focal length vs. aperture

[aliwil]

Hi
I am in the market to buy a telescope. I think I have narrowed it down to 2 telescopes in my price range.
1) 8" (203mm) reflector with 800mm focal length, or;
2) 6" (150mm) reflector with 1400mm focal length.

I understand the greater the focal length the greater the magnification, and;
the greater the apperture the brighter the image.
Both telescopes come with 6.5mm and 20mm eye-pieces.

Any views on which I should buy would be greatly appreciated.

I will probably use it for planets and star cluster viewing, and possibly astro-photography in time.

Many thanks,
Ali

[citivolus (Ric)]

Originally Posted by aliwil

I will probably use it for planets and star cluster viewing, and possibly astro-photography in time.

Open clusters or globulars? The 200mm f/4 would be better for open clusters, but will have coma and need a corrector.

[aliwil]

Thanks, but a coma corrector will blow my budget (i think). will the globulars look any good with the 150mm?

[citivolus (Ric)]

I would suspect that they won't resolve to many individual stars at 150mm, but someone may prove me wrong. The 150mm is also a fairly slow scope at f/9.3.

[Dennis]

If astro photography is going to loom strongly in your future, then the F4 ‘scope will be far more efficient in collecting the photons than the slower F9.3 ‘scope.

Cheers

Dennis

[iceman (Mike)]

I'd definitely get the 8".

But there are 8" f/5 and 8" f/6 scopes.. You don't have to get the f/4 version.

[Starkler (Geoff)]

Originally Posted by aliwil

2) 6" (150mm) reflector with 1400mm focal length.

If this one is considerably shorter than 1400mm physically, which I suspect it is, I advise you keep well clear of these shortened Jones-Bird type newtonians on the market as you will have a world of trouble collimating it and getting half decent views through it.
Be advised they are lemons!

[edwardsdj (Doug)]

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.

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