x 102mm

This means nothing to me.

Also, I see there are Dobsonian, Newtonian, and Gallileo ones.
Assuming these are brands?

More important distinctions I think are:
Tri-centric
Refractor
Reflector
Astronomical (I know what that means)
Terrestrial (I know what that means, too and it's not ET!)
and Equatorial

Any ideas what these mean and why I would want them on my telescope?

Kind of in a rush,
as I'd like to purchase it before Thursday.
(Birthday present)

If I know what these things mean,
then I can maybe find one I want in a price range I can afford.
(the one that has them all is $1,399)

First of all, I think it's a mistake to buy someone something as personal as a telescope without consulting them first. Although it may spoil the surprise, I'd strongly advise including the recipient in the decision-making process. Some of the factors involved are age, amount of technical savvy, physical constraints, and many more. I'll try to give some specific answers:

Minimum decent size: 75mm for a refractor, 114mm for a reflector, but 100mm and 150mm respectively would be better.

Dobsonian is a type of mount, like a cannon. Newtonian is an optical design using mirrors. Galileo is a brand, not a very good one.

Tri-centric: never heard of it in 50 years in astronomy!
Refractor: optical design using a lens
Reflector: optical design using mirrors (same as Newtonian)
Astronomical vs. terrestrial: first good only for astronomy, second good only for birding, etc.
Equatorial: type of mount

This probably has left you more confused than ever, so let me tell you what I would buy: any one of these, depending on budget:
http://www.telescope.com/jump.jsp?itemType=CATEGORY&itemID=9

Finally, where to buy: don't buy from a discount/department/big box store! They don't know anything about telescopes and sell nothing but junk. Buy from a telescope store (look in the Yellow Pages) or online. If you ignore this advice, you're almost certain to get your loved one a piece of crap! Better to delay the gift by a few days, than waste your money.

But please, PLEASE, heed my initial advice of including the recipient in the decision-making process.




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