General options for non-standard setups

Some of the useful things that can be done with non-standard configurations and ICF's are:

• Simultaneous observations of the same line with both high and low resolution. For example, it is possible to look at the whole passband with low resolution, plus a line at the centre with high resolution; once reduced, these two separate subsystems will end up in the X and Y registers of SPECX.

• High resolution spectra of two lines in opposite sidebands. By adjusting both the Frontend LO frequency (to move the lines closer together) and the DAS LO frequency (to centre the subsystem on the two lines), both lines can be observed simultaneously with the highest resolution (125 MHz). Note that there are limitations on the LO tuning step - the subbands can be tuned either in steps of 1.25 or 125 MHz. In the cases where the second subsystem is only tunable in steps of 125 MHz, this is relative to the first subsystem frequency. So in the case of configuration 20044, subsystem 1 can be freely tuned, and subsystem 2 is locked to it, but can be tuned in steps of 125 MHz away. If the specified LO frequency is not a multiple of 1.25 MHz away, the software will round up and change the header to reflect this. But to avoid possible confusion it is recommended that only the allowable offsets are used.

• High resolution spectra of 2 or more different lines widely separated within the IF passband. If it is required to observe say 2 lines separated by 800 MHz, but with high resolution, the Frontend frequency can be set appropriately, and 2 separate 125 MHz subsystems tuned to the line centres. Note that the 1.25 MHz LO quantisation mentioned above will apply. Also the maximum number of correlator channels is still the same, so the effective resolution of 2 125 MHz subsystems will be the same as a single 250 MHz subsystem (ie 0.156 MHz).

• Multiple copies of the same subsystem. It is possible to observe the same line through 2 or possibly even 4 independent correlator sections. This is sometimes useful when there is a problem with one of the correlator banks.