One example for now on how to apply the ratings, and there will be more.

Listing number 1:   

Welcome to my sale of a magnificient and huge semi-professional reflecting telescope. Its a mirror telescope and the details of the superb optics are:

5 1/2 inch parabolic mirror ground to an accuracy of 1 light wave. Thickness of 1/2 inch at its centre and a 0.200 strehl.

This mirror shows superb definition of over a million stars and because it has a focal length of  1300mm, magnification using a x2 Barlow lens is over x500.

The scope sits on an alt-az mounting on a robust 3 section tripod that makes for absolute rigidity.

I have had this scope for more than 5 years and have enjoyed it immensely, but now is the time to upgrade.

Included are 3 eyepieces, a Plossl (10mm), a Huyghens (20mm) and a Ramsden (15mm).

Analysis using the star rating and some things set out in all the Guides to find out how to assess.

No makers name attracts      0*

5 1/2 inches aperture             7*

parabolic mirror                    5*

1 wavefront accuracy         - 35*

1/2 thick:-  

The thickness ratio is 5 1/2 divided by 1/2 = 1:11       6*

1300mm focal length:-

5 1/2 inches = 140mm.

f/ = 1300 divided by 140 = f/9       10*

0.200 strehl                       - 125*

x500 magnification:-

5.5 x 30 = 165.                     0*

alt-az  mounting                 5*

3 section tripod                  0*

Eyepieces:-

Plossl                        10*

Huyghens                   0*     10 +2 divided by 3 averages to 4*

Ramsden                    2*

The maximum magnification with the shortest focal length eyepiece supplied AND using the Barlow is 1300 divided by 10 then multiply by 2 = x260. So what is this guy talking about?              0*

The total star rating is minus 123*.

The number of things considered is 12.

Average overall star rating is minus 123 divided by 12 = minus 10*

'Magnificent, semi-professional'. Deduct 2* = -12*.

Over a million stars. Deduct 1* = -13*

Final rating = minus 13*

Verdict:   Best thing to do with it is point it at the Sun and let it burn. 

ONCE YOU HAVE GONE THROUGH PART FIVE TO STAR RATE WHAT INTERESTS YOU, USE THIS PART TO HELP YOU DECIDE WHAT YOU ARE WILLING TO PAY FOR IT.

How much to bid depends on so many factors but if you are a beginner, an indication of prices is better than you guessing what these things are worth. Here, you will find descriptions of scopes as though they were new but the prices are for them in overall good and used condition.

If you look at all the scopes on Ebay worldwide, there is going to be more than 2000 items of differing types, qualities and makes. You may think the task of organising your thoughts on these is impossible. It is not.

Scopes will be categorised as

top of the range,

very good scopes and mountings,

average types,

poor instruments that look good (appearance) and

'toys'.

TOP of the RANGE (Star Rating = 9.5 to 10*)

The top of the range big aperture scopes and their mountings will be ignored, such as APO Vixens, APO Tele Vue, Astro-Physics and similar quality reflectors that cost in excess of £2000 used.

Nor will  all the Celestrons, Meades and Skywatchers (Helios) and similar be considered. These fall into the category of very good scopes except for any models in those ranges that are made for department store sales.

On Ebay, these will be few and far between. If you see any, don't be surprised if the starting bid is put at £1000 or more depending on age, with a reserve.

With the top of the range telescopes, the manufacturing process is such that each one produced is tested by hand and not allowed out of the factory unless it meets the manufacturers minimum tolerances. With the average scopes, these are mass produced and very infrequently, there will be a 'freak' that is excellent in every respect with performance with a good eyepiece that rivals a scope costing much more. No-one knows why, but please don't bank on getting one.

What are considered are all those refractors and Newtonian reflectors costing up to around £200 brand new, but only as regards the prices for them in used and good condition. Please bear in mind that what follows is not inscribed with fire on tablets of stone.

'TOYS'. (No Star Rating)

Refractors

40mm, 50mm and 60mm f/10 (or greater) refractors with or without stopped down main lenses on alt-az mountings with three or four section tripod. They have 0.96 inch fitting eyepieces that is their identification 'mark' as 'toys'. Advertised brand new with magnifications up to x750. Come with Barlow lens, star diagonal and two or three low quality eyepieces. Usually in a plastic moulded case with a carrying handle or moulded polystyrene inards to a cardboard box plus star maps.

For the person who wishes to be serious about observations, move on. As a present for an 8 to 12 year old, in used and good condition, about £12 to £14 (that's generous) with postage at about £6.

Such scopes are also sold (some with 1 1/4 inch focussers) by chain, department and catalogue stores as well as some  photographic retailers

Reflectors

Mirror diameter 60mm to 80mm and f/10 (or greater). Comes with all the accessories, a largish central obstruction that cuts down the already small aperture, enormous magnification and mounting all as above. Not for the serious observer but as a present for a young person, about £14 to £15 (that's generous) and £6 postage.

