How to Choose a Good Beginner Telescope

Posted by E. Gazelle on February 10, 2009

Here are some useful tips on how to choose a good starter telescope kit for a beginner. When you go shopping for your first telescope, please bear in mind that every telescope has its strengths and weaknesses, even the most expensive models.

For beginners, it is advisable to keep things simple. Get a basic but reasonably good telescope kit and then familiarize yourself with it to avoid fumbling and fiddling during actual observation.

As a beginner, avoid buying too many accessories too soon. Generally, your first starter telescope should be:-

* Reasonably priced – get coupons, best-buys and Great Deals for Stargazers;
* Comes with basic accessories;
* Portable;
* Easy to set up and dismantle; and
* Simple to operate, handle and maintain.

Portability is important as you are more willing to use it compared to dragging a larger and bulkier one around.

Typically, 2.4″ (60 mm) to 3.1″ (80 mm) refractors and 4.5″ (114 mm) to 6″ (150 mm) reflectors are great as beginner telescopes.

If you are at a loss as to which starter telescope kit is best for you, here are some tips and pointers which you may use as a guide in selecting your first telescope.

Understanding the types of telescopes and their parts

Know the differences between a Galileo Refractor and a Newton Reflector Telescopes, and have a basic understanding of the:-

1. Mount;
2. Aperture Size of the telescope;
3. Eyepieces;
4. Magnification;
5. Focal Ratio (f-ratio); and
6. Computerized and Manually Controlled Telescopes.

1. Galileo Refractor Telescopes vs. Newton Reflector Telescopes

Pick one that best suit your observation habits. Beginner refractor telescopes are comparatively more expensive than starter reflectors because of the use of lenses which are more costly to produce than the primary mirror  in a Newtonian tube. Due to this cost factor, larger telescopes tend to be Newtonian reflectors, including the giant Hubble Telescope.

Refractors are generally recommended as a good starter telescope for kids and children as they are more sturdy and can be easily transported. Reflector telescopes, on the other hand, use mirrors instead of lenses to collect light. These mirrors tend to get out of alignment if handled roughly. Therefore, users of reflector telescopes must know how to collimate a misaligned reflector telescope optical tube assembly (OTA).

2. Understanding the Mount

The mount is a very important part of a telescope – it is absolutely necessary if you want steady viewing. There are 2 basic types of mounts: alt-azimuth and equatorial. The beginner telescope with an alt-azimuth mount is more suitable for children due to its ease of use. Adult beginners will find the equatorial mount more versatile and allows them to grow into the hobby.

Remember that the Earth is rotating all the time. In order to keep the object in view for any length of time, you need to constantly adjust your telescope. The equatorial mount is designed to track the movement of objects in the sky and polar alignment is vital.  Telescopes mounted on equatorial mounts must be accurately aligned on the pole to ensure accurate tracking.

3. Understanding the Aperture Size of Telescopes

An astronomy telescope is basically a tool to collect light from a distant source.  This light is then focused to a point where it is then magnified for a clearer view through the  eyepieces.   The ability of a telescope to gather light is directly proportionate to the size of its aperture.

The aperture of a telescope refers to the diameter of either the objective lens of a refractor or the primary mirror of a reflector.  Therefore, the larger the aperture of a telescope, the better it is in collecting sufficient light for you to see distant objects clearly.

In short, aperture size is the true key to the power of a great beginner telescope.

Therefore, telescopes with larger apertures will be the better candidate for viewing deep-sky objects which are much further away from Earth and are usually dim compared to near-Earth objects.

4. Understanding Eyepieces

Barrel sizes or diameters of eyepieces

The eyepieces of most beginner telescopes come in three different barrel sizes or diameters. They are 0.965 inches, 1.25 inches, and 2 inches. Eyepieces with larger barrel sizes are more expensive. The barrel size of your eyepiece is determined by the size of your focuser (for reflectors) or the size of your diagonal (for refractors). The standard barrel size is 1.25″. The 2″ eyepieces are more expensive. For adult beginners, avoid the non-standard 0.965 inches eyepieces.

Usually the barrel size of the eyepiece also determines your field of view or how large a portion of the sky you can actually see through the eyepiece. The smaller ones will show a lesser portion of the sky for a given focal length and apparent field than the larger ones.

Since standard eyepieces come in diameters of 1.25″, it makes sense to buy telescopes that support 1.25″ eyepieces. Please check this carefully as some cheaper models are fitted with eyepieces measuring 0.965″ unless you’re buying it for your kid.

If you are serious about this hobby and wish to buy more eyepieces in the future, do invest in a telescope that uses the standard 1.25″ format. The reason being that your investments in one set of eyepieces can be used for all your future telescopes.

Focal Length of Eyepieces

Besides barrel size, the next important criteria in selecting an eyepiece is its focal length.  Choosing an eyepiece by its focal length is in effect choosing your desired magnification since magnification is measured by dividing the focal length of the telescope by the focal length of the eyepiece.

Eyepieces with long focal length of more than 25mm give lower magnification while those with less than 10mm focal length are known as magnifying powerhouses.

