Your opinions, please...

Alan W. Craft wrote:
Correct me if I'm wrong, but shouldn't the focal length of the telescope
in conjunction with the f.l. of the eyepiece, that is, the magnification,
determine whether or not a telescope is fast or slow?

The terms "fast" and "slow" come, as you may know, from photography,
where they roughly express how quickly a lens will properly expose
film. The lower the f-ratio (or t-ratio, if they differ significantly),
the shorter the exposure need be.

There are two factors involved:

1. The light-gathering power of a lens or telescope is, to
first order, proportional to the area of the aperture.
There are secondary considerations--light throughput,
central obstruction, and so on--but these probably do not
equate to more than a 25 percent difference in linear
aperture.

2. The image scale--that is, the linear size of the image at
the focal point (more properly, focal plane) of the lens
or telescope--is proportional to the focal *length* of the
lens or telescope.

With these two factors in mind, consider two telescopes: a 4-inch
f/12 and an 8-inch f/6. Both telescopes have a 48-inch focal length,
so they both have identical image scales. For example, if you were
to put a piece of ground glass at the focal plane of both telescopes,
and you pointed them at the Moon, both pieces of ground glass would
depict a little Moon approximately 0.4 inches across. Since the
Moon is about half a degree across, we might say that the image scale
is 0.8 inches per degree in both scopes; more usually, we use mm and
say that the image scale is 20 mm per degree.

However, because the 8-inch has twice the aperture, it gathers about
four times (2 squared) the light. Four times as much light is put
into the same size image, so that the image of the Moon on the ground
glass in the 8-inch scope is four times as intense as the image on
the ground glass in the 4-incher. If you were to put pieces of film
in place of the ground glass, the film would be exposed four times as
quickly in the 8-inch scope as it would be in the 4-inch.

Next, consider a third scope: an 8-inch f/12. (A fairly long scope,
unless it's a folded design like an SCT, but never mind that for now.)
This scope has a focal length of 96 inches, twice as long as the other
two scopes, so its image scale is twice that of the others. A piece
of ground glass at its focal plane will show an image of the Moon
about 0.8 inches across. Its image scale is therefore about 1.6 inches
or 40 mm per degree.

Now, in comparison with the 8-inch f/6, it gathers just as much light,
but because the image of the Moon covers four times as much area, the
image is only one-fourth as intense. On the other hand, in comparison
with the 4-inch f/12, it gathers four times as much light, but that
larger amount of light is spread out over four times the area, so that
the image at the focal plane is just as intense in the 8-inch f/12 as
it is in the 4-inch f/12.

To summarize, then, scopes of the same focal *length* have the same
image scale. In contrast, scopes of the same focal *ratio* have the
same image intensity. This in turn determines how fast they will
expose a piece of film and gives rise to the terms fast (low focal
ratio, meaning more light spread out over a smaller image, so more
intense illumination) or slow (high focal ratio, meaning less light
spread out over a larger image, so less intense illumination).

While an 8" f6 would be considered fast when compared to an 8" f10,
the same 8" f6 would at the same time be equitable in "speed" to a 4" f12,
and therefore considered slow.

To continue my thought: However, when a scope is used visually, the
chief consideration is aperture, not focal ratio. The larger the
aperture, the more light gathered, and the brighter the image at any
particular magnification. To be sure, an 8-inch f/6 requires an
eyepiece half as long in focal length as an 8-inch f/12, in order to
achieve the same magnification, but what of that? A simple 2x Barlow
will suffice.

The image will look pretty much the same in an 8-inch f/6 at 100x and
an 8-inch f/12 at 100x, all other things such as optical quality being
equal. On the other hand, at 100x, the view in the 4-inch f/12 will
look significantly dimmer--one-fourth as bright, in fact.

Such slowness is precisely what I am trying so desperately to avoid,
and in order to use the telescope for comet-hunting and widefield DSO
observation, while at the same avoiding glaring instances of coma
and exacting collimations.

If what you want is wide true field of view, and you are willing to
live with less aperture, then you want a combination of short focal
length and wide focuser. For example, the Pronto has a focal length
of just 480 mm and a 2-inch focuser. That yields a maximum true field
of view of around 5 degrees--quite wide. Of course, the larger the
aperture, the more you will see in any given field of view, but it is
pretty hard to make a high-quality fast scope of significant aperture.

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

Alan W. Craft wrote:
I have this "brain cramp" that keeps telling me that you
need low, and even very low, magnification to see most
DSO's, but that's not necessarily true, is it?

