INTRODUCTION

There is no shame in wanting a telescope but knowing absolutely nothing about them. So you haven't the slightest clue. So what? That is what Ebay Guides are for.

These six guides collectively 'blunderbus' the whole telescope buying scene in the least reading and most understandable possible and each should not occupy you for  more than 15 minutes.

They are intended to be practical only for those who know little or nothing about scopes and buying a decent one, 'practical' because only the very basic essential technicalities have been included and the rest have been completely avoided because you don't need to know about those.

I make no apologies for the fact that astronomical telescopes is a huge subject because it combines physics, astronomy and engineering all 'under the same roof'.

 

NOW LET'S GET DOWN TO IT.

After studying this Guide, you will be able to view telescope listings critically.

Those who will not like this Guide are those selling cheap as well as expensive scopes not for the serious observer.

To write just this 1st understandable guide, it will be divided up into several sections:

1      Minimal optics you need to know

2      Types of telescopes and their bits and pieces,

3      What you put them on,

4      What to use to stand all of this on the ground,

5      Things to be careful of,

6      What to ask the seller.

 

1.   OPTICS YOU NEED TO KNOW

Focus the image of the Sun on a sheet of paper and the distance between the lens and the hot spot on the paper is called the focal length. Careful. If you do that the paper will catch fire because you focus light as well as the heat. Lenses as well as curved reflecting mirrors bring light to a focus so both have a focal length designated as 'f' or 'F'.  f = 1000mm means focal length of 1000mm.

The diameter of the lens or mirror is called the aperture. Because telescope lenses and mirrors are circular, they have a surface area exposed to the light they bring to a focus. The bigger the aperture, the more light is focussed.

If you divide the focal length by the aperture using the same units (inches, centimetres, or whatever), you have the stop number, denoted by 'f/'. Photographers know about these because these are the numbers 1.8,  2.8,  3.5,  5.6,  8 and so on marked on camera lenses. f8 or f/8 is a stop number of 8. That is:    focal length divided by aperture = 8.

For a telescope lens or mirror, the higher the stop number, the dimmer the image formed. So, a lens working at f8 will produce a brighter image than one working at f15. Likewise for a telescope mirror.

 

2.   TYPES OF TELESCOPES AND THEIR BITS AND PIECES

The common types are the one that uses a lens up front (the refractor) and the one using a mirror at the bottom of the tube is the reflector (mirrors reflect light). All astronomical telescopes produce an upside down image, and that is normal for them.

Big lenses are more difficult to make than big mirrors, so big mirrors are cheaper. It's unusual to find a refractor with a lens bigger than 6 inches diameter but 6 inch diameter mirrors are very common.

Once the lens or mirror produces its image, you need to hold up a magnifying glass to enlarge it. That is the eyepiece that contains at least two lenses.

Eyepieces have a focal length between 4mm and around 40mm. If you divide the focal length of the mirror (or lens) by the focal length of the eyepiece, you have the magnification.

The eyepiece itself is placed in the focusser being a tube you can rack in and out using gears or slide in and out to focus on the image produced by the lens or mirror. Eyepieces are either 0.96 inches, one and a quarter inches or two inches in their fitting and must be the same as the focussing tube diameter.

The most common reflecting telescope is the 'Newtonian'. It is the simplest and the most reliable for good results. Light comes into the tube, hits the curved mirror at the bottom of the tube and will be brought to a focus. A flat mirror is put in the way to divert the focussed light out of the tube where the focusser is bolted to the outside of the tube near the end where the light comes in.

 

3.   WHAT YOU PUT THEM ON

You put your scope on a mounting of which there are two common types. The one that goes round horizontally as well as up and down is the altitude-azimuth (alt-az) mounting. If you want to follow anything in the sky using this, you have to move it in altitude and azimuth at the same time. If you want to photograph through your scope, you will find that the image in the eyepiece slowly rotates with this sort of mounting and there is nothing much you can do about that.

A variant of this type is called the Dobsonian but still, it is intended for eyeball to eyepiece. There are some fancy computer controls to get these things to track an object in the sky but you will still get eyepiece image rotation and these computer controls are fairly expensive.

The other type is called the equatorial mounting. You line up one axis on the North Pole of the sky and just move the other axis to track what you are looking at or imaging. If set up properly, you have no image rotation in the eyepiece and it 'holds' the image for quite a time, especially if the tracking is done using an electric motor drive and a hand-held controller.

