WHICH SCOPE ?
You could say I tricked you into reading this, because I am not to embark on a pro and contra of different telescope designs. Since these pages are intended for a beginner in ATM-ing, I believe there is only one choice: the Newtonian.
The Newtonian is unique in that it is a simple design and also offers the best all-round performance possible. Other designs are only of interest if you have very specific needs or constraints. And because the Newtonian is a simple design, it also offers the best price/performance and effort/performance ratios.
So what I want to talk about here is the optical and mechanical characteristics of a Newtonian that best fits your needs.
Optical:
ApertureThe smallest newts have an aperture of about 10 cm (4"), these are mostly used for RFTs (Rich Field Telescopes). They also give beautiful planetary and moon images. However don't expect to see detailed deep-sky objects with them, because even if you do see them they will be faint-fuzzies to the letter. Everyone should be able to make a newt of this size. And if the focal length is long enough, then you could even do it without optical tests. (Though I don't recommend this)The medium sized newts have 15 to 25 cm (6-10") of aperture. The 15 cm (6") is especially suitable for a planetary scope. For the longer focal lengths, it is still an "easy" target.
The 20 cm (8") is good all-round choice. Its top performance is often limited by seeing (and can thus perform worse than a 15 cm (6") on planets) but it will show quite a lot of deep sky objects, and the brightest of them will even show structure. This aperture is often chosen for a first scope. With little determinism it can be made successfully by nearly everyone.
A 25 cm (10") is more or less a bigger 20 cm (8") scope. Since it has a bigger aperture, it will show more (details in) deep-sky objects. But it is also a bigger and heavier scope and thus not so easy anymore to build and lug around. Still, when you have a healthy self-confidence it is in reach of a beginning ATMer. Just don't give up, and keep at it, you will succeed.
Apertures of 30 cm (12") and above are not advisable to beginners. Not because it is impossible to make them, with a lot of self-confidence and determinism you can certainly make a scope of this size, but because you probably don't know what you are getting yourself into. It is a lot of work and if you have no observing experience then you truly would not know what to expect from the finished scope. It could be a disappointment. But don't let this stop you. If you are convinced that you want a scope this size, then go for it. Just do it !.
Very big amateur telescopes have apertures of 50 cm (20") up to 100 cm (40") and even above this. They should only be undertaken after you have made a smaller scope first. The experience thus won will save you more time on the big one than you spend on the small one.
A last word: the aperture determines the best possible resolution, the amount of light received (i.e. limiting magnitude) and the useable magnification range. Obvious maybe, but don't forget it when talking about focal ratios.
Focal length
While deciding on an aperture is easy for most people, the focal length is more problematic. I suspect that it is often a non-optical decision, and more driven by the size constraints. From an optical viewpoint, the focal length determines the FOV (Field Of View). The longer the focal length, the smaller the FOV.Often the focal length is determined indirectly, via the focal ratio. I.e. the focal length divided by the aperture, written as f/# where # is a number.
If the scope is intended for visual use only, then the f/# should be as high as possible (i.e. the focal length should be as long as possible) though is it not advised to go to extremes. For a first scope I would recommend not to go beyond f/10.
A scope for imaging (Photo, CCD and Video) usually has a low f/#. But for a first scope f/5 will probably be the limit, and f/6 is probably more reasonable. (At least if you grind the mirror yourself)
Secondary size
The size of the secondary should fit you usage of the scope. A visual scope needs a fully illuminated area with a radius of about 5 mm (1/5"). A imaging scope needs a fully illuminated area which covers the imaging device (For photo's 24 mm x 36 mm (1"x1.5").The obstruction factor is the diameter of the secondary expressed in percentages of the primary diameter. This value should be as low as possible for visual observations. It is argued that value's below 20% are not needed, since the improvement is not visible.
Mechanical
TubeThere are two choices, an open framework tube (called Truss-Tube) and the traditional closed tube. I am unsure about the optical advantages of one over the other. It seems to me though, that the closed tube is preferred by most ATMers. The truss tube is used when a closed tube would become to big and heavy. I have read somewhere that the high-tech look of the truss tube should not encourage its use.My conclusion is that the closed tube is the design of choice, unless other constraints prohibit its use.
Mount
Up until about 10 years ago, the equatorial mount was considered far superior to the Alt/Az. The Alt/Az mount was only used for "lesser" scopes. However, since the aperture of the amateur scopes has steadily increased and the computer driven mounts are within reach of the average amateur the Alt/Az mount has taken the ATM-community by storm. The Dobsonian mount seems to be considered the best choice for beginners and advanced amateur alike.Cell
The cell is used to mount the (primary) mirror in the tube. It should allow the mirror to expand and contract freely. This can be achieved by resting the mirror on support points such that it can slide over them. The lateral movement than has to be constrained by other means. A newer method is to glue the mirror to the support points by using a silicone based glue. The silicone will remain flexible when it is cured. This method seems to have been used with success, even on larger mirrors.The sagging of the mirror should also be taken into account. Sagging can seriously deform the mirror surface. It can be kept under control by using more than three support points, under the condition that all support points carry the same weight. This is achieved by the so-called floating cell. This type of cell often has 3, 9, 18 or 27 support points.
A full thickness mirror (diameter to thickness = 6 : 1) does not need more than 3 support points. And thus allows for a simple cell design.
Conclusion
Some specifications for beginner scopes:Planetary scope: 15 cm (6") f/10
General purpose: 20 cm (8") f/8
Deep sky: 25 cm (10") f/6
For all scopes: Dobsonian mount, full thickness mirror, mirror glued to a three point cell. The radius of the fully illuminated field 5-10 mm (1/5 - 2/5")