Check your Eyepiece Focal Length with a Ronchi Grating (long)
dtayl...@my-deja.com
length of an eyepiece. The best method suggested seemed to be the
classical method of measuring the exit pupil diameter. This works well
for the longer focal length eyepieces but not so well for the short
focal length ones where the exit pupil gets too tiny (less than a
milimeter) to measure easily.
I would like to suggest here another way, which is the magnification
method using a Ronchi grating as the object to be magnified. I tried it
and it worked very well even with short focal length eyepieces. It gave
consistent results that agreed very well with the manufacturers nominal
focal length of the dozen eyepieces I tried and gave an accuracy of
about +/-0.1 mm in focal length.
The setup consists of a flashlight shining through a Ronchi grating (120
lines/mm in my case) as the object which is placed about where the eye
would go if it were looking through the eyepiece. 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.
As there is usually nothing new under the sun, I suspect that others
have already used this method. I would be interested to know if any of
you have tried it or heard of others that have.
Sent via Deja.com http://www.deja.com/
Before you buy.
Charles Olson
dtayl...@my-deja.com
"Charles Olson" <chuck...@worldnet.att.net> wrote:
> I like it!
>
> Chuck
Glad you liked it. Incidentally, I forgot to mention in the original
post that the focal length measuring technique described will also work
for 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.
When measuring the Barlow eyepiece combination it may help to use a
larger screen distance from the Ronchi grating, as the separation of the
principal planes of the combined Barlow-eyepiece combination could
conceivably be larger. I haven't looked into where the principal planes
should be with a Barlow-eyepiece combination yet.
The exact formula for finding the focal length is according to my
derivation:
f = (L-h)/(M + 2 + 1/M)
where h is the separation of the principal planes. So if h is larger
one wants to make L say 100x h to keep the error less than 1%. That is
unless one can determine h by measurement using for example the nodal
slide technique, so that one can put h into the formula directly.
Don Taylor