Re: Distortion

[Tom Droege <droege@wwa.com>]

I would be quite happy to go for a "cheap commercial lens".  I have just not
found one.  35mm format camera lenses are not quite good enough.  OK we
would use them if they were also cheap enough.  Unfortunately, the ones that
would be good for tass are relatively expensive.  4" diameter objectives tend
to cost $2000+.   There is also the problem of getting a bunch of identical 
units.  Collecting camera lenses one is faced with a different problem for
each
lens to attach a focus motor.  The resulting kludges will tend to be
unreliable.
Buying a lens lets one work out one good solution for the system.  

I have slowly been persuaded that the right place to run the Mark IVs is at 
longer focal length than the original proposal.  This adds a mechanical 
mounting problem.  Lenses like a 300mm f/2.8 or a 400mm f/4.5 will require
some sort of up front support.  This again will be a different problem for
each
lens. 

Someone here may know how to buy standard camera lenses at a discount.
So far I have had no luck.  Something like a 400mm f/2.8 which I see in Keh 
for $3959 would be great.  I could live with the corner problems for such a
lens if I could get it cheap enough.   What would they cost if I bought 50?  
Can we get some camera maker to give us a great deal?  I checked with
Carl Akerlof of Michigan who bought some similar lenses for his GRB project.
He said that he was unable to get a deal, and that he tried (with I presume
the buying power of Michigan behind him).  So unless someone really knows
how to do it better, we will proceed with the present scheme. 

I am ready for someone to step forward and say "I can get it for you
wholsale".

Tom Droege 


At 01:24 PM 1/4/98 -0500, you wrote:
>> Tom asked how much distortion we can stand.  The answer: a fair amount.
>> Chris already mentioned that the astrometric solution can include the
>> proper terms for the normal distortions, like pincushion and barrel.  There
>> are enough reference stars in the 4-degree field of view to correct for
some
>> pretty high order terms, assuming the images themselves are clean.
>
>Tom, Arne:
>
>First off, Happy New Year to the both of you.
>
>I haven't been following the Mark IV discussion closely, and I was 
>surprised to read that Tom is having lenses designed for the project.
>I was tempted, when I read that, to ask Tom why special lenses were
>needed, but I guessed that that was something I'd missed, and I didn't
>want you to have to go over it again for me.  Now, however, Arne says
>you can handle a fair amount of distortion, and the photometry is 
>evidently recoverable, so now I have to ask what the advantages of a 
>custom lens over a commercial one are.  To my mind, the possibilities 
>are throughput (i.e. limiting magnitude), astrometric precision, and 
>photometric precision.  Arne thinks one can take care of the last
>two terms, so only throughput is a problem (besides money, of course:
>Tom is very generous, but if there is a cheap commercial lens that gets
>you within a few tenths of a magnitude of where the custom lens would
>be, I'd rather he save his money!).  A short answer will do, even a 
>pointer to a technical note, if there is one.
>
>Sorry to waste your time because of my bad reading habits...
>
>Roland Vanderspek
>
>