Abstract: The ASTRA Spectrophotometer IAU Symposium 210 Modeling of Stellar Atmospheres

A CCD-based spectrophotometer for a new automated 0.5-m telescope at the
Fairborn Observatory, Washington Camp, AZ, USA, altitude 1800 m should begin
operations by Spring 2004. The Citadel ASTRA (Automated Spectrophotometric
Telescope Research Associates) Telescope will permit observations of Vega the
primary spectrophotometric standard, rapid measurements of the naked-eye stars,
sufficient observing time to obtain photometric measurements of the nightly
extinction, and still obtain high quality observations of stars of about 10.5
magnitude in an hour. This multiplexed cross-dispersed instrument should
produce high-quality fluxes at least of 3300-9000 A with a
resolution of 14 A in first and 7 A in second order and full wavelength
coverage except for regions badly affected by telluric lines.

The telescope and spectrophotometer are optimally designed for efficient
operations using input from astronomers and the experience of the design team.
The CCD frames will be reduced to one-dimensional spectra using program CCDSPEC
by Austin F. Gulliver & Graham Hill and photometrically calibrated using
program SPECPHOTOCAL by Barry Smalley that should correct for the effects of
most telluric lines at airmasses < 4. Model atmospheres are the analytical link between the physical properties of stars (M, R, L, and composition) and the observed flux distributions and spectral line profiles. By comparing predictions of model atmospheres with spectrophotometric fluxes (and Balmer line profiles) effective temperatures, surface gravities, and metallicities can be found for a wide variety of stars. Comparisons for the same star between the best-fitting model atmospheres calculated from different codes will provide insight into how well each code reproduces these observations. High-quality elemental abundance studies will permit consistency checks. As the efficiency of convection can produce observable results in the energy distributions, it should be possible to check the results of different convection theories. Investigators will be able to synthesize a variety of indices which could be obtained by filter photometry. The first major observing project will be the revision and extension of the bright secondary standards. The existing grid of good secondary standards will be increased several-fold and include those selected to be near variables of particular interest. Over the lifetime of the instrument, measurements of secondary stars for calibration and extinction will be used to improve the quality of the secondary standard fluxes. In less than a year of normal observing, all isolated stars with V < 7.5 mag. and declinations between +76 and -35 degrees can have their fluxes well measured. Adelman, Gulliver, and Smalley in planning to deal with this potential flood of ASTRA data, realize that they will need help to make the best scientific uses of it. Thus they are interested in discussing possible collaborations. As many studies of variable stars will utilize local spectrophotometric standards, they plan to calibrate such stars as part of the initial effort.