[Laser] Re: variable field of view for noise elimination

[Tim Toast]

hi james and all

The mask or variable iris to narrow the FOV would
probably work just fine for multiple noise sources. I
don't see any real problems with it other than maybe
being able to position it easily and accurately. 
For single noise sources, you could use a fine wire or
needle to block it. Something you could position with
a knob or screws. 
One thought i had for positioning things
electronically, without having to touch the
instrument, is using a modified analog meter movement.
After removing some of the case from the meter, the
pointer needle could be placed in the focal point area
and moved with a small current to micro-position it or
move it out of the way entirely. Although it would
only have the one axis of movement, maybe it would
also work with a small mask attached like a flag to
the pointer. Mounted so the pointer/mask moves left
and right relative to the horizon for example. Since
the pointer would move in an arc instead of a straight
line, the mask might be an elongated slot rather than
a square or round hole. 

With telescopes, it's always much easier to aim if you
use a low power first (wide field of view), then
switch to high power after you get the object
centered. I know with most optical setups here,
eyepieces are not used, but that would be another way
to narrow the FOV or change it if you wanted to. These
setups would use the eyepiece projection technique
which would allow you to change the FOV by moving the
sensor back and forth. A mask or variable iris etc..
could be used as well between the eyepiece and sensor.
The eyepiece need not be expensive either, just a
simple double convex or similar small 1/4 or 1/2 inch
diameter lens could be used, having a focal length of
an inch or so depending on the magnification power
desired. A zoom type eyepiece would be nice for this
so you wouldn't have to change the lens or move the
sensor.
Another benefit of having an eyepiece is just in using
it to aim the equipment - even with the poor image
quality of fresnels, it would work well enough. Back a
few years ago in here, someone mentioned using an
inexpensive eyepiece or small lens mounted right
behind the sensor to help aim the receiver. The diode
blocks the central part of the image when viewed
through the eyepiece, but you could still see the area
outside of it to some advantage. 

One source to find good quality optical parts, is from
these older full sized VHS camcorders. People are
selling them cheap or just throwing them away as the
prices for newer smaller ones get lower. Inside the
lens assembly you'll find a nice electronic iris that
you can control via a small voltage, an assortment of
small high quality lenses to use as eyepieces and a
little optical filter right in front of the ccd chip
that blocks IR and passes visible light. The camera
viewfinder usually has an adjustable focus eyepiece
with a lens and sometimes a small first surface
mirror. On the front of the camera there is an IR pass
filter for the auto focus circuit to look through. The
auto focus assembly also has a pair of good lenses. In
the tape cassette compartment, there are a couple
photo-transistors and one dual-axis IR LED in the
middle to sense the tape ends. Along with all kinds of
little mechanical parts and a rechargeable battery
etc.. The video head itself makes a good precision
bearing ...a gold mine of otherwise expensive and hard
to find parts.
-_-



Tim Toast
http://www.aladal.net/toast/exp.html

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