Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Gaia on-board metrology: basic angle and best focus   The Gaia payload ensures maximum passive stability using a single material,
SiC, for most of its elements. Dedicated metrology instruments are, however,
required to carry out two functions: monitoring the basic angle and refocusing
the telescope. Two interferometers fed by the same laser are used to measure
the basic angle changes at the level of $\mu$as (prad, micropixel), which is
the highest level ever achieved in space. Two Shack-Hartmann wavefront sensors,
combined with an ad-hoc analysis of the scientific data are used to define and
reach the overall best-focus. In this contribution, the systems, data analysis,
procedures and performance achieved during commissioning are presented

33
The categories of this paper are: plasm-ph
3
The categories of this paper are: physics
36
The categories of this paper are: cmp-lg
2
The categories of this paper are: math
14
The categories of this paper are: math-ph
20
The categories of this paper are: acc-phys
8
The categories of this paper are: hep-ex
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
4
The categories of this paper are: cond-mat
10
The categories of this paper are: q-bio
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The categories of this paper are: hep-lat
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The categories of this paper are: econ
16
The categories of this paper are: stat