South Pole Diaries 1998/99

   

   


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

Coming Soon!

 

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