Telescope choices

Other basics for beginning sky watching include a planisphere (a circular star chart with a mask that rotates
to show the stars on view at any particular time) and a simple star atlas. Planisphere's and star atlases
can be obtained from good book stores or via the internet from many of the websites in the Links page.

A planisphere will cost you about $10 to $20 (perhaps as little as $5 if you purchase one of the smaller ones) and will last a long time if looked after (I still use one I bought 18 years ago). Don't forget to buy a planisphere that is specific to your location (they come supplied for different latitudes). Information about these and other other useful items to consider can be found on the Hints and Tips for beginners and beyond page. Star atlases and maps come in a variety of forms, either as maps alone or as part of a book, and can be obtained from most good book stores or via the internet from many of the websites listed in the Links page. Armed with this and a pair of binoculars, you can learn the basic sky sights with little outlay. And the money will not be wasted - the binoculars will always be useful, even if you eventually graduate to a large telescope.

The smallest telescopes, i.e. those with apertures under 75 mm or 3 inches, are almost always refractors.
Telescopes with larger apertures are usually reflectors or Cassegrain's since large mirrors can be made more
cheaply than lenses. The smallest refractors, of 50 or 60 mm (2 or 2.4 inches) aperture, will show the
Moon's craters and dark lowland 'seas'; (the maria). Saturn's rings, Jupiter's cloud belts and it's four main
satellites, some attractive double stars, and various nebulae and galaxies.

Small or medium - sized reflectors have apertures of 100 mm (4 inches) or larger. They will usually fainter
show fainter objects and finer detail than small refractors.

Although there may be a few quality instruments of 60mm aperture my experience has shown that many are
simply not worth bothering with. They are often badly made, have poor tripods and mountings, come
supplied with poor quality finders and eyepieces, and are all too frequently a great disappointment
to the buyer. It is likely that many budding astronomers have had their enthusiasm and interest in astronomy
crushed by having such an instrument.

Some small telescopes may appear to be a good first buy, but there is one very important warning to issue
here. Many low-budget telescopes are either poorly made and/or make misleading claims as to their
performance. In the worst cases the telescopes may be next to useless for astronomy, even though
the finish may appear good. Unfortunately, some leading stores, camera shops and mail order catalogues
sell telescopes which are of very poor build and/or optical quality. As with any purchase of specialist
equipment, always seek the advice of someone who knows what they are talking about before you part
with your money.

One of the main failings of simple lenses is that they produce fringes of false color around objects,
an effect known as chromatic aberration. A lens which is corrected for such false color is termed
achromatic so it is worth looking for this in the description, although it still does not guarantee good
performance. Mirrors do not suffer from chromatic aberration.

WHAT TO LOOK FOR
If possible, make the following checks before buying a small refractor..

• Look down the tube through the main lens. Is there a stop - a disc with a small hole in it - a short way
  down the tube? (This should not be confused with light baffles, which are a series of rings of decreasing
  diameter positioned at intervals down the tube).
  
  Such a 'stop' is an attempt to sharpen the image, like using a small f-stop on a camera. Unfortunately,
  it drastically reduces the effective aperture, and hence the image brightness. Many small telescopes
  have these 'stops' fitted but they are difficult to see; it is a fair bet that a telescope with stops fitted are
  of inferior quality and should be avoided.
  
  Telescopes of this sort will show you only a little more than you can see with the naked eye. If they
  claim to use the full size of the lens, they are being fraudulently advertised.



• If possible, observe the Moon at night. Its edge should be sharp and free from obvious false color.
  Make sure you observe from outside, and not through a window, closed or open.

If possible, make the following checks before buying a small reflector..

• Look down the tube or part where the eyepieces (often mistakenly referred to as lenses) would
  normally be placed. This part is often referred to as the 'focuser' or 'drawtube'. Is there a stop
  - a disc with hole in it - a short way down this assembly?
  
  Such a stop is often an attempt to sharpen the image due to poorly made mirrors. Many small cheap
  reflecting telescopes have these 'stops' fitted and they may be an indication of poor build quality and
  should be avoided.

