Amateur Telescope Making
Below are some simple equations that will determine some of those components.
Basic Telescope Making Formulae
Symbols used in telescope making.
D = diameter of primary mirror
F = Focal length
L = Intercept
N = Secondary size
I = 100% Fully illuminated field size
s' = Secondary offset
The most important measurement in telescope design is the Intercept ( the distance from the centre of the secondary mirror to the focal plane). To determine this distance you must know the diameter of your primary mirror (D). The tube will need to be 40mm bigger than the primary mirror to allow for tube currrents (warm air moving throughout the tube). The focuser will be the most important components that will need consideration, as the profile (height) will also be added into this equation. You will have to allow a small amount for in-travel (to allow for different focus of the eye and eyepiece) .
The smaller that this number can be the smaller the secondary mirror will be and the less central obstruction (obstruction to incoming light) will be, meaning better brighter images.
To determine the secondary size (N) you need to figure out the 100% fully illuminated field (I) you require (the amount of light you want to reach your eye) in most cases for visual use this should be between 6mm - 15mm.
The light cone returning from the primary mirror will not intercept the secondary mirror evenly and so we need to build in some form of offset to counteract this. The offset is always set away from the focuser and towards the primary mirror.
Formulae
1) Secondary size:
N= L x (D-I) ? (F) +I
2) Fully illuminated field size:
I = (NxF) - (LxD) ? (F-L)
3) Secondary offset:
s' = N (D-N) ? 4x (F-L)
If you are considering building yourself that dream telescope, please drop in speak with our experienced telescope makers. We can help you with component selection and design questions. we will run your idea's through our computer design software to check everything before you spend any money. Some freeware and shareware design software is available.