YOU HAVE HAD A NICE AND WORKING TELESCOPE AND MOUNTING FOR SOME TIME BUT NOW FANCY ANOTHER WITH A BIGGER APERTURE. WHAT SHOULD YOU BE CONSIDERING?

YOU SHOULD NOT BE CONSIDERING A SCOPE AND MOUNTING OF LESSER QUALITY THAN YOU ALREADY HAVE.

1.     MOUNTING.

Once you consider a larger aperture scope, you should also ask yourself whether your mounting is suitable for it. That is because there is little point in replacing say, a 114mm reflector on an EQ2 equatorial head by an 8 inch reflector on the same head. The weight limit for the EQ2 will be exceeded and you will find that the EQ2 creaks and groans under the strain of the larger scope AND the heavier counterbalance weights. The extra load on its bearings will do it no good at all AND the Declination shaft will probably bend under the strain of the extra counterweights..

As an indication of which equatorial head to what size scope (having Skywatcher heads in mind):

Refractors of aperture

80mm       EQ2,

90mm       EQ2 but EQ3 better

4 inches    EQ4

5 inches    EQ5 or EQ6

6 inches    EQ6 (about £800 new)

Reflectors of aperture

114mm     EQ3

6 inches    EQ4 but EQ5 better

8 inches    EQ5

10 inches  EQ6

12+ inches    EQ6 too small unless the scope is a CAT and weighs no more than 20kg (44 lbs)  - or - the scope is an open truss type weighing a maximum of 20kg (mirror cell unit has metal tubes joined to it to hold the focuser unit with spider in the correct position. You then cover it with thin black fabric to stop stray light).

2.     UPGRADING (TO) A REFRACTOR .

This is easy. All you do is buy another scope of reputable quality with the aperture you want. If that one takes the same fitting eyepieces as you already have, you need not have the star diagonal that comes with it  (if you already have a good one) or any more eyepieces  unless the price for the scope and extras is the same for the scope without extras.

Do make sure that the objective lens is fully corrected and that lens curvatures are accurate to at least 1/4 wavefront. Check that the rack and pinion focuser is not sloppy and worn if it is a used scope. The minimum aperture you should have is a 3 inches diameter main lens.

3.     UPGRADING (TO) A REFLECTOR.

a) Newtonian.

i)      The tube.  

Personally, I would completely avoid a metal tube because it does not damp vibrations quickly enough and the image will 'dance' even in a slight wind. That can be overcome by thin soundproofing material stuck to the inside. Steel is far too heavy so aluminium is used which is soft and easily dents. Fibre glass tubes tend to be too heavy when made sufficiently rigid (like 4mm wall thickness – I’ve done it, cut sections out to ventilate the tube and then thrown it away). All in all, pvc is my choice. It is readily available, easily worked with, almost acoustically dead and strong.

Later, tube air currents will be mentioned. Such air currents tend to follow the inside wall of the tube. To attempt to keep turbulent air out of the light path, the tube diameter should be bigger than the mirror diameter.

For example, mirror diameters followed by minimum suggested tube diameters are:

6 inch             8 inches

8 inches        10 inches

10 inches      12 to 13 inches

12 inches      15 to 16 inches

unless the tube is an open truss-type. For anything larger than a 6 inch, consider fixing below the mirror cell one or more 4 inch square 12 volt electric fans used to cool your pc, or, more than one fan of smaller dimensions. Those can be run to ventilate the mirror and then turned off when observing. There are plenty listed on Ebay.

ii)     Eyepiece focuser.

To keep the centre of gravity as near to the axis of the tube as possible, one of the low profile focussers is best. What you should avoid is a reflector with a short tube and very long focuser. Apart from optical disadvantages, you will not find it easy to adjust the counterbalance weights on the Dec shaft of the equatorial mount to prevent the scope from falling to one side when the Dec and polar axis clamp screws are slack. Rack and pinion of good quality or helical focussing are both fine. So is a low profile Crayford, it is a matter of preference. The focuser should be a 2 inch with a 1 1/4 inch reducer.

iii)   Mirror Cell.

This is a cradle for the main mirror. It bolts by three bolts from the outside of the tube into three threaded lugs of the cell itself. Three hand screws can be adjusted from the outside of the scope to collimate the main mirror with the flat mirror near the top of the tube.

How many points flotation? Mirror cells support the flat rear of the mirror. The cheaper versions are where the rear of the mirror sits on a flat piece of metal with no air space between the metal plate and the rear of the mirror. The mirror is unventilated and will take a long time to cool to the outside temperature. The better cells have 3 or 9 points that support the mirror and provide space where air can circulate.

If you are thinking about a 6 inch mirror, a 3 point flotation will be fine. Any larger than 6 inches, my preference would be a 9 point cell that supports the mirror as much as possible. However, if the mirror is say, 9 inches and has a thickness ratio of around 1:7, you will get away with a 3 point cell. If the thickness ratio is around 1: 12, definitely a 9 point cell. Barry will make you a mirror cell to any specifications you want.

iv)   Spider.

The spider consists of a number of thin steel strips (vanes) that are usually spot welded to a central mechanism in the centre of the tube that holds the flat mirror with three adjusting screws for collimation. The other ends of the vanes bolt through the outside of the tube Now let us talk about the vanes.

If you look at photos of star fields taken through a telescope, the brighter stars look like a Walt Disney portrayal of stars in that there are a number of spikes jutting out of them. The number of spikes you see directly relates to the number of vanes holding the flat mirror. That is called a diffraction pattern and I am not going into the technicalities of that. If interested in diffraction, please consult a good physics book.

