F-ratios and brightness

could someone explain, in simple terms, why a shorter focal length is better
for DSOs than a longer one, given the same apature?

On Mon, 29 Sep 2003 08:46:25 -0700, "Brian Stephanik"
stepped up to the plate and batted:

could someone explain, in simple terms, why a shorter focal length is better
for DSOs than a longer one, given the same apature?


It has nothing to do with brightness really. For a given aperture, the
shorter the focal length, the wider the field you can see through any
given eyepiece.

The true field of view you get with an eyepiece is calculated by using
the field stop of the eyepiece and the focal length. The field stop is
the "ring" inside the eyepiece that defines the black edge that you
see when looking in your eyepiece.

TFOV = Field stop diameter / Focal length x 57.3

So, let's say a 4 inch sope with a focal length of 900mm with an
eyepiece that sports a 27mm field stop, such as a standard 32mm
plossl:

TFOV = 27/900 * 57.3 = 1.7 degrees

A 4 inch scope with a 600mm focal length and the same eyepiece:

TFOV = 27/600 * 57.3 = 2.6 degrees

So, the shorter focal length scope will be better for those wide DSO's
because it will show you a larger piece of the sky than the longer
focal length scope.

G../0

"Brian Stephanik" wrote in message
...
could someone explain, in simple terms, why a shorter focal length is
better
for DSOs than a longer one, given the same apature?

Hi Brian,

It isn't all DSOs, just big DSOs. And even then, there are other factors.

Let's take two extreme examples: A 10" f/4 and a 10" f/15. Since it won't
cost anything, we'll make them both perfect apos so we don't have to worry
about any other problems --- just the focal ratio.

So, Friday night you take them both out to a dark site. And you turn both to
M33 and put in identical eyepieces. Again, since saa is picking up the tab,
we'll use some nice spendy Naglers :-). With the f/4, you see M33 and some
sky around it to provide contrast. The view is nice, but you hurry over to
the f/15. But, with the same eyepiece, you have a much higher power. Instead
of M33, you have only a small portion of it in view. There is no sky to
provide contrast. Even more critical, the light from it is spread out over a
much bigger area.

It's kind of like going to butter your toast with the butter in a little
packet of butter at the local diner. You have enough to butter one piece
(that's the low power view). But now you have the same single pat of butter
(just as 10" only brings in a certain amount of light) but you have to
spread that butter over every piece of bread in the whole loaf (because they
higher power is spreading it out over a much larger area).

Of course, when you go to view a tiny Planetary nebula, the reverse is true.
With the f/4 you will need to use a barlow and short focal length eyepieces
with shorter eyerelief to get a high magnification. With the f/15, you can
drop in a medium focal length eyepiece with long eye relief and still have
more than enough magnification to see it.

However, if later in the night you go to view M15 and you put in different
eyepieces so both scopes show 200x, it will appear identical in each scope.
The focal ratio will not affect anything because the view at the eyepiece
depends on aperture (which is identical) and telescope focal length divided
by eyepiece focal length. And with different eyepieces, we have make these
the same.

But now, after your observing, you decide to photograph M33. Because we are
running on the saa tab, we order up a couple of prime AP mounts. We will use
prime focus (without an eyepiece, placing the film at the focal plane of the
objective). Just as before, the f/4 focuses the available light into a
smaller area of the film and the photo turns out great. But using the same
exposure time, the f/15 has spread the light out farther and M33 doesn't
show up as well. But again, when we to photograph a tiny planetary, the f/4
makes it look like a star, while the f/15 enlarges it enough to show it is a
planetary.

Hope this helps.

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

On Mon, 29 Sep 2003 08:46:25 -0700, "Brian Stephanik"
wrote:

could someone explain, in simple terms, why a shorter focal length is better
for DSOs than a longer one, given the same apature?

It isn't. A shorter focal length just translates to less magnification for a
given set of eyepieces, and less magnification generally translates to a greater
apparent brightness. This is good for _detecting_ many DSOs, but not usually for
resolving detail.

