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I don't have any references for you but I have an idea. You will need acheap, small telescope that you can focus on an object within 10 feet of the telescope. In addition you need a convex lens (a focusing lens) whose focal length I will discuss with you in a moment. First though, get yourhands on the cheap telescope. Now perform the following test. Focus on apiece of paper that is 10 feet away. Have a friend mark on that piece ofpaper where the edges of the field of view (how much you can see) is. Thisshould form a circle on the paper with some radius. Use an eyepiece inthe cheap telescope that has a field of view that corresponds to a regionabout 4 inches across. Inother words a circle about 2 inches in radius.You now need a convex lens that will produce an image that would fill thiscircle with everything you would want to see at a glance through theperiscope. The idea is as follows.The convex lens creates an image. This image is exactly the same as if you
were to place a photograph 10 feet from the telescope and looked at it
instead. Now you have to decide how big a field of view you want in the
circle. This will tell you the focal length of the convex lens you need.
Suppose you want a 60 degree field of view then you want 30 degrees
above and 30 degrees below the center of the circle. I assume that you
want the periscope to focus on objects that are not close to the
periscope say 20 feet or more away. In that case then The focal length (f)
of the lens needed is found by a little geometry.
+
| + _
2 in | + / \
| + | |
| 30 deg. + | |
--|--------------| |------------
f | |+ 30 deg.
| | +
\_/ +
convex lens +
+
(2 in / f ) = tan(30 degrees)
Now solve for f
f = 2in/tan(30degrees) = 3.46 inches
A lens of focal length of about 3 and a half inches will do.
I note that this is about 90mm. There are camera lenses
in this range. You might go to a used camera store or pawn shop
to get an inexpensive lens.
Some cheap telescopes come with a prisim for looking at things
ovehead so this could be used for the lower right angle turn
of the periscope. a mirror could be used for the upper part.Here is a VERY ROUGH schematic.
/
mirror /<---------- light
/ |
|
camera
lens
|
|
------- Image formed by camera lens
| only 3.5 inches away from the
| camera lens
|
|
|
|
| 10 ft distance between
| image and telescope
|
|
|
|
|
V
| T |
| E |
|L|
|E|
|S|
|C|
|O|
|P|
|E|
eye /
eye <---|------| / telescope prisim
piece /
I would work with what you have access to so do not use the
numbers I have given you as being "written in stone" rather
they are intended as a very rough guide. Please let us know
how things worked out and what solution you came up with.
Good luck! :-)
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