... plane

The PSPC has no "real'' focal plane. An incoming photon releases an avalanche of electrons in a certain depth in the detector chamber. The higher the photon energy, the deeper the formation of the electron shower.

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... mirror

The deviation of the mirror geometry from the ideal one causes a major part of the mirror defocusing.

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... dimension

The distinction between true photons and counted photons ("events'') is not made here.

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... that

The expression "$A:\,\,=B$'' or "$B=\,\,:A$'' means: A is defined by B.

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... PSF

In our usage, the meticulous signification of the term PSF is: analytical model of azimuthally averaged part of the observed PSF.

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... relevance

The term encircled energy for P is not used here. In our case photon counts are distributed, not energy. In this article, p and P are the generic symbols for a photon probability density distribution and P for a probability distribution. So, the reader should be prepared to see that p denotes different distributions in different context. For example, $p(r;E,\epsilon)$ in (6) and in (10) are different.

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... PSF

For two-dimensional circularly symmetric normal distributions, i.e. not azimuthally dependent, the relation ${1\over2}FWHM = median$ is valid. In general, however, both quantities areunrelated.

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... error

Two constitutive parts are to be named; a contribution due to the imprecise attitude reconstruction and a physical part. The latter comes from the temperature dependent misalignment of the axes of star tracker and mirror and other sources.

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... drift

Systematic drift may be caused by thermal boresight deviation or by angular momenta due to the magnetic coils.

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...

The expression "k=a(b)c'' means: integer variable k runs from a to c in steps of b.

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... 

1999 was the year of the latest revision of the report at the time of writing whose history spans some years.

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... eccentricity

There is no great risk to confound eccentricity e and even less $e_{\rm A},e_{\rm B}$ with the mathematical constant $e:\,\,=2.71...$

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