| CSIRO Astronomy and Space Science ATCA Users Guide |
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At the start of observing, a number of procedures need to be executed to set up the array for observng. The following table summarises the tasks, and indicates the frequencies which need the various tasks.
| 16,6&3cm | 15&7mm | 3mm | ||
| Set Power Levels into CABB samplers | • | • | • | See Setting Power Levels into CABB Samplers |
| Set CABB channel ranges | • | • | • | See Set CABB Channel Range for Calibration and Monitoring |
| Flag interference | • | See Flagging Channels | ||
| Check focus | 7mm only | • | See Focus | |
| Calibrate Delays | • | • | • | See Calibrate the Array Delays |
| Check pointing | • | • | See Checking Pointing for mm obs | |
| Calibrate Phases | • | possibly | possibly | See Calibrating Phases |
| Calibrate Gains | • | • | See Setting Antenna Gains |
The rms digitiser quantisation levels (as displayed in the sampler tab of CACOR) should be close to 20 (Range: Approximately 18-22). The values in the atten section of the window show the settings of the CACOR attenuators. The CACOR attenuators have a range of 0-15 dB.
Figure 3.2: CACOR sampler level display
If the levels are not close to 20, enter the following commands in the CACOR interface:
CACOR > tatts[ervo] 20 | set the target for the attenuator servos to 20 | |
CACOR > atts[ervo] on | correlator servos the attenuators to adjust the levels into the samplers |
Wait for the levels to settle to around 20, then turn the adjustment off again with:
CACOR > atts[ervo] off |
It is occasionally impossible to get values close to 20 because there is insufficient range in the CABB attenuator settings. At mm frequencies there is extra attenuation available in the mm receiver. The current settings are displayed in MONICA. To change mm attenuators:
CAOBS > set mm ca0# a b | where # is the antenna
a is the attenuation in a pol (on both frequencies), between 0 and 15 (dB) b is the attenuation in b pol (on both frequencies), between 0 and 15 (dB) |
CACOR generates an average visibility (averaged across the spectrum) that is used for T_{sys} and array calibration. Some care needs to be taken to ensure that the range of CACOR channels used for calculating this average is as wide as possible within the useable passband, but also excludes interference.
At higher frequencies, it is usually appropriate to use the central fifty percent of channels but at cm frequencies, interference means that it is often better to select non-default channels. See Table tvchanranges for information on recommended channel ranges. If these channel ranges are not generating the anticipated visibility in VIS, inspect SPD reference to determine the cause of the problem.
The tvchan command sets the range of channels that are used for
calibrating the array and for T_{sys} calculation. All unflagged
channels are written to the data (RPFITS) file regardless of what the tvchan
setting is.
(Note: Channels that are flagged are written to the file as zeroes. See the section on Flagging Channels).
To check the current setting, execute the CACOR command
CACOR> tvch[an] | The first pair of numbers gives the channel range for the first frequency. The second pair of numbers gives the channel range for the second frequency. |
Figure 3.3: CACOR display showing the currently used tvchannels.
CACOR> tvch[an] default | sets the channel range to use the central 50% of the channels (i.e. channels 513 - 1537). |
CACOR> tvch[an] ch_1 ch_2 ch_3 ch_4 | sets specific channels where ch_1 and ch_2 are the low and high channels for frequency 1 and ch_3 and ch_4 are the low and high channels for frequency 2. | |
CACOR> tvch[an] f# ch_1 ch_2 | sets specific channels for a single frequency, where # is the frequency (1 or 2). |
The following table lists the suggested tvchan ranges for the standard continuum bands.
| Band | Center Freq | ch_1 | ch_2 | ||
| (MHz) | |||||
| 16cm | 2100 | 300 | - | 1000 | |
| 6cm | 5500 | 513 | - | 1537 | |
| 3cm | 9000 | 560 | - | 1149 | |
| 15mm | 17000 | 513 | - | 1537 | |
| 7mm | 33000 | 513 | - | 1537 | |
| 3mm | 93000 | 513 | - | 1537 | |
Table 3.1: Suggested tvchan ranges for each of the available ATCA bands.
