VLA Frequency Page

VLA Frequency Page

This page allow you to specify the frequency at which you would like to observe. If this is a spectral line observation then you may set the line rest frequency and velocity frame. A list of frequently observed spectral lines are provided to simplify this process.

Note: If the Synthesizer setting option is not "set" then the other parameters on this page are irrelevant.

Synthesizer setting option

The Synthesizer Setting Option determines the method used to specify the baseband local oscillator setting. This option determines the significance of the rest of the fields on this Page and the meaning of the "Freq Vel" on the LO Page. The possible selections are:

rail: Final LOs are 100 200 250 150 exactly for IFs A, B, C and D.

center: Final LOs are 125 225 225 125 (IFs A, B, C, D) minus baseband filter center. (i.e. the sky center frequency is independent of bandwidth, which is not true for rail).

set: Use the other options and parameters from this Page together with the "Freq Vel" field on the LO Page to determine the final LO values.
no change: Use the final LO values from the last scan.

Most observations will use Rail or Set. Unless "set" is selected, all other fields on this Page and the "Freq Vel" field will be ignored by the on-line system. OBSERVE therefore will ignore values for these fields unless "set" is selected.

Frequency/velocity option

The Frequency/Velocity switch determines how to calculate the baseband local oscillator setting. This field only has meaning when the Synthesizer Setting Option is selected to "set"; to make a selection here this must be done first. You may either a) select an absolute frequency or b) use other fields on this Page together with the "Freq Vel" values on the LO Page . to define the required sky frequency.

If Absolute Frequency is selected, all other fields on this Page will be ignored by the on-line system. In this case, therefore, OBSERVE will ignore explicit values for other fields.

For Option:	Sky Frequency will be:
absolute frequency	FV (MHz) + signed sum of other LOs
frequency offset	FV (MHz) + LRF (MHz)
radio velocity	LRF (MHz) * DCE * DCS where DCE = (1 - VE/c) VE (km/s) and DCS = (1 - FV/c) FV (km/s)
optical velocity	LRF (MHz) * DCE * DCS where DCE = 1/(1 + VE/c) VE (km/s) and DCS = 1/(1 + FV/c) FV (km/s)

where:

FV is the "Freq Vel" value (from LO Page).
LRF is the Line Rest Frequency defined by the Transition Code or the Line Rest Frequency field.
VE is the component of the Earth's velocity in the direction of the source. This is calculated by the VLA on-line computers.

Transition Code	Rest Frequency (MHz)
H	1420.40575
OH(1-2)	1612.2310
OH(1-1)	1665.4018
OH(2-2)	1667.3590
OH(2-1)	1720.5300
H2CO(101-111)	4829.6596
H2CO(211-212)	14488.4750
H2O	22235.120
NH3(1,1	23694.4955
NH3(2,2)	23722.6336
NH3(3,3	23870.1296
NH3(4,4)	24139.4169
NH3(5,5)	24532.9887

If one of these is selected, then the Line Rest Frequency is filled in. If this is blank, then the Line Rest Frequency is used for the baseband local oscillator calculation.

Line Rest Frequency

The Line Rest Frequency expressed in MHz. These frequencies are used in the calculation of the baseband local oscillator frequencies. This field only has meaning when a) the Synthesizer Setting Option is selected to "set" and b) the Frequency/Velocity switch is set to something other than Absolute Frequency; to make a selection here these must be done first. Entering a value here will override any implied Line Rest Frequency specified in the Transition Code. The VLA on-line hardware constrains IFs A and C to be at the same frequency; similarly B and D. Seting a frequency for IF A therefore also sets all IFs to the same frequency; setting IF B also sets IF A and either B or D sets the other at well. The check the frequency hit Return (Enter) on the keyboard or the "Verify" button on the button bar.