apparent image size

Does using a larger aperture telescope lend to a sense of more image scale? Or is it purely a function of focal length?

If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of 1200mm. Is the 150mm f/8 refractor going to seem to have more image scale? Or is the brain tricked by the higher resolution?
Sarah

I'm not sure if this is a correct analogy, but it may serve as an
illustration...

A 3,000 pound automobile can get from zero to 100 mph in about 18 seconds.
The engine is a standard Detroit v-8 developing 220 hp. A unicycle powered
by an 8 hp tricked out lawn mower engine can also go from zero to 100 mph in
about 18 seconds. The top end speed of both vehicles, much like the image
scale of both scopes below, is the same. You wouldn't want to be on a
unicycle doing 100, and you wouldn't want to look through an F/20 60mm (is
there such a thing?) under any kind of power.

Al


"Sarah Whitney" wrote in message
...

Does using a larger aperture telescope lend to a sense of more image
scale? Or is it purely a function of focal length?

If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of
1200mm. Is the 150mm f/8 refractor going to seem to have more image scale?
Or is the brain tricked by the higher resolution?
Sarah

"Sarah Whitney" wrote in message
...

Does using a larger aperture telescope lend to a sense of more image
scale? Or is it purely a function of focal length?

If I had a 60mm f/20 next to a 150mm f/8, both have a focal length of
1200mm. Is the 150mm f/8 refractor going to seem to have more image scale?
Or is the brain tricked by the higher resolution?
Sarah


Forget the analogies Al. The fact is, F20 and F8 are not the same thing.
F- ratio is a ratio of FL to Aperture. An F20
scope will give a narrow FOV whereas F8 will over double the FOV. FOV is
also dependant on the FL of EP.

FOV is a linear relation to F ratio.

Sarah Whitney wrote:
Does using a larger aperture telescope lend to a sense of more image
scale? Or is it purely a function of focal length?

If I had a 60mm f/20 next to a 150mm f/8, both have a focal length
of 1200mm. Is the 150mm f/8 refractor going to seem to have more
image scale? Or is the brain tricked by the higher resolution?

Possibly!

One thing that resonates with what you're suggesting is that most people
find that Jupiter in the telescope at 100x, say, feels very small to
them. Yet at that magnification, Jupiter is looks about 1.25 degrees
across--or about 2-1/2 times as wide as the Full Moon! Most people would
be surprised to hear that the tiny image of Jupiter they see in the
eyepiece is considerably larger than the Moon they see in the sky by the
unaided eye.

Part of the problem is probably that when you're looking in the eyepiece,
there are no obvious points of reference by which to judge size. But I
have always thought that another part of the problem is that Jupiter at
that size just doesn't seem to show as much detail as the Moon does at
1x. The natural reaction is to want to make Jupiter even larger, to make
it easier to see what detail there is, but the root cause is still the
desire to see more detail.

And since a larger aperture permits one to see greater detail on the
planets, it could very well be that the eye is tricked by that greater
detail into thinking that planets look larger through a bigger scope of
the same focal length.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Tombo wrote:
Forget the analogies Al. The fact is, F20 and F8 are not the same thing.
F- ratio is a ratio of FL to Aperture. An F20
scope will give a narrow FOV whereas F8 will over double the FOV. FOV is
also dependant on the FL of EP.

FOV is a linear relation to F ratio.

I think that is rather an inappropriate way of putting it.

In case it isn't obvious to the original poster, there are two fields of
view: true and apparent. True field of view (TFOV) is the size of the
actual patch of sky that you can see through the eyepiece and telescope.
Apparent field of view (AFOV) is how big that patch of sky *looks* when
magnified by the telescope. Thus, AFOV is, roughly, TFOV times the
magnification. Neither is in a simple relationship to f/ratio.

AFOV is independent of the telescope altogether; you can see that by
looking through the eyepiece when it isn't inserted into any telescope.
You'll generally see the field stop clearly delineating the AFOV.

