Progress on the Implementation of the Bandwidth Transfer
Technique for High Frequency Observations – Report #2
ALMA Technical Note Number: 10
Status: FINAL
Prepared by:

Organization:

Date:

Brian Mason (NAASC/NRAO)

ALMA03 September2014

EOC Memo: Bandwidth Switching Progress Report #2
B. Masonv.Sep 03, 2014

1SummaryThis report summarizes progress on testing the bandwidth-switching (BWSW) observing mode during theAustral Winter 2014 ALMA EOC campaign, following on the earlier report by J.Donovan-Meyer.
Findings:
1. Spectral window to spectral window instrumental phase stability is excellent and present no obstacle
to BWSW observing.
2. However, atmospheric phase (delay) fluctuations are significantly contaminating our measurement of
the SPW-to-SPW phase offsets with the current observing strategy. In fact, we find that compared toignoring the SPW-to-SPW phase offsets determined by current procedures, coherence is decreased by
38% when they are applied and the map noise is increased by 60%. Therefore an appropriate observing
procedure is required to measure the instrumental offsets in practice.
3. A ∼ 12 beam shift is seen in these data when the SPW offsetts are applied. This needs further
investigation and resolution.
Recommendations for further EOC testing:
1. Implement a short 3-scan TDM/FDM/TDM sequence to measure instrumental offsets.
2. Define one or two spectral setups that represent what is likely to be encountered in practice, with somespread in frequency, and use them in this test. I suggest a basic paradigm of: a few (4 - 16) FDMs forthe science target, scattered around the receiver sidebands (include some in relatively low transmission
portions of the band); with TDM’s chosen in two ways: 1) using the same BB’s as the FDM sciencewindows; 2) using standard continuum windows. Both sidebands should be used for both science andcalibration SPWs.
3. Include observations of a science target (not the phase cal) in both spectral modes so that the calibration
can be fully verified.
4. Until BWSW test observations are collected that successfully meet all these criteria, focus on acquiring
data at B9 or lower frequency.

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Test Data

Test datasets are summarized in Table 1.

2.1

SPW-to-SPW Stability: BP Stability dataset

We used the BP stability dataset previously acquired (July 10, 2014; X1fa) to evaluate the stability of phases(primarily) between different base-bands (SPW 0 = BB 1 = TDM; SPW 1 = BB 1 = 58 MHz FDM; SPW
2 = BB 2 = 234 MHz FDM; SPW 3 = BB3 = 1875 MHz FDM). For this observation the spectral setup wasfixed. Three bright QSOs were observed in rotation.
The data were reduced as follows:
Execution
X410
X5f3
X6fc
X1faBand
B9B10B10B9Notes
clean execution with FDM/TDM/TDM BP seq.
various issues; FDM/TDM/TDM BP seq.
only a few good scans, but contains a complete TDM/FDM/TDM BP seq.
July 10, 2014 BP stability test.
Table 1: Test datasets discussed in this report.
1Figure 1: Typical phase offsets betwen SPW PHASE-INF solutions, relative to the SPW0 PHASE-INT,
shown here for antenna DA41. Top panel: 2 GHz TDM window; upper middle panel: 58 MHz FDM window;
lower middle panel: 234 MHz FDM window; bottom panel: 1875 MHz FDM window.
1. The script generator was used and the resulting script run on the data with no significant modifications.
2. A PHASE-INT solution (SOLINT=18.15s=6 integrations) was obtained for the TDM window on all 3
QSOs, pre-applying the BP solution (from the brightest QSO only).
3. A PHASE-INF gain solution was obtained for each source and each SPW; the BP solution was preapplied, as was the TDM PHASE-INT solution with appropriate SPW-mapping.
An example of the resulting PHASE-INF solutions for each SPW relative to the TDM PHASE-INT solution
are shown in Figure 1.
The phase solutions were extracted and analyzed, with results shown in Figure 2 (the mean offset overthe execution for each ANTxPOL) and Figure 3 (the RMS and MAD1 of phases over the execution). Thereare occasionally failed solutions, more common in the narrower band SPWs. Likely this is what generates theoccasional outliers in phase The RMS phase noise is also higher in the narrower FDM windows than in thewider windows, likely reflecting thermal noise in the solutions. The deviations from zero of the SPW0-SPW0
case reflect differences in the PHASE-INF and the mean value of the PHASE-INT solutions.
The SPW-to-SPW phase offsets appear very stable in time over the duration of this 1.5 hour observation,with no signicant gradients evident in the solutions.
1 The Median Absolute Deviation (MAD) is as a more robust estimator of the scatter in a distribution than the RMS. Hereit is renormalized so that the MAD is equal to the RMS for a normal distribution.

