Another f ratio question

[craigcullum]

Hi,

Here's another f ratio question, I understand that a fast telescope produces a wide field of view in comparison to a slow one.

Because of this planetary viewing is better with a slower telescope due to it's limited field and greater magnification.

Why would you not buy a fast telescope, say f/4.5 and compensate with a high magnification eyepiece or a barlow lens?

[Blue Skies]

You'll find it just doesn't work that well. The key is in the focal length, not the aperture here. Take an 8" f/6, with F=1200mm, and an 8" f/4, with F=800mm. A 25mm eyepiece will give you 48x and 32x respectively. A 6mm would give you 200x and 133.3x respectively. No amount of barlowing is going to get you around that. I have seen scopes pushed well beyond their limits and basically the image is rubbish. If its high quality optics, maybe it will be ok, but generally not.

[Rigel003]

There are other issues. In refractors, chromatic aberration is much more difficult to control at shorter focal ratios and reflectors need a much wider diagonal to accommodate the short, fat light cone and this adds to diffraction effects and lack of contrast. Short focal length newtonians suffer from coma.

[Gem]

You are right in a way, but it isn't the most ideal. Using a barlow is adding another layer to the whole equation. The less layers and chances to lose light/contrast/colour the better. The same problem goes the other way too... if you have a long focal length scope (e.g. SCT) and you add a focal reducer it isn't quite as good as getting a shorter length scope.
No one scope is ideal for all things!

[RobF]

At the risk of being accused of oversimplification, low F ratio is valuable for imaging (time to suck down light directly proportional to square of Focal ratio).

Higher ratios better for higher magnification visual work, particular bright objects including planetary/lunar work.

:scared3:


You rarely know what's going to "get you in" starting out, so a compromise can be a good idea. :)

[Satchmo]

So long as you have good quality optics there is nothing wrong with doing that . It is not too hard to keep the central obstruction by the diagonal below 20% for a visual scope. High quality 'amplifiers' like the Televue Powermate series are favored by the planetary imagers. Looking at the equipment used by the top planetary imagers you'll find SCT's and moderately fast Newtonians of F4 to F4.5 well represented.

[Barrykgerdes]

Long focal lengths from small diameter lenses and mirrors are best for any sort of accurate viewing due to the limitations of the curvature on the lenses or mirrors because of chromatic and spherical aberation. This gets more pronounced as the F ratio reduces.

The common SCT's are in fact Low F ratio telescopes being in the order of F2 or F3 but have in fact have had the focal length increased with a convex secondary mirror (Barlow) to make them generally F10. Special corrections in the light path are made by corrector plates and figureing to make the performance like an F10 telecope.

Barry