(There is a wide range of valid frequency settings at mm frequencies).
Occasionally, there will be channels that, because of interference (or other issues), you may wish to completely excise from the data (i.e. not write those channels to the data file).
To check the extent of currently flagged channels:
CACOR> fflag | by itself, makes CACOR display the number of channels that are current unflagged. In each frequency, there is a maximum of 2049 channels. |
Figure 3.4: CACOR display showing the current number of channels that are unflagged.
There is no easy way to discover which channels have been flagged, and flagging will remain even with a change of project (or frequency etc.) so flagging should be checked at the start of observing. If there are more channels flagged than anticipated, flagging should be cleared then reflagged if necessary.
This is done by:
| select the CACOR edit button | brings up the CACOR edit screen | |
| <CR> | exits out of the CACOR edit screen and has cleared all flagging |
To flag the set of known bad channels
CACOR> fflag f# birdies | flags channel known bad channels in frequency # when using the continuum bands | |
CACOR> fflag f# default | flags the central channel (and 1/4, 3/4 channels) in frequency # for 64 MHz modes |
This needs to be done at both f1 and f2.
To flag explicit channels, use the command:
CACOR> fflag f# ch_{m} | flags channel ch_{m} in frequency # | |
CACOR> fflag f# ch_{m}-ch_{n} | flags channels ch_{m} to ch_{n} inclusive in frequency # |
Again, this needs to be done at both frequencies.
If you know exactly which channels have been flagged, they can be unflagged with the
funflag command: formats are similar to the fflag command i.e.
CACOR> funflag f# ch_{m} | unflags channel ch_{m} in frequency # |
The subreflector on each of the ATCA antennas can be raised and lowered to allow the signal to be focussed at the feed. In general, the standard observing focus is constant with frequency (and position on the sky). The exception to this is that the 3mm and 7mm focus position is different on CA01. The following table gives the best focus positions for all antenna at high frequency.
Table 3.2: Subreflector focus positions for each antenna at 7mm and 3mm.
To change the focus, use the command:
CAOBS > focus ca0# height | where # is the antenna and height is the height given in the table focusheights. |
Delay offsets need to be determined and corrected at the start of the observation. Failure to correct for delay offsets will result in decorrelation, and complete data loss.
To calibrate the delays, track a strong unresolved source (e.g. PKS1934-638 or PKS0823-500 for cm
observations; for mm observations use PKS1253-055, PKS1921-293, PKS2223-052 or
PKS0537-441). PKS1934-638 is not strong enough to calibrate at mm frequencies.
Correlation should be apparent before you proceed. Phases in SPD should be stable, though they may be wrapping rapidly and delays should be flat in VIS.) Check that you have correlation in both frequencies and both polarisations.
If there is significant interference (see the amplitude plot in SPD), you will need
to flag interfering channels or use tvchan to avoid the interference.
If working at 3mm or with significant interference, it may be necessary to average over several channels (to improve the signal/noise ratio) with the command:
CAOBS > corr delavg # | where 8 is a good initial value for # - however,
if the delays are large (>50nsec), you should try a smaller value to start with. |
If you still don’t have correlation, check the Monitoring and Troubleshooting section at the end of this chapter, and if this doesn’t resolve your problems get help.
Assuming that the phases in SPD are stable, enter the command:
CAOBS > corr dcal a | dcal calculates the required correction
and ’a’ applies the correction that the dcal finds.
Without the ’ |
This command should correct the delays so that the phases across the band on the SPD plot are flat (or fairly close to flat) and the delay offsets as seen on VIS are close to 0 nsec. If the offsets were large (greater than 100nsec) it may be necessary to do this again.
Note: Given the scale on the delays (tens of picoseconds) it is now possible to see the effect of baseline errors and atmospheric variation in delays.