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Figure 2: Distribution of SPW to SPW phase offsets.
Note that these are the overall phases of the SPWs. Depending on the reduction strategy the SPW-toSPW phases derived may be smaller than this. For instance, a mean phase of each SPW is often determinedfrom the BP observation and carried along throughout the reduction. In general it is the stability of theSPW-to-SPW phases that is important: here we have assessed the instrumental component of that stability.
Since the various SPWs’ data were collected simultaneously and the SPWs were not very different in wavelength coverage, the atmospheric delay changes with time should not be a significant effect. We assess theatmospheric effects in the next section.

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Figure 3: RMS (dashed blue) and RMS-equivalent MAD (solid black) of SPW to SPW phase offsets.

4Figure 4: Difference in phase solutions for the TDM SPW for two sequential BP observations (B9, X5f3).

2.2

Atmospheric Fluctuations

Atmospheric phase/delay fluctuations are significantly affecting our attempts to measure the SPW-to-SPWoffsets. This can be seen in Fig. 4 which shows a comparison of phase solutions obtained in two sequential
TDM BP observations. In the absence of systematics the average TDM phases would be identical and thephase differences (plotted) would be zero.
In X6fc we obtained a TDM/FDM/TDM sequence of BP observations with the aim of being able tolinearly interpolate the offset. However a bandpass observation is too long for this to be effective. This canbe seen in Fig. 5 (from X410).
The BWSW observing mode will need to measure the instrumental SPW-to-SPW offsets in less than thetime for atmospheric phases to change significantly.

5Figure 5: PHASE-INT solutions for each SPW on the BP calibrator (B9, X410) showing that phase fluctuations are large over the time period in question.

6SPW01234567

Ch0 [GHz]
668.0
668.0
668.9
668.9
668.9
668.9
668.1
668.9

BW [MHz]
2000
2000
61
61
61
61
48861

Baseband13123424

Table 2: SPW Setup (post-split). TDM/calibration scans used SPWs 0,1,6 & 7; science/FDM scans used
SPWs 2 − 5. The SPWmap used was [0, 1, 0, 6, 1, 1, 6, 0].