No, indeed it's not. For very small DSOs, you will often want to use
higher magnification.

Of course, you wouldn't want to use a 5mm on a galaxy.

I think there is no "of course" about it. I often want to use a 5 mm
on a galaxy in my 70 mm Ranger. That kind of an eyepiece would only give
me about 96x, typically yielding about a 1/2-degree true field of view,
and nicely framing many DSOs.

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

You are correct in that an 8" reflector is too much telescope for any GP
mount. I would not go down that road myself. Consider instead a far
superior driven Dob solution.

Del Johnson



"Alan W. Craft" wrote in message
...
On Thu, 24 Jul 2003 14:14:36 GMT, "Del Johnson"
....reflected:

The 8" f/6 is not too slow for deep sky objects. In fact, it is perfect
and
the tube is not really that long.

I agree, and would LOVE to have the 8" f6, but
wouldn't the tube overwhelm a GP-DX mount, let
alone a standard GP? As it is, I'm thinking that
the f5 would be a bit cumbersome in its own
right, but certainly less than the f6.

I've seen the pictures. Parks places the 8" f6
on either their 'Precision,' or, more appropriately,
on their 'Superior' equatorial mounting, and not
at all onto their 'Astrolight' which bears enough
of a resemblance to either the GP or GP-DX
as to preclude their collective considerations.


Alan

"Brian Tung" wrote in message

snip

To summarize, then, scopes of the same focal *length* have the same
image scale. In contrast, scopes of the same focal *ratio* have the
same image intensity. This in turn determines how fast they will
expose a piece of film and gives rise to the terms fast (low focal
ratio, meaning more light spread out over a smaller image, so more
intense illumination) or slow (high focal ratio, meaning less light
spread out over a larger image, so less intense illumination).

snip

It is my understanding that the area of the objective primarily determines
the *amount* of light collected by a given instrument. You do not have
"more light" or "less light" available due to strictly a difference in focal
ratio, unless you want to take into account the effects of increased
scattering along the longer focal length of the higher focal ratio scope.
Understand that I am not questioning your assertion of the dependency of
image intensity upon focal ratio, only your implication of the dependency of
the amount of available light (i.e. "more light" "less light").

Richard

With the appropriate eyepiece, an f6 system will give you the widest possible maximum field for a
given aperture, for visual use, assuming you want a 7mm exit pupil (the largest a youthful eye will
accept, usually). It used to be that f ratios smaller than 6, such as 5 or 4, would give you bigger
maximum fields, but with modern ultrawide apparent field eyepieces, such as the 35 and 41 Panoptics
and the 31 Nagler, this is no longer the case. It is a matter of achieving a about a 7 mm exit
pupil, which can be calculated by dividing the focal length of the eyepiece, such as 31, or 35 or
41, in this example, by the focal ration, which is 6 in this example. The 31 Nagler will thus give
only about a 5 mm exit pupil on an f6 system, but it has an 82 degree apparent field, which a lot of
us like. If you want a 7 mm exit pupil with a 31 Nagler, you need about and f.4.5 focal ratio.
As you get older, the maximum exit pupil your eye will accept gets smaller, and so by the time
you are 60, you will be better off with a 5 mm exit pupil.
If you have substantial light pollution where you observe, you will also prefer an exit pupil
smaller than 7, because a smaller exit pupil will darken the sky more.
At least this is my undersanding of the matter.
On a telescope with a mirror larger than 10 inches I wouild go with a focal ratio smaller than
6, and use a Tele Vue Paracorr to correct the coma.
Bill Meyers

"Alan W. Craft" wrote:

On Thu, 24 Jul 2003 08:40:04 +0000 (UTC), William Mc Hale ...reflected:

LarryG wrote:
"Alan W. Craft" wrote in message
...

I'm considering a Parks classic Newtonian,
and to mount on a Vixen GP-DX equatorial
mount. While their 8" f3.5 seems to be just
a wee bit too fast, and their 8" f6 a tad too
slow for DSO's and the like(not to mention
the tube length), I've looked into the
possibility of an 8" f5 custom-made by
Parks and sold via Scope City...

Check out the recent issue of Sky and Telescope about observing DSOs
and the merits of high or low magnification. "Slow" scopes are fine for
many such objects. The conventional wisdom dictating fast scopes simply
doesn't hold up in practice, once the other variable of vision and observing
are factored in.

Not to mention that f/6 is not really all that slow. On an 8" one could
get about 2 degrees out of a 35 mm Panoptic.

