F4 vs F8; 'Fast' vs 'slow'
Portnoy
and the moon. I am thinking of purchasing a 20" F4, but I have a few
questions first:
what does it mean for a scope to be 'fast', and how does that relate to the
F ratio?
I suspect that alot of large aperture scopes have a (relatively) small
focal length, just seems practical. Does a lower F number provide better
viewing of distant galaxies? If so, why?
Tim Tasto
A "fast" f-ratio, like f/4, means a wider field of view and brighter
images for astroimaging. A lot of DSO's are large but very dim,
so the faster telescope with a wider field gives a "better"
view, especially for CCD work.
Thats the short of it!
Tim
Tim Tasto
A "fast" telescope is one that has a "short" focal length or small
focal ratio. The shorter the focal length (small f-ratio), the
"faster" the telescope. I consider f/6 or higher to be "slow",
f/6 to be a "compromise", and anything shorter like f/4 to
be "fast". Other astronomers may have slightly different
opinions.
One final thing, if you buy a "fast" scope, i.e. f/4, read up
and study about collimation. It is critical on "fast" scopes
if you want good images.
Tim
Roger Hamlett
"Portnoy" <po...@nay.org> wrote in message
news:Xns9185D3A07D...@63.240.76.16...
> I currently have an 8" F8 that I primarily use for staring down the
planets
> and the moon. I am thinking of purchasing a 20" F4, but I have a few
> questions first:
>
> what does it mean for a scope to be 'fast', and how does that relate to
the
> F ratio?
(which effectively controls the aperture), allowing you to alter the
f-ratio. A large 'f stop', gives a _low_ f ratio allowing shorter
exposure, and the system is then considered 'fast'. Normally on cameras,
the 'stop' is calibrated in f-ratio numbers, so that the exposure
relationship is easily calculated (without having to include the focal
length in the equation). You can shoot an object that is moving more
quickly, using a shorter exposure. So a scope with a low f-ratio, is a
'fast' scope. At this point, everything else is still variable, so you
could build a f-4 scope with a 8" aperture, or a f-4 scope with a 20"
aperture. Both would be equally 'fast', but the latter would only give
1/2.5th the field of view with everything else left the same (because of
the longer focal length...). A 20" scope with the same field of view as
the 8" example, using the same sized film or CCD, would only be f-1.6!...
However at this point, a f-4 20" scope, could fully illuminate a much
larger 'frame'. Hence you will see such designs for film photography, or
for use with really large format CCD's.
This is why 'wide field' cameras are rather specialised, since making
something at such a low f-ratio requires different compromises to normal.
A typical 'example', would be something like the Meade LX200SC F2.2.
> I suspect that alot of large aperture scopes have a (relatively) small
> focal length, just seems practical. Does a lower F number provide
better
> viewing of distant galaxies? If so, why?
Field of view. Also though, there is then the relationship of the
resolution limits of the atmosphere, and the 'sampling' device being
chosen. The movement in the sky from atmospheric effects, unless you are
in exceptionally clear air, limits the 'useable' resolution to the point
where perhaps something around 1.5 arc seconds/pixel is a 'reasonable'
resolution to aim for. A CCD camera with pixels 10 microns across, would
give this resolution at a focal length of about 40". Now in fact most
(all?) 'large aperture' scopes will have a focal length well beyond this,
which is why you will see 'focal reducers' being an essential 'add on' for
such scopes. The problem is that building shorter focal lengths, increases
the abberations. Also the whole 'equation' changes for viewing 'bright'
objects (the planets), where exposures are short enough, that the movement
is not so significant in the period of the exposure. For planetary use,
focal lengths perhaps 3* greater will typically be used. This gives a
'required' focal length close to that offered by most typical large(ish)
scopes (10 to perhaps 16 inches).
Most typical large scopes, are not _that_ low in terms of their 'f
number'. The main limiting factor is usually tube length, which makes
(say) a f30 12" scope pretty nearly impractical for 99% of applications -
on a Newtonian design, the tube would be about 29 feet long!... However
scopes with these sorts of f ratios, are often make as Maksutov Cassegrain
designs.
On truly 'large' scopes, the focal length will have been selected as a
compomise between the manufacturing difficulties, the desired target, and
the sampling system to be used (eye, film or CCD).
Best Wishes