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WHAT TELESCOPE SHOULD I LOOK FOR?

Before looking at the range of telescopes that is available, consideration should be given as to what the telescope is to be used for.  This is the most important consideration of all.  The worst possible choice is the one that never gets used. The telescope that is too complicated or too cumbersome to set up will spend its time at the back of a shed or garage and never be used.  The first telescope should be easy to set up, easy to use yet give impressive views of the sky.

First we must consider some of the physical characteristics of a telescope and how they affect our requirements.

The main specifications for a really useful first telescope should be: a minimum aperture of at least 90mm for a refractor or 130mm for a reflector and 1,000mm focal length for each.  This combination will provide enough light grasp and permit a high enough magnification to see detail on the brighter planets.

One of the most important attributes for an astronomical telescope is the ‘light grasp’.  This is an expression used by astronomers to describe the process of the main optic directing light from a distant object into the eye.  The pupil of a young human eye is about 7mm in diameter when fully adapted to the dark.  This is equivalent to 38 square millimetres (38mm²).  A 100mm diameter telescope has an aperture area of 7,854mm².  It is therefore capable of directing up to 206 times as much light into the eye of the observer.  Put another way it will enable the observer to see objects about 200 times fainter than could be seen with the unaided eye.  So a larger aperture will allow even fainter objects to be seen.

The focal length is also important.  The FOCAL LENGTH is effectively the length of the telescope.  It is measured as the distance from the main optic to the point where the image is formed.  A short focal length will give a wide field of view but the objects in the field of view will appear small.  A long focal length will give a narrow field of view (small area of sky) but the objects in view will appear larger.  Short focal lengths are best for looking at star fields and larger objects.  Long focal lengths are most suitable for small objects and studying fine detail, for example on the Moon and the planets.

Magnification, strictly speaking, is not an attribute of the telescope it mainly depends on the eyepiece being used.  A telescope of a specific focal length will produce an image of a specific size and this cannot be changed.  For example a telescope of a certain focal length may produce an image of the full moon 10mm in diameter.  A longer focal length will produce a larger image and a shorter focal length will produce a smaller image.  The eyepiece is then used, much like a microscope, to magnify that image.  A larger image to start with will allow the eyepiece to produce a higher magnification.

A long focal length 25mm (low power) eyepiece used on a 1000mm telescope will produce a magnification of 1000 ÷ 25 = 40x.  A short focal length 10mm (high power) eyepiece used on the same 1000mm telescope will produce a magnification of 1000 ÷ 10 = 100x.  However the same eyepieces used on a 1500mm focal length telescope (that naturally produces a larger image) will have magnifications of: 1500 ÷ 25 = 60x and 1500 ÷ 10 = 150x.

To summarise, if the craters on the Moon or the cloud bands on Jupiter are to be studied then a telescope with a relatively long effective focal length should be sought.  A shorter focal length telescope will be more suited to wide field views of the stars.  A good all round first telescope should have a focal length of around 1000mm to 1200mm.

TYPES OF TELESCOPES

REFRACTING TELESCOPES

All refracting telescopes use a glass lens as their primary focusing unit.  This lens is normally made up from two or more lens elements to produce a clearer image and reduce colour distortions caused by refraction as explained below.

Small_Refractor

Lenses use the property called REFRACTION to change the direction of rays of light and direct them towards a desired position.  Refraction occurs when light passes between two different transparent materials such as glass, air and water.

When light passes, at an angle, through the surface of a block of glass the angle is changed.  As the light re-emerges through the opposite side of this material its angle will be changed again back to its original angle.  To utilise this phenomena lenses are produced with a curved surface so when parallel rays of light meet the surface it will present an angled surface to each ray.  The paths of all the rays hitting the lens will be bent towards the centre line of the lens.  As the light emerges from the back face of the lens it is again bent.  If the back surface is convex the same as the front surface then the light will be bent even more towards the centre line of the lens.

 

REFLECTING TELESCOPES

A Newtonian is the simplest type of reflecting telescope. Below is the layout of the optics of a Newtonian tube assembly.  The Newtonian configuration is the simplest layout and therefore generally the cheapest of all the reflecting type telescopes.  Used with the simple Dobsonian mounting this type of telescope can make a very useful and cheap option for a first telescope at around £200 for a 150mm (6″) instrument.

