FAAS Setup

The FAAS system provides a versatile spectrometer for use with SEQUOIA Each pixel has an autocorrelation spectrometer with available bandwidths of 2.5,5,10,20,50, and 80 MHz. The bandwidths from 2.5 to 20 MHz have 1024 channels while the 40 MHz bandwidth has 512 channels and the 80 MHz bandwidth has 256 channels.

To select the bandwidth for FAAS, one uses the CBAND command,
CBAND NMHZ
where NMHZ is one of the following (2.5,5,10,20,40,80). The autocorrelation function (ACF) can be smoothed prior to the fourier transformed to obtain the power spectrum with the CSMOOTH command. This smoothing is applied to all correlators and cannot be individually selected. The command is
CSMOOTH n
where (n=0,1,2,3,4,5,6,7). The options are listed in Table 3.5

Table 3.5

FAAS Weighting Options

Number	Name	$\Delta\nu/\Delta\nu_{sp}$	First Sidelobe	Comments
			(dB)
0	Uniform	1.20674	$\sim-6$	Narrowest bandwidth
1	Riemann	1.71471	$\sim-15$	Good sidelobe balance
2	Hann	1.97932	$\sim-15$	Excellent Rejection
3	Chebyshev	2.01856	$-30$	Not much rejection
4	Blackman	2.22980	$-30$	low first sidelobe,
				low rejection
5	Chebyshev	2.31664	$-40$	low first sidelobe
				very low rejection
6	Chebyshev	2.58034	$-50$	low first sidelobe,
				very low rejection
7	Chebyshev	2.81928	$-60$	low first sidelobe,
				very low rejection

The theoretical noise level for FAAS is given by,

$$\DeltaT = {{2(1.146)T_{sys}} \over {\sqrt{\gamma {{B}\over{N}} \tau}}}$$

where the factor of 2 is for position or beam switched observations, the factor of 1.146 is the sensitivity loss from the 2-bit digitization used in FAAS, $\gamma$ is the bandwidth factor due to smoothing as is given in Table 3.5 ($\gamma$=1.21 for uniform weighting), B is the FAAS total bandwidth in Hz (CBAND=2.5 MHz, etc.), N is the total number of channels, and $\tau$ is the total integration time on MAIN and REF in seconds.

The IF level which is fed into an autocorrelation spectrometers must be within a certain range in order to digitize the signal. During each CAL measurement, a set of attenuators are varied in order to place the IF level within the range set by the CTHRESHOLD command. A user can manually set these levels for the main and reference buffers in the correlator with the CMAIN and CREF commands.

On infrequent occasions, a chip within the correlator boards may fail. A symptom of a failing chip is a non convergence status when setting the IF levels during a calibration (CAL) measurement. If this occurs repeatedly, call the staff. However, an observer can shut down the pixel of the offending chip with the CPIX command and continue observing with one less pixel in the array.

Extragalactic Filter Banks Setup

The QEF spectrometers do not require any significant setup. Occasionally, a bad channel may be present in a given pixel. The capability exists to remove bad channels from filterbank data. The bad channels are replaced by 1$\times$10$^{35}$ in the scan data, so that they will not be plotted by SPA. All bad channels are kept in computer memory while the tracking programs are running and stored on disk when the tracking programs are exited. The previously stored values are loaded when the system is started with //STP, and the file is initialized when the system is started with //ITP.

The current state of the bad channels list can be determined by viewing screen 3. This screen lists all of the bad channels currently in the list (253 bad channels maximum), ordered by filterbank and channel number. The presence of the letter I to the right of the filterbank device name indicates that the list is currently in use for that device.

Channels can be added or deleted, and the whole list can be cleared via user commands entered through the TEK4023 keyboard. To clear the entire file:
CHDEL ALL
To add channels to a filterbanks list:
CHADD F$_n$ ch# ch# ch# or
CHADD F$_n$ ch# TO ch#
To delete channels from a filterbanks list:
CHDEL F$_n$ ch# ch# ch# or
CHDEL F$_n$ ch# TO ch#
Channel additions for several devices can be strung together in the same line. For example,
CHAD F1 13 2 TO 5 F3 8 TO 10 F4 9
adds channels 2, 3, 4, 5, 13 to the F1 list, channels 8, 9, 10 to the F3 list, and channel 9 to the F4 list. Similarly, one can delete channels from several devices at the same time:
CHDEL F3 9 10 F1 4 TO 6

The bottom of screen 3 displays an error message if a mistake is made while adding/deleting bad channels. Thus, it is advisable to select screen 3 before making changes to the list. Because there is no longer any card copy of bad channels currently in use, it is suggested that when the bad channels list is updated, that the user make a copy of screen 3 via the COPY A06 command.

Once the list has been put on disk, it can be used by typing
BADC F$_n$ ON
or
BADC F$_n$ OFF
where F$_n$ is a legal filterbank identifier (see above) and the last parameter informs the system whether the bad channel elimination is being turned ON or OFF for the filterbank.