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3.2. Setting up

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.

Set focusSee Section 3.2.1
Set power levels into CABB samplersSee Section 3.2.2
Set CABB channel rangesSee Section 3.2.3
Flag interferenceusually not necessarySee Section 3.2.5
Calibrate delaysSee Section
Check pointingXSee Section
Calibrate phasesSee Section
Calibrate gainspaddleSee Section

3.2.1. Focus

The subreflector on each of the ATCA antennas can be raised and lowered to allow the signal to be focussed at the receiver feed. However, changing the subreflector position will change the path length (obviously) which will change the geometric phases and delays, so, it should only be done when required, usually when the frequency changes. Extra care is needed with calibration with programs that change focus during the observation.

A good time to do this is at the start of the observing and it can be done while driving to source.

Default focus positions have been determined for each band.

 caobs> focus default drives the subreflector to the default focus height for that band
 caobs> focus ca0# heightdrives the subreflector on antenna # to the given focus height

Table 3.1. Subreflector Focus Positions for the Antennas.


Note: As the antenna tips over, it deforms slightly, and the detected focus position changes by up to 1mm. This is normal and will not significantly effect the data.

3.2.2. Setting Power Levels into CABB Samplers

The rms digitiser quantisation levels (as displayed in the sampler tab of cacor) should be close to 20 (Range: Approximately 18-22). To set the levels, attenuation can be added or removed. The values in the atten section of the window show the settings of the cacor attenuators. They have a range of 0-15 dB.

cacor sampler level display

Figure 3.2. cacor sampler level display

If the levels are not close to 20, the ATTS button in the cacor GUI can be activated to enable cacor to automatically add or remove attenuation.

This can also be done in the cacor interface:

 cacor> tatts[ervo] 20set the target for the attenuator servos to 20
  (most of the time, this command is not needed as tatts will usually already be set to this level)
 cacor> atts[ervo] oncorrelator 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 

Fairly obviously, the algorithm is unable to “improve” the levels if the attenuators are at their limits (0 or 15).

For 16cm observations, strong RFI in the band will often make the sampler levels vary significantly from cycle to cycle, making it difficult (and often impossible) to set the attenuators appropriately. In this case, you can set the attenuators manually to values that are known to be good, as listed in Table 3.2. To set the attenuators manually, you will need to use the att command. For example, to set the CA03 attenuators as per Table 3.2, you would need to give the commands:

 cacor> att1 ca03 6 13sets the CA03 IF1 A and B pol attenuators
 cacor> att2 ca03 2 2sets the CA03 IF2 A and B pol attenuators

See attf ca0# n m for more complete information about how to use this command.

Table 3.2. Attenuator settings for the 16cm band

1 A546786
1 B9313598
2 A1042984
2 B742594

If the sampler levels are very low (approx 0), there is probably a problem which will need to be checked.

There are another set of attenuators in the receivers (as opposed to the correlator) which can also be used to set the levels.

These are controlled from caobs.

 caobs> show atten[uation]shows the current receiver attenuation levels in the caobs status window

 caobs> set atten ca0# atta attbsets the receiver attenuator levels on antenna # to attenuator levels a and b

3.2.3. Set CABB Channel Range for Calibration and Monitoring

cacor generates an average visibility (averaged across the spectrum) that is used for Tsys 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 3.3 for information on recommended channel ranges for 2048 MHz continuum bands. If these channel ranges are not generating the anticipated visibility in vis, inspect Appendix C to determine the cause of the problem.

The tvchan command sets the range of channels that are used for calibrating the array and for Tsys 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 Section 3.2.5 for more information).

Current settings are displayed in the cacor GUI, as shown in Figure 3.3.

cacor display showing the currently used tvchannels.

Figure 3.3. cacor display showing the currently used tvchannels.

It is also possible to check the current setting with 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.

Setting the range is done with the cacor command:

 cacor> tvch[an] defaultsets the channel range to use the central 50% of the channels (i.e. channels 513 - 1537).
 cacor> tvch[an] ch1 ch2 ch3 ch4sets specific channels where ch1 and ch2 are the low and high channels for frequency 1 and ch3 and ch4 are the low and high channels for frequency 2.
 cacor> tvch[an] f# ch1 ch2sets specific channels for a single frequency, where # is the frequency (1 or 2).

Table 3.3 lists the suggested tvchan ranges for the standard continuum bands. On any given day (or interference environment) other ranges may be more appropriate.

Table 3.3. Suggested tvchan ranges for standard ATCA Continuum bands.

BandCentre Freqch1ch2

(There is a wide range of valid frequency settings at mm frequencies).

3.2.4. Checking Delavg

To calculate the delay of an individual IF, cacor checks the phase in each channel in the tvchan range (just defined) and calculates a linear fit of these phase measurements as a function of frequency; the slope of this line is the delay.

If the noise on the phase is high (due to a faint source or poor weather, for example) or there is some strong unavoidable RFI in the tvchan range, then the least-squares line of best fit may not accurately represent the real slope due to the delay.

