[pure-silver] Re: Tech Pan officially discontinued, substitu
- From: Michael Briggs <MichaelBriggs@xxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sat, 18 Sep 2004 01:38:54 -0500 (CDT)
On 17-Sep-2004 Richard Knoppow wrote:
>
>>
>> On 17-Sep-2004 Richard Knoppow wrote:
>>>> Polyestar film base is a spin off of the Cold War -- it
>>>> was developed by Kodak
>>>> for the CORONA spy satellites because film on acetate
>>>> base
>>>> was ripping in the
>>>> camera.
>>
>> "On April 15 [1960], Discoverer XI went into orbit and the
>> recovery system
>> malfunctioned again, unfortunate, as it was the first
>> perfect camera operation
>> due to Eastman Kodak's change from acetate-base to
>> polyester-base film."
>>
>> --Michael
>>
> This is around the time polyester base began to be used.
> However, while Kodak may have employed it for this film it
> was not developed because of this project. The material at
> Dupont's site states that polyester films, namely Mylar, was
> developed in the early 1950's from the Dacron project.
I was trying to say that polyester film base was invented for the Corona
project, not polyester. Developing a film base seems to be more challenging
that it might naively seem. Replacing nitrate base took years despite the
obvious problems with nitrate. The requirements include transparency, chemical
and mechanical stabilty and that the gelatin adhere.
The CORONA project was declassified in the late 1990s. In the February 1997
Physics Today the CIA official in charge of the program, Albert Wheelon, wrote
about the project. As part of the description of the initial design, he
described the film as "A fine-grain film was required to realize the resolution
potential of the Itek camera. Eastman Kodak had developed such film for the
U-2 program. For the Corona program, it developed an acetate-based film that
was 70 mm wide and three thousandths of an inch thick. The film was relatively
slow but gave 280 lines per millimeter over the entire field of view at high
contrast. At two-to-one contrast, it give 170 lines per millimeter." Later
he describes problems in the early part of the program and the solutions
imlemented, including: "In addition, the acetate-based film became embrittled
in space and broke several times. Fortunately, Eastman Kodak had developed a
polyester-based film that corrected this deficiency."
Photographers might also be interested in the lenses used in the first spy
satellites: "Their [Itek] Corona design used a 24-inch focal length with an f-5
Tessar optical design. ...... The telescope lenses were close to diffraction
limited ....." After several years "The basic camera was improved by going to
a Petzval f-3.5 lens design. The attitude control capability of the Agena
improved with time. [Meaning that the spacecraft pointing stabilty was
improved.] Through these upgrades, the resolution was improved from 25 feet to
6 feet."
This is getting far off the original question. While polyestar for film base
was originally developed for 70 mm roll film, acetate has remained the most
common base for roll film. Kodak decision to stop making one type of polyester
material was a factor in their decision to stop production of Tech Pan. I hope
Kodak continues to make or buy the thicker polyester that they use for their
sheet films.
--Michael
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