Having entered the rest frequency, the user must then enter the source's velocity either with respect to the local standard of rest with respect to the sun, or with respect to the Local Group of galaxies. This is done by either typing VLSR, VSUN, VOPT, or VLGR followed by the source's velocity in km/sec. The observer must enter which sideband-upper or lower-is being used by typing either USB or LSB. For SEQUOIA, most observations are made at USB with the exception of the SiO maser line at 86.24 GHz for which LSB is more appropriate.
Once all of the above parameters have been appropriately set, Doppler steering will update the frequency synthesizer every 10 seconds-as long as the user is in the SPEC line mode. However, should the user switch to the CONT mode, the synthesizer will lock and stay locked at the last setting prior to entry into the continuum mode. To return to Doppler steering, it is necessary to reenter the spectral line mode. The Doppler corrections are based on the MAIN position commanded RA and DEC regardless of whether the user is currently pointing at the MAIN or REF position. One can manually turn the Doppler steering on or off with the DOP command.
For frequency switching observations, it is also necessary to enter the offset from the main synthesizer frequency. This is achieved by typing FOFF (offset in MHz). Note that this entry, unlike all other frequency entries to the tracking programs, is in units of megahertz. See Chapter 4 for more details.
Finally, for test purposes, it is possible to lock the synthesizer frequency to an arbitrary value by typing SYN1 or SYN2 followed by frequency in GHz (limited to the range 0-1 GHz). The same frequency should be entered for each keyword in order to avoid peculiar effects associated with the original purpose of these two keywords. As long as the synthesizer is locked up in this manner, both of the frequencies actually being output to the synthesizer will be displayed on line 21 of the screen 1 display. In order to return the synthesizer to its normal behavior type either USB or LSB (whichever sideband is normally used). When this is done, the synthesizer will unlock and the special display on screen 1 will disappear. Also, for frequency switching tests, it is possible to manually switch between offset frequencies using the keyword FLUKE (OFF-, MAIN, OFF+) where OFF-, MAIN, OFF+ represent the negative frequency offset, no offset, and positive frequency offset using the offset magnitude given by FOFF (in MHz). Table 3.4 summarizes the radiometer keywords.
truein