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GBT Current Performance - Summer 2006

Pointing and Focus

The following table lists the current GBT pointing and focus performance. The current metrics consist of using astronomical calibrators to measure the blind pointing, offset pointing, and tracking (see below). N.B., this performance is only achieved during benign conditions when the thermal gradients are small (several hours after sunset until several hours after sunrise) and the wind is low (< 3.0 m/s).

Note A: blind pointing corresponds to the one-dimensional rms pointing error (maximum of azimuth and elevation) or rms focus error after performing only a single point/focus observation for your observing run to remove any DC offsets. It is blind in that other than the first point/focus observation no local offset observations are performed.

Note B: offset pointing corresponds to the two-dimensional rms pointing error or the rms focus error after performing a local offset point/focus observation. The pointing calibrator is typically less than 10 degrees from the target source.

Note C: tracking corresponds to the two-dimensional rms pointing error while tracking a source. This measures any short-period pointing fluctuations (such as feedarm vibrations). Pointing offsets that vary smoothly with time have been removed.

For additional information about the GBT pointing and focus performance see PTCSPN 25, PTCSPN 26, PTCSPN 28, PTCSPN 49. The performance may be significantly worse during the daytime and has not been well quantified. The one-dimensional rms pointing error due to wind can be approximated by

arcsec,

where S is the wind speed in m/s (see PTCSPN 26).

Surface Accuracy

The GBT active surface is adjusted using several different techniques. Photogrammetry was used to set the initial surface at a given elevation to determine the best zero offset position of each actuator. A finite element model (FEM) is used to adjust the active surface to improve the performance as a function of elevation. Out-of-focus holograph as recently been used to improve the FEM model at higher freqencies (for additional information see PTCSPN 47, PTCSPN 50, .PTCSPN 51). From 290 MHz to 50 GHz the gain is approximately constant with elevation with an rms surface error of 390 microns. Below is a plot of the measured aperture efficiency versus frequency. N.B., this performance is only achieved during benign conditions when the thermal gradients are small (several hours after sunset until several hours after sunrise).

For a measure of the GBT performance that includes the weather and receivers see: A Performance of the GBT

-- DanaBalser - 21 Jul 2006