Re: ATM My thoughts on spatially resolved PMT photometry: long

[Andy Saulietis <iss@pvtnetworks.net>]

Dominic writes,

>A PMT could do all of this and in many cases outright outperform a 
>CCD if designed to do so. Herein lies the problem. PMT astrophotometry 
>systems were not designed from the onset to do spatial imaging, and 
>they are still not a "cool" peripherial for a PC, but they could be. 

Actually, there was a spatial imaging PMT developed early on and
was used in early forms of real time video cameras. It
was called an 'image dissector' tube and consisted of
a photocathode followed by a magnetically focused section
that produced an 'image' of the electrons produced by 
the optical image on the photocathode. At the electron focus
was a *small* pinhole thru which part of the electron image
was allowed to pass thru, these electrons went into a conventional
photomultiplier chain & the amplified current was used to
read out the current at that point in the image. Magnetic
coils were used to 'move' the electron image around, allowing 
the image to be scanned. These dissector tubes could achieve
resolutions of several thousands of lines/field, and had linear
very noise free gains of several million fold. The method
of electron scanning was very flexible. These tubes formed
the basis of early strap down star trackers, which we at NASA
used in the Shuttle Program, other variants were used in
unmanned spacecraft for imaging and tracking. They have
been since been replaced in the 80's by CCD's which are simpler to
build and much lighter, but are far less sensitive, with less
dynamic range for real time tracking applications, and only recently
have approached the spatial resolution possible with IDT. Some
of these IDT star trackers are *still* in use on the Shuttle.

The IDT's biggest problem was complexity, expense, and weight,
AFAIK they are on longer being manufactured.


Andy Saulietis
ISS Enterprises
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