VLA Source Editing Page

VLA Source Page

The Source page sets general information about the Scan, Source name, observing mode, VLA frequency band, source position etc.

Tipping procedure This mode asks for a tipping scan, a series of system temperature measurements at various elevations from which the atmospheric opacity may be derived. This is important mostly for K and U band observations, where the opacity can be significant and variable. Tipping scans are rather different from standard scans:
- The position (RA, Dec) given in the source card is not the position to which the telescopes are pointed. Rather, the RA gives the azimuth at which the tip should be performed, while the Dec determines whether the tip is done from low to high elevation, or from high to low elevation. To determine the azimuth, the RA is converted to degrees by multiplying by 15; the important point here is to avoid shadowing in the more compact (C, D) configurations. If the Dec specified is closer to 23 degrees, the first tipping pass ascends from low to high elevations; if the Dec is nearer to 55 degrees, the first pass descends from high to low elevations.
- By default, each tipping pass consists of 7 samples in elevation, spaced uniformly in air mass between 23 and 55 degrees elevation. This can be modified, but not within OBSERVE: see VLA Scientific Memo No. 170, or talk to one of the VLA support staff.
- About 8 minutes (excluding slew time required to reach the initial position) is required for each standard (7 sample) tipping pass. If more than 8 minutes is allocated, the VLA will simply go on to the next scan as soon as the tipping pass is finished.
- OBSERVE understands the initial and final positions of the antennas, so slew times should be fairly accurate. However, if, say, 12 minutes were allocated to a tipping scan, so that the scan ended in the middle of a tipping pass, OBSERVE will not be able to predict where the telescopes will wind up, and may give erroneous slew times (either short or long).
Referenced pointing. Referenced pointing is an observing technique that can improve the (primary beam) pointing of the telescopes in the array. The standard pointing for some antennas may be in error by some tens of arc seconds, a large fraction of the primary beam at K and Q bands. Observations in those bands can benefit substantially from pointing up on a nearby source. There is little or no advantage to using referenced pointing at lower frequencies.
The mode "Determine ref pntng" solves for the pointing offsets for each antenna by comparing the signal ON and slightly OFF a point source (generally the phase calibrator); these are 5-point scans similar to those familiar to single-dish observers. The initial ON position is calculated based on the standard pointing model for the VLA, which is determined every month or so. There are several special considerations for this mode:
- Pointing offsets are generally determined using broadband continuum measurements at X-band: use NRAO default 'XX'.
- Use an integration time of 10 seconds. Multiples of 10 seconds are also OK but you will need to increase the dwell time accordingly.
- A pointing scan requires a dwell time of at least 2 minutes, assuming 10 second integration times. Scans longer than this will yield multiple 5-points.
- Local lore says the pointing source should be stronger than 0.3 Jy to give good pointing solutions.
- Both the pointing model and referenced pointing lose accuracy near the zenith.
The pointing solutions thus found should only be applied to sources within about 30 degrees in AZ/EL of the pointing calibrator. This generally means referenced pointing scans should be made approximately every hour.

Once referenced pointing corrections are determined, they must be applied via the "Referenced Pointing" field towards the bottom of the Source page. Simply set this to "yes" to apply the last referenced pointing solutions. Do NOT set "Referenced Pointing" to "Yes" in any scan in "Determine ref pntg" mode!!
Referenced pointing scans are NOT usable for phase and/or amplitude calibration. That must be done separately, presumably with the newly-determined pointing corrections applied (Referenced Pointing=yes on the Source page).
Auto phasing mode. VLBI users of the VLA in phased array mode will need to use the Autophasing mode on a calibrator. To apply the results of an autophased calibrator on another source, use the Apply Last Phase mode. VLBI users of a single VLA antenna should use the Single Dish VLBI mode.
Solar mode The Solar observing mode sets up many things that are necessary or observations of the Sun: attenuators, solar noise tubes, etc.
Other modes All other modes are primarily for use by VLA staff for test observations.

User Default

This field allows the selection, for this source, of a User Default from your own list. The User Defaults are defined on the Lists Page. The User Default Choice will allow selection of one of the User Defaults. If a User Default is selected, any explicit values in it are copied to the target scan. To see the contents of your User Defaults, invoke the scan editor by double clicking in the scan in the User Default list shown by the List editor.

NRAO Default

This field allows the selection of an NRAO Default for this source. The NRAO Defaults are loaded with the program and are listed on the List Editor Page. The NRAO DEfault Choice will allow selection of one of the NRAO Defaults. If an NRAO Default is selected, any explicit values in it are copied to the target scan. A value will be used from the NRAO entry if no value is explicitly defined for any field. The values in an NRAO default may be examined, but not changed, using the Scan Editor.

