making astronomy accessible to all
BUYING A TELESCOPE
PART ONE

A word on buying your first telescope....

DON'T BUY IT (...yet)!
The sage advice of a seasoned observer may save you a wasted £100 or two. Take your time to buy your first telescope and take advantage of other people's equipment and expertise before you buy.
Here are a few lessons from my mistaken purchases.....

£14 to open the heavens?

I was captivated by the sight of Jupiter in the winter sky behind my house and so bought my first telescope at a cost of £14 from a local supermarket. The telescope came with promises of over 300X magnification and multiple eyepieces, erecting and Barlow lenses as well as a starfinder. Needless to say the performance of the instrument was well below my expectations. The tripod may as well have been made of rubber for all the support it gave; the focusser flopped about uselessly in the tube when I tried to focus. In use any object or star flew out of my field of vision as soon as the telescope was touched. Nonetheless, I managed to see Jupiter's moons and the rings of Saturn (after much searching) and the Ring Nebula. The frustration of losing the object almost as soon as it became visible drove me to my next purchase.

Ebay, and the £100 scope.

This was the scope to end all scopes; an equatorially mounted 6inch Newtonian reflector, with a full set of quality eyepieces in cool black. It was unbranded, simply sold as an astronomy telescope and made in China. As soon as I clicked the 'commit to buy' button, I was certain that I would be seeing the Apollo 11 landing site with the footprints on the Moon, and producing pictures that would make the Hubble Space telescope team weep. However, the reality was again slightly short of my expectation.
The equatorial mount was a vast improvement, having spent an hour reading the instructions and setting the scope up and finding out where all the bits fitted. I trained it on a telegraph post on the horizon and was slightly surprised to find that not only could I not clearly read the sign on the post, but the surrounding image seemed remarkably dull even in full sunshine.
Undaunted, with my finder scope aligned I set out to polar align my new purchase and view Jupiter. The ease of focus and comparative steadiness of the mount were immediately apparent, but the planet still hopped out of view as soon as the focusser was touched. Also, as I now know by looking through other people's scopes, the optics were not good (as is often the case with cheaper telescopes). Nonetheless some faint impression of Jupiter's belts was seen and the rings of Saturn were a delight. Having found Jupiter and Saturn, my new found enthusiasm plataeued a bit while I searched for new targets and re-read the instructions over and over. Pretty soon, I discovered skymaps.com, I couldn't wait for the Orion Nebula to come up from below the horizon, I even got up at 4 am one morning to have a look at it, and the Andromeda Galaxy (wow - another whole galaxy just out there, if only I could find it). Lunch hours were spent browsing the minority sections of the bookshops. This was to be my ultimate downfall however as I purchased a copy of Patrick Moore's 'Sky at Night' magazine which showed dozen's of galaxies; now there were more than enough targets to focus my telescope on. But there was a problem, try as I might I could not locate a single galaxy! I started to struggle and my fledgling interest would have stopped right there, but for my local astronomical society...

An example of a good Scope! (not a supermarket one):- Skywatcher ST80, a small simple refractor. priced at £110. This scope is not for sale in your supermarket - but it will perform!


A word on limits

With the benefit of hindsight; it is easy to see that I was pursuaded into purchasing items on the basis of their benefits, but without consideration for their limitations. Unless you know a little about telescopes these can be difficult to establish. Even more difficult can be the understanding of what objects you will want to see, and what this implies in terms of the limits of the instrument you will need. A faint galaxy will appear as no more than a tenuous puff of cloud or smoke, unless you know what to look for you may never find it!

Resolution

With a telescope, we refer to the minimum angle that can be seperated, or differentiated as the 'resolution'. In practice, this is the narrowest angle between two stars that a telescope will split into two. Since there are a great number of optical double stars (and multiple star systems) in the Milkyway, this is reasonably easy to check.
This can be expressed mathematically as:
Resolution (in arcseconds) =116/Aperture* (Rayleigh criterion)
Or
Magnification (in number of times) = Aperture* in mm X 2 (approximate Dawes Limit)
*For refractors, this is the diameter of the objective (front) lens, for reflectors this is normally the size of the main mirror.
The sales literature on my original purchases therefore, claiming over 300X magnification (from a 65mm objective lens) were entirely misleading. In practice, most telescopes 3inch or less will show little detail on Jupiter, and only larger features on the moon.

