Telescopes - What should I look for?

There are many aspects about a telescope that should be considered before purchasing one.  This post will be an introductory primer to some of these considerations.

Let's get some terms out the way first.

Aperture - This is the size of the mirror or lens that collects the light from the sky.

Focal Length - This is the length the light has to travel in the telescope before being focused at the eyepiece.

Focal Ratio - This is the focal length divided by the aperture.

Reflector - This type of telescope collects light on a parabolic mirror and reflects it back toward a secondary mirror which delivers the light to an eyepiece.

Refractor - This type of telescope collects light with a lens which, in turn, delivers the light to an eyepiece further down the tube.

Catadioptric - This is essentially a composite telescope that collects the light with a lens and uses interior mirrors to bring the light to the eyepiece at the other end of the optical tube.

There are many different builds of telescopes but the basic types are listed above.  In later blogs individual builds of telescopes will be investigated more thoroughly.

A refractor tends to have very clear images as light travels through less optical material.  However, it focuses different colours of light at different points causing chromatic aberration; distorting the image.  This can be corrected by using a doublet or triplet apochromatic telescope.  These telescopes are very expensive per inch of aperture.  However, a low aperture apo refractor is perfect for planets and large expanse nebulae.  On a motorized mount you will be able to get amazing photographs.

A catadioptric telescope tends to have a very long focal length and comes in a compact optical tube.  This is very handy for transportation as it can fit in the trunk of a car quite easily.  The price range is moderate per inch of aperture so it might be a good advanced scope to invest in.  The planets will be amazing and deep-sky treasures will be yours for the taking.  The focal ratio is typically quite high (f/10 etc.) so you will have to take longer exposures if you are interested in astrophotography.  A focal reducer can be added to take it down to f/7 or so to alleviate this issue slightly.

The first telescope I started to work with consistently is an 8" Reflector on a Dobsonian Mount.  A Dobsonian mount is essentially an alt-azimuth mounted telescope that supports rotating the optical tube on two axes, the azimuth(side to side) and altitude(up and down).  A image of this is shown below.

8" Reflector with Dobsonian Mount

The aperture size determines how much light is gathered by the telescope.  The higher the aperture, the more light that is gathered.  With greater light gathering ability you will be able to see fainter objects.  The focal length is an important factor in determining how much magnification you can get out of your telescope.  The magnification is simply calculated by dividing the focal length of the telescope by the focal length of the eyepiece.  The higher the focal length of the telescope the more magnification you can get with each eyepiece.  For more information about eyepieces please see my previous post linked below.

http://simonjastronomy.blogspot.com/2017/01/nova-summary-january-2017.html

There are several warnings about going out to get the highest aperture, longest focal length telescope.

1. The higher the aperture the heavier the telescope.  You will not use a telescope that you can not carry conveniently.  It will collect dust around the house.
2. Magnification can only take you so far.  If the magnification is too high you will be magnifying the atmospheric disturbances and will likely see nothing but blurry objects.  Do not trust shopping mall telescopes that say they will give you 500x.  You will see nothing but blurry light.

An 8" telescope with a 1200mm focal length is a medium range telescope that can be used to find all the deep-sky Messier Objects and, at the same time, have great views of the planets.  The Dobsonian mount is very easily transported and offers the most affordable telescope per inch of aperture.  I can setup and take down my telescope in less than 30 seconds.  The drawback to this mount is that it does not track automatically.  You will not be able to take long exposure photographs using it.  As the Earth rotates you will get star trails in your images.

Several people have asked if a telescope can be used for terrestrial viewing.  They certainly can.  I've pointed my telescope at the mountains and had great views.  Whatever you do DON'T POINT IT AT THE SUN.  You will destroy your eyes and your telescope.  There are ways to do this safely as outlined in a previous blog post listed below.

http://simonjastronomy.blogspot.com/2017/01/explore-universe-sunspots.html

If you are considering purchasing a telescope I would be more than happy to offer my advice and point you in the direction of some expert dealers.