You could achieve the same result using a longer focal length EP with a slower
telescope. All that matters is magnification, not the actual focal length of the
telescope.

_________________________________________________

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

On Mon, 29 Sep 2003 18:22:50 GMT, Chris L Peterson
stepped up to the plate and batted:


All that matters is magnification, not the actual focal length of the
telescope.

Actually, since maginfication is determined by the focal length of the
scope it does matter.

G../0

On Mon, 29 Sep 2003 15:07:32 -0400, guid0 wrote:


Actually, since maginfication is determined by the focal length of the
scope it does matter.

No, magnification is determined by the ratio of scope focal length to EP focal
length. Magnification matters, focal length doesn't. The magnification can be
adjusted by choice of EP more easily than by changing the telescope focal
length.

Certainly, as Jon pointed out, there may be practical reasons for matching
telescope focal length to your available EPs to give a good range of
magnifications and reasonable exit pupils. My point was that you can't say (as
did the original poster) that shorter focal lengths are better than longer ones
for DSOs.

_________________________________________________

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

On Mon, 29 Sep 2003 23:59:58 GMT, Chris L Peterson
wrote:

On Mon, 29 Sep 2003 15:07:32 -0400, guid0 wrote:


Actually, since maginfication is determined by the focal length of the
scope it does matter.

No, magnification is determined by the ratio of scope focal length to EP focal
length. Magnification matters, focal length doesn't.

What I meant is thant since the scope's focal length is a factor used
in determining magnification, it becomes intrinsically important when
you state that magnification is important. You can't have one without
the other.

The magnification can be
adjusted by choice of EP more easily than by changing the telescope focal
length.

Certainly. I'd rather have a few eyepieces than a variable focal
length scope to change mags but cat owners do change their fl. to vary
the size of the field for a given scope and a given eyepiece.

Certainly, as Jon pointed out, there may be practical reasons for matching
telescope focal length to your available EPs to give a good range of
magnifications and reasonable exit pupils. My point was that you can't say (as
did the original poster) that shorter focal lengths are better than longer ones
for DSOs.

Agreed. I took his question in the sense of a shorter fl. being better
for larger DSO's like M31 or NGC7000. The majority of DSO's doesn't
really need so large a field and magnification becomes more important
in resolving actual details on the subject.

G../0

"guid0" wrote in message
...

Agreed. I took his question in the sense of a shorter fl. being better
for larger DSO's like M31 or NGC7000. The majority of DSO's doesn't
really need so large a field and magnification becomes more important
in resolving actual details on the subject.

As a point of interest, here is a link to some bad astronomy that I just hit
through Google:

"A shorter focal ratio means that objects are bigger and brighter when
observed"

http://www.wpo.net/glossary.html

Try this instead:
Holding aperture constant, individual image details (galaxy cores in a
cluster, details in Juptier's main belts, details on the surface of the
moon) are either bigger and dimmer, or smaller and brighter (_not_ bigger
and brighter) .

Although capable of wider fields of view than a long focus scope, the short
focus (aka "fast") scope provides smaller and brighter _individual_ image
details at the focal plane for a given aperture. As you apply an eyepiece to
magnify the image at the focal plane, the two scopes will equalize in image
detail and brightness as you approach the same magnification.

Fast scopes are only optically beneficial _visually_ by yielding wider
fields of view, owing to their ability to achieve lower magnifications. In
fact, there's a downside to the fast scope. If an obstructed design, it will
have a larger obstruction. The larger obstruction will persist through ever
increasing magnifications, thus robbing contrast which would exist otherwise
between individual details. So for high powers with obstructed scope
designs, a slow scope is not without advantage.

As an aside, in my thinking, this is the allure of the 5" apochromatic
refractor. As an unobstructed scope it doesn't suffer any loss of contrast
at high powers, and yet it can be had in the F7 range, which means when
using 2" eyepieces, it can achieve spectacularly wide fields of view. The
resolution limit of a 5 inch aperture is around one arcsecond, which is
about as good as the seeing allows (on average) in many places. Certainly
the views of planets in such a scope are quite good, as are wide field views
within the arms of the Milky Way.