NRAO defaults are available for a few spectral line cases:

XGHI: Extragalactic HI
HI: Galactic Hi
OH: OH masers

A summary of the continuum defaults are given in the following:

Name	A/C (MHz)	B/D (MHz)	BW(MHz)
PP	327.5	333.0	3.125
LL	1464.9	1385.1	50
L1	1364.9	1435.1	50
L2	1515.9	1365.1	25
L3	1515.9	1435.1	25
CC	4885.1	4835.	50
XX	8435.1	8485.1	50
UU	14964.9	14914.9	50
KK	22485.1	22435.1	50
QQ	43314.9	43364.9	50
LP	1464.9	333.0	50, 3.125
PL	1464.9	333.0	50, 3.125
VP	327.0625	327.0625	3.125
VL	1664.3	1664.3	12.5
VC	4985.0	4985.0	50
VX	8414.9	8414.9	50
VK	222334.0	22235.0	50
21	1464.9	1385.1	50
18	1664.9	1635.1	50
HH	1417.5	1467.5	25
44	73.8	73.8	1.56
4P	73.8	333.0	1.56, 3.125
P4	73.8	333.0	1.56, 3.125

J2000 Position

The two Text Fields following "2000.0" allow entry of the J2000 RA and declination of the source as hh mm ss.s and +dd mm ss. If these fields are filled in then this becomes the position of the source and the epoch will be taked to be J2000. Hitting a Return (Enter) on the keyboard or the Verify button will cause the values to be read, checked and the other positions to be updated. The "*" after the 2000.0 label indicates that the source standard Epoch is J2000.

Position Of Epoch

The two Text Fields following the epoch of the observations (e.g. 1999.9) allow entry of the RA and declination of the source at that epoch as hh mm ss.s and +dd mm ss. If these fields are filled in then this becomes the position of the source and the epoch will be taked to be of date. Hitting a Return (Enter) on the keyboard or the Verify button will cause the values to be read, checked and the other positions to be updated. The "*" after the epoch label indicates that the source standard Epoch is of date.

B1950 Position

The two Text Fields following "1950.0" allow entry of the B1950 RA and declination of the source as hh mm ss.s and +dd mm ss. If these fields are filled in then this becomes the position of the source and the epoch will be taked to be B1950. Hitting a Return (Enter) on the keyboard or the Verify button will cause the values to be read, checked and the other positions to be updated. The "*" after the 1950.0 label indicates that the source standard Epoch is B1950.

Integration Time

This is the integration time in seconds. Allowed values are:

0 ( 10 sec)
1 ( 1 2/3 sec)
2 ( 3 1/3 sec)
3 ( 3 1/3 sec)
4 ( 5 sec)
5 ( 5 sec)
6 ( 6 2/3 sec)
7 (8 1/3 sec)
8 (8 1/3 sec)
9 (10 sec)
10
multiples of 10

Band And Bandwidth

The frequency "Band code" and bandwidth can be specified for each of the VLA's 4 IFs (A,B, C, D).

The most common case is for all IFs to be at the same band. For this case, select the band for IF A; all other IFs will be modified to be at the selected band. The VLA hardware constrains IFs A and C to be at the same frequency; similarly B and D. To choose a band for IFs A and C only, select a new band for IF C; to choose a band for IFs B and D only, select one of these. A blank Band Code means no band is specified.

Possible band codes with standard frequency limits are:

Code	Low Frequency	High Frequency
4	48 MHz	96 MHz
P	306 MHz	340 MHz
L	1.34 GHz	1.73 GHz
C	4.50 GHz	5.00 GHz
X	8.00 GHz	8.80 GHz
U	14.4 GHz	15.4 GHz
K	22.0 GHz	24.0 GHz
Q	40.0 GHz	50.0 GHz

If one IF pair has a Band Code of 4 or P, then the other pair may be at any band. Any other combination of Bands is not possible in the VLA electronics. More specific frequency information is specified in the Frequency and LO pages.

The total bandwidth in MHz for each IF is specified in the appropriate Choice. For the Spectral Line cases, both the total number of channels produced by the correlator and the channel widths available depend upon the selection of the Correlator Mode , and the Hanning Smoothing option (both on Correlator Page). Both of these options should be correctly set before selecting the bandwidth. In the case of spectral line observations, the Bandwidth display shows both the maximum number of channels and the minimum channel bandwidth.