Single Vane attached to the underside of the focuser. I would avoid this one. The diffraction pattern is an odd one and because the optical flat needs to be held rigidly, a single vane is not up to the job unless quite thick (at least 4mm) and that will obstruct the amount of incoming light, and usually, you need all the light you can get.

Double Vane. This one holds the optical flat mechanism across the diameter of the tube and is, effectively, two vanes. Again, the diffraction pattern is not pleasing but is better than the single vane. The two vane spider is not as rigid as others unless vane thickness is at least 3mm.

Three Vanes. A pleasing diffraction pattern and you should take no objection to having three vanes. Thickness of vanes about 1mm.

Four Vanes that effectively span tube diameters at 90 degrees to each other. The vane material can be the thinnest of all (0.5mm) and the rigidity is maximum. The diffraction pattern is pleasing.

Curved Vanes. Before his death, Ken Novak developed and produced curved vane spiders with one vane having less curvature than the other. I had one of his curved vane spiders in a 12 inch scope. The diffraction pattern was a lot less than with any straight vane spider and photographs of fields with bright stars showed almost point-like images. It did not hold the flat mirror as rigidly as the four vane spider that replaced it.

v)     Mirror.

There is no point spending more money unless your new mirror of whatever aperture has these specifications:

1/4 wavefront or better,

strehl ratio 0.950 or better,

between f/4.5 and f/8 depending on your preference of focal length,      

parabolic and diffraction limited,      

thickness ratio 1:9 or better,

minimum aperture of 6 inches.

vi)    Eyepieces.  

List any Huyghen, Ramsden and Modified Ramsden eyepieces you have on Ebay and replace those by Plossls, Erfles and/or Orthoscopics. I find that Plossls and Erfles are easier on the eye than the Orthoscopics I have.

b)     Non-Newtonian Reflectors - generally called Catadioptric.

The optical layout is like this:

Light comes towards the scope and first passes through some sort of glass plate. It goes down the tube and is reflected from some sort of mirror back up the tube to a flat mirror that takes the light and diverts it into the focuser just like in the Newtonian. The flat mirror can be stuck to the inside of the glass plate or supported by a spider.

Alternatively:

Light passes through some type of glass plate, goes down the tube and is reflected by a large curved mirror with a hole in it back up towards the glass plate. It hits a small curved or flat mirror stuck to the inside of the glass plate and is reflected back down to the large curved mirror and through the hole in that mirror to the focuser. That way, you look at things from the same position as you would when using a refractor and you will need a star diagonal.

Alternatively:  Same set-up but with no glass plate.

The idea:  

What these 'cats' do is fold the light path so that you can have a shorter tube for large f/ numbers. For example, for one of these with an aperture of 8 inches working at f/10 (focal length = 80 inches), the tube length can be 24 inches or so and not nearly 7 feet as with a Newtonian. That way, you cram a long focal length into a short tube that is unlikely to flex if made of the right material and is easy to transport.

More importantly, it is possible to use a main spherical mirror that behaves like a good parabolic mirror if the front glass plate is slightly curved so that it acts as a correcting plate (a Schmidt plate). Thing is, money saved in not having a hand-figured parabolic mirror can be swallowed up by the quality control required for the correcting plate.

The Makers:

Celestron were the first to put these into mass production and Meade followed. There are other makes but those two are the most well known and best quality.

The Types:

Depending on the optical set-up, these 'cats' are called CATS, Schmidt-Newtonian, Cassegrain and Maksutov. They are all very useable but the Maksutov is noted for crisp pin-point star images on a par with good refractors. If not well mechanically constructed, the Mak does not travel well and will spend most of its life on an optical bench being collimated and re-collimated by an optical engineer.

The advantages:

A folded light path with a shorter tube means less weight and a 'lesser rated' equatorial head. Also, the tube is air tight, meaning that re-silvering should be around every 20 years or so if you keep an eyepiece in the focuser when the scope is not in use. Lastly, air currents.

A Newtonian has a tube open at both ends. That allows warm air rising from the ground to rise up inside the tube. That warm air is a tube air current and can affect the light path causing 'bad seeing' inside the tube.  A closed tube CAT does not suffer from tube currents to the same extent unless the tube is made of heat conducting material. Even if that is so, the whole tube tends to stabilise and tube currents are minimal.

The Disadvantages:

I'll let you decide on that when you compare prices for a Newtonian and a CAT of the same aperture. Also, the CAT f/ ratios are in the order of 10 but focal reducers are available that will cost you.

 4.    UPGRADING YOUR TRIPOD.

For maximum stability, you will not beat a 6 inch diameter steel tube with wall thickness at least 6mm., nearly filled with stones and firmly cemented into the ground to a depth of a minimum of 2 feet.. The next best is a steel tube on three adjustable feet on which the equatorial head sits (a pier). If you wish to continue with a tripod, replace your aluminium tripod with a thick-walled stainless steel one. The cheapest stainless tripods I have come across are from skies_unlimited but there could be bargains on Ebay.

5.    WHAT CAN YOU MAKE YOURSELF?

‘Making’ your own scope really means buying the bits and putting them together because ‘the bits’ are now not as ridiculously expensive as they were in the 1950s and 1960s.

Practically, the only thing you should try is the reflector telescope tube, which should be the same length as the focal length of the main mirror. The strongest and lightest one I ever made (in 1965) was four rings of 1 inch thick pine held rigidly by 4 thick struts of pine along the length with the fillers being strips of 2 ply, 10mm wide with gaps of 3mm between them to keep the Newtonian tube ventilated. The exterior was gently sanded round, veneered and French polished.

PART TWELVE DEALS WITH SELLING A TELESCOPE ON EBAY