-Stephen

You could achieve the same result using a longer focal length EP with a
slower telescope. All that matters is magnification, not the actual focal
length of
the telescope.

If there were an unlimited range of eyepieces available, this would be true.
However the limit of eyepiece focal lengths is about 50mm in the 2 inch sizes
and practically speaking about 40 mm in the 2 inch widefields, long focal ratio
scopes are indeed limited.

An F10 scope with a 40mm eyepiece will have an 4mm exit pupil whereas an F6
scope with the same eyepiece will have nearly a 7mm exit pupil.

jon

"Jon Isaacs" wrote in message
...

[Someone] wrote"
You could achieve the same result using a longer focal length EP with a
slower telescope. All that matters is magnification, not the actual focal
length of
the telescope.

If there were an unlimited range of eyepieces available, this would be
true.
However the limit of eyepiece focal lengths is about 50mm in the 2 inch
sizes
and practically speaking about 40 mm in the 2 inch widefields, long focal
ratio
scopes are indeed limited.

I know what you mean, but just to clean this up a little, the 40mm reference
is for 1.25" eyepieces, although I've seen it said that the 32mm Plossl is
about the limit, but none of this is entirely accurate data. To the point,
the maximum field of view for a given eyepiece is based on the field stop of
the eyepiece, with the 1.25" maximum stop being 27mm, and the 2" maximum
field stop being 46mm.

The two primary factors in determining field of view are field stop of the
eyepiece and focal length of the telescope. With these we can determine the
true field of view outside of any other factors:

True field of view = field stop of eyepiece / focal length of telescope * 1
radian

As a significan example, a 1.25" 24mm eyepiece (Panoptic for example) might
have a 27mm field stop (which it does), as might a 1.25" 32mm eyepiece
(Plossl, which it also does). So the field of view is the same for these
two, which shows that field is not strictly limited by the relationship
between the focal length of the scope and the focal length of the eyepiece.
There is no doubt that the long focus scope is limited in field. Of course
that _can_ be said for anything. Nothing is boundless afterall. So the
corrected statement is that a long focus scope is _more_ limited in field of
view than a short focus scope of same aperture (which I'm sure you also
meant).

IOW, holding the largest 1.25" field (27mm field stop) constant, shortening
the focal length of the telescope will increase the amount of sky one can
see, regardless of the magnification (or the focal length) of the eyepiece.
But this has nothing to do with the OP's topic.

There are several fun and interesting relationships in optics. Three primary
factors are the focal length of the telescope, the focal length of the
eyepiece, and the field stop of the eyepiece. From these one can derive
magnification and true field of view, but again, this has nothing to do with
the OP's topic.

As an aside, it is unfortunate that not all eyepiece manufactures list the
field stop, so we have to try the method: apparent field of view (which they
do list) divided by the magnification of the eyepiece in a scope of given
focal length.

To try to answer the specific question, the key to determing "brightness" is
exit pupil. Since one can quickly calculate the exit pupil by dividing the
eyepiece focal length by the focal ratio of the telescope, it seems
reasonable that this might become directly linked. But, one only need
consider that an alternate method of determing exit pupil is to divide the
aperture of the telescope, by the magnification of the eyepiece when used in
that scope.

No matter how you slice it, this requires the aperture value of the
telescope. In the end, exit pupil determines the brightness of the sky
background (at any given time), and aperture determines the brightness of
light sources (all of the time). And, although niether is necessarily
readily quantifiable, the relationship holds as aperture increases. So for a
given eyepiece, more aperture will increase the background sky brightness,
_and_ it will increase the brightness of light sources. The ratio of
aperture to focal length, the focal ratio, really has nothing to do with it.
It is merely a convenient factor.

-Stephen