If possible, make the following checks on the telescope's mounting..

• Lightly tap or push the telescope up and down and side to side. Is there is any excessive movement or
  play in any of the parts about which the telescope is pivoted?
  
  Many telescopes are completely let down by poorly made mountings. A bad mounting can make
  a telescope difficult, even impossible, to use properly. The slightest movement of the telescope can
  cause it to vibrate badly, in a breeze for example. Also, such play can make aiming the telescope
  accurately a very frustrating process. If there is any excessive play or undue stiffness in any part
  of the mounting the instrument should be avoided.



• Move the telescope about as if you were going to point it at an object. Is the motion smooth, with only
  moderate friction?
  
  If the movement is stiff or jerky the telescope will be difficult to aim. If the movement is too slack
  the problems listed above will arise.
  
  The mounting is the area where many telescopes will fail a test. Quite often I have seen quite
  well made telescopes rendered next to useless because the mount is of poor quality!

If the telescope fails any of these tests, do not buy it, or return it for a full refund.

TELESCOPE TYPES - ADVANTAGES AND DISADVANTAGES
Broadly speaking, telescopes may be divided into three types. The general advantages and disadvantages
of each type are listed below.

REFRACTORS - ADVANTAGES:

• Optics hold their alignment (collimation) well.
• No reflecting surfaces (mirrors) to maintain.
• Image definition and contrast is generally good.
• No central obstruction (as in reflectors).
• Simple eyepieces work well.
• Good for solar, lunar and planetary views.
• Good for resolving close double stars.

REFRACTORS - DISADVANTAGES:

• Only smaller aperture instruments are easily portable.
• Bulkiest type of telescope per inch of aperture.
• More difficult to mount rigidly.
• Not always suitable for photography without filters.
• False color fringes appear with cheaper lens systems.
• The eyepiece is often in an awkward position.
• Costs more per inch of aperture than any other type of telescope.

REFLECTORS (NEWTONIAN) - ADVANTAGES:

• The cheapest instrument per inch of aperture.
• Suitable for visual and photographic use.
• Can be readily purchased in sizes much larger that other types.
• Eyepieces are usually conveniently placed.
• Compact - does not need a tall mount.
• Good choice if faint objects (via a large instrument) are the target.
• Small reflectors are good instruments for the beginner.

REFLECTORS (NEWTONIAN) - DISADVANTAGES:

• Mirror surfaces are prone to deterioration over time.
• The secondary mirror/mount blocks some light and reduces image contrast.
• Mirrors are sensitive to optical alignment, especially if the telescope is frequently moved.

CASSEGRAIN REFLECTORS (SCHMIDT, MAKSUTOV ETC) - ADVANTAGES:

• Compact and portable.
• Eyepiece is seldom, if ever, in an awkward position.
• Mirror coatings are well protected.
• Good for visual and photographic use.
• Usually come equipped with computer controls as standard.
• Competition between makers promotes good quality/price.

CASSEGRAIN REFLECTORS (SCHMIDT, MAKSUTOV ETC) - DISADVANTAGES:

• Larger secondary mirror lowers image contrast.
• Secondary mirror tends to produce a blind spot at low power.
• Large instruments can be very expensive.

MAGNIFICATION
The magnification of a telescope depends on the eyepiece used. (A table detailing the magnification yielded
by different eyepieces appears further down this page.) Telescopes usually come with a selection of eyepieces
that offer low, medium and high powers. Do not get carried away by advertisements for small telescopes that
claim magnifications of many hundreds of times. Too high a magnification will show less rather than more,
since an over-magnified image will be faint and indistinct.

I have seen some small telescopes advertised as being able to magnify up to 400 times when in reality, even
under exceptionally good conditions, the telescope could never deliver a good image at anything
over 200 times magnification. The figures quoted for some telescopes are excessive and very misleading.
So, my general rule of thumb is.. If you see a telescope advertised as able to magnify, say, 300 times.
Halve that figure and you will be closer to the truth!