Non-Ebay sellers are also those as mentioned above.

POOR SCOPES THAT LOOK GOOD (Star Rating in the minus stars to plus 2 stars)

Refractors

Usually, the listing for the brand new item will concentrate on what you will see through it, the appearance of the scope and generally, will give only the minimum information about the optics and nothing about optical quality. Aperture will be up to 90mm  f/8 (or greater) described as huge, enormous, professional and the like with x500+ magnifications. The main lens is likely to be a very cheap cemented achromat that is either not corrected, severely under corrected or hugely over corrected. The mounting is usually an alt-az or a very flimsy German equatorial on a wobbly tripod. Plenty of free Moon maps, charts of the solar system and similar.

The serious observer should give this type a miss. Used in good condition, about £18-£20 and £8 postage.

Reflectors

Aperture is up to 114mm and described in the same way in the listing for the new item as huge, professional, etc. The mirror will be spherical and thin. As to prices, much the same as the refractor in this category.

With these scopes, the finish is good because that is what production costs have been spent on.

AVERAGE SCOPES (Star Rating = 2.5* to 6*)

Listing for new items in this category will likely give you some information on the optics. Some will mention wavefront, perhaps the strehl ratio and the mirror thickness ratio. Sometimes, the seller does not know the difference between these and a full description of the optics is something like:

F1000mm,   f1:10,   f10-0.5,   f/114/f0.89.  A suggested translation of that is 1000mm focal length, 1:10 thickness ratio, focal ratio 10, 0.5 wavefront, 114mm diameter and a strehl ratio of 0.89. Often, all the essentials will not be stated.

In these listings for new scopes, there is likely to be a load of hot air, sales blurb and drum banging but if the optics are of average quality, that doesn't matter.

Emphasis is on the accessories that come with the scope and optional accessories at a knock-down price and/or free postage if you buy those at the same time as the main scope.

For all of these scopes, star charts, maps of the Moon and wall posters are usually included free of charge.

As an indication concerning equatorial mountings, many sellers refer to EQ numbers. Bear in mind that such a mounting has a load capacity that should not be exceeded and, say, an EQ3 mounting from one manufacturer may not be the same as, say, a Skywatcher EQ3 mounting and is likely to be inferior.

PRICES FOR USED AVERAGE SCOPES

Refractors

Here, you will find apertures around 76mm to 90mm working at f/10 or more, but there are few, mainly because it is cheaper to produce reflectors with some quality to them. If you come across anything like this, about £45 to £55 and postage around £15.

Reflectors

Apertures of 4 inches to 8 inches   Mounting should be equatorial with or with the option of electric drives and polar finding telescope. Used and in good condition only.

4 inch to 5 inches  ---

 £40 to £55   

cost of re-aluminizing mirrors, about £40 if scope is more than 4 years old

5 inch to 6 inches  --- 

£55 to £65    

same sort of re-aluminising cost

6 inch  --- 

£65 to £85 

re-aluminising cost about £65                

8 inch  --- 

£95 to £115

re-aluminising cost about  £80.

[The reason for 4 years as against 8 to 15 years mentioned elsewhere is because the quality of aluminising is consistent with a telescope kit made down to a price].

THE PRICE YOU SHOULD BE WILLING TO PAY

TAKE THE MIDDLE OF THE PRICE RANGE THAT IS APPROPRIATE TO WHAT INTERESTS YOU.

GO THROUGH THE PART FIVE RATING CHECK.

FOR THESE STAR RATINGS REDUCE THE PRICE BY THE PERCENTAGES SHOWN:-

MINUS STAR RATING :      100% REDUCTION AND POSTAGE PAID BY SELLER or SHOPKEEPER PAYS YOU TO TAKE IT AWAY. YOU WANT IT ONLY BECAUSE YOU COLLECT JUNK.

0 TO 3 STARS:      85% REDUCTION if that is what you want. The reason is that with such low quality, the private seller will be glad to see the back of it and a non-private seller will be delighted to sell one.

3.1 TO 6 STARS:        3.1 end of the range 75% REDUCTION

At this end of the star rating range, you will be cutting someone’s losses if you buy it.

6.0  end of the range     25% REDUCTION

the reason being that whilst not the best of scopes, it is not awful.

6.1 TO 8 STARS:         6.1 end 20% REDUCTION

Again, moving into better quality instruments is indicated by a lesser reduction.

8 end 10% REDUCTION

You are now nearly into good scopes.

8.1 TO 10 STARS:          0% REDUCTION

PART SEVEN STARTS OFF WITH EXAMPLES OF WHAT YOU HAVE READ HERE.