Setting the Velocity Scale

         

If you are doing spectral-line work, you will, at some stage, need to set the velocity scale. Refer to § 13 for some discussion on velocity types and velocity reference frames. You will need to know whether you want the `radio' of `optical' velocity definition, and whether you want the LSR or heliocentric (barycentric really) reference frame. The relevant velocity information is entered into the SU (source) table with SETJY. You can specify different values for different sources. I will dicuss here only those adverbs specific to velocities.

Many spectral-line users will end up running a task called CVEL (see § 13 which makes Doppler corrections (the ATCA does not use on-line Doppler tracking) to the spectra. Optionally, you can enter ther velocity information at that time, rather than now.

• Specify the source name with sources. Naturally you are only interested in your program source(s), not any of your calibrators.
• Specify the velocity (km/s) of the source with sysvel and the channel that this velocity refers to with aparm(1) (see discussion in § 13.5). You should know this from when you computed your central observing frequency when you observed. If you look in the SU table with PRTAB, you will see that the velocity has been recomputed to refer to the reference pixel. Make sure it did it correctly.
• Indicate which velocity reference frame the above velocity is measured in with veltyp. AIPS offers LSR (for Galactic work usually) and Heliocentric (for extra-galactic work usually) only.
• Indicate whether the above velocity was measured in the `radio' or `optical' definition with veldef.
• For SETJY to be able to work out the width of one channel in the appropriate velocity type, you must specify the rest-frequency (Hz) of the spectral-line that you are observing with restfreq, and the freqid so that the correct observing frequency can be selected. If somehow you have managed to create a spectral-line data base with an IF axis of length other than unity, you must also specify the correct IF with bif and eif. Otherwise leave these at 0. Note that you should use the sum of restfreq(1) and restfreq(2) to enter the rest-frequency; see the example below.

sources='NGC9434',' ' Select program source

optype=' ' Use specified source parameters

freqid=1 Select observed frequency

bif=1 with this combination eif=1

sysvel=5353 Specify velocity at

aparm(1)=257 this channel in

veltyp='helio' or 'lsr' this reference frame and

veldef='radio' or 'optical' this velocity definition

restfreq(1)=1667.0E6 Rest frequency of line

restfreq(2)=0.359E6 Rest frequency of line

Use PRTAB after running SETJY to ensure the parameters you have entered have been correctly transferred to the SU table.

Last update : 11/04/95

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