Focal Ratio not important if you don't do astrophotography?

Hi all,

Is Focal Ratio an important consideration if you're never going to be using the
telescope for astrophotography?

Thanks, Excalibur

Is Focal Ratio an important consideration if you're never going to be using
the
telescope for astrophotography?

Hi:

It can be. With a very high focal ratio instrument, it may be diffcult to
obtain widefield views even with long-focal-length eyepiece. With a very low
focal ratio scope, it may be difficult to get high magnification, even with
barlows and short focal length eyepieces.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
Like SCTs and MCTs?
Check-out sct-user, the mailing list for CAT fanciers!
Goto http://members.aol.com/RMOLLISE/index.html

"Excalibur" wrote in message
...
Hi all,

Is Focal Ratio an important consideration if you're never going to be
using the
telescope for astrophotography?

Thanks, Excalibur

I've always thought that f/ratio was little more than a source for
confusion. IMHO, it would be better to talk about aperture and focal
length. Aperture determines light grasp (image brightness) and potential
resolution. Focal length determines image scale and, when the eyepiece is
added into the equation, true field.

Clear skies, Alan

Alan French wrote:

I've always thought that f/ratio was little more than a source for
confusion. IMHO, it would be better to talk about aperture and focal
length. Aperture determines light grasp (image brightness) and potential
resolution. Focal length determines image scale and, when the eyepiece is
added into the equation, true field.

I agree with Alan. There are some times that the f/ratio becomes a
factor itself but only at the extremes, e.g., when coma or difficulty
collimating become significant. Otherwise the focal length determines
the things we care about -- magnification and field of view. I think
this is true with astrophotography as well, where plate scale is
determined by focal length.

Mike Simmons

In article ,
Excalibur wrote:

Is Focal Ratio an important consideration if you're never going
to be using the telescope for astrophotography?

Yes!

A low f-ratio gives you a physically smaller instrument (given the
aperture, of course) and also offers wider fields of view. The
disadvantages are that image quality may suffer, and you also need
more expensive eyepieces which are able to handle the low f-ratio
without degrading the image quality further (by using a Barlow lens
you can, from the eyepiece's point of view, increase the f-ratio;
OTOH then the Barlow need to be of a quality good enough to handle
the low f-ratio).

A high f-ratio will much more easily give you diffration-limited
images, and you can also use less expensive eyepieces which don't
work well with a low f-ratio. The disadvantage is that the telescope
will be physically larger and bulkier.

So if you want a portable instrument, and/or wide fields of view
(e.g. for deep-sky observing), choose a scope with low f-ratio. But
if you want images as sharp as possible (e.g. for planetary observing),
choose a scope with high f-ratio.

--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/

"Excalibur" wrote in message
...
Hi all,

Is Focal Ratio an important consideration if you're never going to be
using the
telescope for astrophotography?

Thanks, Excalibur

It's of some importance.

High f-ratio generally leads to better optical quality, both inherently
(aberrations are easier to correct) and because eyepieces work better,
especially simple eyepieces.

With an f/10 telescope, all eyepieces will work well. With an f/4
telescope, plan on spending $300 per eyepiece if you want images sharp out
to the edge of field.

Low f-ratio makes it easier to obtain really low powers and wide fields.
With an f/10 telescope, plan on using a 2-inch focuser and 40-mm eyepiece
for your low-power deep-sky viewing, and even then, it's not the lowest
practical power. With an f/5 telescope, a 20-mm eyepiece in a 1.25-inch
focuser will do the job, and a 32-mm eyepiece will give you an even brighter
view.

More about this in my telescope book.


--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope