Ka-band observing guidelines

Compiled by A. Harris from Zpectrometer data and related email exchanges with B. Mason and T. Minter

Elevation effects

In short, observe between elevations of 20 and 80 degrees (below 75 degrees if possible) for reasonable telescope performance, and at as high an elevation as possible for atmospheric transmission considerations.  Both pointing and the surface degrade at high elevations.   The pointing model is most accurate from 20-80 degrees elevation, where data for the model are taken.  The surface correction tables have very few observations at high elevations, and the surface may distort from deformations in the backup structure, so the surface may deteriorate at high elevation.  The feed arm is vertical at 77.8 degrees, and stresses in the backup support structure change rapidly at elevations near this.  A practical lower limit is 20 degrees elevation for reasons of gain, pointing, and atmospheric transmission (transmission scales as t_z^AM, where t_z is the zenith transmission and AM is the airmass, 1/sin(elevation)).  

Wind effects

We measure 10% flux errors in repeated scans on pointing sources for winds of about 3 m/s.  This is lower than the general GBT recommendations but is consistent with observations reported in  http://wiki.gb.nrao.edu/pub/Dynamic/DynamicProjectNotes/dspn1.pdf  Observations seem to be profitable up to winds of 6-8 m/s.  Deviations from nice smooth pointing and especially  focus curves are good ways to gauge the effect of wind on effective antenna gain.

Surface distortion from solar heating

The surface distorts markedly during rapid changes in ambient temperature.  Daytime observations can have aperture efficiencies below 70% of the nighttime values (a factor of two in observing time).  Daytime observations should use the AutoOOF (automatic out-of-focus mapping to measure and correct surface distortions); see http://wiki.gb.nrao.edu/bin/view/PTCS/AutoOOFInstructions   The suggestion is to run AutoOOF every hour or two; this is insufficient during rapid temperature changes near dawn or sunset, when we have easily measured varying antenna gain on short timescales.  The wiki page suggests that a useful diagnostic is watching for a sidelobe in the elevation scans in the pointing curves (AutoPeak); a noticeable sidelobe is a sign that an an AutoOOF is indicated.

Questions or comments?  Please contact Andrew Harris.