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F-ratios and brightness



 
 
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  #1  
Old September 29th 03, 04:46 PM
Brian Stephanik
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Default 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?


  #2  
Old September 29th 03, 05:12 PM
guid0
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Default F-ratios and brightness

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
  #3  
Old September 29th 03, 06:56 PM
Chuck Taylor
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Default F-ratios and brightness

"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/


  #4  
Old September 29th 03, 07:22 PM
Chris L Peterson
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Default F-ratios and brightness

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
  #5  
Old September 29th 03, 07:49 PM
Bob May
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Default F-ratios and brightness

Focal length determines the image scale at the focus point of the scope.
A focal length of about 108" gives a image scale of 1" to the degree.
Shorter focal lengths give more angle per inch while longer ones give less
angle per inch.
Thus, if you are using a particular eyepiece, lets say a nice eyepiece which
has an aperture stop of 25mm, this means that if you have a 108" focal
length scope, the FOV of that EP will be 1 degree of sky. At 54" focal
length, the same EP will show 2 degrees of sky.

--
There are more Democrats on the Calif. Special Election than Republicans!
Go count if you don't believe me!
Bob May


  #6  
Old September 29th 03, 08:07 PM
guid0
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Default F-ratios and brightness

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
  #7  
Old September 29th 03, 09:03 PM
Stephen Paul
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Default F-ratios and brightness

"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?


The simple answer is, for visual applications it's not. Aperture counts for
90% (arbitrarily high value pulled out of the air) of a telescopes
capabilities at the eyepiece.

For imaging, as has been indicated, shorter focal lengths allow wider fields
of view for a given objective, so the prime focus of the objective doesn't
spread the light of a source out across as large an area, which means more
light per square unit measure of objective surface, hence for prime focus
photography it is indeed brighter, requiring less exposure time. The trade
off is you don't get large details, although you do get more degrees of sky.
For wide field images, use short focal lengths, for planets go l-o-n-g.

On the other hand, I'm starting a rumor in hopes to get either confirmation
or correction, that longer focal lengths, or more importantly larger
(slower) f-ratios are more forgiving of atmospheric disturbances when
observing planets, because the range of good focus at the focal plane,
doesn't exceed the accomodation of the human eye. And, the same can apply to
imaging, although not to as great an extent since a camera can't accomodate
with its fixed image plane position.

-Stephen Paul

  #8  
Old September 29th 03, 11:02 PM
Dan McKenna
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Default F-ratios and brightness

In Spie # 1236, Advanced Techniques Optical Telescopes, 1990 on page 475
in "Seeing monitor based on wave front curvature sensing" F. Roddier et al

df=0.678*wave length*(focal length/diameter)^2 *(diameter/r0)^5/6
df=delta focus (rms) r0=Fried parameter, 10cm= 1 arc second 5 cm= 2 arc second
etc.(in v)

This is from a paper that describes a seeing monitor that uses variations in
focus.

The focus variation goes as the square of the f ratio if you hold the diameter
and seeing constant.

Dan




Stephen Paul wrote:

"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?

----------------------Snipage
occurs------------------------------------------------
The simple answer is, for visual applications it's not. Aperture counts for
90% (arbitrarily high value pulled out of the air) of a telescopes
capabilities at the eyepiece.



On the other hand, I'm starting a rumor in hopes to get either confirmation
or correction, that longer focal lengths, or more importantly larger
(slower) f-ratios are more forgiving of atmospheric disturbances when
observing planets, because the range of good focus at the focal plane,
doesn't exceed the accomodation of the human eye. And, the same can apply to
imaging, although not to as great an extent since a camera can't accomodate
with its fixed image plane position.

-Stephen Paul


  #9  
Old September 29th 03, 11:55 PM
Mike Ruskai
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Default F-ratios and brightness

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?


Unless you're taking a picture, it isn't. For visual observing,
brightness is determined solely by magnification. There's no such thing
as "faster" in visual observing.


--
- Mike

Remove 'spambegone.net' and reverse to send e-mail.


  #10  
Old September 30th 03, 12:42 AM
Jon Isaacs
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Default F-ratios and brightness

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
 




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