Telescope Buyers' Guide

This is a telescope buyers' guide, constructed out of one or more members' experiences. Hope that it helps many people find the right telescope for them.

There are quite a few important factors that determine the capabilities of a telescope:

Telescope Parameters

• Telescope type
• Aperture
• Focal Length
• Optical Quality
• Type of mount
• Motorization / Computer Control
• Compatibility with standard eyepieces
• Cost

We shall now see how each parameter affects the capabilities of a telescope.

Telescope Type

There are many types of telescope designs. Telescope designs can be broadly classified as:

• Refracting telescopes or Refractors
• Reflecting telescopes or Reflectors
• Catadioptric telescopes

Refractors use lenses to create an image of the faint / distant object. Reflectors use mirrors to focus an image of the object. Catadioptric telescopes use a combination of mirrors and lenses to create an image.

What are the pros and cons of each design? We will discuss refracting and reflecting telescopes as they are popular.

Refracting Telescopes

Refracting telescopes, as mentioned earlier, use only lenses to create an image of the object. Refractors are usually expensive because a well-engineered combination of several lenses is required to create a good image.
In principle a refractor can be made using two lenses, but such a refractor will suffer from a horrible defect called 'Chromatic Abberation'. A combination of two lenses in the place of one can remove this defect to some extent. A combination of two such lenses is called an "Achromat". But to remove chromatic abberation further, many more lenses are required. Thus, there are two types of popular refractors:

• Achromatic Refractors
• Apochromatic Refractors

An achromatic refractor will give a fairly good image. Apochromatic images give the produce the most crisp images, and are ideal for photography of celestial objects. But both designs are expensive and are often available only with small "apertures".

To summarize: Refractors are ideal for astrophotography, i.e. for photographing celestial objects.
They are usually not preferred for visual observing because:

1. They are expensive
2. They usually have small apertures [We will shortly see what this means]

Pros:

1. Very crisp image (if the refractor is an apochromatic type)
2. Are usually portable
3. Ideal for astrophotography

Cons:

1. Good apochromatic refractors cost a lot
2. Refractors are usually of small aperture, and hence, one can view only planets and bright celestial objects with them
3. Refractors can usually produce lesser magnification because they usually have shorter focal lengths

Reflecting Telescopes

Reflecting telescopes produce an image by using a mirror. The image produced by reflectors is usually observed with an eyepiece that is made of lenses, as in the case of refractors. There are two popular types of reflecting telescopes:

• Newtonian reflector
• Schmidt Cassegrain

Newtonian reflectors are the most popular for visual observing of celestial objects.
Cassegrain telescopes are usually used for astrophotography, when refractors are incapable of offering sufficient magnification.

Newtonian Reflectors

This is the most popular design for visual observing. Newtonian reflectors have two mirrors - a "primary" concave (actually parabolic) mirror and a "secondary" flat mirror. The primary mirror gathers the light from the celestial object and focusses it. The secondary mirror only changes the direction of the light, so that it is suitable for viewing.

To summarize: Newtonian reflectors are the most suitable for visual observing.

Pros:

1. They are cheaper
2. They come in a large range of apertures, starting from small 4-inch apertures to more than 30-inch aperture
3. They provide good magnifications
4. They are ideal for visual observing
5. Can be made by hand with a few basic tools

Cons:

1. Image quality usually not as good as apochromatic reflectors
2. Are usually designed to produce inverted images

Schmidt Cassegrain

This design is usually preferred for astrophotography of small celestial objects. This design accomodates long focal lengths in a shorter tube length, making it less bulky. A favourite amongst astrophotographers and amateur spectrographers.

To summarize:Schmidt Cassegrains are suitable when you want large magnifications, especially for photography.

Pros:

1. Large magnifications with small sizes
2. Ideal for photographing small objects

Cons:

1. Difficult to make
2. Made usually only by foreign companies, thus making them expensive

Aperture

Aperture is a measure of how much light your telescope can gather. While in terrestrial telescopes, the magnification of distant objects is more important, in astronomical telescopes, the light-gathering power is more important than magnification.

Astronomical telescopes are mostly used to enhance the visibility of objects to the eye. While viewing planets requires magnification, viewing the universe beyond the solar system requires aperture.

Aperture is usually quoted in inches of diameter of the primary focussing element, i.e. the diameter of the "objective" (the bigger) lens in the case of refractors, and the diameter of the primary mirror in the case of reflectors.

The light-gathering ability of a telescope varies as the square of the diameter of its aperture. Which means that a 8" telescope is 4 times more powerful than a 4" telescope, which in turn is 4 times more powerful than a 2" telescope.

To summarize: Larger aperture helps you view more objects, and more details in objects.

Pros:

1. Larger aperture show more detail
2. Larger aperture increases contrast
3. Larger aperture shows fainter objects, more distant galaxies
4. Large aperture is ideal for deep-sky observing (seeing galaxies, nebulae, star clusters etc.)

