Asimov,

I believe you have misinterpreted the geometry. The detector array is
some distance behind the aperture array, perhaps one meter or more. The
concept is that of a simple shadow mask. There are 52000 shadows on
32000 detectors, so the details of the computation are probably a bit
complex.

I recommend a paper by Scott Barthelmy, which is available at

http://swift.gsfc.nasa.gov/docs/swif...SBarthelmy.pdf


73,
Gene
W4SZ

Asimov wrote:
Hi,

Say, I've got a question about something I've read about the Swift
gamma ray detection satellite. It's detector consisted of an antenna
composed of 52,000 lead blocks arranged by computer generated
pseudo-random locations and glued to the back of a sheet of 4x8
plywood. The actual detectors where located in the spaces between the
blocks. It wasn't mentioned but I think these were probably crystals
which give off a pulse of electricity every time they are struck by a
gamma ray.

Anyways my question is, in your opinion, what is the purpose of the
pseudo random arrangement of this "apperture mask" antenna? I suspect
it has something statistical related to the nature of ambient noise
signals. I tried making a pc simulation of this but I get the same
answer whether the array is in rows or pseudo-random. I tried with
noise but I saw no reduction. I haven't tried adding a signal to the
noise in the simulation yet though...

A*s*i*m*o*v

... Three types of lies: lies, damn lies, and statistics - Mark Twain.

"Gene Fuller" bravely wrote to "All" (11 May 05 21:02:18)
--- on the heady topic of " Apperture Mask Antenna"

GF From: Gene Fuller
GF Xref: aeinews rec.radio.amateur.antenna:30137

GF Asimov,

GF I believe you have misinterpreted the geometry. The detector array is
GF some distance behind the aperture array, perhaps one meter or more.

The original release mentions that the gamma rays can only penetrate
inbetween the lead blocks but doesn't mention how far back the
detectors were located or how they were arranged.


GF The concept is that of a simple shadow mask. There are 52000 shadows
GF on 32000 detectors, so the details of the computation are probably a
GF bit complex.

"The mask together with a set of gamma-ray detectors, totaling 32,768
pieces of cadmium-zinc-telluride each measuring four square
millimeters are the main components of Swift's Burst Alert Telescope
(BAT). BAT will locate hundreds of bursts to better than 4-arcminute
accuracy and provide enhanced sensitivity to faint bursts that earlier
detectors have missed."

Maybe it is a little less complex than we might assume...


GF I recommend a paper by Scott Barthelmy, which is available at

GF http://swift.gsfc.nasa.gov/docs/swif..._SBarthelmy.pd
GF f

"For an image of the coded aperture mask, refer to:
http://swift.gsfc.nasa.gov/news/coded_app_mask.html."

That's all I have...

A*s*i*m*o*v

.... "Beware of all enterprises that require new clothes." -- THOREAU