Correct me if I'm wrong, but shouldn't the focal length of the telescope
in conjunction with the f.l. of the eyepiece, that is, the magnification,
determine whether or not a telescope is fast or slow?

While an 8" f6 would be considered fast when compared to an 8" f10,
the same 8" f6 would at the same time be equitable in "speed" to a 4" f12,
and therefore considered slow.

Such slowness is precisely what I am trying so desperately to avoid,
and in order to use the telescope for comet-hunting and widefield DSO
observation, while at the same avoiding glaring instances of coma
and exacting collimations.

Please excuse my ignorance on the matter if I've overlooked something.

Alan

Brian,
A fine discussion! Keep those posts coming. The only thing I don't like
about your posts is that you can spell (for example: aperture, its, too), which
embarrasses the rest of us.
Let me just add that the maximum brightness possible varies inversely with
the maximum true field obtainable visually.
So, the image an 8 inch will be four times as bright as in a 4 inch, but if
both have eyepieces that yield the same maximum exit pupil (say 7, but it could
be 6, or 5 for an older observer), then the visual field area in the 8 inch will
be one-fourth of that in the 4 inch.
Bill Meyers

Brian Tung wrote:

Alan W. Craft wrote:
Correct me if I'm wrong, but shouldn't the focal length of the telescope
in conjunction with the f.l. of the eyepiece, that is, the magnification,
determine whether or not a telescope is fast or slow?

The terms "fast" and "slow" come, as you may know, from photography,
where they roughly express how quickly a lens will properly expose
film. The lower the f-ratio (or t-ratio, if they differ significantly),
the shorter the exposure need be.

There are two factors involved:

1. The light-gathering power of a lens or telescope is, to
first order, proportional to the area of the aperture.
There are secondary considerations--light throughput,
central obstruction, and so on--but these probably do not
equate to more than a 25 percent difference in linear
aperture.

2. The image scale--that is, the linear size of the image at
the focal point (more properly, focal plane) of the lens
or telescope--is proportional to the focal *length* of the
lens or telescope.

With these two factors in mind, consider two telescopes: a 4-inch
f/12 and an 8-inch f/6. Both telescopes have a 48-inch focal length,
so they both have identical image scales. For example, if you were
to put a piece of ground glass at the focal plane of both telescopes,
and you pointed them at the Moon, both pieces of ground glass would
depict a little Moon approximately 0.4 inches across. Since the
Moon is about half a degree across, we might say that the image scale
is 0.8 inches per degree in both scopes; more usually, we use mm and
say that the image scale is 20 mm per degree.

However, because the 8-inch has twice the aperture, it gathers about
four times (2 squared) the light. Four times as much light is put
into the same size image, so that the image of the Moon on the ground
glass in the 8-inch scope is four times as intense as the image on
the ground glass in the 4-incher. If you were to put pieces of film
in place of the ground glass, the film would be exposed four times as
quickly in the 8-inch scope as it would be in the 4-inch.

Next, consider a third scope: an 8-inch f/12. (A fairly long scope,
unless it's a folded design like an SCT, but never mind that for now.)
This scope has a focal length of 96 inches, twice as long as the other
two scopes, so its image scale is twice that of the others. A piece
of ground glass at its focal plane will show an image of the Moon
about 0.8 inches across. Its image scale is therefore about 1.6 inches
or 40 mm per degree.

Now, in comparison with the 8-inch f/6, it gathers just as much light,
but because the image of the Moon covers four times as much area, the
image is only one-fourth as intense. On the other hand, in comparison
with the 4-inch f/12, it gathers four times as much light, but that
larger amount of light is spread out over four times the area, so that
the image at the focal plane is just as intense in the 8-inch f/12 as
it is in the 4-inch f/12.

To summarize, then, scopes of the same focal *length* have the same
image scale. In contrast, scopes of the same focal *ratio* have the
same image intensity. This in turn determines how fast they will
expose a piece of film and gives rise to the terms fast (low focal
ratio, meaning more light spread out over a smaller image, so more
intense illumination) or slow (high focal ratio, meaning less light
spread out over a larger image, so less intense illumination).

While an 8" f6 would be considered fast when compared to an 8" f10,
the same 8" f6 would at the same time be equitable in "speed" to a 4" f12,
and therefore considered slow.

To continue my thought: However, when a scope is used visually, the
chief consideration is aperture, not focal ratio. The larger the
aperture, the more light gathered, and the brighter the image at any
particular magnification. To be sure, an 8-inch f/6 requires an
eyepiece half as long in focal length as an 8-inch f/12, in order to
achieve the same magnification, but what of that? A simple 2x Barlow
will suffice.

