Table of Contents
SEQUOIA provides large instantaneous bandwidth which allows an observer to measure two spectral lines simultaneously. New IF modules have been constructed to process the 15 GHz wide output from the SEQUOIA dewar and to mix the IF signals into one or two 50 MHz outputs which are fed into the new array of autocorrelation spectrometers. These modules are daisy chained to the old modules which feed the ExtraGalactic Filter Banks and operate at a different baseband frequency. Observers can switch back and forth between these modules with the IFTYPE command. IFTYPE 1 sets the QEF IF modules and IFTYPE 2 activates the DCC (Dual Channel Correlators) IF modules. Table 1 shows the available configurations of the new correlator system.
Table 1. Dual Channel Correlator Configurations
| Mode | Pixels | IFs/pixel | Spectra | Bandwidth (MHz) | Channels/pixel | Channel Spacing (kHz) | Comment |
| 6450 | 32 | 2 | 64 | 50 | 1024 | 50 | CBAND 50 |
| 6425 | 32 | 2 | 64 | 25 | 1024 | 25 | CBAND 25 |
| 6412 | 32 | 2 | 64 | 12.5 | 1024 | 12.5 | Not yet implemented |
| 3250 | 32 | 1 | 32 | 50 | 2048 | 25 | Not yet implemented |
| 3225 | 32 | 1 | 32 | 25 | 2048 | 12.5 | Not yet implemented |
| 3212 | 32 | 1 | 32 | 12.5 | 2048 | 6.25 | Not yet implemented |
The APOINT program assumes that only a single IF configuration is defined. To set up an auto-point observation with the new correlators, one needs to be sure that the new IF modules are selected (IFTYPE 2) and that 1 IF configuration is defined. The following set of commands issued at the vt100 console in the control room or the remote mterm window, sets a single IF configuration with the dual IF module with 50 MHz bandwidth per pixel with the SiO maser line frequency:
IFTYPE 2 CONF 2 RESET CONF 1 COR CBAND 50
LINE SIO-4 LSB
START QTUNE
Note that the LINE function (in addition to the FREQ,VLSR,VSUN,VOPT, USB,LSB commands) takes an additional argument which assigns the frequency (or VLSR) to the designated IF. If this argument is absent, then the command assigns the value to both IFs. The CONF 2 RESET command deactivates the second IF configuration if previously defined to allow APOINT to run properly.
If one wishes to observe 2 lines simultaneously (either within 85-100 GHz band or 100-115 GHz band), they need to set 2 configurations with the CONF command and assign a frequency or VLSR to each IF. For example, the following sequence of commands sets up an observation to measure 12CO and 13CO simultaneously with 25 MHz bandwidth,
IFTYPE 2
CONF 1 COR CBAND 25
LINE CO-1 USB (see note 2)
START QTUNE
To configure DCC LO B at 13CO
CONF 2 COR LINE 2 (13)CO-1 USB CONF 1
The last command in the above sequence sets the displayed configuration to CONF 1. It is imperative that the user set the first configuration to the displayed configuration for the data to be stored properly. When crossing the 100 Ghz boundary, it is important to move both LOs (LINE without the 1 or 2 argument). This is because the 1st LO is set by the value of LO A (LINE 1). If both LOs are set to SIO (as they would be in the above example after typing IFTYPE 2), and the observer types LINE 1 CO-1, the 1st LO will be changed from 80 Ghz to 120 Ghz. Leaving LO B (LINE 2) at SIO would require a synthesizer frequency of 35 Ghz which is out of range. This will result in a SYNTHESIZER OUT OF RANGE error and probably a PPC REPORTS INVALID ARGUMENT error. Both IFs must be observed with the same bandwidth setting. The next example sets up 2 IFs both observing 13CO but with the two IFS in series to gain additional bandwidth but with some overlap to facilitate the merging of the 2 IFS into a single spectrum:
CONF 1 COR CBAND 50
LINE 1 (13)CO-1 USB 1 VLSR 1 100
CONF 2 COR LINE 2 (13)CO-1 LSB 2 VLSR 2 -10 .
The different sideband settings for each IF is required to eliminate the discontinuity at the edge of the bandpass. At 50 MHz, each IF has 136 km/s velocity range. So the first IF in the above example extends between 32 and 168 km/s while the second IF extends between -78 and 58 km/s. There are approximately 26 km/s of overlap which corresponds to 195 channels. There are actual fewer channels than this as each spectrum is trimmed at the edges to account for the normalization of the autocorrelation function.
For a regular PS or DPS observation with such multiply defined configurations, two scans will be stored per observation within the T file. The first scan corresponds to CONF 1 and the second scan corresponds to CONF 2. For an OTF map, two OTF files are written to the /raw/dcc directory for each scan, defined as data taken between 2 reference measurements. The first configuration has the extension .loa and the second configuration has the extension .lob.