Now you ask 'what's so important about having no image rotation?' Nothing if you are using an eyeball. If you are imaging with a camera, a CCD imager or anything else, rotation during a time exposure blurs the recorded image.

 

4.   WHAT TO USE TO STAND ALL OF THIS ON THE GROUND.

Dobsonian mountings stand on the ground by themselves. Alt-az and equatorial mounts can be fixed to tripods or steel tubes (a pier) with three feet bolted on.

 

5.  WHAT TO BE CAREFUL OF.

The Telescope:

(i)  Refractors

If you see a small diameter (40, 50 or 60mm) lens with a black disc directly in front or behind it, this shows that it is a shoddy piece of work that cannot form a sharp image over its entire surface but only the middle portion can. So the edges are blanked out thus reducing its aperture. But it is still advertised as a 60mm or whatever telescope. If the focal length of the lens is say, 1000mm, a 60mm lens will be working at an f number of 1000 / 60 = 16.67  producing a very dim image that will be made even dimmer by any eyepiece. If that 60mm lens is stopped down to 45mm, you are talking about f22.

(ii)  Eyepieces

Eyepieces of  2 inches and 1 1/4 inches (32mm) barrel diameters are common whilst the 0.96 inch barrel diameter eyepieces are being phased out by most of the big manufacturers. Anything not 2 or 1 1/4 inches is to be avoided and that goes for a telescope whose focusser takes only 0.96 inch occulars. Some of the cheap telescope makers provide their own non-standard diameter eyepieces and you will find it very difficult to use accessories with a standard fitting.

(iii)  Focussers

The focussers of cheap (and sometimes the more expensive) refractors have a lot of slop in them. They are of plastic with plastic gears or plastic and metal gears (the metal chews up the plastic). Rack the focusser in and out and it wiggles its way in and out of the tube. That is completely useless except as a toy for a child. The focusser will not hold the eyepiece exactly parallel with the lens up front. The test is to look at a star. If it is seen as a point, fine. If all the lenses or the mirror and the lenses are not parallel, the star shows a 'tail' or the image is triangular.

(iv)  Reflector

A good reflector normally has a mirror shaped to bring all light that hits it to the same focal point. That shape is a parabola and the mirror is said to be parabolic. Parabolic mirrors are first made spherical and the parabolic shape is produced by hand. Some scopes have spherical mirrors but that is OK provided that the f/ number is no less than 10. If that is the case, the eyepiece will not pick up the imperfections in the image produced by the spherical mirror.

The reflector has a flat mirror immediately beneath the focusser that directs the light producing the image into the eyepiece. That mirror is placed in the centre of the tube and gets in the way of light from falling on the main mirror, but it is never as big as the main mirror. If you are looking at a 3 inch reflector, subtracting the area of the obstruction is likely to reveal that it is only a two and a half inch scope, a negative sales point not to be brought to your attention.

Mirror accuracy is expressed in terms of the wavefront of light. Excellent is one-tenth, very good is one eighth, very useable is one-quarter, barely useable is one-half  to one wavefront and a complete waste of good money is a mirror with an accuracy of more than 1 wavefront of light. Practically, your eye will not distinguish image quality produced by a one-tenth and a one-eighth mirror.

A good quality mirror should be of low-expansion Pyrex glass, not window glass because the reflecting surface should not twist and turn with changing temperatures. For a plate glass finished 10 inch diameter mirror, the actual glass slab (the blank) costs the mirror maker something like £25. If a Pyrex blank were used instead, by itself, that would cost about £100 and a Zerodur glass blank (the lowest expansion glass) would cost around £300 for the same aperture. 

Mounting:

Sloppy bearings are a curse. The alt-az mounting that flops everywhere with screws or bolts that come undone is a nightmare. But if you tighten them too much, nothing turns and the telescope is 'paralysed'. These are cheap and shoddy with a bad design. There are some equatorial mounts using slow motion drives that are so sloppy, it is a wonder that they turn anything. The slop is further increased by those long flexible handles that themselves have play in them. With mountings such as these, you will spend more time with a screwdriver and spanner (wrench) than looking at the sky.

What you put it on:

The best is a pier, but most have a tripod in the kit. The cheaper kits have tripods that flex under the weight of the scope and mounting. The stronger ones are usually of two or three section legs and can still be unstable. In any case, most flimsy ones are not heavy enough to keep upright in a wind. Whilst you are fighting a sloppy focusser, trying to keep the slow motion drive fixed on the object, the rubbish tripod is trying to throw the whole lot on the ground. No wonder many people put their scopes on ebay within a couple of weeks of buying them.

