[Laser] Riflescope, Telescope, Eyepiece, One Pixel Camera

[TWOSIG at aol.com]

In a recent post Stan, WA1ECF, posed this question:

If I am using the optics of a rifle scope, how much blockage of the distant 
signal will occur with the reticle, either cross hairs or mill dot or whatever ?


A short answer was given by PauLC, W1VLF, which I can see one situation in 
which I have to disagree with his answer.  I would like to make additional 
comments.  

First let me state that I have assumed that the modulated light source is a 
point source in line of sight.

If you are thinking about using a telescope, or binoculars, or a riflescope 
in front of a detector, be aware that the purpose of the eyepiece is to 
collimate the reduced area light beam, and therefore its focal distance is at 
infinity behind the eyepiece.  The light is spread over the exit aperture of the 
eyepiece.  If you have a detector that is about 5 to 7 millimeters across, that 
may be OK for binoculars and telescopes that have fairly short eye relief.  A 
riflescope, however is a long eye relief tool and has a very large exit 
aperture, often larger than the objective, and that may also give you less light.

As a general rule, I think you can get better performance from your detector 
if you remove the eyepiece and put the detector in the objective's focal plane.

If you are working with a small detector, there may be advantages to working 
through the eyepiece of a telescope (such as higher magnification).   It would 
require that you add yet another lens to make a one pixel camera.  ( The 
telescope with its eyepiece was designed to work with an eye, which is a living 
camera. )  In that case, the cross hairs of a riflescope will form an image at 
the focal plane of the camera along with the image of the target and its 
surroundings.  If you align the system so that the cross hairs are on the distant 
light source, the cross hairs will block the light from the transmitter.  If the 
"aim" is off the exact center, then the cross hairs will block none of the 
light from the source.  

The cross hairs are at the focal plane of the riflescope objective, not in 
the light path, like the mirror on a Newtonian telescope.  A 20% obstruction in 
a Newtonian reduces image brightness  by 20%.  In this setup, crosshairs that 
cover 20% of the field of view would either block a point source completely, 
or not at all.  




James 
N5GUI