[Laser] Re: street light EME

[James Whitfield]

From: <F1AVYopto at aol.com>
Subject: Re: [Laser] Re: street light EME


".....In this condition our 24 cm mirror with its 50 cm  focal length gives
to much
light on the 1 mm² photodiode.  "


The Moon as seen from the Earth is a little more than 8 mR across.  The
image from a 500 mm lens or mirror will therefore be about 4 mm in diameter.
The area of the image should be more than 12.5 square mm, which should give
you less than 8 percent of the image on your photodiode.

I am assuming two things:  First, that your 24 cm mirror is, or is of
quality similar to, a telescope for amateur astronomy.  Second, that you
want to narrow the effective field of view of the instrument.  Increasing
the focal length of the telescope, perhaps with a Barlow lens seems to be
what you need to do.

Here is a suggestion that might be easier to try, and I think it will
appropriately narrow the field of view.  It will certainly reduce the amount
of light that the sensor will receive.  Place an opaque mask that has a
small hole in front of the sensor.  I suggest you try one with a 0.5 mm
diameter hole ( should reduce the light by a factor of 4 ) and a 0.25 mm (
factor 16 ).  If you can, it may help to refocus the instrument so that the
Moon's image is on the mask.

With the F/2.1 mirror you have, the sensor needs to be no more than 1 mm
from the mask with the 0.5 mm hole.  It could be 3 mm from the 0.25 mm hole.
If you were to try this on a telescope with at longer focal ratio, the
geometry would allow greater distance from the mask to the sensor.  I am not
sure if there is any advantage in doing so.  ( It might if photodiodes are
more efficient in converting photons over a larger surface area. )

I came up with this idea trying to narrow the field of view of much larger
photosensors.  I hope that it will assist your efforts.

James
 n5gui