Re: Lens Shields

[Chris Albertson <chrisja@jps.net>]

Tom Droege wrote:
> 
> I presently run the Mark III with long paper shields that I hang on the end
> of the lenses.  This is an attempt to get rid of the stray light the
> bounces off surfaces that the Mark III has to look near as it is on the
> ground.  These seem to help a lot but I have no good measurements.  There
> is just too much going on in my neighborhood to make any controlled test.
> 
> Anyone have any rules of thumb about shielding the front of a telescope
> like the Mark IV?
> 
> I am presently trying to design the roll off roof house that the Mark IV
> will live in.  Each inch of the light shield tends to add 3" to the
> footprint of the house.  I thus want to make the shield as short as
> reasonable.  Elliot once mentioned a focal length as a starting point.
> This would add a lot more length than I would like to the house.
> 
> Tom Droege

Tom,

In terms of results per $ a lens shade is cheaper then more glass
or silicone. 

The most effective shape is square.  If you must make
it short at least make it the optimum shape. Cut a square hole in
a flat black mask and place it as far in front of the lens as you
can.

Even if there were no lights there is still the sky and it has
some brightness.  If a little piece of sky that is outside the
image area can "see" the lens it will illuminate the side of the
optical tube, bounce back into the optics and raise the general
level of "background" light hitting the CCD chip.  Baffles inside
the optical tube can help too.  Five optical elements means 10
air/glass surfaces.  If coated each will reflect 1%, It adds up
even with just on-axis light.  This is a camera lens not a 
telescope and will be suject to lens fair.

The sky has some uniform brightness.  How much of it do you want
to let into your optics.  With no shade one hemisphere can see the
from element.  With a square mask at infinity only light from the 
image area hits the lens.  Three lens diameters may be a good
compromise.  A 1 diameter shade sees something like a 60 degree
cone and is not to effective.  If the system is sky brightness 
limited then the shade directly address the system's leak link

I have not been able to test it yet, but I built a baffle system
for The Mk III camera.  There is a rectangular hole 24 inches 
from the from element and two more smaller masks between it and 
the lens.  If I stand in from and close one eye when I get just
a little off-axis I can't see the lens.  This baffle system is
about 12 lens diameters.

-- 
   --Chris Albertson             home: chrisja@jps.net        
     Redondo Beach, California   work: chris@topdog.logicon.com