The best simple guide to the true, usable magnification of an instrument is: twice the aperture in millimeters,
or 50 times for each inch of aperture. (A guide to telescope performance appears further down this page.)
Of course, if the telescope's aperture is stopped down, the maximum usable magnification is
correspondingly reduced.

The atmosphere itself places a limit on the highest magnification you can use, because air currents make
the images of stars and planets unsteady, an effect known as 'seeing'. No matter how large a telescope you
own, from a normal ground-level site the maximum usable magnification will be about 300 to 400 times.
Beyond this, an eyepiece just magnifies the distorting effect of the atmosphere, creating a useless
"boiling" image.

Telescopes, like cameras, have 'f' numbers. The focal length of a telescope is the length of the light path
from the main lens or mirror to the eyepiece. The focal ratio (or f/ratio) of a telescope is it's focal length
divided by its aperture. For example, a telescope of 100 mm aperture with an 800 mm focal length is an
f/8 instrument. A simple guide to the focal ratios of telescopes is listed in the table be low...

TELESCOPE FOCAL RATIOS ('f' number)

The above table does not account for all telescopes, some will fall outside or between the sizes listed above.
Extremely short focal ratios of less than f/3, and long focal ratios above f/20, are seldom encountered.

Focal length (and ratio) is not a critical consideration, but it does determine what objects an instrument is
best suited for observing. For example, f/4 to f/6 telescopes are preferred by people who like to observe
deep-sky objects such as nebulae and galaxies. Such instruments are sometimes referred to as being 'fast'.
However, others who like to view the Moon and planets may opt for f/7, f/10 or more, this being
sometimes considered to be one of the better focal ratios for planetary viewing.

TELESCOPE PERFORMANCE
So, what are different size telescopes actually capable of? The table below lists the parameters of most of the
commonly encountered telescopes available to the amateur astronomer today.

TELESCOPE PERFORMANCE FIGURES

Most of the figures in the above table are self explanatory.Clear aperture is the size of the instrument,
the measurement of its main mirror or objective lens. It is from this figure that all the other parameters
of the telescope are dictated.

The larger the instrument, the more light it gathers and this then affects the faintest stars visible.
Also, the larger the instrument, the greater its ability to resolve small or close together objects, otherwise known
as the telescope's resolving power or 'Dawes limit'. There are practical limitations to this factor so the
practical resolving power is quoted as well.

Many people get misled by 'magnification' or 'power' thinking that more is better. Up to a point this may be true
and in theory any telescope can magnify an object thousands of times but in reality the usable magnification
of an instrument falls far short of this. Again, the size of the instrument is a crucial factor and will dictate
the optimal magnification to be used to look at an object and the maximum magnification that a telescope
is capable of. Anything more than that and image quality will almost certainly start to suffer.

The figures quoted above are for a good quality telescope in excellent condition used when sky conditions
are very good - a set of criteria that is seldom met!

MOUNTINGS
Altazimuth: The simplest type of mounting, used by many small refractors and reflectors, is the altazimuth
design. (See also the description of the Dobsonian mount further down this section.) This requires you
to move the instrument simultaneously about both axes (altitude and azimuth) to keep an object in the field
of view. In essence, the telescope can be moved up and down (altitude, or angle above the horizon - 0°to +90°)
and left and right along the horizon (azimuth, or angle along the horizon - 0° to 360°, North, East, South, West,
and back to North, and vice-versa).

Equatorial: Larger telescopes often incorporate an equatorial mount, which needs to be set up more
carefully with the polar axis pointing to the north celestial pole, near Polaris (assuming that you are
observing from the Northern Hemisphere).

In reality the equatorial mount has 4 axes. Two of these are generally adjusted then fixed before
the telescope is used. These two axes align the mount so that the poleward pointing axis is aligned parallel
to Earth's axis. These 2 axes are in effect the altitude and azimuth axes. The other two axes can then
be used to move the telescope to correspond with Right Ascension and Declination co-ordinates.
An equatorial mount is more expensive and complex but has the advantage that objects can be kept within
the field of view as the Earth rotates by turning the telescope around the poleward pointing axis only.

