F number

In article ,
says...
"eric" wrote in message
...
Hi Eric,

Low F is more bright, and Hi F give more contrast. [SNIP]

The image brightness depends on aperture, not f/ratio. F/ratio has no
relation to contrast, although an obstructed instrument of high or "slow:
f/ratio is likely to have a smaller obstruction, resulting in a slight
increase in contrast.

Absolutely. It's all too easy to look at the focal ratio and forget
that it's just that - A /ratio/. For someone new to telescopes (and
particularly familiar with camera lenses), it's easy to get things
backward. A camera lens set at different f stops is varying aperture
with a constant aperture, while comparing two, e.g. 6" telescopes of
different f ratios is a difference of focal length, not aperture.
Varying either against the other is what changes the f ratio.

I suspect some people might try an 8" f8 scope with eyepiece X, then
an 8" f4, also with eyepiece X and come to the conclusing that the f4
is instrinsically brighter. However, it's actually just operating at
half the manification and therefore four times the brightness. Take
an eyepiece of X/4 and try it in the f8 and it will be just a bright.

--

-- Len Philpot - --
------ ----- http://philpot.org/ --

My thinking:

F ratio is an important factor in astronomical scopes. But it can be confusing
and just about every rule of thumb has plenty of contradictions.

One rule of thumb that is often wrong:

A slow focal ratio scope has a less curved primary mirror. True for standard
Newtonians but SCTs and MAKs use fast primaries (F2 or so) and a magnifying
secondary.

But in general, focal ratio does have some real meaning. It is an indication
of potential aberrations, the difference between an F4 and an F6 Newtonian or a
F6 and an F9 achromat is big...

But the most important factors are that it is an indication of maximum possible
FOV and more importantly maximum exit pupil.

For example, the limitations of the ETX-125 are easily described by pointing
out the 1.25 inch focusr and F15 focal ratio. This means max FOV will be
around 0.8 degees with a 2 mm exit pupil...

Jon

Unfortunately, Sam Wormley quoted the following from website:
Generally, the following information about focal ratios can be
helpful:

* f/10 or higher - good for observing the moon, planets and double stars
(high power)
* f/8 - good for all-around viewing
* f/6 or lower - good for viewing deep-sky objects (low power) "

It's unfortunate because the above statements tend to perpetuate some
pernicious myths of visual observing. One, is that long f/ratio telescopes
aren't well-suited to deep-sky observing. Another, is that short f/ratio
scopes aren't suited to planetary observing. And a third, is that deep-sky
observing is best done at low power. None of the above could be less true.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond

"Jon Isaacs" wrote in message
...
My thinking:
[SNIP]
But the most important factors are that it is an indication of maximum
possible
FOV and more importantly maximum exit pupil.

For example, the limitations of the ETX-125 are easily described by
pointing
out the 1.25 inch focusr and F15 focal ratio. This means max FOV will be
around 0.8 degees with a 2 mm exit pupil...

Jon,

IMHO, it is far more direct to understand maximum possible true field from
focal length and maximum field stop diameter. Not all f/4 telescopes allow
a wide field of view.

Clear skies, Alan

Bill Ferris wrote:
Unfortunately, Sam Wormley quoted the following from website:

Generally, the following information about focal ratios can be
helpful:

* f/10 or higher - good for observing the moon, planets and double stars
(high power)
* f/8 - good for all-around viewing
* f/6 or lower - good for viewing deep-sky objects (low power) "


It's unfortunate because the above statements tend to perpetuate some
pernicious myths of visual observing. One, is that long f/ratio telescopes
aren't well-suited to deep-sky observing. Another, is that short f/ratio
scopes aren't suited to planetary observing. And a third, is that deep-sky
observing is best done at low power. None of the above could be less true.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond


That's a fair criticism, Bill. The page I quoted strayed from a mathematical
description to one based on historical experience with instruments *not* of
superior quality. High quality optics should perform well independent of the
focal length to aperture ratio.

Eric wrote:

I'm trying to get my head around how the f number affects things

Can someone fill this in for me?

Given all else remains the same...

low F ---------- vs ----------- hi F

Thanks
Eric
Thanks all, i now have a better idea of the significance of the f number.
Eric

On Sat, 20 Nov 2004 17:31:26 -0500, "matt" wrote:

it is not pretty especially for modest size instruments due to the fact that
the pixel illumination decreases too much and enters the low s/n area that
creates problems with stacking.

To some extent, although external binning can significantly remove this
limitation. But it doesn't change the fact that the angular size of the Airy
disk is independent of focal ratio.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Im sorry nobody has given you a simple straightforward answer. That's
pretty typical of this heady, sometimes headless, group. It's called
the
O-ring syndrome. Do a Google search and you will find many "useful"
explanation for what you seek. Good luck.
Mark



Eric wrote:

I'm trying to get my head around how the f number affects things

Can someone fill this in for me?

Given all else remains the same...

low F ---------- vs ----------- hi F

Thanks
Eric

On Sat, 20 Nov 2004 02:14:16 GMT, Eric wrote:

I'm trying to get my head around how the f number affects things

Can someone fill this in for me?

Given all else remains the same...

low F ---------- vs ----------- hi F

A capital F usually denotes focal length. It seems to me that you're
really asking about focal ratio, as in f/4.5, f/10, etc.

I will assume that's what you mean.

By all else being equal, I mean that the aperture of the telescope and the
quality of the objective are the same. Also the same is the field of full
illumination when talking about an obstructed instrument.

The lower the focal ratio:

1) The higher the magnitude of image aberrations, including spherical
aberration, coma, astigmatism, and chromatic aberration (the latter only
with refractors, of course).

2) The wider the field of view for a given eyepiece, and the lower the
magnification.

3) The less time taken to expose an extended object (planet, nebula,
galaxy, etc.) on film or CCD. Star exposure time depends on aperture
only, and is not affected by focal ratio.

4) The shorter the tube in a refractor or Newtonian (or any other
single-reflection focal system).

5) The larger the central obstruction in a Newtonian. The same is true of
compound systems (SCT, MCT, Cassegrain, etc.) in practice, though it's not
a given (secondary power/position can be changed).

Take the opposite of each item, and that's what you get with a higher
focal ratio.


--
- Mike

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