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 FOV and f ratio
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## FOV and f ratio

#1
October 1st 04, 10:56 PM
 Matt St. Helens external usenet poster Posts: n/a
FOV and f ratio

Hi:

What kind of FOV can I get from an f 8.75 80mm refractor? What is the
relation that tells
you this?

#2
October 1st 04, 11:00 PM
 Florian external usenet poster Posts: n/a

hat kind of FOV can I get from an f 8.75 80mm refractor? What is the
relation that tells you this?

Hi Matt,

Have a look at the eyepiece chart on the Tele Vue site...

http://televue.com/engine/page.asp?ID=3D107

The formulae are listed and there is a calculator you can use to get
such info for all Tele Vue eyepieces.

-Florian

#3
October 2nd 04, 01:16 AM
 Jon Isaacs external usenet poster Posts: n/a

Hi:

What kind of FOV can I get from an f 8.75 80mm refractor? What is the
relation that tells
you this?

1.25 inch focuser?

I believe the internal barrel diameter is around 28 mm for a 1.25 inch
eyepiece. This is the maximum field stop diameter which determines the maximum
possible Fielf of view.

The focal length of the scope in question is something like 8.75*80mm=700mm

The field of view will be 28mm/700mm = 0.04 radians x 57.3 deg/rad = 2.3
degrees.

2 inch focuser will be significantly greater..

jon

#4
October 2nd 04, 03:26 AM
 Matt St. Helens external usenet poster Posts: n/a

I believe the internal barrel diameter is around 28 mm for a 1.25 inch
eyepiece. This is the maximum field stop diameter which determines the

maximum
possible Fielf of view.

The focal length of the scope in question is something like

8.75*80mm=700mm

The field of view will be 28mm/700mm = 0.04 radians x 57.3 deg/rad = 2.3
degrees.

2 inch focuser will be significantly greater..

Thanks for the answer. It is 1.25". If I upgraded to a 2" focuser with a
would I still get the increase in FOV? According to that formula, I would.
Is that the case?
Do I need 2" EP's to get the increase in FOV? Even if I don't, 2.3 degrees

#5
October 2nd 04, 06:26 AM
 David Knisely external usenet poster Posts: n/a

Matt St. Helens wrote:

What kind of FOV can I get from an f 8.75 80mm refractor? What is the
relation that tells
you this?

Well, if it uses an 1.25" eyepiece focuser (eyepiece outside diameter is
1.25"), the maximum field stop (the opening at the front of the field lens of
the eyepiece) diameter for such eyepieces is around 27mm or so. The field
stop formula will yield the true field:

TFOV = (180/Pi)*(EFSD/Fl), where EFSD is the eyepiece field stop diameter, Fl
is the telescope's focal length, and 180/Pi is the number of degrees in a
will yield a maximum true field of view on the sky of about 2.2 degrees. A
24mm Tele Vue Panoptic eyepiece would give you 29x and that maximum 2.2 degree
field, which would be good for examining large objects at low power. If you
replaced the focuser with one for 2" OD eyepieces, the maximum you might hope
for would be about 3.8 degrees (47mm field stop). Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club:
http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
**********************************************

#6
October 2nd 04, 04:07 PM
 Matt St. Helens external usenet poster Posts: n/a

Thanks for the answers. It seems the EP field stop is the main limiting
factor.

#7
October 2nd 04, 07:46 PM
 Larry G external usenet poster Posts: n/a

On Fri, 01 Oct 2004 21:56:59 GMT, Matt St. Helens
wrote:

Hi:

What kind of FOV can I get from an f 8.75 80mm refractor? What is the
relation that tells
you this?

There is a Telescope Optical Parameters Calculator available for
http://www.vvm.com/~piscescs/telecalc/telecalc.html

Cheers,
Larry G.
#8
October 3rd 04, 02:59 AM
 Canopus external usenet poster Posts: n/a

Larry G wrote in message ...
On Fri, 01 Oct 2004 21:56:59 GMT, Matt St. Helens
wrote: What kind of FOV can I get from an f 8.75 80mm refractor?