Dobsonian: In recent years the Dobsonian mount has become increasingly popular as a low-cost portable
alternative to equatorials. It incorporates a modified altazimuth design, and is best suited to reflectors used
with low power eyepieces for wide-angle viewing of the sky, in which precise tracking is not essential.
Despite this last remark some Dobsonians, especially larger ones, are fitted with a driving system and
computer control. This driving system may also be fitted at a later date to the unmodified mount.

This type of mount is usually made from wood. The base of the Dobsonian mount is rather like two stacked
discs: one is in contact with the ground and the other is able to swivel about upon this. This enables
the mount to pivot left and right in azimuth. Upon this moving disc is a 'U' shaped structure which has two
large semi-circular grooves at the top parallel to each other. Two circular discs mounted along the length
of the telescope tube complete the set-up. These two discs on the telescope tube mean that the telescope
can then be placed on the mount, the discs sitting in the two semi-circular grooves on the mount.
This enables the telescope to point up and down in altitude.

The Dobsonian mount has become so popular since the mounting is simple to make (and use), well within
the realms of D.I.Y. skills. Also, the telescope and mount can be quickly assembled together
(and dismantled) making the whole thing easily portable. Not only that but a large reflecting telescope can
be used on a Dobsonian mount and since the azimuth pivot is close to the ground there may be no need
for steps or stools to reach the eyepiece. Also, because the telescope's Centrex of gravity is closer to
the ground the 'footprint' of the base can be smaller than that of a tripod mounted telescope.

Such is the popularity of this mount that many telescopes are sold simply as 'Dobsonians' but that is to say
that they are actually reflectors (usually) using a Dobsonian mount. The Dobsonian takes its name from
the inventor, John Dobson.

GOTO: A most recent addition has been the advent of the GOTO mount/telescope.
Many of these telescopes are becoming more affordable and available. They are popular in that they are,
in effect computer controlled. In its simplest form the GOTO is a telescope (usually a Cassegrain)
integrated with a hi-tech Dobsonian mount. A handheld computer control unit contains a database and
simple up/down, left/right control buttons (plus a few other buttons) and digital readout.

A simple process of aligning the telescope with two stars can then be made to tell the control unit how
to control the telescope so that it can track an object or find one - polar aligning, 'GOTO', and find -
without too much fuss! GOTO telescopes are sold with the ability to be tripod or pillar mounted, or they
can simply be placed on any steady, flat surface. The portability and versatility of such an instrument is
fairly obvious.

Mountings, warnings: Department store and mail-order-catalogue telescopes often employ notoriously
unstable, badly made and clumsy desktop tripod mountings. There is no point in buying a telescope
with a shaky mounting, as you will be unable to see anything properly, particularly when it is windy outside.
Also, remember that comfort and ease of use are vital. You will not enjoy using a telescope if you have to
kneel down and crane your neck round to look through the telescope.

It must be said that in recent years the quality of some telescope mountings have improved markedly but
there are still a lot of poorly made tripods and mountings out there. Regrettably, I have seen several
telescopes that in themselves were well made and fairly good value for money - only to be completely
let down by their tripods and mountings which were next to useless.

Most small refractors have so-called 'slow motions', which are gears, often with flexible cables, linked to
the axes. They allow you to follow objects by turning knobs. Beware of stiff slow motions which are more
trouble than they are worth. The more expensive mounts have motor drives, which track an object without
any effort. These are particularly useful when observing planets.

EYEPIECES
Eyepieces are the most important accessories you will buy. Regardless of how good a telescope's lens or mirror
may be, its performance will be seriously degraded if the eyepieces are of poor quality. Small telescopes, mostly
60mm refractors, but others as well, frequently come supplied with poor quality eyepieces. These can be replaced
later, at a cost of course, but it is worth considering the eyepiece quality before you part with your money.
Many small telescopes come supplied with high power eyepieces which are unusable because the power offered
by them exceeds the ability of the telescope. An eyepiece frequently sold with many small and cheap telescopes is
a 4mm - all but useless in practice - but it does make a telescope package seem more attractive!

