
Measuring eyepiece focal length
I am interested in measuring the focal lengths of various eyepieces with
reasonable accuracy, maybe 5% or less. I am aware of that one can just measure
the exit pupil but measuring the exit pupil accurate, especially for a short
focal length piece seems difficult.
Thoughts, suggestions, links...
jon


Measuring eyepiece focal length
In article <20040221055224.20229.00000121@mbm04.aol.com>, Jon Isaacs
wrote:
I've had a go at using Don Taylor's method which he gave details of in
February 2000: http://groups.google.co.uk/groups?th=16bb70d0cac07219
I've written a web page describing a variation of Don's method:
http://www.eastervivian.clara.co.uk...fl_method.html
My accuracy is not very good, particularly at the shorter focal lengths.
I hope you find a better method.

Jim Easterbrook <http://astro.jimeasterbrook.me.uk/>
N51.36 E0.25


Measuring eyepiece focal length
Jon Isaacs wrote:
Here is a method that worked very well for me when I wanted to check on the focal
lengths of my eyepieces. It gave results accurate to about 0.1 mm in the focal
length
even with short focal length eyepieces.
It basically works in reverse of the classic method of measuring exit pupils.
Instead of measuring the exit pupil we place an illuminated grid of closely spaced
lines lines where the exit pupil would normally be and use the eyepiece lens to
project the grid onto a distant screen. For the grid I used a ronchi grating. Now
instead of measuring a tiny exit pupil we measure the vastly larger projected grid
spacing on the screen.
The setup consists of a flashlight shining through a Ronchi grating (120
lines/mm in my case) which is placed about where the eye
would go if it were looking through the eyepiece. Be sure to shield any stray
light from the flashlight that doesn't go through the gratiing so that it doesn't
fall on the screen.
The eyepiece is then
carefully moved toward or away from the Ronchi grating (I used an old
rack and pinion focuser to aid this but it wasn't essential) until the
lines are sharply in focus on a screen (I used a white wall of the room)
a couple of meters away. One then measures the center to center spacing
of the projected Ronchi lines. For the longer focal length eyepieces I
measured the distance covered by 5 lines and then divided that by 5.
To find the focal length one first calculates the magnification, M = D/d
where d is the spacing between lines on the grating itself (1/120 mm in
my case) and D is the spacing as magnified by the projection. I then
calculate the focal length of the eyepiece as f = L/(M+2) where L is
the distance from the Ronchi to the projection screen. This is an
approximation (but a good one) in that it assumes that the separation
between principal planes of the eyepiece is small compared to the
distance to the screen. Thats why you want the screen to be on the order
of at least 100 times the eyepiece focal length from the Ronchi grating.
I also tried this method using a transparent millimeter scale instead
of the Ronchi. That works OK for the longer focal length eyepieces. If one doesn't
have either of these a single wire of known thickness might also serve.
The technique can also be used to measure the focal length of
Barlow + eyepiece combinations. In that case it gives the effective
focal length of the eyepiece as used with the Barlow. By dividing that
result into the focal length measured for the eyepiece by itself, one
can get the Barlow's amplification factor as used with that eyepiece.
A more exact formula for finding the focal length is according to my
derivation:
f = (Lh)/(M + 2 + 1/M)
where h is the separation of the principal planes.
I have been trying to think of something that would work as a substitute for the
Ronchi grating. A single wire of known thickness ought to work, but I would want
to check the thickness with a micrometer.
Have fun if you try it,
Don Taylor
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