 

What is written in the Ebay listing:

 

THIS IS A MAGNIFICENT AND HUGE PROFESSIONAL OPTICAL INSTRUMENT

Professional astronomers use observatories with reflecting telescopes with a minimum diameter of around 40 inches (1024 cm). The finder for such a scope would be something like an 8 inch refractor. Enough said.

THE OPTICS ARE DIFFRACTION LIMITED

Never mind what 'diffraction limited' actually means. Let me just say that having diffraction limited mirrors is great, marvelous, magnificent and not 'Yuk!!' For new, expect to pay £150 plus for the optics alone depending on aperture starting at 4 1/2 inches (usually £200 ish for the 6 inch mirror). That's just the parabolic and flat mirrors. Spherical mirrors cannot be made diffraction limited and don't let anyone tell you that statement is wrong. Spherical mirrors cannot be made diffraction limited unless you can change the laws of physics.

FULLY GUARANTEED

Against what? Plagues of boils, civil riots or blue ice falling from planes?

WITH A TRIPOD THAT IS AS SOLID AS THE ROCK OF GIBRALTER

How many sections does it have and is it braced across the legs? Stable tripods are of single piece braced legs or two-section legs that adjust for height. Any more sections than this should be suspect for stability unless they are tubular stainless steel with at least 3mm wall thickness (and those will cost you).

MAGNIFICATION UP TO X 600

Please read the note further on.

MADE OF SOLID BRASS WITH BRASS DRAW TUBES AND COVERED IN THE FINEST LEATHER MOUNTED ON AN ORNATELY CARVED BALL AND CLAW STAND

You will enjoy polishing it frequently.

EXPLORE THE GLORY OF THE HEAVENS WITH THIS MAGNIFICENT OPTICAL INSTRUMENT MUCH FAVOURED BY ALL PROFESSIONAL ASTRONOMERS. SO MANY FREE ACCESSORIES YOUR HEAD WILL SPIN. THE TELESCOPE BODY IS FINISHED IN GLOWING WHITE AND RED USING THE LATEST TECHNIQUES DEVELOPED BY NASA FOR TREATING THE SURFACES OF ITS ORBITING CRAFTS. UNSURPRISINGLY, THIS PROFESSIONAL INSTRUMENT WOULD GRACE ANY ROOM IN YOUR HOUSE AND IMPRESS YOUR FRIENDS. GET THE BEST EVER VIEWS OF PLANETS, STARS, STAR CLUSTERS AND OTHER GALAXIES. EXPLORE THE MOON USING OUR FREE MOON MAP THAT SHOWS THE LANDING SITES OF THE MOON MISSIONS AFTER THE FIRST LANDING IN 1969. THIS TELESCOPE AS WELL AS ITS FREE ACCESSORIES IS FULLY GUARANTEED FOR LIFE. YOU WILL NOT BE DISAPPOINTED. BUY ONE NOW WHILST STOCKS LAST BECAUSE THE AVAILABLE STOCK IS RAPIDLY DWINDLING AND IS NOT EXPECTED TO LAST VERY LONG AS THIS MAGNIFICENT INSTRUMENT IS IN SUCH HIGH DEMAND ALL OVER THE WORLD.

Excuse me. Can I ask you a question? Is this a refractor, a reflector or a simple cardboard tube? It would be very nice to know something about it.

 

NOW BE CAREFUL

Refractors benefit from a hood that acts like a lens hood on a camera lens. It is just a tube painted dead black on its inside that screws into the front of the telescope and prevents stray light from hitting the main lens. Usually, the image quality will benefit because the image will be more 'contrasty'.

Reflectors don't need this sort of device because the tube itself does that job (the mirror is at the bottom of the tube).

Very often, you will see photos in listings of cheap efforts that look very impressive because the business end is of a much larger diameter than the rest of the tube.

Now unscrew the hood and the tube that contains the optics is just a narrow aluminium or plastic thing that in itself, would not impress your dog.

Is such a hood worthwhile? It blocks stray light and if long enough, will cut down the amount of dew (water droplets) forming on the lens on damp nights if the length is something like four times the diameter of the lens. Thing is, with the well designed scopes (Celestrons, Meades, etc), the designers compute the hood length to balance field of view with different eyepices against preventing dew formation.