Magnitude

The magnitude of a star is a grading system of star brightness invented in Greek times and modified through the ages. In essence the star Vega was chosen as the reference star (Magnitude 0) and stars graded in size or apparent brightness down to the faintest stars visible to the naked eye (Magnitude 6). The resultant scale is a logarithmic system, which relates brightness to generally accepted levels of 'difference' as observed by the naked eye.
In urban areas, it is rare to see stars dimmer than magnitude 3 or 4. In country areas, magnitude 4 or 5 can be seen with the naked eye. But we can all see magnitude 9 or 10 when aided with a pair of binoculars or a telescope. The increased size of the optical instrument (objective lens diameter or primary mirror diameter) allows us to view darker objects. However, you will need an instrument larger than 6inch (150mm) to be able to see objects darker than magnitude 12.
Galaxies and nebula (clouds of dust and gas in space) tend to have their magnitudes stated in terms of 'total surface brightness' which means that it will be a great deal dimmer in the eyepiece of an instrument than a star or planet of equivalent magnitude. For this reason, you may actually find a galaxy in your binoculars or telescope, but be dissapointed to be unable to see more than a very faint fuzzy patch or cloud.

f numbers and magnification

When browsing telescope adverts, you will frequently come across the f number and focal length in the description. The focal length of a telescope is the distance behind the objective lens (or in front of the primary mirror) that focus is achieved. For most commercially available domestic telescopes this ranges from 400mm to 2m, and it is specified by the manufacturer for each telescope. The f ratio (f number) is simply the focal length divided by the aperture. So a 2m focal length Cassegrain telescope with a 200mm aperture is an f10 telescope. The smaller the f number, the more field of view the telescope offers and therefore the more light will be incident upon the focus. Small f number scopes (f6 and below) are therefore termed fast scopes.
The magnification of a telescope depends upon the eyepiece in use, in this way the magnification can be changed to suit the targets being viewed. The magnification is calculated as the focal length of the telescope divided by the focal length of the eyepiece. So a 2m Cassegrain telescope fitted with a 20mm eyepiece has a magnification of 100X.

This is not a cheap or unknown scope: the Skywatcher 150mm reflector - £280. An additional cost maybe, but a scope that will last a lifetime, and a good standard for portability, ease of use and good optics.


Binoculars

Some first time observers overlook the benefit of a good pair of binoculars. These can open the skies quickly, simply and relatively cheaply. If you consider the relative merits of binoculars and telescopes; binoculars come out on top in all but the most demanding tests. For that reason seasoned observers us binoculars at least as much as their telescope. You will also find that a good pair of binoculars will be far cheaper than a good telescope.

Every year Lidl's sell Bresser binoculars. At less than £30 they are a bargain and excellent starter Bins. But you will need to pay at least double for a decent pair.


Goto Scopes that don't

Once you have viewed Jupiter and Saturn, you may be tempted to purchase a Goto scope which will advertise that it can show you the heavens automatically, with over 2000 items in its catalogue. However, they may neglect to point out that you will need to set up your Goto scope first, and that in order to do that successfully you will need to find and set at least 2 stars chosen at random from its database. If you cannot identify these stars, you will be unable to get your new scope to work!
Take a look outside with a star map before you buy a Goto scope, and make sure you can find Polaris and name about 12 of the most prominent stars to avoid disapointment. There are many mobile phone apps that will allow you to do this (without having to learn anything) or take a look at the MKAS Winter Stars presentation or the BBC's Stargazing Live Star and Moon guide .

What to do with your telescope

The range of uses a telescope can be put to are almost limitless, but most users tend to fall somewhere between two camps:
- Occasional viewers (get it out to look at lunar eclipses, nice clear nights and show M31 to our mates). This group are typically hobbiests who enjoy the 'photonic' connection with the universe around us, and are motivated purely to look at things they haven't seen before.
- Dedicated observers (planetary, solar, stellar or imagers). This group are typically purpose driven, looking for data or advancement in a chosen skill (such as spectroscopy, planetary observing or imaging)
Clearly the first group will not take out a mortgage to buy a custom made diffraction limited astrograph. But the cost of your new found hobby may well be determined by how far towards the second group you are drawn.

A telescope can also be used to project images of the sun, monitor a patch of space for supernovae (nova patrol), monitor sun spots, produce images of space looking for changes (asteroid and comet detection), measure atmospheric disturbance, carry out spectroscopy or extinction and light pollution surveys, variable star photometry etc. The depth to which your new found interest will take you is very much dependent on you, however the primary means for exploring it (aside from the educational institutions that teach the subject) is likely to be your local astronomical society.

