GBT Precision Telescope Control System (PTCS) Update

Todd Hunter & Melinda Mello

During the summer GBT maintenance season, engineers performed a comprehensive service effort on the active surface hardware. Repairs were performed on a number of actuators with broken cables, unstable LVDT modules, and other maladies. At the same time, the scientific team reviewed the past measurements of how the surface deforms on large scales in response to daytime thermal changes. Out-of-focus (OOF) holography maps taken throughout a clear winter day indicate that although surface deformations become significant whenever the Sun is above ~ 10 degrees elevation, they change slowly while tracking a single source. This led the team to implement a new feature in the active surface software manager that allows an additional set of Zernike terms to be added to the existing elevation-dependent gravitational terms that have been in place for high-frequency observers since late 2006. With this component available, the team then developed an initial, staff-operable version of the OOF technique that allows the surface deformations to be measured and quickly corrected. The end-to-end functionality was tested on the sky at Q-Band during a sunny afternoon. The telescope beam profiles on a bright quasar changed from asymmetric to Gaussian, the peak antenna temperature increased, and two obvious sidelobes were eliminated. Thus, this technique has the potential to bring the daytime GBT aperture efficiency at high frequencies closer to its nighttime level.

An observer tool called “AutoOOF” is in final testing and is expected to be available to observers in a shared-risk mode this winter to further explore the effectiveness of the technique. AutoOOF is an Astrid procedure (i.e. similar to AutoPeakFocus) that acquires a quick set of in-focus and out-of-focus on-the-fly continuum maps on a quasar using the currently active receiver (Q or Ka-band). It then automatically launches the analysis software developed by Bojan Nikolic, a former Green Bank scientist who continues to collaborate with the PTCS group. This software produces a set of fits to the current surface deformation. The fits are described by Zernike terms (the so-called "thermal" terms) and are displayed in Astrid as surface images. The default correction is sixth order; however, the Astrid OOF plug-in allows the cautious observer to examine and compare the surface images that result from both a fewer and greater number of terms before choosing which to send to the active surface. The total time to run the procedure and apply the corrections is about 20 minutes. Because surface deformation becomes ever more important at higher frequencies, additional research continues on the initial Mustang OOF maps obtained last spring. The use of multi-pixel Mustang maps has the potential to significantly speed up the process of measuring and correcting the surface during 90 GHz observations.

Watch for other PTCS project news in the coming months.