Material of Eyepieces: Glass vs. Plastic

Depending on manufacturing quality, glass optics generally out-perform plastic lenses in terms of image quality and durability although glass optics are more expensive.

5. Understanding Magnification

You can easily calculate the magnification of any telescope using different eyepieces by dividing the focal length of the telescope by the focal length of the eyepieces.

For example, using a 25mm eyepiece (i.e. focal length of 25mm) on a telescope with a focal length of 700mm will give you a magnification of 700 ÷ 25 = 28 times. A 9mm eyepiece will give you a magnification of about 77 times.

Therefore, using the same eyepiece, a telescope with a focal length of 700 produces larger images compared to a telescope with a shorter focal length such as 540mm.

What is the Ideal Magnification

• Low magnification of between 20 to 50 times are ideal for wide-field views of extended deep-sky objects under dark sky condition.

• Magnification of up to 100 times are great for locating objects and for most deep sky observations.

• Magnification of between 75 to 200 times is suitable for viewing the moon, planets and more compact objects such as globular clusters or wide double stars.

• Magnification of more than 200 times calls for absolutely clear sky conditions and excellent optics to make the additional magnification deliver.

Whether a particular telescope can deliver clear, sharp images under high magnification depends on its overall construction and the quality of its optics.  In a telescope with small aperture size that can collect only so much light, a high magnification (i.e. low eyepiece focal length) will only spread that limited amount of light collected into a larger, blurry image.  In astronomy telescopes, clarity is more important than high magnification.

Moreover, do remember that our Earth is not static. It rotates on its own axis and revolves around the Sun. Such movements will be correspondingly magnified by the telescope. If you are using a manual telescope with high magnification, you’ll have to adjust your telescope more frequently to get your object into view. Besides the Earth’s movements, you must also consider its unstable layers of atmosphere. A telescope with very high magnification will magnify such disturbances.

In short, lower power provides better viewing experiences most of the time.

6. Understanding Focal Ratio (f-ratio)

The focal ratio (or the f-ratio) of a telescope is calculated by dividing the focal length of the telescope by its aperture size.  For example, a telescope with an aperture of 100mm and a focal length of 700mm has an f/ratio of 700/100 = f/7.

An understanding of the difference between telescopes with high or low f-ratio will help you in selecting the telescope that will best suit your needs. Generally, your choice will depend on where and what you would like to observe.

Exit Pupil

Your pupils, when fully dilated, will be about 5mm to 7mm in diameter at most. In order for you to see clearly, the images must not be larger than your pupil size or else whatever extra that is delivered by the larger magnification will be lost.

Therefore, to ensure that a particular eyepiece delivers optimum results, you’ll need to determine the exit pupil, which is obtained by dividing the focal length of the eyepiece by the f-ratio of the telescope.

• For example, a 25mm eyepiece (low magnification) with an f/5 telescope will give an exit pupil of 5mm – within your pupil size.

• For high magnifications, the ideal exit pupil size is between 0.5mm and 1.0mm. Therefore, an eyepiece of 5mm (high magnification) with an f/5 telescope will deliver an exit pupil of 1.0mm, which is within the practical limits of magnification.

Low Focal Ratio Telescopes – Newtonian Reflector Telescopes

Simple Newtonian reflecting telescopes usually have low focal ratios of between f/4 and f/8. They are sometimes described as rich-field telescopes and are ideal for wide-angle views of extended objects such as galaxy clusters and nebulae. If you want a taste of most items on the celestial spread, the popular Newtonian reflector of between f/5 to f/8 is your top choice as an all-purpose beginner telescope. Newtonian telescopes with low focal ratios of between f/4 to f/5 give excellent results in locations with dark skies. If you are observing in light polluted cities, opt for a starter telescope with higher focal ratio, e.g. from f/9 to f/11.6. If you wish to observe in both sky conditions, a beginner reflector telescope of between f/5 and f/7 will be a good choice.

High Focal Ratio Telescopes – Galilean Refractor Telescopes

Refracting telescopes generally have higher focal ratios of between f/8 and f/15 due to their relatively smaller apertures. They are renowned for their sharp and exquisite image quality. Their smaller field of view and good contrast at low magnification make them the best telescopes for observing in light polluted skies. Refractors are recommended for observing the planets, the moon and its craters, the sun (with solar filters fitted) and double stars. A larger refractor of at least 70mm will give sharper images such as the rings of Saturn, the moons of Jupiter and a clearer view of the craters on the moon.

7. Computerized or Manually Controlled Telescopes

Whether to choose a computerized or a manually controlled telescope as your beginner telescope is really a matter of personal preference and budget. Some beginners prefer to get to the bottom of things while others prefer the automated mode, money not being an issue. Once you are familiar with setting up a computerized telescope, it will take you no more than 5 minutes to auto-track the night sky. Once that is done, all you need to do is to enter the name of the objects and the telescope will continuously rotate to keep it in focus. This auto-tracking feature of the computerized models make them excellent for extended viewing for research purposes as well as for astrophotography.