NO APOLOGY FOR STARTING WITH THE BASICS. WITHOUT THESE YOU ARE GOING NOWHERE, SO THERE HAS TO BE A TIME WHEN YOU NEED TO GET FAMILIAR WITH FOCAL LENGTHS, F/RATIOS AND THE REST  THAT WILL BE KEPT TO THE BAREST MINIMUM JUST TO GET YOU STARTED.

YOU NEED THESE BASICS TO TAKE A CRITICAL VIEW OF WHAT IS IN THE LISTINGS FOR TELESCOPES THAT ATTRACT YOUR ATTENTION, SO AGAIN, NO APOLOGIES.

I AM GOING TO ASSUME THAT THE ONLY THING YOU KNOW ABOUT TELESCOPES IS THAT YOU SEE THINGS BIGGER WITH THEM, AND THAT IS ALL.

THE ONLY THING YOU NEED TO BE ABLE TO DO IS USE A CALCULATOR TO ADD UP, TAKE AWAY, DIVIDE AND MULTIPLY, BUT NOT NOW.

After studying this, you will be able to view Ebay telescope listings critically. It is designed to instill in you the need for critical examination rather than looking at photographs in the listing and  being overwhelmed by appearance.

Necessarily, some attitudes taken are generalized but these will be examined closer in subsequent parts for a more accurate approach.

This part gets you started, and that is all.

To write this so that it is understandable and not full of technical gobbledegook, 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,

1.    OPTICS YOU NEED TO KNOW

With a pair of old spectacles, 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.

(For the more technically minded: that is as good as saying that parallel rays of light from infinity converge to the focal point and the distance between the focal point and the centre of the spectacle lens along the optical axis is the focal length).

Careful. If you do that the paper will catch fire because you focus heat. Why that happens will come later when observing the Sun is considered in another Part.  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. It is the distance between the lens and the hot spot being the image of the Sun.

The diameter of the lens or mirror is called the APERTURE. Because telescope lenses and mirrors are circular, they have a circular surface area exposed to the light they bring to a focus. The bigger the aperture, the more light is focused. The area of the 'light grabbing surface' really matters and being circular, its area is something x its radius x its radius again.

Let’s get this one straight from the start but without the real maths. Let the radius of the main mirror be 1. The area of it will be something multiplied by 1 x 1 =say, 1. If the radius is 2, the light gathering ability (because it depends on the area) is the same something x 2 x 2 = say, 4. For a radius of 3, the light gathering is that same something x 3 x 3 = say, 9.  So a mirror of radius 3 gathers 9 times the light of a mirror of radius 1, and it does. That extra light gathering capacity is 300%

If you divide the focal length by the aperture using the same units (inches, centimeters, or whatever), you have the STOP NUMBER, denoted by 'f/'. These are the same as 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. The f/ number is some indication of the brightness of the image formed.

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.

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 (remember it by ‘mirrors reflect [reflector] 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. Some sellers call eyepieces 'occulars' or just 'lenses', but it means the same.

The eyepiece itself is placed in the FOCUSER 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 focusing 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 (THE FLAT) is put in the way to divert the focused light out of the tube where the focuser 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. That looks like the letter 'T'. The telescope is on one arm of the 'T' that rotates whilst the other part of the 'T' points at the North Pole of the sky and can also rotate. 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 itself:

(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. Just be careful here. Image brightness produced by an f/12 lens is not half the brightness of the image from an f/6 lens. It is a lot less then half.

(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 focuser takes only 0.96 inch oculars. 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)  Focusers

The focusers 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 focuser 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 focuser 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.

If the focusser is made of neoprene, that is one of the hardest plastics and is used for vehicle suspension bushes. Even so, a neoprene gear rotating on a metal gear will eventually get mashed. Suspension bushes made of neoprene do not rotate.

(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 focuser 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 aperture 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 (NOT  the wavelength of light). Excellent is one-tenth, very good is one eighth,  useable is one-quarter, not useable is one-half  to one wavefront and a complete waste of good money is a mirror with a so-called 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 but your camera will.

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 'paralyzed'. 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 focuser, 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.

HERE IS THE LISTING YOU SHOULD LAUGH AT AND THEN MOVE ON TO SOMETHING WORTHWHILE (LIKE MAKING A HOT DRINK FOR YOURSELF) IF YOU ARE SERIOUS ABOUT BUYING A TELESCOPE.

THE WONDERFUL OPTICS IN THIS TELESCOPE OF IMPRESSIVE SPECIFICATIONS COMPRISE A SURFACE SILVERED PRIMARY MIRROR 129 mm IN DIAMETER WITH A FOCAL LENGTH OF 516mm GIVING f/4 FOR THE BRIGHTEST IMAGES. THIS SPHERICAL MIRROR IS MADE FROM HIGH QUALITY FLOAT GLASS AND IS 12 mm IN THICKNESS. BEING THIN, IT COOLS TO THE OUTSIDE TEMPERATURE QUICKLY. TWO EYEPIECES ARE INCLUDED AND THE MOUNTING IS A STRONG ALT-AZ STRUCTURE SUPPORTED ON A FOUR SECTION TRIPOD FOR MAXIMUM RIGIDITY.

If you have got this far, it's likely that you have something on your mind that has not been answered. That is, what shall I buy and what is good? What should I avoid and for a good one, what should I pay for it?

All I can say at this point is press on with the Guides. Your questions will be answered.

Part Two deals with questions you should be asking the seller.

As an aside, if anyone is interested and has some extensive knowledge of cosmology, I'd certainly appreciate some feedback on five consecutive papers published on the web dealing with a modification to the M-theory based creation theories. You will find it by putting in the following site into the very top box on your Google browser that I can't set it out fully here because that would break Ebay rules.

It is http:// three w's/angmalta.net/clients/alan/creationfivepapers/