# Thread: Apparent field of view of an eyepiece

1. ## Apparent field of view of an eyepiece

What is a good way for measuring the apparent field of view of an eyepiece?
Thanks!

2. ## Apparent field of view of an eyepiece

On Tue, 09 Jan 2007 20:06:40 -0600, Kevin <jkevn@nospam.msn.com> wrote:

The empirical method is the find a star near Declination 0 degrees.
Set your telescope so that the star is on the eastern edge of the
field. Then time it as it traverses the field, until it disappears
off of the western edge.

Then, you calculate your true field thusly:

True Field (deg) = Transit Time (minutes) * 1 deg / 4 minutes

This result will give the true field in arc degree(s). Transit
Time should be measured in minutes.

Next, calculate the Magnification of your telescope:

Magnification (x) = Focal Length Objective (mm) / Focal Length Eyepiece
(mm)

Finally, to get the Apparent Field

Apparent Field (deg) = True Field (deg) * Magnification (x)

Actually, these are all approximations, but should be good enough
for non-precision work.

Cheers,
Larry G.

--
Your mind is a terrible thing to waste - TURN OFF YOUR TV!

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3. ## Apparent field of view of an eyepiece

Kevin posted:

OK, here is how you do it:

MEASURING THE APPROXIMATE
APPARENT FIELD OF VIEW OF AN EYEPIECE

MATERIALS: 1. A Meterstick, Yardstick, or other linear device whose
length is accurately known, which can be hung vertically on a wall, and
whose exact middle or center is accurately marked. This could also be a
narrow strip of paper of known length with its exact middle and ends
marked clearly. This object will be known as the observing "target".
2. A method of holding and properly supporting an eyepiece rigidly in a
horzontal position (like a bracket attached to a camera tripod), but
which can be manually moved towards or away from a measuring target.
3. A tape measure.

STEP #1: Mount the vertical "target" (ie: the Yardstick or its
substitute) on the wall so that its exact middle is will be about same
height above the floor as the center of the eyepiece. For a meter
stick, the midpoint will be the 50cm mark, and for a yardstick, it will
be the 18 inch mark. Mark this midpoint with a visible marking like a
small piece of tape or a black felt tip marker, so the middle can be
easily seen from a distance.

STEP #2: Mount the eyepiece at a height above the floor which is exactly
the same as the mid-point of the target, so that the observer can look
into the eye lens with the eyepiece optic axis or barrel horizontal and
parallel to the floor. Make certain the eyepiece is as horizontal as
possible, and that it can be easily moved towards or away from a nearby
wall from as little as two feet from the wall to as much as six feet away.

STEP #3: place the eyepiece straight out from the wall from where the
observing "target" is located. Look into the eyepiece with *both* eyes
open and merge the images of the eyepiece field of view and the target.
Make the center of the superimposed eyepiece field centered on the
mid-point mark of the observing target as closely as possible, and keep
above the floor).

STEP #4: Look at the top and bottom of the target, again with both eyes
open. Try to make the top and bottom edges of the eyepiece field match
the top and bottom edges of the target on the wall by carefully moving
the eyepiece towards or away from the wall. Make certain when moving
the eyepiece that it remains pointed exactly towards the center of the
observing target, and that its height above the floor does not change.
Once the edges of the eyepiece field match the top and bottom of the
target, take the tape measure and measure the distance from the back of
the eyepiece just beyond the eye lens (ie: where your eye was sitting
when you were looking through the eyepiece) to the middle of the target
on the wall. If the target has a length of "2Y" and the distance to the
wall you measured is "D", then the apparent field of view of the
eyepiece is then:

AFOV = 2*ATAN (Y/D), where Y is *half* the total length of the target
and ATAN the arc-tangent (or inverse tangent) function. For example, if
you were using a yardstick (36 inches in length, or Y = 18.0 inches) and
your eyepiece field matched its length at a distance of 37.0 inches from
the center of the target, the apparent field of view of the eyepiece
would be about 51.8 degrees. Do the measurements several times and
average the numbers to reduce the variations induced my random
measurement errors.

Clear skies to you.

--
David W. Knisely KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 14th Annual NEBRASKA STAR PARTY *
* July 15th-20th, 2007, Merritt Reservoir *
**********************************************

4. ## Apparent field of view of an eyepiece

Kevin wrote:

Q. How can I measure the apparent field of view of my eyepiece?

A. Here's a method that's worked for me. You need a tape measure and
the ability to use your two eyes for slightly different purposes.

As mentioned in the answer to the preceding question, if you look
through an eyepiece just on its own, without a telescope, you'll usually
see a sharp edge to the field of view. That's because you're seeing the
field stop, which lies at the focal plane of the eyepiece. If the
barrel itself is being used as the field stop, the edge might not be as
clean, but you can still use this method.

Set up in front of an object of known height (or width) h. I use a
doorway that's 80 inches tall. Holding both eyes open, look through
the eyepiece with one eye. Be sure to keep the eyepiece level with
the midpoint of the object. You should see an indistinct field with
the eyepiece eye, and the object with your other eye.

Now, move back and forth until the object is just as tall (or wide)
as the field of view. Measure the distance d between your *eye* (not
eyepiece) and the object. The apparent field of view is then measured
directly as

h
aFOV = 2*atan ---
2*d

Here's a rough ASCII diagram:

___
/ ^
/ |
/ |
/ |
/ |
/ |
/
eye<EP h
\
\ |
\ |
\ |
\ |
\ |
\ v
---

|<-- d -->|

For example, my 6 mm Radian yielded a distance d of 69 inches, so the
aFOV was 2*atan(80/138) = about 60 degrees. I estimate the error on
this method to be on the order of 1.5 degrees, plus or minus.

--
Brian Tung <brian@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

5. ## Apparent field of view of an eyepiece

Thank you very much for the excellent suggestions.

What is interesting is that, if I hold the eyepiece at arms length so I
actually see a small image of a tape measure or an object with a measured
width in the eyepiece, and I adjust the distance of the eyepiece so the
known width matches the apparent field of view, and I calculate the apparent
field of view angle, this seems to correspond well with the method of using
both eyes with superimposed images.

"Kevin" <jkevn@nospam.msn.com> wrote in message
news:LbKdnb4UWt-y1znYnZ2dnUVZ_oupnZ2d@giganews.com...