[time-nuts] Re: The Collapse of Puerto Rico’s Iconic Telescope [April 5th, 2021 New Yorker]

[Dana Whitlow]

I have not yet heard what GB wants to do, but if it's long range work
(return time of
10's of sec or longer), the problems related to T/R switching, Tn
degradation of the
receiver, etc, get a lot more tractable than one might suppose.

At Arecibo, the S-band radar was intended solely for long range work.  A
big part ot
T/R switching was to use separate (optimized) feeds for transmitting and
receiving,
and swap feeds during turnaround.  The receive feed was covered by a
carefully-
fitted sliding cover during transmit times, which provided excellent
isolation.  There
was a microswitch to sense cover position and which would block
transmission if
it was not satisfied.

Feed horn turnaround time was about 7 seconds as I recall.

The transmitter was basically operated CW, and the "imaging" technique was
"range-
Doppler".  So, the transmitter was run for slightly under the round-trip
travel time, then
feed turnaround was initiated, timed to finish shortly before the leading
edge of the
return was expected.  The receiver would do its thing for the duration of
the return.
Note that it was also necessary to completely shut down the beam current of
the
transmit Klystrons during receive so that shot noise on the beam did not
spoil the
receiver's noise temperature.  After the receive interval was finished, the
feeds were
again swapped and a new cycle was started.  During each transmit period the
crew of scientists would have time for a preliminary look at the
just-recorded data
to see if anything was going wrong.

OTOH, Arecibo's 430 MHz ISR for ionospheric studies clearly had to have
almost
instantaneous turnaround time, and arrangements were rather more complicated
(more so than I can take time to describe at the moment).  But they worked
well,
and loss in the receive state of some big PIN switches in the path was
tolerably low.
The first 35-40 dB of TX-RX isolation came from use of a "turnstile
junction", with
the remainder coming from the PIN switches.

For both radars, the non-participating receivers would also be covered, but
with
a simpler flap arrangement, some powered by small DC motors and some by
small pneumatic cylinders.

Dana
(retired from Arecibo Dec 2016)


On Wed, Mar 31, 2021 at 2:30 PM ew via time-nuts <time-nuts at lists.febo.com>
wrote:

> During my days at UTC I was among other things responsible for our
> Industrial Laser Group. We sold some 50 KW CW CO2 lasers! We had the only
> aero dynamic window that allowed  a vacuum on one side. When not in use you
> put a pencil through it. Patented.
>                  Bert Kehren In a message dated 3/31/2021 3:17:21 PM
> Eastern Standard Time, richard at karlquist.com writes:
>
>
> On 3/31/2021 11:57 AM, Lux, Jim wrote:
>
> >
> > In some ways it's like high power laser labs. It's not the direct beam
> > you worry about - nobody is going to put their hand in the beam path.
> > It's the stray reflection when something gets bumped and falls across
> > the optical bench and reflects a stray beam at 0.01% power into your
> eyes.
>
> I did consulting at Coherent Laser Group working on CO2 lasers.
> In that lab, actually, many people had the misfortune of accidentally
> putting their hand or arm in the invisible beam.  You could see the
> scars on their arms.  There were also accidents where the laser
> burned the ceiling, etc.  Very scary place to work; fortunately,
> I never got injured.  They made lasers up to 1kW optical power out.
>
> Rick N6RK
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