GBT keyholexs

Wednesday 12 November 2025
This documentation is courtesy of Frank Ghigo.

With the slower az slew rate due to cold weather, the size of
the "keyhole" becomes bigger.  This can be a problem when
doing AutoPeaks at the lower frequencies, where the scan rate
is high.

The angular scan rate (Rang) is the azimuth rate (Raz)
times cosine of the elevation.  So given a desired scan rate
and a maximum allowed azimuth rate, the maximum allowed elevation is
   El(max) = arccos( (Rang/Raz)

I have made a table, below, giving the default scan rates used
for AutoPeak for the various receivers, and the corresponding
elevation limits for the max az rates for the cold and warm conditions.
The AutoPeak rate is in arcmin/minutes.
You can see that with the L-band receiver, the keyhole is down
to 74 degrees for the cold Az rate limit.

        Receiver   Rate  El(warm)  El(cold)
                   ('/min) (deg)    (deg)
        Rcvr_342   360     80.00    68.43
        Rcvr_450   360     80.00    68.43
        Rcvr_600   360     80.00    68.43
        Rcvr_800   320     81.08    70.83
       Rcvr_1070   260     82.69    74.36
         Rcvr1_2   260     82.69    74.36
         Rcvr2_3   180     84.82    78.98
         Rcvr4_8    80     87.48    84.66
        Rcvr8_10    40     88.54    86.91
       Rcvr12_18    38     88.59    87.02
  RcvrArray18_26    18     89.12    88.15
       Rcvr26_40    20     89.07    88.03
       Rcvr40_52    12     89.28    88.48
       Rcvr68_92     6     89.44    88.82
         Mustang    12     89.28    88.48


El(warm) = max elevation for warm weather azimuth slew rate (36 deg/min)
El(cold) = max elevation for cold rate (17 deg/min)