The Advanced Technology Solar Telescope: 
Science Goals and Instrument Description

Thomas R. Rimmele
National Solar Observatory (Sacramento Peak)

High-resolution studies of the Sun’s magnetic fields are needed for a better
understanding of solar magnetic fields and the fundamental processes
responsible for solar variability. For example, the generation of magnetic
fields through dynamo processes, the amplification of fields through the
interaction with plasma flows, and the destruction of fields are still poorly
understood. There is still incomplete insight as to what physical mechanisms
are responsible for heating the corona, what causes variations in the
radiative output of the Sun, and what mechanisms trigger flares and coronal
mass ejections. Progress in answering these critical questions requires study
of the interaction of the magnetic field and convection with a resolution
sufficient to observe scales fundamental to these processes.

The 4m aperture ATST will be a unique scientific tool, with excellent angular
resolution, a large wavelength range, and low scattered light. With its
integrated adaptive optics, the ATST will achieve a spatial resolution nearly
10 times better than any existing solar telescope. The ATST has been highly
ranked by the latest Decadal Survey of Astronomy and Astrophysics and the
NAS/NRC study of ground-based solar astronomy.

The ATST will provide:
· Unprecedented angular resolution of 0.03 arcsec in the visible and 0.08
arcsec at 1.6 microns to enable us to clearly resolve and study the
fundamental astrophysical processes on their intrinsic scales and to verify
model predictions.
· A high photon flux for accurate and precise measurements of physical
parameters, such as magnetic strength and direction, temperature and velocity,
on the short time scales involved.
· Access to a broad set of diagnostics, from visible to thermal infrared
wavelength.
· Low scattered light observations and coronagraphic capabilities in the
infrared to allow measurements of coronal magnetic fields.

Development of a 4-m solar telescope presents several technical challenges.
The large heat flux makes thermal control of optics and telescope structure a
paramount consideration. To achieve diffraction-limited performance, a
powerful adaptive optics system is required that operates from the visible to
infrared wavelength using solar structure as the wavefront sensing target. Low
scattered light is essential for observing the corona but also to accurately
measure the physical properties of small structures observed on the disk.
Contamination control of the primary and secondary mirrors must therefore be
addressed. An initial set of instruments will be designed as integral part of
the telescope during the D&D phase. A strawman telescope design and instrument
concepts will be discussed. Examples of recent high resolution observations
with adaptive optics will be given.