Telescope loan

One ideal solution is to join a society that has a small fleet of instruments that they can make available on loan to its members. Mid Kent Astronomical Society is just such a society, if you are interested in borrowing equipment, please speak to the head of instruments and imaging , or look at the equipment for loan section of our website (this area is open only to registered members of the society).

The right way to choose

In order to choose the scope that's right for you you need to do some research and try some of the telescopes you are interested in. This can be difficult to organise through a shop (although some do offer try before you buy services). The easiest way is go along to your local astronomical society's observing sessions and try their scopes. Not only can you see what money can buy (and what you can do with it) but most likely you will find someone who can advise you. But don't neglect to do your homework first, and DON'T BUY IT (yet.......)

PART TWO

How much to spend?

Assuming you read have part 1, and have done some investigation and been to a few observing meetings, you should now be able to answer this question yourself. However, the first item to purchase should be a pair of binoculars. A cheap pair will cost less than £30 a decent pair will cost £100 or more. 10X50's will be ideal; they are likely to come with you to all your observing events but may need replacement eventually (particularly if you sit on them). For a small reliable beginner scope you should allow at least £100 (e.g. Skywatcher Startravel 80mm, dependent upon your planned observing targets), but the scope that meets with your expectations may cost over £1000, read on to narrow down your requirements. The first item you need is your mount, but what mount you choose depends on the telescope you require. Consider first what you want to view, and therefore what resolution you need:-
For general planetary observations, you will need high magnifications (long focal lengths), but not necessarily large optics (since planets are bright). Most telescopes will perform this task. For more demanding planetary viewing (Mars viewing, belts of Jupiter, Saturn's ring divisions) you will need both aperture and magnification. Planetary specialists tend to prefer long focal length refractors for this task.

For star clusters and faint narrow targets, catadioptric designs (Schmidt Cassegrain, Maksutov or Maksutov Cassegrain reflectors) are ideal. They will also double as planetary scopes.

For viewing fainter diffuse objects (Deep Sky - galaxies, nebulae and faint clusters), larger aperture instruments are necessary (over 150mm). Keep in mind that catadioptric designs (cats) lack the contrast of the refractors, but large aperture refractors are heavy and expensive.

For imaging, refractors are usually the optics of choice. This is because a small refractor can be coaxed into revealing faint widefield detail due to a CCD's greater sensitivity than the human eye, but with the penlty of 'halos' around the stars. However; 'cats' will also serve well allowing faster optics (more light) and hence reduced exposure times with the penalty of less contrast. For both types, filters can be used to improve contrast.

The mount

Finally, we arrive at the first piece of kit you must obtain (although it may come with your chosen telescope). If you have used other people's equipment, you will now appreciate that the stability (steadiness) of your mount is crucial. Therefore, having considered the items above you must now translate this into a mount specification.
The typical uses (and main types of mount) are as follows:
Viewing and star hopping

The dobsonian mount for a Newtonian reflector is one of the most common manual viewing platforms. It is simple, unpowered and cheap to manufacture and use. The mount, designed by John Dobson in the 1960's as a simple mount to construct by amateur telescope makers (ATMs) is almost universally used for large aperture Newtonian Reflectors. However Skywatcher and one or two other manufacturers have now produced GoTo dobsonian telescopes.

Planetary or single target

Almost any tracking mount will do, but cheaper scopes will use 'Altitude Azimuth' mounts (Alt Az mounts; these track by moving up and across) which are not suitable for imaging deep sky objects. For longer single target work (viewing planets or faint objects), a German Equatorial Mount (GEM) will be useful. The higher the powers you are viewing at, the greater your stability requirements will be. Generally, you will need to achieve half your maximum resolution as a stability factor (in angular movement). For viewing, this is very difficult to specify, so try this on a borrowed mount with a high-powered eyepiece to give yourself a sense of how the instrument will perform in use.

Deep Sky

Galaxies, nebulae, faint star clusters and most faint fuzzy objects are collectively known as Deep Sky Objects (DSO's). The primary catalogue for these is the Messier Catalogue, which numbers these objects from M1 to M110. Very few of the nebula and galaxies when viewed with higher aperture instruments show more than faint patterning. The brightest and largest in the northern hemisphere - the Andromeda Galaxy (M31), reveals little of its structure even when viewed through a 20 inch dobsonian, although Thomas Grubb thought he saw some feint spirality in M31 when viewing it through Wiliam Parsons' 6 foot aperture reflector. For this reason, a lot of DSO enthusiasts turn to imaging.

