The SFASU Observatory


Opened in 1976 to satisfy the needs of the introductory astronomy course on campus, the SFASU Observatory also has a long history of professional astronomical work.  The research telescopes include a 10-inch Meade LX200 Schmidt-Cassegrain, an 18-inch Lincoln Labs Ritchey-Chrétien, and a custom 41-inch Classical Cassegrain.


The 41-inch Telescope and Three Channel Photometer


















                                                           Figure 1

Both the telescope and the primary instrument package have been built in the shop facility of the Department of Physics and Astronomy at SFASU.  Dr. Markworth along with the departmental machinist built the telescope in 1983/84 with the three-channel photometer following in 1988-90.

The three-channel photometer is a modified version of the UT photometer Edward Nather (1992, see Figure 1).  The target star is placed in the stationary central channel.  The left and right channels are moveable over a wide range.  A periscope arrangement of mirrors picks off a portion of the field-of-view and redirects it to the offset stage.  The right channel is reserved for a nearby comparison star of constant light and the left

channel is used for sky readings.  The final periscope mirror of the left channel is perforated to allow light to pass simultaneously to the pre-aperture viewer as well as to the photomultiplier.  The pre-aperture viewer contains a Santa Barbara Instrument Group (SBIG) ST-7 CCD camera. Under the control of Cyanogen Production MaxIm DL Imaging software, the camera images a nearby field star, while MaxIm DL sends corrections to the telescope motors to keep the telescope tracking properly.

Three Hamamatsu R647-04 photomultiplier tubes send their signals to a Keithley CTM05 counter/timer card in the computer.  The spectral response of the tubes is shown in Figure 2.  Peltier cooling units lower the temperature of the photomultiplier tubes during summer operation to decrease thermionic noise to acceptable levels.



Stage Number

Serial Number

Dead Time (ns)














Figure 2


One of four filters can be rotated into the field-of-view just behind the aperture plane.  Currently, the Johnson-Morgan (UBV) system filters are installed.  The fourth position is presently clear.  These include a visual filter (Schott BG 18 plus OG 515), a blue filter (Schott BG 25 plus GG 400), and an ultraviolet filter (Schott UG 1).  The spectral response of these filters or filter combinations is shown in Figure 3. [Excel Data File]

Figure 3

Taken together the spectral response of the photometer is presented in Figure 4.

Figure 4

As noted above pulse counting photometry is achieved using the Keithley CTM05 counter/timer card.  The card has a stable crystal time base and five programmable counters.  A gating scheme is employed to sample the output of the photomultipliers in increments of 0.1 second.  These 1/10th second readings are accumulated into user specified integration intervals.  The short gating intervals ensure that the accumulator will not overflow while looking at brighter sources and also offer the possibility of computing standard errors.  The system just described regularly achieves standard errors of 0.15% or less, making it among the most precise photoelectric photometers in the world.

Telescope control software written in Visual Basic by W. D. Bruton of SFASU interfaces to The Sky from Software Bisque, allowing the telescope to move to any object in the database of The Sky.  The interface also allows for easy acquisition of stars in the offset stages.  The controller handles all telescope motions, the X-Y position of the two offset stages, the three filter wheel motors, telescope focus, and dome rotation.  During operation AstroVid cameras are positioned on the eight-inch finderscope as well as the post-aperture viewer of the center stage for ease in acquiring and centering the target star.