Just think. The diameter of the lens is 60mm. Now screw a tube on the end 500mm long. Look through the eyepiece and you will see a small circle containing the image surrounded by black, which is the out of focus inside of the 500mm tube.

With the cheap efforts, it is more likely that when the scope is made down to a price, someone takes a flying guess concerning the length of the hood with only appearance in mind.

 

6.   WHAT TO ASK THE SELLER.

Unless the listing is clear:

a) what is the clear aperture? You will know the focal length so work out the f/ number. For a small refractor, anything more than 10 - move on. For a reflector, anything more than 10 - move on.

b) what accuracy is the lens or mirror in wavefronts of light? The shopkeeper does not know or there is nothing in the literature? Move on.

c) what is the focusser, plastic or metal? All thin plastic or the seller cannot say. Move on.

d) what eyepieces are supplied and are they one and a quarter inches or two inches? They are 0.96 inches or some odd dimensions - treat it as a toy.

e) what sort of bearings does the mounting have? It uses bolts and screws but no proper bearings that can be adjusted. Know where your tool kit is.

f) are slow motion drives worm and wheels made in metal or are they plastic? Do they have much backlash and play? If they are friction drives - passable if made well. If they are cast aluminium, since when can you cast an accurate gear instead of using a proper gear cutting tool on a lathe?

g) is the tripod braced across all of its legs to make it more rigid? It has 27 sections so it folds up to 4 inches for convenient storage. Leave it in storage. It will be worse than useless. If the scope comes with a table-top tripod, ask the seller if it also comes with a heavy, rigid table to put it on. People who settle for table-top tripods usually end up in the garden on their hands and knees with their heads in an ant nest.

h) are counter-weights supplied with the equatorial mounting? No. The design of this mounting is unique so that counter-weights are not required. It is an alt-az mounting, not a German equatorial. However, it could be a fork mounting and the photograph will show that.

i) is the mirror spherical or parabolic? There is nothing in the literature. It is a cheap, thin spherical mirror.

j) what sort of guarantee is given and do you have to pay postage to and from the other side of the world when you return the scope to the manufacturers? The seller says 'if anything goes wrong, just e-mail me.' And then?

h) does the equatorial mounting make things easy for you by having a polar telescope fitted into it so you can more easily line up on the North Pole of the sky? A polar scope makes life so much easier but if the mounting does not have one, you need to know exactly what to do. Does this scope come with full understandable instructions that are not just a translation from Mandarin into German and then into French and finally, into English? For example: place utmost mount to the ground screw turning frequently at your leisure until right thinking says good.

i)  if this thing is being imported from Plotzk in Tibet, will import duties from a non-EU country be added to the price? So what is the final price and how much postage forms part of that? Is it £60 postage, £40 cutoms duty and £4.99 for the full telescope kit? If you must, holiday in Plotzk and pick one up locally.

j)  is the primary and secondary mirror Pyrex? It is of the highest quality. It is plate glass and will twist and turn at different temperatures. The mirror is made of a circular slab of glass and is curved on one side which supports a very thin layer of reflecting surface coated on it. Unless the slab is 'rock solid' under most conditions, the reflecting surface coated on it will not function properly.

k) Strehl. Please see the 3rd Guide.

A dealer in an ebay shop should be able to answer all of these questions. If he does not know, it may be very convenient for him not to know and you are on your own. A private seller is unlikely to know most of the answers.

You may see listed on ebay a rubbish scope on a worthwhile mount or vice versa. It may be profitable to buy what is on offer and throw something away just to get the other piece.

 

MAGNIFICATION.

Many scopes, especially the cheaper ones with a small aperture are advertised with big magnifications as the big sales feature. Those not in the know think that is great. It is not.

You do not need big magnification because in the English climate you need only 30 times the aperture of your lens or mirror in inches. That is the maximum magnification that the English climate will allow most of the year. On a handful of exceptional nights, 50 times the aperture. Any more is unusable.

In any case, with a floppy tripod and fifth rate mounting, the image is wobbling everywhere and magnifying it even further will just magnify the imperfections in you equipment. That is what magnification does.

For planets, no more than x230 but for star fields, use the smallest magnification your scope can handle. The lower the magnification, the wider the field of view and vice versa.