Imaging or high stability requirements

If you are intending to purchase a driven mount (one that tracks the object you are viewing) for imaging, the minimum increment (step size) is important, it will need to be under 1/10th the maximum resolution of your telescope or around 1/10th of your field per pixel ratio.
- So for a high quality 6inch reflector (or refractor), with a maximum resolution of 0.8 arcseconds, you will need a mount with step size of 0.08 arcseconds.
- For a camera with a 10MB APS/C sensor (a typical DSLR), working at prime focus on a 621mm focal length telescope the field of view would be approx 2 degrees and field per pixel will be 2 arcseconds. The mount will need to provide 0.2 arcsecond steps to track accurately.
Quite obviously, whatever your mount stands on must be capable of providing a secure and inflexible base for your mount. This is not normally a consideration for a standard tripod, unless the ground is soft and uneven, but for a permanent setup, the pier must be of sufficient stability to support your mount.

Keep in mind that seeing conditions in the UK will limit your opportunities more than your choice of equipment. For conditions to yield less than 2 arcseconds of wobble on a star image would be exceptional for the UK (some say it is fairly exceptional to see a star at all!)

Remember; the best telescope that you ever buy will be the one that you use the most!

Appendix

A list of suppliers

There are a huge variety of manufacturers and distributors of equipment. With the use of the internet, you should be able to find a good local supplier (see our suppliers link pages for ones members have used). Be wary of making serious purchases over the internet from untried suppliers; a good stockist will sort out any problems you have quickly and efficiently, whereas the cost to courier an item out of the country (with insurance) is likely to be over £50 and runs the risk of further damage.
Some experienced members of the society look for a new (different) item first on a secondhand website, such as AstroBuySell, however, you will need to take care with secondhand purchases, as there is nearly always some point of compromise on the performance of the item thus purchased! Most members will have a piece of equipment made by Skywatcher (if not an actual telescope) - they generally produce reliable and cheap equipment tailored to a particular use. Some of their ranges outperform other manufacturer's offerings at twice the price.

For your guidance, a list of well-known good astronomical manufacturer's names is included below so that you can 'recognise' the names in the business.

TypeBestMiddleCheapest
BinocularsFujinon, Swarovski, Cannon, ZeissHelios, Celestron, OpticronPractika, Bresser
TelescopesVixen, Takahashi, Televue, TMBWilliam Optics, Celestron, Moonfish, TAL, Meade, GSO (also sold as AstroTech, AltairAstro and Orion)Skywatcher, Bresser, Revelation
EyepiecesPentax, Takahashi, Televue (Nagler), VixenMeade,Celestron, William Optics, BaaderSkywatcher, Revelation
MountsVixen, ParamountSkywatcher, Celestron, Meade


Many other manufacturers produce equipment, but if you are considering a large purchase; do some research. If however you get an offer that looks too good to be true; take time to evaluate your potential purchase preferably, with someone who has expertise in the area you are interested as the consequences of a poor purchase may last longer than your interest in astronomy. So if you know anyone who would like to buy an old supermarket scope or an ebay special see me, otherwise if buying new equipment these suppliers have all given good service to members:
Ian King Imaging
Telescope House
F1 Optical
Modern Astronomy
First Light Optics

Buying new may cost you a little more than an Ebay deal; but you will usually save more in the longer term and have the security of good aftersales support. Do ring these guys up and ask their advice (and tell them you read about them on this website!) -their advice is free and they are all experienced astronomers.

Further reading

Observing:
Turn Left at Orion: Hundreds of Night Sky Objects to See in a Home Telescope - and How to Find Them
Book by Guy Consolmagno and Dan M. Davis
Probably the best 'quick' guide to observational astronomy written for all ages.

Simple Stargazing
Book by Anton Vamplew a former member, interesting and written in Anton's irrepressible style!

Imaging:

Infiniti Rising
Book by Nik Szymanek
Excellent introduction to the subject and full-page images

Rick Saunders 'Full and proper kit' *Don't click this yet...
free online pdf very detailed - 155 pages, this takes you through all the equipment and processes necessary for an advanced standard of astro-imaging.
(*can take quite a while to download, don't click on this until you have time to wait for it).


EQUIPMENT REVIEWS

Here you will find links to Equipment reviews:
» Sky and Telescope Magazine




This article is by Noel Clark

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