NATIONAL RADIO ASTRONOMY OBSERVATORY
TO: Operators and Observers
FROM: Ron Maddalena
SUBJECT: Program to help with DBLBO observing
The DBLBO observing procedure, as most know, makes an observation with the reference beam of the nutator. The observer must specify the correct offset between the signal and reference beam in the H2 and V2 fields of the P-line in their observing file. (See the DBLBO documentation in the 140-ft manual for more details). However, although the nutator rack may say the offset is X arcmin it could be very different. I suggest strongly that observers should not trust the theoretical offset but that they should measure the difference between signal and reference beam.
I have written a program that should help you measure the offset between the
signal and reference beam. The program resides in the directory ~oper140/bin and
is called testdblbo. The program, which should be run by the operators, asks for
some user input and instructs the operator to do two pointing measurements, one
in the signal beam and the other in the reference beam. Here is a sample use of
testdblbo in which I used actual observations (my responses are in bold-italics):
fahd> testdblbo (or: ~oper140/bin/testdblbo )
Helps the operator measure the parameters to use for DBLBO.
Enter the expected beam throw in arcmin:
Enter subreflector position (22.5, 202.5, ...):
The theoretical H2,V2 fields are:
Enter source position you will use for pointing:
Enter RA (1950) DEC
POINT using the above coordinates. Enter the derived LPCs (make sure the detector is in SIG):
8.7 - 13
Now, switch the detector to REF and perform a POINT using the following coordinates:
Enter the LPCs from the second POINT:
The H2,V2 fields the observers should use for DBLBO in their observing files are:
(The following is for informational use only. Do not use the below answers in the observing file.)
Measured beam throw: 8.66 arcmin
Measured angle: 25.7 degrees
In this example, the operator types in 8 arcmin and 22.5 degrees for guesses for the beam throw and nutation angle. Next, the operator enters the source position for POINT and executes the observation with the detector switch on the nutator rack set to SIG. Then, the operator enters the measured LPCs from the measurement. The program then tells the operator to set the detector switch to REF and gives a position the operator should place into the panel. The operator performs the second point in the reference beam and enters into the program the determined LPCs. The program then derives the values for the H2 and V2 fields that the observers should use in their P-lines of their observing files. In this example, the results for H2 and V2 are -31.2 and 346. The actual beam throw was 8.66 arcmin at an angle of 25.7 degrees, results that should not be entered anywhere in the observing file.
All suggestions and questions are welcome.