Re: Lens Shields

[Tom Droege <droege@wwa.com>]

OK, the present Mark III shields are about three diameters.  This seems to
do some good.  This is about what I can afford for the Mark IV.

While you say a lens shield is cheap, if the size of the house has to be
increased then the cost goes up as the cube of the cost to build the house.
 In general the building that houses a telescope has tended to cost more
than the telescope.  Sad but true.  I  was at Yerkes a couple of years ago.
 All that building for that "little" telescope.  A quick calculation puts
the volume of the building 1000x that of the telescope.  So while silicon
and glass is "cheap" building volume is not so cheap.  

It would be nice to design a fold out shield, but that is just one more
thing to go wrong. 

Tom 

At 05:21 PM 5/25/98 +0000, you wrote:
>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
>
>
>