A general theory of low order field aberrations of decentered optical systems has been given by Shack & Thompson ([1980]). In particular it has been shown that the general field dependence of third order astigmatism can be described by a binodal pattern, known as ovals of Cassini. Only for special cases such as a centered system do the two nodes coincide and the field dependence reduces to the well known rotationally symmetric pattern with a quadratic dependence on the distance to the field center. These general geometric properties have been used by McLeod ([1996]), starting from equations by Schroeder ([1987]), for the alignment of an aplanatic two mirror telescope. We shall recall this method briefly.

In a Cassegrain telescope the absence of
decentering coma in the center of the field does not imply that the
optical axes of the primary (M1) and the secondary mirrors (M2)
coincide. The axes of the primary and the secondary mirrors must
intersect at the coma free point, but the axis of M2 may still form an
angle
with respect to the axis of M1. For this case
McLeod showed that the components *Z*_{4}and *Z*_{5} of third order astigmatism of a two mirror telescope
with the stop at the primary mirror for
a field angle
with components
and
are given by

For a centered two mirror telescope Wilson ([1996]) has
derived expressions for
the low order field aberrations including third order astigmatism (*B*_{0})
showing the dependence on fundamental design parameters and optical
properties of the total telescope and on the position of the stop
along the optical axis.
This gives more physical insight into the characteristics
of field aberrations of two mirror telescopes. Similarly we
derive, for decentered two mirror telescopes, explicit expressions for
third order astigmatism, i.e. for the parameters *B*_{1} and *B*_{2},
and discuss the field dependence of astigmatism for
fundamental types of two mirror telescopes.

In a centered optical system the field center, projected towards the
sky, is the direction parallel
to the optical axis of M1. The formulae for the parameters *B*_{0},
*B*_{1} and *B*_{2} are initially derived for this reference system.
However, in a decentered system the image of an object in
the field center is not in the mechanical center of the
adapter, where the instruments are located. For all measurements
in the telescope, the practical field center is the center of the
adapter. The evaluation of the measurements of field astigmatism has
to take this difference of the origins of the reference systems into
account. The structure of the Eqs. (1) and
(2) will remain the same, but the parameters *B*_{0},
*B*_{1} and *B*_{2} will change and
will denote the
field angle with respect to the center of the adapter.

One case of practical interest is the collimation at the Cassegrain focus of the VLT where the stop is at the secondary mirror and the telescope is only corrected for spherical aberration. We apply our formulae to measurements of astigmatism in the field of the Cassegrain focus to determine the misalignment angles and . In addition, the possibility to change the misalignment angles by well defined values by rotating the primary mirror around its vertex allows a measurement of the accuracy of this collimation method.

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