Alfa_ang - IDL procedure for calculating the optimum rotation angle for ALFA

Alfa_ang is a GUI for the rotang routine. The purpose of this routine is to calculate the optimum rotation angle for ALFA given the RA and Dec of a source and the date and time of the observation. The routine also returns the azimuth and zenith angle of the telescope at the requested time, thus it can be used to find the correct rotation angle and azimuth for the Fixed Azimuth Drift Scan observing mode used by the AGES drift precursor observation.

This document covers the use of the alfa_ang GUI, there is also basic documentation for rotang covering the standard command-line options and detailed documentation covering the other command line parameters, including the output parameters, and describing the minimisation algorithm.

To run alfa_ang at Arecibo:

• If you are using the AGES account, simply type alfa_ang at the command prompt
• If you are not using the AGES account, start IDL (type idl at the command prompt) then enter the following commands:
  IDL> @phil
  IDL> @corinit
  IDL> addpath,'/share/ages/IDLSTUFF'
  IDL> alfa_ang

This brings up the ALFA rotation angle calculator window:

The top line is used to specify the time and date of the observation. The Date (YYMMDD) box is used to enter the date in the format YYMMDD (e.g. 041119 for the 19^th of November, 2004). The Requested time (HHMMSS) box is used to enter the time in HHMMSS (e.g. 000000 for 00:00, 063000 for 06:30 or -003000 for -00:30). The Form of time radio buttons are used to selected which of HA (for Hour Angle), AST (for Atlantic Standard Time) or LST (for Local Siderial Time) the requested time is given in.

The second line is used to specify the R.A. and Dec. of the object to be observed. the RA (HHMMSS) box is used to enter the R.A. in the format HHMMSS (e.g. 012900.0 for 01:29:00.0) and is in J2000 coordinates. The Dec (DDMMSS) box is used to enter the declination in the the format DDMMSS (eg 154700.5 for 15:47:00.5) and is also in J2000 coordinates. Finally, the Calculate button sends the inputs to rotang and the Exit button exits the program.

The results are returned in their own Results window:

The top line gives the date of the observation and the R.A. and declination of the source. Below this are given the Julian Date (JD), Local Sideral Time (LST), Atlantic Standard Time (AST), Hour Angle (HA), azimuth angle (Az), zenith angle (ZA), ALFA rotation angle (Rot Ang) and the declination separation of the beams (Dec Sep). A graphical indication of the beam positions on the sky is also shown in a 15 by 15 arcmin window. The '+' signs indicate the actual beam position and the numbers beside them indicate the beam number. Finally, the Close button is located below the beam position display, this button is used to close the Results window. It is possible to have more than one results window open at once, selecting Close will close only the window in which the button is located.

If the source is below 20 degrees or above 2 degrees zenith angle, then a warning line will be displayed just below the top line of the Results window and a warning pop-up will appear:

Alfa_ang was written by Robert Minchin. Rotang was written by Robert Minchin and incorporates code from Phil Perilat and from the Cornell EGG.

Original page written by Robert Minchin, 22^nd November 2004. Last modified 22^nd November 2004 by Robert Minchin.