To reduce the noise, or to average out the effect of the RFI, setting delavg in the correlator averages together that number of channels in the tvchan range before calculating the slope.

However, care needs to be taken! If the phases are wrapping quickly, averaging over a large number of channels will give a meaningless value, and delays determined will be incorrect.

The current value of delavg is in the status bar of the cacor interface.

cacor showing current delavg setting

Figure 3.4. cacor showing current delavg setting

Setting delavg is done with the cacor command:

 cacor> delavg #Averages # channels.

Figure 3.5. 

An spd plot with a slow changes in phases

The phases are not wrapping at all in the A pol, and wrapping approximately every 250 channels in the B pol. Averaging over 32 channels would still give enough points across the band to do a linear fit to this data

Figure 3.6. 

An spd plot showing the phases wrapping very quickly

The phases in both pols of this graph are wrapping over a small number of channels. It is possible to check just how quickly the phases are wrapping by viewing a smaller number of channels:

spd> chan f# 400 500

3.2.5. Flagging Channels

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> fflagdisplay the number of channels that are currently unflagged. In each frequency, there is a maximum of 2049 channels.

cacor display showing the current number of channels that are unflagged.

Figure 3.7. 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:

 cacor> fflag f# defaultflags the central channel (and 1/4, 3/4 channels) in frequency #

To flag the set of known bad continuum channels

 cacor> fflag f# birdiesflags channel known bad channels in frequency #

This needs to be done at both f1 and f2.

To flag explicit channels, use the command:

 cacor> fflag f# chmflags channel chm in frequency #
 cacor> fflag f# chm-chnflags channels chm to chn 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# chmunflags channel chm in frequency #

3.2.6. Array Calibration

Once the array is working, the system can be calibrated.

Calibration will not fix a broken system.

The calibration involves calculating and applying the delay offsets (for mm observations), setting the phases to zero degrees and correcting the visibilities to the appropriate flux levels.

vis display showing output during ATCA calibration

Figure 3.8. vis display showing output during ATCA calibration Calibrate the Array Delays

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 loss of data.

In the 64MHz zoom configuration, delay calibration is more complex as it generally needs to be done in two stpes. (First delay calibrate a zoom channel, then in the continuum. See Section for more information)

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 and PKS0823-500 are 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 carefully craft the tvchan range (or, if bad enough, actually flag interfering channels) to avoid the interference in the calibration data.

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 Section 3.4.6 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 dcal calculates and applies the required correction>

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. It may be necessary to do this again.

(Early versions of the CABB software required an “a” switch at the end of the dcal command for the correction to actually be applied. The delays are now applied automatically by the command. Any switch other than an a will result in the delays not being applied.)  64MHz Delay Calibration

Delay calibration is slightly more complex in 64MHz mode.

This issue is that the phases in spd wrap more than once in a single (64MHz wide) continuum channel. The solution for this is to solve the delays for a single continuum channel using zooms, which will unwrap the continuum data sufficiently to be able to determine the delays in the continuum bands.

To use the zooms for delay calibration, set calband to z z:

 cacor > calband z zUses zoom data in both frequencies. For a hybrid correlator config, use f z

Do the delay calibration as normal.

 caobs > corr dcalAs always, ensure that there is a working system before doing the dcal

Once the majority of the delay error has been corrected for, the whole continuum band needs to be used for a second delay calibration:

 cacor > calband f f 
 caobs > corr dcal Checking Pointing for mm obs

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 [1] 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 .

  • 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 a1[a2 ...a6]defines antennas to be included in pointing calculations
 caobs > set point_a 12345would use all antennas except CA06

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 if1[if2... if4]defines IFs to be included in pointing calculations;
 caobs > set point_if 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 5 GHz is the first null at 10 GHz.)

 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 centre 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. Calibrating Phases

Phase Calibration is somewhat “cosmetic”, however, it can be 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 pcalpcal calculates the required correction and applies the correction that the pcal offsets.

(Early versions of the CABB software required an “a” switch at the end of the acal command for the gains to actually be applied. They are now applied automatically by the pcal command. Any switch other than an a will result in the phase corrections not being applied.) Setting Antenna Gains

Once the delays have been set and pointing checked (for mm observations), the antenna gains need to be set. To set the gains:

 caobs > corr acal [#1 #2] acal calculates the required correction. #1 and #2 are the fluxes of the calibrator at the first and second frequencies respectively.

(Early versions of the CABB software required an “a” switch at the end of the acal command for the gains to actually be applied. They are now applied automatically by the acal command. Any switch other than an a will result in the gain corrections not being applied.)

If using PKS1934-638 or PKS0823-500, the flux densities are known by caobs and do not have to be entered, i.e.

 caobs > corr acal 

Gain settings are used for calculating Tsys. This can be recalculated off-line, but some care needs to be taken.

The Tsyss should be checked in the cacor GUI. Any outlier readings should be checked.

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 Tsys values (in cacor) are appropriate. If there are any problems, check the Troubleshooting section Section 3.4.6 or get help.

[1] MoniCA Pointing Information: MoniCA → Navigator → misc → pointing