Data Reduction

After having enabled the PISCO context (command SET/CONTEXT PISCO), the basic reduction of PISCO data involves merely one command, REDUCE/PISCO, which starts a routine (pisco.prg) solely consisting of other MIDAS commands. So, the whole procedure can be followed when the ECHO facility is enabled (see description for ECHO/ON or ECHO/FULL).
The essential steps are: (1) division of the data by the integration time (stored in descriptor O_TIME), (2) subtraction of the sky and the offset (using a sky measurement, if available, or a dark measurement), (3) calibration (division by calibration file) and (4) Fourier transformation. As is described in Appendix A of the PISCO Operating Manual, the last step, a simple Fourier transform using the MIDAS command FFT/POWER, is the core of the reduction of PISCO data.
The procedure is designed to handle an entire set of data by operating on an input catalog and producing a single output table in which each row holds the results for one data frame in the input catalog. There are no free parameters except for a mode, which can be set to 1, 2, or 3 depending on whether the two sky channels X and Y shall be reduced separately, together, or separately and together, respectively. Normally, the X and Y channel should be treated together.

 

Table E.1: Format of output table produced by REDUCE/PISCO

column label	content
IDENT:	identifier, copied from descriptor IDENT of raw data file
UT:	universal time (hours), copied from descriptor O_TIME(5) of same data file
X_INTENSITY:	mean of 1st line from data file, divided by integration time O_TIME(7), sky subtracted
Y_INTENSITY:	mean of 2nd line from data file, divided by integration time O_TIME(7), sky subtracted
POLXY:	degree of polarisation in channels X+Y (mode=3)
ERRORXY:	error of :POLXY
ANGLEXY:	position angle of E-vector in channels X+Y (mode=3)
POLX:	degree of polarisation in channel X (mode=1)
ERRORX:	error of :POLX
ANGLEX:	position angle of E-vector in channel X (mode=1)
POLY:	degree of polarisation in channel Y (mode=2)
ERRORY:	error of :POLY
ANGLEY:	position angle of E-vector in channel Y (mode=2)

 

The program attempts an error estimate from the noise level in the Fourier-transform. The polarisation angles still have an arbitrary zeropoint and need to be transformed to a standard system by means of observations of standard stars. Apart from the comparison with calibration data, the results of REDUCE/PISCO should be final.

The output table is a standard MIDAS table so that all applicable table commands can be used to further process the results. The columns of this table are shown in Table E.1.

=01 =11 =1992