T4 0.75 meter APT

Observing Program

The T4 0.75m APT began high-precision observations of solar-type stars in April 1993 at the Fairborn Observatory APT site on Mt. Hopkins and continued operations there until July 1996. The telescope was relocated during the summer of 1996 to Fairborn's new site at 5500 ft in the Patagonia mountains near Washington Camp, Arizona. Operation of the 0.75m APT is funded by NASA, NSF, and Tennessee State University.

The T4 APT is dedicated to long-term photometric monitoring of approximately 75 lower-main-sequence (solar-type) stars. The spectral types of these stars range between F6 V and K3 V and span a broad range of ages. These stars were monitored contemporaneously for many years by the Mt. Wilson HK Project to determine their levels of magnetic activity. The APT observations are made differentially with respect to three comparison stars carefully chosen for high photometric stability. Extensive quality control and calibration measurements with the APT result in an external precision of about 0.0013 mag for a single observation and 0.0001 to 0.0002 mag for seasonal means. At this level of precision, the telescope is able to follow long-term brightness changes in Sun-like stars that are analogous to the subtle long-term brightness changes in our Sun observed with space-based radiometers. The Sun's brightness varies by roughly 0.001 mag in step with the sunspot cycle. The APT measurements of solar-type stars, combined with the observations of the stars' magnetic activity, should provide greater insight into the relationship between magnetic cycles and brightness changes in a variety of stars.

Several of the stars on the observing menu of the 0.75m APT have been discovered to have planets in orbit around them. Since these discoveries have been made indirectly with radial-velocity techniques, complementary observations are required to confirm the existence of these planets. For instance, the observed radial-velocity variations might actually arise from surface magnetic activity or stellar pulsations, both of which would be expected to give rise to small but measurable photometric variations. Therefore, the precise brightness measurements made with the 0.75m APT are important for understanding the nature of the observed radial-velocity variations and confirming the reality of the planetary discoveries. The 0.75m APT is also monitoring these stars in search of the signatures of planetary transits. Detection of transits, combined with the radial-velocity observations, allow the determination of the planet's true mass, its size, and its mean density. This provides important clues to the planet's composition and origin. The long-term photometric observations of these stars will also reveal which of them have the stable energy output required for life to exist on suitable planets.


  • Pyrex 30-inch Cassegrain optics from Star Instruments
  • Primary mirror focal ratio is f/2
  • Secondary mirror diameter is 8 inches
  • Effective focal ratio is f/8
  • Horseshoe equatorial mount design by Fairborn Observatory
  • Mechanical fabrication by Rettig Machine Shop of Redlands, California
  • Micro-stepping Berg belt to disk & roller drives on both axes
  • High-precision photometer from Fairborn Observatory
  • Stromgren by filters
  • Neutral density filters with attenuation of approximately 1.25, 2.5, 3.75, and 5.0 mag.
  • Selectable diaphragm wheel with diameters of 30, 45, 60, & 90 arcseconds
  • EMI 9124QB photomultiplier detector running at 1200V and +4 degrees C
  • Photomultiplier, voltage divider, preamp, and filters in sealed environmentally controlled chamber
  • ATIS93 automatic control system by Fairborn Observatory
  • External precision of a single observation is approximately 0.0013 mag, 0.0002 for seasonal means