Light_path_Newtonian

Light from a distant object enters the open tube and is reflected back up the tube by a parabolic (concave) mirror.  Because the mirror is curved the light is focused into a point where an image is formed.  To enable the observer to study the image without blocking the light entering the tube, a second small ‘flat’ mirror is mounted at an angle of 45° at the top of the tube to direct the light out through a hole in the tube.  A focusing unit is fitted to the hole to hold and adjust the eyepiece.  The eyepiece is a simple microscope used to magnify the image formed by the mirror.  Using a variety of eyepieces with different focal lengths, higher or lower magnifications can be obtained.

There are other variations of the reflecting telescope layout such as the Cassegrain.  This design replaces the flat angled secondary with a small convex mirror that redirects the light back down the tube and through a hole in the centre of the Primary Mirror where the eyepiece is mounted.

REFRACTING TELESCOPES

A first telescope must be easy to use, portable enough to move around and set up and be within a modest budget.  The budget available is important but if possible at least £230 should be spent on a new telescope or the pro-rata amount for a second hand instrument (say £100 for a telescope that costs £250 new).  Avoid the models that are sold in high street stores as they tend to be poor quality.

Some of the best manufacturers to look out for are: MEADE, CELESTRON, ORION, SKY WATCHER, TAL, KONUS and BRESSER.  Suppliers of these telescopes can be found in the adverts in popular astronomy magazines such as ‘Astronomy Now’ and ‘Sky at Night’.

 

There now follows a few examples for consideration.

Evostar_120

The Skwatcher Evostar 102

This telescope represents the ultimate first refracting scope for the beginner.  It has an aperture of 102mm and a focal length (FL) of 1000mm.  It is supplied with the tripod, two eyepieces, even a camera adaptor.  The MRP is about £269.

The telescopes in this range are:

Evostar – 90             90mm   FL 900          £199   A little small (ok)

Evostar – 102           102mm FL 1000        £269   Perfect

Evostar – 120           120mm FL 1000        £349   A bit expensive

Evostar – 150           150mm FL 1220        £549   Big & expensive

Most manufacturers listed above have a similar range.  The telescopes in these ranges are typically supplied on a tripod and with an equatorial mounting.  They usually have two eyepieces (25mm and 10mm) and sometimes include a Barlow Lens (see page 6).  All are supplied with a 90° Star Diagonal.  This is a mirror set at 45° to direct the image into a comfortable position for viewing through the eyepiece.

The smaller examples are usually supplied with a 30mm aperture finder whereas a 50mm would have been better but this can be upgraded later.  There are other telescopes at 60mm to 70mm aperture that are not bad if only a small budget is available (£60 to £100).  Their capability is really limited to observing the Moon and the moons of Jupiter.

The details of these telescopes can be checked out on the websites and can be purchased through mail order.  They do need to be assembled but this is generally an easy task.

REFLECTING TELESCOPES

Reflecting telescopes are generally cheaper than the equivalent sized refracting telescope.  This is because they use a mirror as the main optic and not a more expensive lens.  A mirror only has one surface to be ground and polished but the typical refractor, that has two or sometimes three lens elements, has four or six surfaces to be ground and polished.

The cheapest and simplest reflecting telescope is a Newtonian tube assembly (see page 6) mounted on a Dobsonian mount.  The mount is a simple Alt azimuth with a turntable for rotation and a trunnion for up and down movement.  These are very easy to set up and simple to use.

Because the Newtonian has a secondary mirror at the top of the tube there is a small loss of light so a 150mm will give a just slightly brighter image than a 100mm refractor.

Celestron_Dobsonian

The150mm FL 1220mm Celestron Starhopper £230

Other models in this range:

Starhopper                         200mm (8″) FL 1220mm         £335

Starhopper                         250mm (10″)    FL 1270mm   £559

Starhopper                         300mm (12″)    FL 1500mm   £949

Most manufacturers listed previously have a similar range.

A major advantage, beside the cheapness, of the Dobsonian is its simplicity of use.  It just needs to be placed down on a flat surface and it is ready to use.  A finder scope is attached to the main tube to help find a desired object.  Once the object is located in the main telescope it can be tracked by moving the tube gently, up or down and around while looking through the eyepiece to keep the object central.

The Newtonian tube assembly can also be fitted to an equatorial mounting.  This does make the telescope more expensive but can make it easier to track objects across the sky.  The combination gives the advantage of a large aperture telescope on a mount that can easily be driven to track objects.  Most Dobsonian telescopes use a fairly long focal length tube assembly whereas a shorter focal length is generally favoured for the equatorially mounted Newtonian.

