Our new chopping secondary does a good job of improving the baselines obtained with the heterodyne receivers. It should probably be used by any observers not requiring a "designated offset".
Currently, one should not chop faster than 5 Hz, or with a throw larger than 9 arc minutes. For heterodyne observations, you might as well chop more slowly than this; there appears to be no improvement in the quality of the data if you chop faster than about 1 Hz, and the duty cycle degrades rapidly is you chop faster than about 1 Hz. When you choose the chopping frequency, don't choose a nice even number like 1 Hz, which is evenly divided by 60 Hz, because there is some 60 Hz pickup in the system. Choose a strange frequency like 1.123 Hz instead.
The chopper is normally controlled by the computer. To do this the VAX sends instructions over a GPIB bus to an HP function generator, which produces the waveform that the chopper is criven by. The VAX sets the amplitude, frequency and offset of the squarewave prodeced by the function generator. The UIP's SECONDARY command allows these parameters to be specified.
Occasionally you may get the message
Cannot access the GPIB
- cannot set the secondary mirror parameters. *** SET THEM
MANUALLY ***
when you issue the SECONDARY command. This means
that there is a problem with the GPIB control. Nine times out of
ten, you can fix this problem by issuing the UIP command
SECONDARY/STOP several times. If the error is still reported after
you've given this command about ten times, then the problem is serious.
If you are adventurous, you could try power cycling the HP function
generator, and the SCSI to GPIB converter on the VAX (then try
SECONDARY/STOP again). If you aren't adventurous, you can manually
program the HP function generator using the numbers suggested by
the SECONDARY command.
To manually stop the chopping secondary from chopping, press any one of the buttons on the top row of the HP function generator for at least 2 seconds. You will hear a beep, and the little square-wave symbol will disappear. To stop it under computer control, issue the SECONDARY/STOP command.
To manually program the chopper, first issue the UIP's SECONDARY command with the appropriate parameters (see the UIP help on this command). The SECONDARY command will tell you what values should be entered into the HP function generator's front pannel. To initiate square-wave chopping, press the square-wave button momentarily. Please note that the SECONDARY command should be issued before you start taking chopping scans (using the CHOP_SLEWY command) even if you know what values should be entered into the HP. The SECONDARY command doesn't merely report information to you. It also programs the hardware that determines when the AOS should integrate, and it tells CHOP_SLEWY the separation of the beams on the sky. Also, if you execute CHOP_SLEWY commands from within a command file, you should include a SECONDARY command within the command file.
back to CSO documentation page
Taco (rtm@tacos.caltech.edu)
Nov. 28, 1995