Although this is a completed thread, you may find the following recent
article helpful. The eyepiece field stop size is a physical
constraint, but other characteristics of commonly available eyepieces
- the combination of the apparent field and focal length - may be
something to consider before filling out your eyepiece collection. See
-

Seronik, G. Oct. 2004. Exploring Low Power Limits. Sky & Telescope.
108(4):125-128

- reciting (at page 126) many of the basic equations discussed in this

Longest useful eyepiece focal length = telescope f/ratio * your
maximuim exit pupil size

Telescope magnfication = focal length objective / focal length
eyepiece

Exit-pupil diameter = Objective aperature size / magnification

true FOV = field-stop diameter*57.3/telescope focal length

true FOV = apparent FOV / magnification

When filling out your eyepiece collection, this note runs you through
some eyepiece math to determine what FOV you will see in each
eyepiece, supplementing the math behind the great online calculator
that Larry G referred you to at TeleVue -

http://www.vvm.com/~piscescs/telecalc/telecalc.html

Although the largest physically obtainable field-of-view is limited by
the field-stop diameter, the combination of the apparent field and
focal length of commonly available eyepieces may be the asthetic
constraining factor you encounter.

For example, using the 29mm eyepiece field stop in a 1.25" eyepiece
barrel, a 700mm objective focal length and an objective size of 80mm,
based on the longest useful eyepiece focal length and your (human)
eye's exit-pupil diameter, is:

telescope f/ratio * your maximuim exit pupil size:
(700mm/80mm) * 7mm = (8.75f/ratio * 7mm) = ~61 mm:

which yields a theoretical maximum FOV of:
(61mm/700mm)*57.3= ~ 5.0 degrees

As noted in this thread, the 29mm eyepiece field-stop does constrain
the 61mm theoretically useful limit:

(29mm/700mm)*57.3= ~2.4 degrees

- and can be worked around by "upgrading" to a 2" inch eyepiece size -
something that isn't an option for a smaller 80mm refractor. For
example, I have a 52mm Anatres Erfle 2" lens with a 46mm eyepiece
field-stop:

(52mm/700mm)*57.3= ~4.2 degrees
(46mm/700mm)*57.3= ~3.8 degrees

- a bit closer to the 5.0 degree theoretical limit.

But with the commonly available larger focal length 1.25" barrel
eyepieces today, let's say -

a 32mm eyepiece with a 44deg apparent field, and
a 40mm eyepiece with a 43deg apparent field

you only get -

AF / M = TF where M = Obj_fl/EyeP_fl

44 degrees / (700mm/32mm) = 44deg / 21.9x = about a maximum of 2.0
degrees
43 degrees / (700mm/40mm) = 43deg / 17.5x = about a maximum of 2.4
degrees

- which are pushing up against the 29mm eyepiece field-stop constraint
limit at ~2.4 degrees.

These limits can be seen by running the Televue online eyepiece
calculator and sorting the results by "True Field".

Although David Knisely's summary is right-on, on a practical daily
level, you also might consider what eyepieces are commonly available
and what are the rated focal length and apparent field of each

As Seronik's article in the October issue of _Sky & Telescope_ points
out, there are inherent trade-offs in eyepiece design between the size
of the apparent field and the focal length.

As a result, the 43 degree AF, 40mm eyepiece yielding the 2.4 deg true
field represents about the maximum FOV that you can buy with a 1.25"
barrel. Expensive ultra-wide eyepieces with higher apparent fields
inherently have shorter focal lengths. Thus, with the ultra-wides, the
you end up with the same maximum true FOV, just at a higher
magnification. Examples include:

- a Meade Superwide 24.8mm with 67 deg AF gives a FOV = ~2.4 deg at
28x
- a Meade Ultra wide 14m with a FOV of 84 deg gives a FOV = ~1.7 deg
at 50x

The main reason for filling-out your eyepiece collection by trying to
go as low as you can go is for star-hopping, particularly for non-GOTO
alt-az mounts. It's easier to star hop looking through a 2.5 degree
"toilet tube" binocular-sized field-of-view, than with an ~1.0-1.5
degree "soda straw" true FOV using medium a focal length eyepiece
around 15mm-20mm with a 52 apparent FOV. The low-power eyepiece gives
you a star-hopping FOV bridge between your scope's finder and
higher-power observing magnifications.

Expensive specialty ultra-wide eyepieces are nice for better "framing"
of large extended objects in the eyepiece at higher magnifications,
but IMHO, something to put off buying until you are really seriously

In summary, for a 1.25" barrel, 29mm eyepiece field-stop, 80mm
objective, 700mm fl, refractor, the FOV limits a

Theorectical human exit pupil limit - ~5.0 degrees
Eyepiece field-stop limit - ~2.4 degrees
Available eyepieces asthetic limit - ~2.0-2.4 degrees at 22x and 18x,
depending on what magnification you are most comfortable star-hopping
with.

- Canopus

P.S. - Hope this post isn't too repetitive.

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