[RASMB] IF/vs. intensity scans

[Kristian Schilling]

Borries,

let me first say I was most impressed by our last communication (to me, not 
RASMB) dated 5 AM your time. The early birds gets the worm...

Yes, can confim your results. There is often some difference between sample 
& reference in the magnitude of some 0.1 mm. There are two possible 
explanations:

1) the two sectors in the cp are not identical
2) the cp is misaligned in the housing due to an erranous position of the 
leading bar inside the housing.

The second should be a considerable error that should be visible by eye, so 
I tend to think the main contribution should be from the cp.

As you know, we use independently produced cps (Ti) and observe the same 
tolerances. I should think that errors within 0.1 mm are within the 
tolerances achievable in manufacturing.

We always keep the same assemblies (housings, windows, cps) together & from 
run to run, the bottom positions are well reproducible. As there is a 
larger contribution due to rotor speed (the rotor stretches about 1 mm @ 
60krpm), we always acquire meniscus & bottom positions at the used rotor 
speed for each single run & put it down in the journal. If time is 
critical, you need to scan only the bottom vicinity; the meniscus position 
can be well determined in the fringe display where you know what is ref & 
what is sample.

This procedure also handles possible differences in the positions of the 
rotor holes mentioned by Tom Laue. We think it is an acceptible effort, and 
often, this extra scan turns out also to be useful in some other way.

As to interference: We always measure out radial positions in the fringe 
display as you can see details there that are no more visible in the 
transformed scan. The fringe pattern will end at the boundary of either 
channel, a grey zone indicates the boundary of the other channel and the 
transition from grey to black the boundary of the other channel. The 
problem is, you can't tell which is ref & which is sample, so intensity 
scanning is the only means of getting that information.

Might be worth a suggestion to Beckman though to do something about that 
ingenious light green display of the marked position in the fringe display. 
It is only legible when you scoll to a black region... Might be time to 
find a better solution...

Greetings from Berlin,

Kristian



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