Re: Distortion

[Glenn Gombert <gleng@infinet.com>]

Tom,

        You might want to take a look at the Celestron/Pro-Optic 500mm F/5.6
lense (90mm) that has been advertised in S&T and Astronomy for 5 years or
so. I has an option of a 350mm /F4 converter that can be purchased as a
"add-on". I have been using one of these with my SBIG ST-6 for some years.
The price of the basic unit is on $209.95 in my last issue of S&T from
Adorama on page #137.

        If you would like to try one out let me know and I would be glad to
send you one to evaluate.

Best Reguards,
Glenn G. 

At 02:47 PM 1/4/98 -0600, you wrote:
>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
>>
>>
>
>
Glenn Gombert <gleng@infinet.com>