TFOV, on the other hand, is dependent on the focal length of the telescope
and the field stop opening of the eyepiece. To first order,

field stop opening
TFOV = ------------------------ * 57.3 degrees
telescope focal length

The 57.3 "magic number" comes from the number of degrees in a radian.
(My next Astronomical Games column will explain this formula, among
other things.) Strictly speaking, then, TFOV doesn't depend on the
telescope's focal ratio, either, or the focal length of the eyepiece.

Practically speaking, there may be some dependence on the focal length
of the eyepiece, because shorter focal length eyepieces tend to have
smaller field stops, and longer focal length eyepieces wider field
stops. If a short focal length eyepiece had a very wide field stop,
it would probably have too wide an AFOV and be showing significant
aberrations near the edge of the field. Conversely, a long focal length
eyepiece with a small field stop would be like looking through a soda
straw.

Nonetheless, it is misleading to say that either FOV depends on the
focal ratio of the telescope. To first order, they don't depend on that
at all. Yes, there is some dependence between aperture and focal ratio,
but that is an inverse relationship--focal ratio tends to fall as
aperture increases--and that makes focal length depend less on focal
ratio than it would otherwise (if all telescopes had the same aperture,
for instance).

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Does using a larger aperture telescope lend to a sense of more image
scale?

Yes...
With eyepieces providing the same magnification power per inch of aperture,
the viewed image in a larger telescope will appear a larger scale than that
seen through a smaller scope.

FOV is a linear relation to F ratio.

For a given eyepiece, FOV is linear in relation to Focal Length of the
telescope, not the focal Focal Ratio.

An 5 inch F10 scope and an 10 inch F5 scope will have the same focal length and
same FOV for any specific eyepiece.

A 10 inch F10 scope will have 1/2 the FOV but twice the magnification with that
same eyepiece.

Jon

"Brian Tung" wrote in message
...
Tombo wrote:
Forget the analogies Al. The fact is, F20 and F8 are not the same
thing.
F- ratio is a ratio of FL to Aperture. An F20
scope will give a narrow FOV whereas F8 will over double the FOV. FOV
is
also dependant on the FL of EP.

FOV is a linear relation to F ratio.

I think that is rather an inappropriate way of putting it.

In case it isn't obvious to the original poster, there are two fields of
view: true and apparent. True field of view (TFOV) is the size of the
actual patch of sky that you can see through the eyepiece and telescope.
Apparent field of view (AFOV) is how big that patch of sky *looks* when
magnified by the telescope. Thus, AFOV is, roughly, TFOV times the
magnification. Neither is in a simple relationship to f/ratio.

AFOV is independent of the telescope altogether; you can see that by
looking through the eyepiece when it isn't inserted into any telescope.
You'll generally see the field stop clearly delineating the AFOV.

TFOV, on the other hand, is dependent on the focal length of the telescope
and the field stop opening of the eyepiece. To first order,

field stop opening
TFOV = ------------------------ * 57.3 degrees
telescope focal length


Nonetheless, it is misleading to say that either FOV depends on the
focal ratio of the telescope. To first order, they don't depend on that
at all. Yes, there is some dependence between aperture and focal ratio,
but that is an inverse relationship--focal ratio tends to fall as
aperture increases--and that makes focal length depend less on focal
ratio than it would otherwise (if all telescopes had the same aperture,
for instance).




You are too far over the top. Most people want a straight forward
explanation. Mine is more than
sufficient. Most people are in there right mind and not going to actually
measure the F-stop in the EP.

FOV IS a linear relationship. F10 has TWICE the FOV as F20, so cut the
crap!

JS9 wrote:
With eyepieces providing the same magnification power per inch of aperture,
the viewed image in a larger telescope will appear a larger scale than that
seen through a smaller scope.

Although that's true, I don't think that's what Sarah was asking. It
stands to reason that at double the magnification, a scope with twice
the aperture will naturally show a larger image scale, because it really
*has* a larger image scale. The question is whether there is a visual
effect of greater image scale even when the magnification is the same.
(At least, that's how I understood the question.)

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

An 5 inch F10 scope and an 10 inch F5 scope will have the same focal
length and
same FOV for any specific eyepiece.

Refractor or mirror? Can you mix the 2?
That is not what I have been taught. F ratio no matter what determines FOV.