Spectral Line Data Calibration

The CAL routine used for data calibration requires a calibration chopper. Typing START CAL initiates measurement of the voltages on the ambient load, the sky, and with the receiver blanked, which are then used to generate a calibration scan by computing T$_{sys}$ in each individual channel of the filterbanks. The cal scan is stored in the T4 register of the disk data storage area and can be examined via the SPA data reduction program by typing T4 PLOT at the TEK4010 terminal. A total power measurement of Tsys is also computed via the DVM and is displayed on screen 1. All future data scans are calibrated either with the channel-by-channel (T4) calibration scan by typing the keyword CALI (brate) or with the single T$_{sys}$ value displayed on screen 1 by typing the keyword RAW. If, for some reason CAL fails one can enter T$_{sys}$ manually using the keyword TSYS ( Kelvin) and observe in RAW mode.

For SEQUOIA, broad band system temperatures are calculated for each pixel and displayed on Screen 15 only if the QEF program is running on the QEF PC in the annex. If this is not running, then the one system temperature is calculated from the DVM which is connected to a spare IF cable from pixel 11 (the center pixel). This value is displayed on screen 1 and screen 15 for each pixel. As long as one is using channel by channel calibration (CALI mode), in which case the calibration is performed within each filter bank, then the state of the QEF PC for calibration purposes is irrelevant. For the FAAS system, the spectra are calibrated by the broad band QEF measurements in RAW mode and by the correlator system temperatures displayed on SCREEN 15 WINDOW 3 in CALI mode.

At some observing frequencies, most notably at $^{12}$CO (1-0), a significant correction factor to the computed Tsys in CAL may be required (see FCRAO report $\char93$ 220) to accurately calibrate your data. The magnitude of this factor depends on the difference between the chopper temperature and effective atmospheric temperature, T, and the effective atmospheric opacity. (For example: $^{12}$CO (1-0) observations through 3 airmasses requires a correction factor of 1.3 for zenith = 0.4 and T = 20 K.) If you can make a reasonable guess at the zenith opacity and T, enter them via the keywords TAUZ and DELT and use the keyword CFAC (ON) to turn on computation of the correction factor which will be updated on screen 1 at the beginning of each scan, and will multiply the filterbank data prior to storage. If you don't use the correction option [i.e. CFAC (OFF)] then a standard source should be monitored vs airmass to determine your own correction curve vs El.

If you wish to correct your data to a standard scale of T$_R^*$ (T$_A^*$ / $\eta_{FSS}$) it is suggested that you use $\eta_{FSS}$ = $\eta_{MOON}$ as given in FCRAO report $\char93$220 or its successor.

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Table 4.2

CAL Keywords

CALTIME (seconds)

The integration time spent on ambient, sky, and blanking. To insure adequate signal-to-noise we suggest a time between 3 and 5 seconds.

CFAC ON/OFF

Turns the option on/off to multiply the value of Tsys by a correction factor, CF, that accounts for the difference between the chopper temperature and the effective atmosphere temperature DELT . The value of CF is computed from DELT and the zenith opacity TAUZ by

$$CF = 1 + { {DELT} \over {T_{amb}} } (e^{TAUZ A} - 1)$$

where A = airmass. The variables DELT AND TAUZ must be given by the user; The DIP program can be used to estimate TAUZ . CF is displayed on screen 1. With CFAC ON, CF will be 1.00 and will be updated as your source rises or sets even if you never run the CAL program again. With CFAC off, CF = 1.00 always.

CALI or RAW

Determines if the data is calibrated using the Tsys found from the DVM (RAW) or if the data is calibrated using the individual channel calibrations stored in T4 (CALI). If CALI is selected an asterisk will appear beside cal scan T4 on screen 1.

SSB / DSB

Determines if the data is calibrated assuming the system is single-sideband SSB or double-sideband DSB . If one has the sideband filter tuned single-sideband then use SSB . If one is observing spectral lines with the sideband filter tuned double-sideband use DSB , and lines will be multiplied by two.

START CAL

This activates the routine and results in the Tsys from the DVM being displayed on screen 1 and the calibration scan T4 being updated to show the channel-by-channel Tsys.

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