Monday 11th January - Ice Halos
From
John Storey.....
Today
we saw the most spectacular ice halo imaginable. All the ones
I'd seen previously, and even been moved to write a paper
about with Max, paled into insignificance. We got the full
treatment - 22 and 46 degree haloes, sun dogs, a horizontal
arc that went the full 360 degrees, a brilliant circumzenuthal
arc, 120 degree spots, that funny football shaped thingy on
top of the 46 degree arc, a cross 180 degrees from the sun,
and some funny white arcs that completed the circumzenuthal
arc but I'm sure weren't supposed to be here. The whole station
went berserk with people firing off rolls of film. I tried
to grab some images on the webcam by using a flag as an occulting
disc, but not very successfully. There are some excellent
photos and a description on the web page (http://www.spole.gov).
This may be the best halo display ever recorded on film.
A
little previously I had a go at bringing the Orbcomm satellite
communication system up. When I came to plug the transceiver
into the serial port on the general purpose PC, I found the
serial connector was sort of dangling loose inside the machine
so I had to take the whole computer apart. I was just musing
to myself about this interesting interpretation of "plug
and play" (sort of "find the screwdriver, dismantle,
lash things together, and then play") and speculating
out loud about how I could contrive a gruesome end for Bill
Gates when in walked the two ASA psychologists. They said
they just wanted a look inside the AASTO, but I'm sure they'd
been tipped off. I tried to put on my best "I really
do like Windows 95" expression but I think they know.
Anyway,
believe it or it or not the Orbcomm box is alive and claims
to be looking out for passing satellites, but it hasn't actually
found one yet. Andre - is there an ephemeris somewhere? How
many satellites are up there that I can talk to? Also, is
that eggbeater antenna really 50 ohms?
Last
night, Fred Mrozek, Peter Gillingham and I brought the whole
top-end spider of SPIREX inside, put a plastic bag over the
secondary to minimize condensation, and plugged the PZTs into
the driver electronics to stop them arcing over as the unit
warmed up.
We
then confirmed Mick's diagnosis that PZT #3 is stuffed. We
swapped cables around and made sure it was actually the PZT
itself. Actually the strain gauge works, but not the PZT itself.
While
fiddling about the whole thing suddenly stopped working. We
decided it was a fuse and tried to dismantle the box to replace
it. This was a totally baffling experience and I must have
wasted 45 minutes undoing unimportant screws and pulling back
promising looking rubber strips. If it wasn't for the fact
that one of the world's top opto-mechanical engineers was
working with me and was equally baffled I would have felt
very silly.
I finally prized off the black plastic bit on the mains inlet
and there sat not one but two fuses. The manual says 1.6 A
slow blow so we stuck in a couple of 2.0 A slow blows which
is ok if you don't worry too much about significant figures.
So
then all the lights worked again, but when we plugged in the
new PZT it didn't work. Arrggghhh! Not to put too fine a point
on it, the strain gauge bridge that provides position feedback
wouldn't balance.
At
2 am we gave up and went to bed.
Today
we puzzled over the possible reasons why a brand new PZT strain
gauge might not balance. The book says just to twiddle the
front panel "zero" knob, but that simply didn't
work. Ripping the plug-in circuit board out and examining
it, it seems there are two amplifier stages. The front panel
"zero" pot works on the second stage, and there
is a second, internal zero pot, P2, on the first stage. Aha!
So if the first stage is too far out of whack it will saturate
and adjusting the second stage won't help. So, trim P2 and
we're in business. P2 was at 19 turns from the CCW stop. However,
turning it this way and that had remarkably little effect.
Do you think there's any chance of scoring a circuit diagram
out of PI?
The
new bridge is 700/701 ohm, compared to the previous unit which
is 706/707 ohms, ie a 1% difference. A drawing of the strain
gauge assembly shows that both bridge elements measure the
same thing, ie it's not differential. It's therefore the absolute
value that matters, not the difference. Since the new strain
gauge is 1% lower in resistance than the old one, we figured
that reducing the other two resistors in the bridge (which
are on the circuit board) by the same amount might just work.
Sure
enough, a couple of 75K resistors in parallel with the existing
705 ohm resistors (which are described in the book as 703
ohm) and bingo! - a nicely balanced bridge.
I'm
not sure what kind of resistors I used but they were blue
and looked very expensive and have written on them: BET RN60D
7502FJ NA60.
I
put P2 in middle of its travel, for want of a better idea,
and adjusted the front "Zero" pot as per the manual.
Tonight
we'll take the secondary mirror assembly apart and put the
new PZT in. This evening I've just received some very helpful
emails from mcba, Thomas and John McMahon which I think answer
just about everything, so I'll make great progress tomorrow.
Al
Fowler turned on Abu's closed cycle cooler and it went ker-chug
ker-chug all day but is not cooling down the system fast enough.
Al is now convinced that the cooler head is stuffed and is
waiting for Al Harper to bring a new one in.
A
major problem is looming with the flight schedule, in that
they plan to close all flights between McMurdo and ChCh from
18 Jan until 26th, while they shift the airfield onto the
ice runway.
My
only other activity today was to make a nice gangway across
to the AASTO from the snow. Until now you've add to fall down
the bottom of a snow drift, pick yourself up and then climb
up the ladder. The new method conserves gravitational potential
energy and reduces the risk of breaking a limb. I also tidied
up around the AASTO and straightened the thermometer so it
looks nice on the webcam.
John
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