Celestron_Newt_Equat

Celestron C8-N 200mm FL 1000 Equatorial Newtonian

Models in this range:

Celestron C8N           200mm                  (8″)         FL 1000mm   £299

Celestron C10-N        254mm                  (10″)       FL 1200mm   £639

Most manufacturers listed previously have a similar range.

The small GOTO computerised telescopes are very good optically but are notoriously difficult to set up.  They need to be levelled, set to face north, the time and date entered and the location coordinates entered.  The computer then selects two bright stars for the telescope to be aligned on.  Once aligned the computer will automatically find and track thousands of objects stored in its memory.

ETX90PEtripod_1

Meade ETX90 PE £499

These telescopes have an advanced optical system known as a Maksutov-Cassegrain.  This system uses curved mirrors and a lens to extend the effective focal length to an amazing 1250mm.  At 90mm aperture, its light grasp is small for this focal length so deep sky objects are faint and difficult to see.  However it does give beautiful views of the brighter planets and the Moon.

Celestron have a similar model, the Nextstar 4 at £340.

 

Guide to the parts of a typical telescope.

 

TELESCOPE JARGON

ALT AZIMUTH A type of telescope mount that has a vertical axis (Azimuth) and a horizontal axis (Altitude).  Both axes need to be moved to track an object through its apparent arc across the sky.

APERTURE The diameter of the main objective. This normally is used to describe the size of the telescope.

BARLOW A lens unit inserted into the focuser that effectively doubles or trebles the magnification of the eyepiece.

DECLINATION The up / down movement on an equatorial mounting.

DEWSHIELD A tube fitted to the open end of the telescope to help reduce dew forming on the lens.

DOBSONIAN A very simple and cheap telescope mounting usually used with a Newtonian reflecting telescope tube.

DUST COVER The cap that fits over the end of the telescope to protect the lens or mirror. These often have a hole with a removable cap fitted, By just removing the small cap the amount of light entering the telescope can be reduced to lessen the glare when observing the full Moon.

EQUATORIAL MOUNTING A type of telescope mount that has one axis tilted to point at the celestial pole.  By using this design of mount an object being observed can be tracked by moving just one axis.  A must for astro - photography.

EYEPIECE A small microscope fitted to the telescope to magnify the image formed by the main optic.

FINDER A small telescope fitted to the main telescope to help find the object to be observed.  A finder has a wide field of view and low magnification and is often fitted with cross hairs.

FOCAL LENGTH The distance from the objective to the point where the image is formed.

FOCAL RATIO The ratio obtained by dividing the focal length of the objective by its diameter, usually written as f (number).

FOCUSER or FOCUSING UNIT The holder for the eyepiece.  It carries adjusting knobs to make minor adjustments to the position of the eyepiece to give correct focusing.

GOTO This is a computerised drive system fitted to some modern telescopes.  It enables the telescope to automatically find and track thousands of objects in its database.

LIGHT GRASP The amount of light that a telescope can direct into the observers eye (the bigger the better).

MAGNIFICATION The ability to increase the apparent size of the object using different eyepieces.  A short focus eyepiece produces higher magnification, calculated by dividing the focal length of the main optic by the focal length of the eyepiece: 1000mm ÷ 10mm = 100x magnification.

MAKSUTOV A complex design of telescope that, through an optical trick, produces a very long effective focal length in a short tube.  These are very good for planetary observation.

NEWTONIAN A simple reflecting telescope designed by Sir Isaac Newton.

OBJECTIVE The main light gathering optical unit of a telescope, the large lens or mirror.

OTA (Optical Tube Assembly) The Tube of the telescope without the mounting and stand.

REFLECTOR The type of telescope that uses a mirror as its main optic, usually called a ‘Reflecting Telescope’ or ‘Reflector’.

REFRACTOR The type of telescope that uses a lens as its main optic, usually called a ‘Refracting Telescope’ or ‘Refractor’.

RIGHT ASCENSION (RA) The rotational movement on an equatorial.  The equatorial axis that can be tilted to align with the Earth’s pole.

SCT (SCHMIDT CASSEGRAIN TELESCOPE) A complex design of telescope that, through an optical trick, produces a long effective focal length in a short tube.