Eyepieces are interchangeable, and provide different magnifying powers, determined by their focal length.
They are marked with numbers such as 25 mm, which indicate their focal length. The longer the focal
length, the lower the magnification but (generally) the larger the field of view. Lower powers are best
for observing faint, diffuse objects such as comets, nebulae and galaxies, while higher powers are best
for the Moon, planets and double stars.

The magnifying power of an eyepiece can be found by dividing its focal length into the focal length of
the telescope. Hence a given eyepiece will provide higher magnification on a telescope of long focal length
than one of short focal length.

Below is a table which gives you a guide to the magnification yielded by different eyepieces when used
in conjunction with telescopes of different focal length. Not all the available focal lengths of telescopes
or eyepieces are shown here but the table does list most of the common sizes encountered with amateur
sized instrument s...

There are many designs of eyepiece available to day. The cheapest acceptable eyepiece for amateurs is
the Kellner design; it offers a useful field of view, spanning up to 45°. The designs known as Orthoscopic,
Plossl and Erfle are progressively more expensive alternatives, offering better optical performance or
different qualities - at a price!

Barlow lenses are a common addition to any astronomer's collection of eyepieces. These are used
in conjunction with another eyepiece - the eyepiece fitting into the Barlow lens - which is then fitted to
the telescope. The most common type of Barlow lens, the '2 x', halves the focal length of a given eyepiece
to magnify the image by a factor of two. So, using a 40mm eyepiece with a 2 x Barlow lens gives you
a 20mm eyepiece. The next most common type of Barlow lens is the '3 x', this reduces the focal length of
a given eyepiece by a third, effectively tripling the magnification of an eyepiece. More expensive are
'zoom' or 'variable' Barlow lenses which usually vary in the range from 2 times to 3 times.

People looking through an astronomical telescope for the first time are usually surprised to find that
the image is upside down. This is a basic characteristic of a telescope, and for daytime use is corrected
by using extra lenses or what is termed an erecting prism. It makes little difference which way up
an astronomical body appears, so astronomers usually accept the basic upside down view.

Some small refractors are provided with a star diagonal (an encased prism) to turn the image the right way
up, but left and right are reversed. Bear this in mind when looking at the Moon and planets.

Don't blame your eyepieces for a poor image if your telescope is simply pointing out of a window;
the difference in air temperature inside the room and outside causes turbulence, which distorts the starlight.

FINDERS
A telescope should have a small finder scope attached to its main tube. This is a low-magnification telescope
used for aiming the main instrument. A typical finder has a magnification of 6 and an aperture of 30 mm,
described as 6 x 30, although larger finders often come as standard or are available as an option.

The cheaper instruments often have 5 x 24 finders, which invariably have stops in them restricting
the working aperture to about 10 mm. They will help locate the brightest objects, but little else.

CHECK YOUR TELESCOPE
There are a number of simple tests you can perform on equipment to test its quality. Tap the tube
lightly. The image should take no more than three to five seconds to stop shaking. Some people
hang a weight from the Centrex of the tripod to improve its stability and to damp down vibrations.

The only easy test for optical quality is to observe a star image after allowing the telescope to reach
outside temperature, which may take an hour, and on a night when the seeing is excellent.

Using the maximum usable power (see the telescope performance table above), examine the image
of a moderately bright star. When perfectly focused it should be a point or very small disc of light
surrounded by faint rings. In poor seeing this appearance may is to achieve with even a good telescope.

Defocus the image on both sides of the focus point. In both cases it should become an evenly illuminated
and circular disc, although in the case of a reflector you will see the shadow of the secondary mirror
in the middle. If it is impossible to achieve perfect focus, and the image appears as a short line on either
side of focus, the telescope is suffering from the optical defect called astigmatism.