The 8 MHz filter is only selectable in Continuum mode. This mode is the normal default; it is, however, only true when the Correlator Mode is blank or set to Continuum.

When a new bandwidth is selected for an IF, OBSERVE will make checks on the validity of the current values of the Channel Count and the Starting Channel, both of which are set on the Correlator Page. If they are invalid, they will be marked as not present. WARNING! If you have already selected values on the Correlator Page then you should remember to check these values for validity after making the change here.

Frontend/Subreflector file

The VLA on-line system filename defining the setup parameters for the Front Ends and Subreflector for each antenna. The filename may contain only letters and numerical digits. If left blank, it is written to the OBSERVE file as such. The VLA on-line system will use the default filename for this source. The default filename is "SYS?ROT", where ? represents the observing band.

Most observations do not require this field to be entered. The appropriate default file has all requisite information. If a non-standard file is specified here, it must be created in the on-line computers before this observing program begins. This can only be done directly on these computers, and normally it can done for you by the VLA operators. You should inform them well in advance at (505)-772-4251.

For your information, this file for each band contains (for each antenna) the subreflector focus and rotation commands, the front end filter selection, the transfer switch position, the option to turn on the noise tube, the option to switch the noise tube, and the option to apply the system temperature correction on-line.

IF/Receiver file

The VLA on-line system filename defining the setup parameters for the IFs and Receivers for each antenna. The filename may contain only letters and numerical digits. If left blank, it is written to the OBSERVE file as such. The VLA on-line system will use the default filename for this source. The default filename is "SYS?IF", where ? represents the observing band.

For your information, this file for each band contains (for each antenna, for each IF) the peculiar delay, the peculiar phase, the noise calibration temperature both for the normal noise tube and for the (stronger) solar noise tube, the antenna efficiency, and an optional flag indicating that this IF is no good.

Reference Pointing

Use 'yes' to apply the latest referenced pointing corrections to this scan. Use 'no' if you don't want to apply referenced pointing corrections.

About 'Referenced Pointing':

Referenced pointing is an observing technique that can improve the primary beam pointing of the telescopes in the array. Referenced pointing can significantly improve K and Q band observations. There little or no advantage to using it at lower frequencies.

Before you apply referenced pointing corrections to a source, you must determine the pointing offsets with an observation of a strong point source. See the explanation under Observing Mode, specifically the "Determine ref pntg" mode. If the determination of referenced pointing failed for some or all antennas, those antennas revert to the standard pointing: turning on referenced pointing will not make things worse, even if the determination failed.

You should NOT apply referenced pointing for scans which are determining referenced pointing, i.e. use mode "Determine ref pntng". If you do, the offsets newly determined in that scan will be discarded.

Offset Mode

The VLA has several modes of observation which involve diferent celestial positions. These are specified here and the values are:

blank : Standard mode, one source per source card. Use this unless you know you want one of the other possibilities!

Fast Switching: Switch between the positions given on the Source and the Fast Switch Page.
Mosaic: Observe a set of positions along a line, as specified on the Raster page.
Holography: Staff or special use only; used to measure the antenna surfaces and beam shapes.

About Fast Switching:

Fast Switching is used to switch rapidly between two sources using an offset card, rather than a long series of source cards. The position of the "primary" source is specified on the Source page; that of the "secondary" source, on the Fast Switch Page. This mode avoids the twenty second start-up time encountered with source cards, and allows for total cycle times as short as 40 seconds. The FS mode also gives much shorter observe files, which are much easier to alter in real time. These fast cycle times generally imply one should use short integration times, e.g. 3.3 seconds. The cycle times and secondary source position are specified on the Fast Switch Page.

The source names in the resulting data file will be derived in an obvious way from the source positions. The "primary" and "secondary" sources will therefore be stored correctly as different sources in AIPS.

About Mosaic:

The Mosaic mode is used to observe a series of positions along straight line on the RA/Dec grid. As with Fast Switching this avoids the twenty second start-up penalty associated with individual source cards, allowing very short scans. The times per position, the position angle of the line, and the number and separation of the samples along that line, are specified on the Raster page.

The source names in the resulting data file will be derived in an obvious way from the source positions. The differnt positions along the line will therefore be stored correctly as different sources in AIPS.

About Holography:

This is a special-use mode for staff members. The options are much like those for the Mosaic mode, but using Az/El rather than RA/Dec coordinates. All pointings are given the same name, that given on the Source page.

Comment

This is a text field for the observer to place any comment text desired. "Begin at the beginning. Go on until the end. Then stop." A comment "card" will be written on the OBSERVE file immediately before the source "card" if the comment field is not empty.