2.3

Complete B9 Reduction & Imaging

The best of our test datasets is X4102 , the clean B9 execution obtained on July 30, 2014. This is a ∼ 1.5 hr.,
B9 execution with ∼ 0.6 mm PWV, Tsys values in the range 1100 − 1500 K, and a synthesized beam width of0.2400 × 0.1600 . The observation, in outline, proceeded as follows:
1. BP observation of 3c454.3; narrow-SPW setup; 16 minutes
2. Short BP obs. of 3c454.3; wide-SPW setup; 3 min.
3. Long BP obs. of 3c454.3; wide-SPW setup; 16 min.
4. Flux cal. on J1924-292; wide-SPW setup
5. Main observing cycle:• Phase cal. on J1924-292; wide-SPW setup; 30 sec.
• Target source obs: J1923-210; narrow-SPW setup; 5 min.
The duplicate wide-SPW BP observation was inserted to check the consistency of the phase solutions. In
all approximately 25 minutes of integration were collected on the target source J1923-210. The ALMA
sensitivity calculator (2nd octile weather, 1155 K Tsys, −21◦ dec. source, 60 MHz BW, 668.8 GHz dual-pol,27x12m, 10x7m) predicts 6.1 mJy/bm RMS sensitivity for the integration time achieved on this target. TheSPW setup is summarized in Table 2; results presented are from SPW 4, which was referenced to SPW 1(same BB), but results from SPWs 2 and 3 were similar.
The CASA manual script generator generated a script, but significant modifications are necessary forthe script to run and calibrate the data. Modifications mostly followed the procedure reported in the first
EOC BWSW report by J.Donovan-Meyer, with some tailoring of the solution intervals in time and frequency(generally increasing the amount of averaging in each). The flux and amplitude calibration were transferred
directly from the TDM SPWs.
The target source was split out into its own band-averaged continuum MS. A version of the MS was
also made which did not have the SPW-to-SPW phase offsets (table phase offsets inf) applied, since weare suspicious that these may be substantially contaminated by changes in the atmospheric delay. This
“no-SPW-offs” MS did have the TDM phase-cal phase offsets applied (directly, with no correction for thedifferences in the SPW phases themselves).
A map of the fully calibrated dataset including the estimated SPW-to-SPW phase offsets is shown in thetop left panel of Figure 6. Also shown (top right) are the residuals to a uv modelfit assuming a point source
model. The bottom panels show the same results after a single round of phase self-calibration using the modelobtained from the UV model fit. While some residual is still evident in the map center that could likely beimproved by further iterations of self-cal, the phase self-cal improves results considerably. In particular themap RMS is close to what is predicted by the sensitivity calculator and the residuals are much less stronglypatterned. A map was also made from the MS with no SPW offsets. This map qualitatively resembles thepost-self-cal map but with lower peak flux density.
2 uid://A002/X881d57/X410

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Reduction
SPW-offs”, after self-cal
No SPW-offs

Peak Flux[mJy/bm]
241
462
386

RMS Noise[mJy/bm]
10.4
6.5
7.1

∆ Position[arcsec](0.07, −0.09)(0.07, −0.09)(0.01, −0.03)

Table 3: Imaging results for three different reduction approaches. The expected noise level from the sensitivity
calculator is ∼ 6.1 mJy/bm. Peak flux and position are from a uv model-fit.

Table 3 shows peak flux densities & position shifts (from uvmodelfit) for all three approaches— thenominal calibration, where the TDM phase-cal phases were transferred and (measured) SPW-to-SPW phaseoffsets applied; that, after a single round of (phase) self-cal; and result that ignore the SPW-to-SPW phaseoffsets. The nominal reduction retrives only 52% of the flux that self-cal recovers; this flux is spread overthe map, increasing the RMS noise by 60%. Neglecting the SPW offsets (but using the TDM phase calsolutions) does better, recovering 84% of the peak flux density. These results support the hypothesis that
our measurements of the SPW-to-SPW phase offsets are significantly contaminated by atmospheric delaychanges.
The peak flux obtained in the best (self-cal) case is 462 mJy/bm. The ALMA source catalog contains
measurements of J1923-2104 up to 353 GHz. A power law fit to these data predicts a flux density 32%
higher, 609 ± 22 mJy/bm (statistical error). Part of this discrepancy could be due to the large frequency
extrapolation.
A roughly half-beam position shift is seen when the SPW phase offsets are applied; this needs to beinvestigated further, with better data (i.e. good measurements of the system SPW phase offsets), and SPWs
spread across the receiver band (which could increase the shift). The magnitude of the shift is comparable towhat was seen the earlier BWSW EOC report (Donovan-Meyer). It could be due to an atmospheric phasegradient over the array “frozen-in” to the data by our sub-optimal SPW-offset measurement procedure. It
could be due to some more subtle system or CASA bug. We will want to follow this up with near-future
EOC data that have better offset measurements and SPWs spread across the receiver band.

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Figure 6: Dirty maps (left) and residuals to a point source uvmodelfit (right). The maps made using thefull calibration procedure including application of the estimated SPW-to-SPW phase offsets applied are seenat (top); at (bottom) is the same, but after a single phase self-calibration. Dirty maps (left) are on a linear
min-max scale; residual maps are on a common linear scale ranging from -20 mJy/bm to +20 mJy/bm.

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