Hi Ali,
I agree with Starkler 100%. Those short tube Newtonians with the built in barlow/corrector at the bottom of the focus tube should be avoided.
If the tube is much shorter than the focal length, the focal length is actually achieved with a built in barlow at the bottom of the focus tube. These are typically matched to the scope, of low quality, and can't be changed for something better.
When purchasing a Newtonian reflector, what you are looking for is for the optical path to consist of just two mirrors. Any lenses are a bad thing.
Unlike apeture, focal length can be changed by adding a barlow or focal reducer.
Quote:
Originally Posted by aliwil
Hi 
I understand the greater the focal length the greater the magnification, and;
the greater the apperture the brighter the image. |
Magnification is determined by the combined focal lengths of the telescope and eyepiece:
magnification = telescope focal length / eyepeice focal length
You can achieve any magnification for a given telescope focal length with the appropriate eyepiece.
If you are interested in planetary observing I would make sure you get a driven equatorial mount as you work at high magnification and tracking the planet manually is difficult.
Have fun,
Doug