SKYDIP - calculate sky properties from SCUBA skydip data

Description:

This application takes raw SKYDIP data and calculates tau, eta_tel and B by fitting. Sky brightness temperatures are calculated for different airmasses and then fitted with a model of the sky.

Usage:

skydip in sub_instrument t_cold eta_tel b_fit out model_out

Parameters:

IN = NDF (Read)

The name of the skydip data file, or, if SCUBA_PREFIX is set, the number of the observation (raw demodulated data only). The input data file can be the output from reduce_switch or the raw skydip data. The file can be located in $DATADIR.

B_ERR = REAL (Write)

Error in the fitted value of B. Set to bad if the fit has failed.

B_FIT = REAL (Write)

The fitted value of the B parameter (Filter transmission). Set to bad if the fit has failed.

B_VAL = REAL (Read)

The B parameter (filter transmission). This efficiency factor must be between 0 and 1. A negative value allows this parameter to be free.

CVAR = LOGICAL (Read)

This parameter governs whether the points are fitted with a constant variance for all points (true) or the variance derived from the scatter in the individual integrations (false). The value used for the fixed variance is the mean of all the calculated variances.

ETA_TEL = REAL (Read)

The telescope efficiency. If available the current telescope value is used as the default. Values must be between 0 and 1.0. A negative value allows this parameter to be free. For data taken before 26 April 2000, the default values supplied for 850 and 450 are provided by the routine rather than being read from the FITS header.

ETA_TEL_ERR = REAL (Write)

Error in the fitted value of ETA_TEL. Set to bad if the fit has failed.

ETA_TEL_FIT = REAL (Write)

The fitted value of ETA_TEL. Set to bad if the fit has failed.

GOODFIT = LOGICAL (Write)

Flag to indicate whether the fit was good (TRUE) or bad (FALSE).

MODEL_OUT = CHAR (Write)

The name of the output file that contains the fitted sky temperatures.

MSG_FILTER = CHAR (Read)

Message filter level. Allowed values are QUIET, NORM and VERB. Default is NORM. There are no verbose messages.

OUT = CHAR (Write)

The name of the output file that contains the measured sky temperatures.

RESIDUAL = REAL (Write)

Absolute difference between the model and the data (in Kelvin). i.e. $\sum{ abs( Data - Model)}$

SIGMA = REAL (Write)

Standard deviation of the residuals of the fit (Kelvin).

SUB_INSTRUMENT = CHAR (Read)

The name of the sub-instrument whose data are to be selected from the input file and fitted. Permitted values are SHORT, LONG, P1100, P1350 and P2000

TAUZ_ERR = REAL (Write)

Error in the fitted value of TAUZ. Set to bad if the fit has failed.

TAUZ_FIT = REAL (Write)

The fitted sky opacity for the selected sub instrument.

T_COLD = REAL (Read)

Temperature of the cold load. The default value is taken from the input file. (This parameter is not required for data that has been processed by reduce_switch.)

T_HOT = REAL (Read)

Temperature of the hot load. The default value is taken from the input file. (This parameter is not required for data that has been processed by reduce_switch.)

WAVELENGTH = REAL (Write)

The wavelength of the selected sub instrument.

XISQ = REAL (Write)

The reduced $\chi^2$ of the fit. The nature of the error determination forces the reduced $\chi^2$ to be approximately 1. The SIGMA parameter provides a better guide to the goodness of fit.

Examples:

skydip jun10_dem_0002 short $\backslash$

Process the short sub-instrument using the default value for T_COLD and allowing ETA_TEL and B to be free parameters. No output files are written.

skydip 19970610_dem_0003 long eta_tel=0.9 out=sky model_out=model $\backslash$

Process the long wave sub-instrument with ETA_TEL fixed at 0.9 and B free. Write the sky temperature to sky.sdf and the fitted model to model.sdf.

Notes:

If the input file is not found in the current directory, the directory specified by the DATADIR environment variable is searched. This means that the raw data does not have to be in the working directory. In addition IN accepts a number. This number is converted to a demodulated data filename by prepending it with information specified in the SCUBA_PREFIX environment variable. This filename expansion only works for demodulated data (ie data containing '_dem_'). The '_dem_' is assumed and should not be present in SCUBA_PREFIX.

If the CVAR parameter is true the reduced $\chi^2$ value can not be used to compare the goodness of fit between datasets.

Both RASTER and DISCRETE skydips are supported.

See Appendix for more information on skydips.

Related Applications

SURF: extinction, sdip, skysum