Optimization of the Cherenkov Telescope Array for Intensity Interferometry

Dainis Dravins, Lund Observatory, Sweden

The planned CTA, Cherenkov Telescope Array, is an advanced facility to study
high-energy gamma-ray sources through observations of atmospheric Cherenkov
light.  Recent evaluations by different European research bodies (ASTRONET,
ESFRI, ASPERA) have placed CTA among the highest-priority projects for the
coming decade, enhancing the prospect of its timely realization.

Although the CTA design is not yet finalized, current thinking envisions a
large number of telescopes (between 50 and 100), spread out over a square
kilometer or more, with telescopes of different sizes, ranging between
perhaps 25 and 5 meters, for a total collecting area on the order of 10,000
m**2.  This unprecedented collecting area for optical night-sky observations
will enable also other types of observations, in particular a use as an
intensity interferometer to achieve ultrahigh-resolution
(sub-milliarcsecond) optical imaging.

As a (small) part of the CTA design study, also the requirements for its use
as an intensity interferometer are to be examined.  If modest additional
specifications can be defined, such might be incorporated into the design
provided they do not compromise its main purpose of studying gamma-ray
sources, and do not add significant complications or costs.

Issues include: Detectors - use of only the central pixel(s) of the camera
or adding separate detectors?  Electronics: Will time tagging of photon
pulses to accuracies of 1 ns–100 ps be assured until the signal arrives to
some computing location?  Detector optomechanics: How to focus on stars at
infinity rather than on Cherenkov light in the upper atmosphere?  How does
one incorporate additional optical elements such as color filters in front
of the detector?  Telescope design: An isochronal optical design (such as
paraboloid) permits higher time resolution and is preferred over others
(such as Davies-Cotton).  Is the optical image quality an issue?  Should
telescopes be placed in some optimal geometric pattern for best coverage of
the interferometric (u,v)-plane; is this perhaps important only for the
larger telescopes, or will there be so many telescopes that the (u,v)-plane
is filled-in anyway?