Choosing a telescope

This is one of the hard things to do for beginners as choices are plenty and often beginners do not have the technical knowledge required to choose a telescope. Further, beginners often think telescope is the first and most important thing needed for astrophotography. ( IT IS NOT, the mount is the most important as its no good to have a fantastic telescope but poor accuracy in tracking )

Lets look at simplest factors that should help make decisions. I will keep the explanations as simple as possible and we will get into types of telescopes and see 3 main parameters which help determine the choice of telescopes.

Types of Telescopes

Reflector : ( A popular type is Newtonian/Dobsonian )

• Uses Mirrors

• Usually has large aperture and hence bulky

• Great for visual astronomy as it collects maximum light

• General simplest design with mirrors, hence cheapest

Refractor : ( A popular type is apochromatic )

• Uses Lenses

• Small focal length refractors are very portable

• Large focal lengths look quite long and slender.

• Great for astrophotography

• Long focal length can be expensive, small focal length can be cheap.

Catadioptric : ( Popular types are Schmidt and Maksutov Cassegrain )

• Uses both mirrors and lenses

• Small in length and looks broad

• Can be a good balance between visual and astrophotography.

A simple example of these types using diagrams is available here

Three Main Parameters of Telescopes

Focal Length : Think magnification ( zoom ). Usually specified in mm. Longer focal lengths means more magnification.

Aperture : Light gathering power. Also measured in mm. The thicker/broader the body of the scope, the more light will enter.

Focal Ratio : Focal Length / Aperture. Indicates how "fast" the telescope is or how long will you need to keep shutter open to gather light at that focal length. Faster telescopes are better for astrophotography as it means it gathers more light relative to duration of exposure.

The expensive telescopes are fast, have large focal length and gather more light. ( Focal length can be adjusted with a focal reducer )

Additional information can be found here.

So to summarize things to note :

Choose telescopes suitable for the purpose:

1. Planetary imaging needs large focal lengths and can be slow as planets are bright.

2. Deep Sky Objects can have small to medium focal lengths but needs fast telescopes.

3. We don't always need large focal lengths as it depends on the size of the object we are imaging. Large focal length may mean the object does not even fit in the field of view.

4. The larger the focal length, the more the precision needed in polar alignment.

5. If you want to zoom in on nebulae, galaxies and clusters, choose large focal length.

The below chart plots all messier objects and most planets in apparent brightness and apparent size. As you can see 3 main clusters form and therefore needs 3 combinations of focal length and focal ratio. In short no one size fits all telescope. Closest are fast large focal length Schmidt Cassegrains with focal reducer. Now you know why these are the most expensive. Have a look at this telescope and see the three parameters. It is fast and has large focal length ( which can be reduced if needed )

And here is a highly recommended wizard to choose a telescope or a binocular

With a large variety of telescopes and binoculars out there, it can be daunting to choose what you need. This Product Wizard will navigate the vast selection of products to help you easily select the best telescope or binocular to meet your personal needs.