COST
So what's all this going to cost you? A decent pair of 8 x 40 binoculars can be purchased for about £50,
but can be as much as £90. A reasonable 80 mm refractor will set you back about £150 to £200, while
a good 4 inch reflector can cost as little as £250. Remember that a telescope is a precision optical
instrument and so you should be prepared to pay as much as for a good camera. Moneywise, for larger
and more complex instruments, well, the sky's the limit!

LEGAL ISSUES
If you use a credit card to purchase equipment worth $200 or more, on which the workmanship is shoddy or
the goods are damaged in the post, you can try to resolve the matter with the retailer, but if that fails write to both
your local consumers association office and your credit card company.

If you feel you have been misled in any way, or that the specification of the telescope has been falsely
described, you should complain to your local trading standards office. Indeed, it is your duty to do so.

AND THE BEST TELESCOPE IS...
So what is the best telescope for you? The simple answer is: it's the one that you think you'll use the most
often. Ease of set-up, simplicity of use and portability should be key factors in your decision. Move up to
larger equipment only when you have proved to yourself that you can get out to observe on a regular basis.
Also, make contact with your local astronomical society who may be able to give you advice on equipment
and its use. Their address should be available from your local library or the internet.

A SAFETY WARNING
Many telescopes are supplied with eyepiece filters. They soften the glare of the Moon, Sun and the brighter
planets. Unfortunately, they are often of low quality, and the solar filters in particular can be extremely
dangerous. They can easily crack under the focused heat of the Sun, and should be avoided at all costs.
Remember it only takes a fraction of a second of unfiltered sunlight to cause permanent blindness. The
only completely safe way to observe the Sun is to project its image onto a white card. Never observe it
through the main telescope or the finder, even if it has a filter.

BINOCULARS - A GOOD FIRST CHOICE
There are many beautiful sights, such as the star fields of the Milky Way, star clusters such as the Pleiades
and the Hyades, and ghostly comets, which can only be truly appreciated in low-power, wide-field
binoculars. These are compact, easy to use pieces of equipment, and many observers use them
to complement their telescopic observations.

Binoculars are usually marked with figures such as 8 x 40, 7 x 50 or 10 x 50. The first figure is
the magnification, and the second is the aperture of the front lenses in millimeters. For general observing,
7 x 50 or 10 x 50 binoculars are equally useful. If you find the weight of 50mm binoculars a problem,
go for 40mm or even 30mm models.

Avoid binoculars with magnifications greater than 12, which will be difficult to hold steady, unless you have
need for a specialist pair. Zoom binoculars should be avoided too, as they generally have narrow
fields of view and poor optics. Good binoculars will have colored coatings on the optics, similar to
non-reflective coatings on eye glasses, which improve the image brightness by increasing the transmission
of light.

Some cheap mail-order binoculars economize by using prisms that are too small, which severely limit
the field of view. Look into the front lens; you should see a small circle of light through them. If the circle
is cut off or square, you are losing light. Also be aware that very cheap models may have spurious bulges
in the barrels to give the impression that they contain prisms when they do not. Such instruments are simply
opera glasses with a straight-through optical system, having a very restricted magnification and field of view.

Similar information about binoculars can be found on the Hint and Tips for beginners page.

ABOUT THIS DOCUMENT
With so many instruments available it is difficult for a newcomer to astronomy to make an informed choice. It is hoped that
this document will help the reader decide what will be the best instrument for them to buy. Whatever is purchased will always
be a compromise since it is impossible for a single instrument to have the benefits of another equally useful piece of equipment.
There are many reputable outlets for astronomical equipment, some of which may be found by exploring some of the links
on this page.

This document was adopted from a leaflet distributed during the 1996 UK National Astronomy Week,
c/o the Jodrell Bank Science Centrex, Macclesfield, Cheshire, UK. It was written by Steve Tidey of the Association for
Astronomy Education. Additional and updated material was written by Derek Haselden. Please feel free to copy and
distribute copies of this document but please acknowledge the authors. Also, please do not alter the content without the
permission of the authors.