Hi I am a new member. I am have always loved looking up at the stars in the sky and have always wanted a telescope but for whatever reason have never gotten one.
I have now decided that instead of buying one I would like to build one. So I have been looking on the net at homebuilt reflector telescopes and have decerend that it shouldnt be too difficult to build myn own amatuer star gazing reflector telescope. But the topic has raised a few questions I would like answered so if anyone could help I would be very appreciative.
1. How do you decide what your focal ratio should be?
Long focal ratio scopes (f/5 - f/12) are best suited to planetary work. They give very good contrast on fine detail (like planetary detail and close double stars.) The drawback is that they have relatively narrow fields of view (they can only see a very small bit of the sky even at minimum magnification.) Short focal ratio scopes (f/5 - f/2) are best suited to wide-field / deep-sky work - looking at galaxies and nebulae (faint fuzzies.) These have less good contrast and exhibit comatic aberation (blurred focus) towards the edge of the field of view. This isn't a problem if you are looking at faint-fuzzies but is a problem if you want to look at the planets. Your best bet, for your first telescope, is to compromise and go for an f/6 primary mirror.
2. Is the diagonal mirror a specific type of mirror or is it simpley a flat mirror placed at an angle and is that angle 45 degrees?
The secondary mirror is an extremely accurate flat. It is also a "first surface" mirror (like the primary.) If you are buying your primary mirror then it will most likely come with a suitably sized secondary eliptical flat. If you are making your own primary then you should buy a good flat - they are not so easy to make yourself.
3. Does the diagonal mirror go infront of the focal point of the primary or exactly at the focal point?
The secondary mirror is placed in front of (before) the focal point of the primary so that the telescope reaches its focus point outside the side of the telescope - somewhere inside the focuser tube.
4. What should the focal length of the eyepiece be?
You can change the magnification of the telescope by changing the focal length of the eyepiece. Magnification = Focal Length of Telescope divided by Focal Length of Eyepiece. The maximum useful magnification you can use with your telescope is approx. 50x per inch of aperture - so an 8" scope can magnify up to about 400x. You also have to work out the minumum magnification based on the size of your eye's pupil (usually about 6 or 7mm.)
5. can the secondary mirror (diagonal mirror) be mounted within a piece of glass or perspex instead of on a spider? Would this be an improvement or a hinderence to the design of the telescope?
It is possible to mount the secondary mirror at the centre of an optical window (like a Schmidt-Cassegrain) but the glass plate has to be of optical quality (expensive and very accurately made) and will introduce colour aberration (colour fringes) to the image. A spider is the simplest method of mounting the secondary mirror - they're easy to make!
6. Is a bigger primary better than a lower one and what are the advantages of each? Is it better to have a long thin telescope or a wide short one?
A 6" or 8" primary mirror will give you really good views of the sky and will serve you well for many years... until you look through someone else's bigger telescope. Then you develop an over-riding urge to get a bigger telescope. This is called aperture fever! Everyone suffers from it!!
7. Where can I buy a primary mirror?
Oldham Optical UK -
www.oldham-optical.co.uk/
Orion Optics UK -
www.orionoptics.co.uk/
Lots of other places overseas (Michael O'Connell got his 16" mirror from a supplier in asia - good price - good optics.)
OR you could investigate making your own primary mirror! Check out the mirror kits from Galvoptics UK -
www.galvoptics.fsnet.co.uk/
and a very useful ATM website
stellafane.org/
They are the questions that are plaguing me at the moment and would be very greatful if someone could answer them for me. Thanks
Shane
Keep asking the questions - there are no silly ones except the ones that don't get asked!
Phil.
  