Cons:

1. Larger aperture costs more
2. Larger aperture makes telescopes bulky

Focal Length

Focal length, usually measured in mm, refers to the focal length of the primary focussing element (objective lens in refractors, primary mirror in reflectors). Larger focal length gives more magnification with the same eyepiece. Smaller focal length gives more field-of-view with the same eyepiece, but less magnification.

To observe planets in great detail, an objective with a long focal length and high optical quality is preferred. To observe deep-sky objects, an objective with a short focal length is preferred.

Most telescope users would like to see both planets and deep-sky objects, so intermediate focal lengths are preferred. A focal length of anywhere between 40" (about 1000mm) and 64" (about 1600mm) is preferrable. Magnification can always be increased or decreased by changing the eyepiece.

Astrophotographers who want to photograph small objects will prefer long focal lengths and go in for Schmidt Cassegrain telescopes to reduce the bulkiness of the telescopes.

To summarize: A focal length between 1000mm to 1600mm is ideal for most observers

Pros:

1. Long focal length will increase magnification
2. Long focal length will reduce optical defects (abberations) like Coma
3. Long focal lengths are easier to make and usually reduce cost

Cons:

1. Longer focal lengths cause telescopes to be bulky (unless they are Cassegrains)
2. Longer focal lengths expose defects in the objective element
3. Longer focal lengths limit the field of view

Optical Quality

For viewing at high magnifications, high optical quality is required. Obsession, Pegasus Optics, Discovery, Orion and Celestron are known for their high-quality optics. Optical quality is important for viewing details at high magnifications. Long focal length telescopes and telescopes used for photography should have good optical quality.

Apochromatic Refractors offer the best optical quality. Richie-Chretien telescopes and Meade RCX offer very high optical quality, but are equally expensive. Schmidt Cassegrains and Reflectors are of moderate optical quality.

Amongst the mass-produced mirrors, Celestron and Orion are said to have the best optics, followed by Meade. These are, however, rather heavy on the pricing for Indian markets.

To summarize: High optical quality is important for photography, and for observing tiny details in objects.

Pros:

1. Always beneficial

Cons:

1. Costs more

Type of Mount

There are two basic classes of telescope mounts:

• Equatorial Mounts
• Altazimuth Mounts

Equatorial mounts are required for photographing celestial objects, and for tracking objects as they drift across the sky.
Altazimuth mounts are ideal for observing as they are simple to operate.

A type of Altazimuth mount called the 'Dobsonian Mount' is very popular for visual observing. Most large Newtonian Reflectors use Dobsonian mounts.

Pros of Equatorial Mount

1. Tracking of sky objects is possible
2. Astrophotography is possible

Cons of Equatorial Mount

1. Expensive
2. Heavy
3. Cumbersome to use for visual observing

Pros of Altazimuth Mount

1. Cheaper
2. Easy to operate
3. Ideal for visual observing

Cons of Altazimuth Mount

1. Cannot track, and hence unsuitable for astrophotography

Motorization / Computer Control

Motorization is usually required for astrophotography setups, except when doing wide-field astrophotography. Computer control is hardly ever required, unless you want to slew to objects automatically, rather than finding them manually.

To summarize: Motorization is required for astrophotography, except for wide-field astrophotography.

Compatibility with Standard Eyepieces

Eyepieces come with two standard outer diameters: 1.25" and 2". Telescopes with focussers that do not comply with these sizes should be avoided, because you might not be able to buy separate eyepieces that are compatible with your telescope.

Buying a Telescope - Recommendations

For Visual Observing

It is ideal that a beginner should not focus on planetary observing unless he is planning to do research on them. The planets, and bright "stellar" planetary nebulae are limited in number and hence only few objects can be studied with a planetary (long-focal-length) telescope.

Newtonian Reflector telescopes on Dobsonian or other Altazimuth mounts are ideal for visual observing.

For Planetary Observing

A telescope with very high optical quality and long focal length with moderate apertures is ideal to study planets and planetary nebulae in great detail.

For General Purpose Observing

Choose an aperture as large as your budget can accomodate. Choose reasonable optical quality upto 6" of aperture, good optical quality for upto 12" and very high optical quality for larger telescopes. Buy an eyepiece of very short focal length (4.8mm), of a very good design (Nagler / Panoptic / Ethos) if your budget permits. For lower budgets, cheaper Barlow lenses and low-grade eyepieces can help instead. Focal length could be in the range of 1000mm to 2000mm.

Beginner's Telescope

Choose an aperture of 4" - 8", unless you plan to observe faint objects, or your budget can fit in more. Choose a moderate focal length. A focal length that is 8 to 10 times the aperture diameter should do. Ensure that the focusser is either 1.25" or 2" in barrel diameter.

For Astrophotography

An apochromatic refractor and/or a Schmidt Cassegrain telescope of high optical quality, with Motorized equatorial mount are ideal for astrophotography. For Schmidt Cassegrains, choose an optimum aperture and high optical quality.

Feedback

The author(s) would love to hear feedback regarding the articles posted here. Feedback can be either sent using the Contact Page or via email to akarshsimha at gmail dot com.

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Akarsh Simha

Student, BTech Engg. Physics,

IITMadras. 

ATMed 8" f/8 telescope

Nikon FM10 Camera