The program observe is the tool with which the observer communicates his or her instructions (i.e. where to point, how long to point there, what LO settings to use) to the VLA. The program observe creates an observe file (a series of 80 column ``cards'' written in ASCII) which is sent to the VLA operator on duty. There are several documents that describe how to use observe; the one most appropriate for spectral line users is ``OBSERVE- A Guide for Spectral Line Observers'' by Elias Brinks. This can be obtained from the anonymous ftp account (see the OSS for details). Potential observers are recommended to consult that document to learn how to use observe. We present, here, a concise interpretation of the output of observe in light of the information presented in this document. For this purpose we have included a copy of a single scan (pointing at a target) as produced by observe for an observation of extragalactic H I.
0134+329 1 21 10 00 01 34 49.8320 +32 54 20.520 LL C 5555 14.60 //DS 2AC H 30 7 7 7 7 //LO -3.2 -3.2 3610 3610 //FISZTB -352.000 -352.000 1420.405752 1420.405752
The parameters of a particular observation are presented to the VLA on four cards: a source card, an LO (Local Oscillator) card, an FI (Fluke Synthesizer Settings or Fine Tuning), and a DS (Data Selection) card. The source card contains information about the source, including its name and position. The LO card specifies the settings of the local oscillators. The FI card contains the information necessary to set the Fluke Synthesizers -- the rest frequency of the observed line and the velocity of the source. Finally, the DS card specifies, among other things, the correlator mode and number of channels.
Source Card:
The source card is given in the first line of a given scan in the observe file. In order, the information presented is the source name, qualifier code, stop time, right ascension, declination, observing band, calibrator code, bandwidth code for the four baseband IFs, and the flux density. In this example the source is ``0134+320'', a primary calibrator. We have given this a qualifier code of ``1'' and this scan will end at 21:10:00 (LST). The position is given in 1950.0 coordinates (had we specified J2000 coordinates the position information would have been followed by a ``C''). The ``LL'' specifies that we will be observing at L band in both IF pairs. If one were observing at K band, for example, this would be ``KK''. The ``C'' describes the positional accuracy of this calibrator (check the ``VLA Calibration Manual'', which can be accessed on the WWW, for a description of the available calibrators). The bandwidth code (cf. Table 2.4 or Appendix A of this document) is given as ``5555'' which means that we will be using bandwidth code ``5'' for all four baseband IFs. Finally, the ``14.60'' is the approximate flux density of this source. The calibrator code and flux density are only provided for known calibrators.
DS Card:
The second line of a scan contains the Data Selection (DS) card which begins with the notation ``//DS''. The first set of parameters specifies the correlator mode, in this case ``2AC''. The second entry specifies the array processor options, in this case it is an ``H'' for on-line Hanning smoothing. Other options are ``B'' for bandpass normalization, ``S'' to produce a digital sum of all baselines, or ``L'' to generate a lag spectrum. The observer can chose any or all of the first three (``H'', ``B'', and ``S'') options or the ``L'' option. To summarize:
| Array Processor Options for the DS Card | ||
| Options: | ``H'' | Hanning Smoothing |
| ``B'' | Bandpass Normalization | |
| ``S'' | Produce Digital Sum | |
| ``L'' | Generate a Lag Spectrum | |
The next entry is the
integration time in seconds, in this case it is set to 30 seconds.
The last four entries represent the number of channels, n, in each IF
given as 2
. In this case we will be using 128 channels. One
can also specify a starting channel number which would precede the
number of channels in the observe file. If the observer would
like to use all channels these fields can all be left blank.
LO Card:
The third line of a given scan is the Local Oscillator (LO) card which is used to specify the appropriate LO settings. The identification of the card is given by ``//LO''. The first two entries are the settings for the Front End LOs for IFs AC and BD, respectively, in GHz (see chapter 2.2 of this document). The minus sign is used to indicate that the frequencies are ``up-converted''. The second set of numbers are the settings for the L6 frequency synthesizers (i.e. 2-4 GHz LO) in MHz. Note that the use of the 50 MHz front-end filter is the default; had we specified the use of the 25 MHz front-end filter we would see a ``1111'' following the 2-4 GHz LO settings. In the case of the 12.5 MHz front-end filter we would see a ``2222''. These numbers represent the filter codes for each of the four IFs.
FI Card:
The fourth line of a given scan in the observe file is the Fine Tuning (FI) card. This card specifies the settings of the fluke synthesizers (see chapter 3.2 of this document) and is identified by a ``//FI''.
In the third column, the ``S'' specifies that the frequencies should be set to the values given hereafter. An ``N'' in this position specifies that the flukes should not be changed from the values which they had in the preceding scan (see chapter 3.6.1). The next entry is used to specify the frequency/velocity switch which determines how the fluke frequencies are calculated. In this case we used the optical velocity definition as indicated by the ``Z''. Other options are: ``F'' for absolute fluke frequency; ``O'' for frequency offset; and ``V'' for radio velocity definition. The next entry specifies whether the fluke frequencies are the sum of the LOs (in which case the entry will be left blank) or are to be set at the band centers (``T''). The last entry specifies which rest frame to use for Doppler calculations. ``T'' is topocentric, ``B'' is barycentric, ``L'' is LSR, and ``G'' is geocentric. If this is left blank, ``T'' will be the default. In the case of extragalactic HI observations the first columns of this card read //FISZTB; for Galactic HI the codes normally used are //FISVTL.
For completeness, the valid entires for columns three through six are summarized below:
| Third Column: Fluke Settings- | |
| ``S'' | set frequencies to values given hereafter |
| ``N'' | do not change flukes from previous values |
| Fourth Column: Fluke Calculations- | |
| ``Z'' | use optical velocity definition |
| ``F'' | absolute fluke frequency |
| ``O'' | offset frequency |
| ``V'' | use radio velocity definition |
| Fifth Column- | |
| ``blank'' | fluke frequencies are the sum of the LOs |
| ``T'' | set fluke frequencies at band center |
| Sixth Column: Rest Frame- | |
| ``T'' | topocentric |
| ``B'' | barycentric |
| ``L'' | LSR |
| ``G'' | geocentric |
| ``blank'' | defaults to topocentric |
The velocity of the source in km s
is given in the next two
entries (one entry for each IF pair).
Finally, the rest frequency
of the transition, in this case H I, is given in MHz.
The observer must provide a set of these cards for each scan of the observation. This can be done with the observe program. The LO and FI cards should be identical for a given source and its affiliated phase calibrator. For example, assume you wish to observe five galaxies which are at five different redshifts. You must pick a phase calibrator to go along with each galaxy. The galaxy and its phase calibrator should be observed with the same LO and FI cards. Similarly, you should observe your flux density/bandpass calibrator at each of the five different velocities.
Specific questions about your observe file should be initially directed towards the data analysts or your staff contact at the AOC.