In my last post, I discussed the laser measurements we do to determine position. I failed to mention that in order to find the location of the target* we had to measure the target and some reference block with our laser, measure the block and the beam** axis with the lab's laser, and then mesh the two together.

I failed to mention this because over the last... well, for a while now, some of us have been convinced that there was a problem with the laser measurements for the target/block from our laser. The block looked mostly reasonable, but the target was more than twice as tall as it should be.  Controlled experiments were done, and we tried to analyze every detail, but we always came back to the 'fact' that something wrong with the angles the laser had given.

Some thought the apparent problem was due to the laser being rotated on its side during the measurement and thought we weren't interpreting the angles corrected. I was convinced that I was treating the angles in the correct way and the problem had nothing to do with the rotation of the laser (some of you know how stubborn I can be when I get something into my head).  

My argument: The laser measures theta in the vertical direction, but starts in the xy plane, so we have to use 90-theta to get spherical theta. Phi is measured in the horizontal direction, but is positive when it moves right and it should be positive when it moves left towards the y axis, so we use -phi to get spherical phi. I was insistent that these still held when the laser was on it's side, it's only the xyz system that rotates. When you make the xyz system with your hand, you can rotate it any way you want and it's still xyz if you turn your head the right way.

Yesterday, I was chatting with the guy who wrote all of the laser code and we realized that phi seemed to behave correctly when we didn't include -phi. I meant to check whether using theta instead of 90-theta made a difference but I got distracted. I wish I'd checked.

Today, someone treated theta and phi as normal spherical coordinates and the target had the correct dimensions. I realized that I should have made that 2 minute check yesterday and spared many of us a day long email discussion about the matter. I checked the block and target, treating theta and phi with no corrections and they both look reasonable (see figure below - but ignore the comet-like tail on the target, that's what happens when the laser's out of range).

So I was wrong that the angles from the laser were wrong... they were actually 'corrected'****, which is different from what we expected. But I was right that it had nothing to do with the rotation. I'll take that.

*thin piece of calcium*** foil that we bombarded with calcium*** nuclei
**billions of calcium*** nuclei moving at 40% the speed of light, focused into a spot a few mm in diameter
*** both Ca40 and Ca48 were used
**** I'm 99.8% sure... no one else has confirmed my thoughts, but the author of the code admitted it was possible

Correction: the code was right,  the code was fine, but before the measurements the 0,0 mark was changed by 90 degrees, causing all the confusion.  Ah well, at least we know how to treat it and I can move on to the physics soon.