The image will look pretty much the same in an 8-inch f/6 at 100x and
an 8-inch f/12 at 100x, all other things such as optical quality being
equal. On the other hand, at 100x, the view in the 4-inch f/12 will
look significantly dimmer--one-fourth as bright, in fact.

Such slowness is precisely what I am trying so desperately to avoid,
and in order to use the telescope for comet-hunting and widefield DSO
observation, while at the same avoiding glaring instances of coma
and exacting collimations.

If what you want is wide true field of view, and you are willing to
live with less aperture, then you want a combination of short focal
length and wide focuser. For example, the Pronto has a focal length
of just 480 mm and a 2-inch focuser. That yields a maximum true field
of view of around 5 degrees--quite wide. Of course, the larger the
aperture, the more you will see in any given field of view, but it is
pretty hard to make a high-quality fast scope of significant aperture.

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

Richard Jarnagin wrote:
It is my understanding that the area of the objective primarily determines
the *amount* of light collected by a given instrument. You do not have
"more light" or "less light" available due to strictly a difference in focal
ratio, unless you want to take into account the effects of increased
scattering along the longer focal length of the higher focal ratio scope.
Understand that I am not questioning your assertion of the dependency of
image intensity upon focal ratio, only your implication of the dependency of
the amount of available light (i.e. "more light" "less light").

Right--I was unclear. I do mean a higher (or lower) collected light
to image area ratio, which essentially reduces to image intensity.

I'm sure the increased scattering due to a longer focal length is
quite negligible. What would be there to scatter the light, aside
from the air?

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

"Brian Tung" wrote in message
...
Richard Jarnagin wrote:
It is my understanding that the area of the objective primarily
determines
the *amount* of light collected by a given instrument. You do not have
"more light" or "less light" available due to strictly a difference in
focal
ratio, unless you want to take into account the effects of increased
scattering along the longer focal length of the higher focal ratio
scope.
Understand that I am not questioning your assertion of the dependency of
image intensity upon focal ratio, only your implication of the
dependency of
the amount of available light (i.e. "more light" "less light").

Right--I was unclear. I do mean a higher (or lower) collected light
to image area ratio, which essentially reduces to image intensity.

I'm sure the increased scattering due to a longer focal length is
quite negligible. What would be there to scatter the light, aside
from the air?

I agree that the effect would be quite negligible, and that was my point...
there is essentially no difference in the amount of light delivered to the
focal plane by scopes of like aperture and design but of different focal
ratios. Only the image scale and the resultant intensity are affected. I
know you already know this, Brian. I just didn't want someone to become
confused by the "more light" "less light" thing.

Richard

Bill Meyers wrote:
A fine discussion! Keep those posts coming. The only thing I don't like
about your posts is that you can spell (for example: aperture, its, too),
which embarrasses the rest of us.

But don't you see--it takes a fine speller to know a fine speller, does it
not?

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

On Sat, 26 Jul 2003 18:34:27 GMT, "Del Johnson" ...reflected:

You are correct in that an 8" reflector is too much telescope for any GP
mount. I would not go down that road myself. Consider instead a far
superior driven Dob solution.

Celestron, Orion, Skywatcher, et al are placing 8" f/5's
on Chinese GP-copies. Of course, that's Celestron and Orion.
Also, Meade doesn't seem to be above doing that sort of thing
themselves, thus...

Who am I to be so stringent?

8^)

Dobsonians are attractive, but at this point not as attractive
as equatorially-mounted alternatives, as cost is a factor in my
decisions, but not primarily so.

Del Johnson



"Alan W. Craft" wrote in message
.. .
On Thu, 24 Jul 2003 14:14:36 GMT, "Del Johnson"
...reflected:

The 8" f/6 is not too slow for deep sky objects. In fact, it is perfect
and
the tube is not really that long.

I agree, and would LOVE to have the 8" f6, but
wouldn't the tube overwhelm a GP-DX mount, let
alone a standard GP? As it is, I'm thinking that
the f5 would be a bit cumbersome in its own
right, but certainly less than the f6.

I've seen the pictures. Parks places the 8" f6
on either their 'Precision,' or, more appropriately,
on their 'Superior' equatorial mounting, and not
at all onto their 'Astrolight' which bears enough
of a resemblance to either the GP or GP-DX
as to preclude their collective considerations.

Alan