HOW TO START
Before starting on a design, the constraints should be clarified. Once they are clear, it is usually a piece of cake to make the design. The most common constraints are financial, transport capabilities and experience. Other constraints may be available time, observing location, availability of machinery etc. A vague feeling of the kind : "I like to make a telescope" can often be solidified by making conscious decisions and clarifying the constraints. Writing them down, however obvious, can help a great deal in getting rid of uncertainties.
A list of possible constraints:
- How much are you prepared to spend for the completed, ready to use telescope.
- What transport capabilities do you have. (Or will it be stationary)
- What is your experience, maybe not in telescope building, but when it comes to DIY.
- How much time will you have available for ATM-ing.
- When must the scope be completed.
- Where will you use the scope.
- What tools are available.
- Is there anybody to help ?, maybe join a local club (could be astronomy, woodworking, metal working, electronics, computers, DIY, anything that might be of interest to ATM)
- Where will you work ?
If money is a major constraint, write down how much you want to spend (and maybe when). Make a rough breakdown of what you want to spend on each part. Don't forget the eyepieces, and set something apart for "small stuff". The first breakdown will be inaccurate, but you will be able to refine your estimations when going through the design steps later. If you cannot the estimate the relative costs of the necessary items, just assign equal amounts to everything. The refinement during the design will take care of a more precise breakdown.
You can build any needed part yourself, so the decision of what to buy and what to build is up to you. You will need the following parts or services for a Newtonian telescope :
- Primary mirror
- Primary mirror cell (the 'holder')
- Aluminizing the primary (only for self made mirrors)
- Secondary mirror (and aluminizing)
- Secondary holder
- Spider (for the secondary)
- Focuser
- Tube
- Eyepieces
- Finder (telrad)
- For a self made mirror : Foucault tester (and or caustic tester)
- Mount (tracking or not, alt/az or equatorial)
- Paint
- a lot of small stuff (screws, nails, Teflon, brushes, bolts etc)
- Transport container and or cover
- A (very) dim light (during observing sessions)
- Star maps
- Books on what to observe
- If you use mail order, think about "packaging & shipping".
And if you need arguments, once everything is in place, the costs of observing are almost nil. Of course you may get a bad case of aperture fever, or you might get caught by the ATM-bug :-)
Tool's can be a major constraint, but they are not a valid reason not to build the scope yourself. The minimum tools needed are: hammer, screwdriver, saw, drill (may even be a hand-drill). Of course almost every other tool will come in helpful, but they are not necessary.
Under acknowledgement of the constraints, most people decide first on the aperture. This sets the magnitude limit for visually visible objects and the maximum attainable resolution. IMHO a 10" is about as far as a beginner can go whether or not you grind your own mirror. Larger scopes place heavy (!) demands on the mount and become more cumbersome to use and to transport. Not to mention the financial burdens.
Next the focal length is decided upon. The focal length is the biggest factor concerning the maximum possible FOV. For prime focus photography, the focal length is the only important factor.
The last two optical decision are the size of the area in which you want full illumination (this will depend on your use of the scope) and how far the focal plane should be from the primary optical axis. Using these values you can calculate the size of the secondary, diameter of the focuser and length of the tube.
More information, as well as how to perform the calculations, can be found on the other pages. I suggest you start with "The Newtonian Design".