Mark IV Lens System

[Tom Droege <droege@wwa.com>]

I have asked Elliot Burke to work on a lens system for the
Mark IVs.  He hopped right on the project and has a preliminary
design.  It will use a 4 element lens.  By taking advantage of the
fact that we will always use a lens with a filter, he can design
at lower cost, and possibly better than a camera lens.  It is 
(at least) a quadratic problem, and so should get a lot easier
as the bandwidth is restricted.  

By buying 50 or so at one time we will get a price (we hope) that
is favorable compared to camera lenses.  I have also specified a
field of view that is right for our CCD.  We know camera lenses are
not so good in the corners.  I find a 400mm f/2.8 in Keh for $3959, 
a 400mm f/4.5 is $1219.  We hope to beat the lower price.  Note 
that this is high risk, and a lot of money for me.  We want to get it
right.  I know this is hard.  Now is the time to speak up if you know
anything.  It would be sad if say 600 or 300 would be a much better
solution.

Another advantage is that we will get identical units.  This will allow
a standard focus motor design, and other such improvements.  A 
motly collection of camera lenses would be a pain.

The design that I have specified is a 400 mm f/4.   Is this "right"?

I have also specified the following band passes:
Bessell B    390-470 nm
Bessell V    490-585 nm
Bessell R    570-690 nm
Bessell  I    730-950 nm

Note that I just estimated these from the curves that came with the
filters from Omega Optical.  I am sure that the cut off will not be so 
sharp that a few nm one way or the other will make much difference.
If someone knows (or can look up) the exact pass bands let me know
and I will correct my spec to Elliot.  I used half max at the edges. 

We hope that such a lens will get the survey to mag 16 at a darker
location, or we can cover more area with shorter exposures.  This
should be close to finding moving objects.   

Tom Droege