Please do not go magnification crazy at the cost of everything else. Magnification is the least important thing. Surprisingly, at a magnification of x60, the field of view is so small that you will find it difficult to find the full Moon in your eyepiece even if you carefully line up the scope on the Moon.

If you want to go crazy, go aperture crazy and you won't go wrong. A 10 inch scope is better thn an 8 inch scope which is better than a 6 inch scope and so on, provided that they are all of equal quality. But then, the SECOND GUIDE goes into this more fully.

The English climate is referred to but if you do not live in England, take the English climate as your own and you will not go too far wrong unless you live in a desert or other dark sky location.

I'M NOT BEING ABUSIVE WHEN I SAY PLEASE READ THE MAGNIFICATION PARAGRAPH A SECOND TIME AND GET MAGNIFICATION OUT OF YOUR HEAD.

Having a 60mm to 6 inch scope that is listed with capable magnification more than x250 is like buying an E-type (XKE in the States) Jaguar where the 4.2 litre engine has been replaced with a 1 litre engine. OK if you can live that long to get to 60 mph but any more than that, don't hold your breath.

If you really want a magnification of x750 and see something worthwhile, this is what you need.

1. A 4 mm focal length eyepiece, preferably of Lanthanum glass.

2. A reflector of focal length 3000 mm.

3. A mirror diameter of at least 25 inches, and that will be working at f 4.7 at f = 3000 mm.

4. A heavy mounting, either a custom made fork or German equatorial capable of taking the weight of a 25 inch Newtonian unless it is of Dobsonian design.

5. A thick walled steel tube about 8 to 10 inches diameter and around 6 feet long. At least 4 feet of it should be filled with stones and rubble and then set 5 to 6 feet into a hole in the ground which should be filled with quick drying cement.

Remember that atmospheric clarity will limit your 25 inch to a magnification of 25 x 30 = 750 but on a handful of nights, 25 x 50 = 1250.

It would be better to spend say £200 on a much used 6 inch scope made by a reputable manufacturer than £200 on a brand new 8 inch or even 10 inch scope made by the Hang em High Company Inc. Ltd. based in Conemwell Bay, Antartica. If a scope has had a lot of use, that should tell you that the seller has loved using it. People give up on low quality instruments. Remember that lenses and mirrors do not wear out just because light has been reflected or passed through them. However, surface silvered mirrors do require re-silvering every 8 or so years and that will cost in the region of £50 + depending on size and what needs to be done.

SO WHAT SHOULD YOU BUY? If you are uncertain and wish only to give astronomy a try, buy a pair of  7 x 50 binoculars and a reclinable garden chair. That way, if you don't take to astronomy, you will have a useful optical instrument and a sunbathing device.

 

LET ME INTRODUCE YOU TO MY PRIDE AND JOY. (An introduction to the Second Guide)

Come on in and sit yourself down. I'll make us both a coffee and then I'll introduce you to my pride and joy.

It's a Coulter. I managed to locate one in Florida and had it shipped. Ever since I saw the adverts. in Sky &Telescope in the 80s, I wanted one. Well. I've got the biggest one they ever made. A Dobsonian with a 29 inch mirror. It's fantastic.

1alienx: That's the one with the eyepiece focusser 10 feet off the ground. The whole thing is more than 11 feet tall. You have to use a gantry on wheels or a ladder to get to see what it's pointing at.

Why do you think I've got walking sticks and both legs in plaster?

 

THESE ALWAYS HAD THE REPUTATION FOR BEING GOOD MAKES.  Meade, Celestron, Vixen, Televue, Helios, Tal, Skywatcher and a few others that you can see advertised in Sky & Telescope or the UK magazine Astronomy. Excellent quality parts can be obtained at relatively reasonable prices from Orion Optics, Crewe, Cheshire and Beacon Hill Telescopes of Cleethorpes where Barry Watts will make things to your specifications. Both of them have websites but you will have to phone Barry Watts because he prefers to talk with you regarding your needs.

IF YOU HAVE GRASPED JUST 50% OF WHAT IS IN THIS GUIDE ON THE FIRST READING AND ARE AWARE OF THE OTHER 50%, YOU HAVE ONLY THE ESSENTIAL BASICS FOR LOOKING AT LISTINGS. WITHOUT MORE, YOU SHOULD ONLY LOOK AND NOT BID.

TRUST ME.  YOU SHOULD CONTINUE WITH THE SECOND GUIDE.

Thanks