Figure 3.5: VIS display showing output during ATCA calibration
If observing at mm wavelengths, the antenna pointing should be checked and corrected before phase and amplitude calibration. A pointing scan has six separate pointings: first on the nominal on-source position, followed by four pointings at the nominal half-power points (+/- Az, +/- El), then a pointing back on the on-source position. A separate task, CATAG, calculates the pointing offets and feeds them back to the system to be implemented.
Check the pointing(2) page in MONICA to check that the appropriate CATAG parameters are set:
User Antenna Mask should include all antennas that need to be used for pointing.
User IF Mask should be set to include IFs that are required. 1234 uses all 4 IFs. 12 uses the two polarisations in the first frequency.
34 uses both polarisations in the second frequency. The setting 1234 can only be used if the frequencies f1 and f2 satisfy abs(\frac{f1}{f2}-1)\leq 0.15.
Integration Cycles defines the number of cycles spent on each step of the pointing solution. This will usually be set to 2 or 3.
Modifying these parameters is done in caobs
CAOBS > set point_a a_1[a_2...a_6] | defines antennas to be included in pointing calculations;
e.g. CAOBS |
Note: Pointing using the selfcal algorithm (default mode) requires 4 or more antennas to calculate amplitudes. If using 3 or fewer antennas,
holography mode is needed. To enable holography, set the Integration Cycles to the negative (e.g. -2) value.
CAOBS > set point_if if_1[if_2...if_4] | defines IFs to be included in pointing calculations; |
e.g. CAOBS > set point_a 1234 would use both frequencies, both polarisations. |
(Note: If the two frequencies are different by more than 15%, the system will not let you use the second frequency. This is because the beam size is sufficiently different that problems can arise - think about the situation of a 5 and 10 GHz frequency pair. The half-power point at 5GHz is the first null at 10GHz.)
CAOBS > set point_p # | where # is the number of cycles spent on each pointing step. # is usually 2 or 3. If using holography mode, this should be -2 or -3. |
After these parameters have been set, ensure that your schedule file has a pointing scan set up. This scan will need to have
’Scan Type’ set to ’Point’ and the ’Pointing’ parameter set to ’Update’. To start the pointing scan,
you must use the ’start’ command in CAOBS (’track’ will simply track the on-source position):
CAOBS > start # | where # is the scan number of the pointing scan. |
Watch the amplitudes in VIS to check progress. You should see the amplitudes drop to approximately half the amplitude of the nominal center position. The difference between the plus and minus pointings give an indication of the error in the pointing.
After the pointing scan has finished, check the errors that are found in the MONICA pointing page. If any of the ’Last Az Correction’ or ’Last El Corrections’ is greater than 10”, a second pointing should probably be considered.
For more information on offset pointing, see the Reference Pointing Guide.
Once the delays have been set and pointing checked (for mm observations), the antenna gains need to be set. This calibration is somewhat cosmetic, however, it is very useful for on-line monitoring.
To set the gains:
CAOBS > corr acal #_1 #_2 a | acal calculates the required correction.
’
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If using PKS1934-638 or PKS0823-500, the fluxes are known by CAOBS and do not have to be entered, i.e.
CAOBS > corr acal a |
Do not use PKS1934-638 or PKS0823-500 for calibrating the array at mm wavelengths: they are not strong enough.
Gain settings are used for calculating T_{sys}. This can be recalculated off-line, but some care needs to be taken.
Phase Calibration is even more ’cosmetic’ than gain calibration, however, it is sometimes useful to set the phases to zero at the start of observations to help judge the phase stability. To set the phases to zero:
CAOBS > corr pcal a | pcal calculates the required correction.
’ |
At this point, the array should be set up and ready for observing. Check that all the products in VIS look correct and that the T_{sys} values (in CACOR) are appropriate. If there are any problems, check the Troubleshooting section or get help.
Users Guide last modified on 2011-04-27 15:49:06