Optical Images of Stellar Surfaces
Images a thousand times sharper than from Hubble
Current methods in optical astronomy barely start resolving stars as
surface objects (not mere points of light), their disks perhaps
flattened by rapid rotation, changing their size under pulsation, or in
the process of ejecting gas clouds.
Improved resolution will reveal the diversity of stars but requires
new techniques. Conventional methods are limited by turbulence in air,
which us why ambitious space missions have been proposed for both ESA
and NASA. However, their complexity means they will take time to become
reality.
How can CTA make optical images?
The first kilometre-scale optical imager, enabling mapping of stellar
surface structures, may be realised with CTA, using a technique known
as intensity interferometry. The great advantage of this technique is
that it is not sensitive to either atmospheric turbulence or
imperfections of the telescope mirrors. The reason is that pairs of
telescopes are connected only electronically (not by optical light),
with time resolutions of nanoseconds, corresponding to light-travel
distances of some metres. Requirements are large-area light collectors
with very fast detectors - precisely the features of CTA!
What we can Observe
Intensity interferometry is suitable for observing hot stars and gas
clouds surrounding them. Some examples are shown in the figure below,
which shows types of objects that could be studied with CTA intensity
interferometry.
 |
| Objects that could be studied using CTA as an intensity
interferometer. Top row: Stellar shapes and surfaces affected by rapid
rotation; Middle row: Disks and winds of hot gas around active stars;
Bottom row: Stellar surroundings such as shells, obscuring clouds, and
jets. |
How sharp could images be?
Computer modelling has been performed to understand how sharp images can be obtained with CTA, and a result is shown below.
 |
| A simulation of the possibilities for a ground-based intensity
interferometer. Here, the reconstructed image of a binary star (of
assumed magnitude 6, barely visible to the naked eye) is shown. The
resolution approaches an angle of some 30 microarcseconds, a thousand
times better than feasible with the Hubble Space Telescope. |
Further Reading
Dravins et al.: Stellar Intensity Interferometry: Astrophysical targets for sub-milliarcsecond imaging, Proc. SPIE 7734, 77340A (2010); http://arxiv.org/abs/1009.5815
Nuñez et al: Stellar Intensity Interferometry: Imaging capabilities of air Cherenkov telescope arrays, Proc. SPIE 7734, 77341C (2010); http://arxiv.org/abs/1009.5599
LeBohec et al.: Stellar Intensity Interferometry: Experimental steps toward long-baseline observations, Proc. SPIE 7734, 77341D (2010); http://arxiv.org/abs/1009.5585
Jensen et al.: Stellar Intensity Interferometry: Optimizing air Cherenkov telescope array layouts, Proc SPIE 7734, 77341T (2010); http://arxiv.org/abs/1009.5828