General Considerations and Observing Scenario for BandwidthSwitching Observing Mode
ALMA Technical Note Number: 11Status: FINAL
Prepared by:

Organization:

Date:

Brian Mason (NAASC/NRAO)

ALMA04 September
2014EOC Memo: General Considerations & Observing Scenario for Bandwidth Switching
Observing Mode
B. Masonv.Sep 04, 2014

1Introduction

This memo presents some general considerations ALMA Cycle 3 Bandwidth Switching (BWSW) oserving
mode, the defining characteristic of which is the collection of calibration data in a different spectral configuration from the science targets.
In order to help identify assumptions and implications for ALMA subsystems, including the analysis
pipeline, a BWSW observing scenario is presented (§ 3).

2General Considerations

2.1

Spectral Constraints

FT

It is helpful to consider the set of spectral constraints that exist. For cycle 2 (ALMA Cycle 2 Proposer’s
Guide v1.8 & Technical handbook, v1.1) these are:
• There may be up to 4 basebands (BB), each by definition 2 GHz wide.
• There may be up to 4 spectral windows (SPW) per baseband.

• Within one baseband, all SPW’s must use the same correlator mode; these SPWs may however use
different amounts of spectral averaging in the correlator.

2.2DRA

Because of the prohibition on mixed correlator modes, BWSW can be triggered by use of just four highresolution FDM modes spaced sufficiently far apart across the receiver sidebands. This minimal BWSW
scenario will have four distinct BB, each with one high resolution FDM SPW.
The maximum number of SPWs in cycle 2 is 16 (4 SPW per BB), subject to data rate limitations.
Question: What are the C3 Spectral Constraints?

Possible Spectral Configuration Sequences

For a given science spectral setup which utilizes all correlator resources and does not provide adequate SNR
for calibration, it is necessary to specify what precise spectral setup to use for the calibration observations1 .
Some obvious possibilities are:
1. Keep BB’s fixed; replace narrow windows with a single TDM window (one TDM window will cover theentire BB). Alternatively:
2. Same as #1, but instead of keeping the BB’s fixed, shift the BB as feasible to optimize atmospherictransmission & stability while maintaining coverage of science SPWs.
3. Use standard continuum windows in the low-opacity/high transmission regions of the receiver band (orpotentially, one standard continuum window per BB).
Options #1 & #2 provide the most direct measurement of phase and amplitude corrections in the science
SPWs with the penalty that the integration time required to achieve a given SNR in the TDM window on
the calibrators would be higher than using the standard continuum bands.
Option #3 would likely provide the highest SNR measurements of the calibration sources. For many
ALMA receivers, however, the standard continuum tunings lie at one edge of the receiver bands and/orrequire very large extrapolations in frequency to reach the far edges of the receiver bands (as much as 70 100 GHz for Bands 7 - 9). It is as yet unclear what effect such large frequency extrapolations would have on
the gain corrections corrections— in particular, on the real part of the gain corrections vs. time. Assuming theprimary flux calibrator is also observed with the standard continuum setups, the flux calibration would need
1 In this memo, I refer to the science SPW setup as the “FDM setup” and the calibration SPW setup as the “TDM setup”.

1

Band
34
67899bδfsep /GHz
1212151212616

δfextrap /GHz
18.5
1842
68
95
77
77

F
2.18
2.12
5.94
8.01
11.2
18.1
6.9

Notes
Standard cont.SPWs
Widest spacing available.

Table 1: SNR degradation factor F of flux calibrations extrapolated from standard continuum bands to thefar edge of the receiver band, for each ALMA band.

to be bootstrapped to the FDM windows, probably onto the source used for the SPW-offset measurement
sequence. As described in § 2.3, the required extrapolation in frequency is both model dependent and quite
noisy at the band edges.
For these reasons the recommendation is to keep the TDM BB’s approximately the same as the FDM
BB’s, with some freedom to shift them to optimize transmission and sensitivity.

Accuracy of Flux Density Extrapolations Across the ALMA Bands

FT

2.3

orDRA

Using a simple monte-carlo simulation, I evaluated the accuracy of flux calibrations extrapolated from thestandard continuum bands to the far edge of the receiver band for each of the ALMA receiver bands. In thissimulation the source of interest is assumed to have a simple power-law spectrum whose spectral index isestimated from the standard continuum band measurements and extrapolated to the frequencies of interest.
When available, continuum bands in both the USB and the LSB are used. The picture here is that there
is one source which has been measured with both the TDM and the FDM setup (the boostrap source), andthat the TDM data can be accurately calibrated with respect to an absolute flux standard. We wish to use
those TDM measurements to predict the absolute flux density of the bootstrap source in the FDM windows.
Results are summarized in Table 2.3 and are not sensitive to the true source spectrum within the expectedrange. When available the use of both USB and LSB continuum bands greatly improves results. At the bandedges, particularly for the higher frequency receivers, the extrapolated flux density becomes quite uncertain.
The SNR of the extrapolated flux density is found to be well represented by a degradation factor F given by:
1δfsep
SN Rextrap. = SN Roriginal ÷ F = SN Roriginal √(1)
2 δfextrap
F =√

2δfextrap
δfsep


(2)

Here δfsep is the difference in frequencies between the most widely spaced “standard continuum” SPWs andδfextrap is the difference between the central frequency of the standard continuum bands, and the frequencyof the center of the farthest-outlying science SPW. SN Roriginal is the SN R in one of the two sidebands (orhalves of the total band) used to determine the source spectrum. The SNR is defined as Strue /σ, where σ isthe measurement noise (for the TDM bands) or RMS of the extrapolated flux values (for the FDM bands).
Assumptions & Caveats: This analysis assumes systematic errors (e.g., relative calibration accuracy
between USB and LSB) are smaller than measurement errors; and that the bootstrap source has a spectrum
well-approximated by a power law over the range of interest.

2.4

SPW Phase Offsets

Individual SPWs have overall phases which vary between SPWs and need to be measured; test observations have demonstrated these SPW to SPW offsets to be stable over ∼ 1.5 hours. The main challenge
to determining these offsets observationally is the time variability of the atmospheric phase delay. For aclean measurement of the instrumental phase offsets between spectral windows, the recommended observing
sequence is to observe a single source as follows:21. TDM setup bright source measurement
2. FDM (science setup) bright source measurement
3. TDM setup bright source measurement
The complete cycle of 3 measurements needs to be finished in less than the timescale for the atmosphericphases to change uncorrectibly; i.e., the complete cycle should take roughly the same time (or less) as thetime between phase calibrator observations when observing the science target(s). The sequence should be as
symmetric in time about its midpoint as feasible in order to cancel linear phase gradients. It is permissible,and likely desirable, for the TDM and FDM measurements to have different scan lengths.
The scans should be long enough to determine the SPW offset to some specified accuracy (e.g., the samenumber as used for determining the phase calibrator observation parameters). Due to the high noise level in
the FDM measurements, this TDM/FDM/TDM SPW offset sequence should be collected on a bright source,
probably the bandpass (BP) calibrator.

Observing Scenario

FT

1. TDM SPW choice: the TDM (calibration) SPWs will be chosen to approximately cover the BBs of
the FDM (science) SPWs, with some freedom to shift to optimize transmission and sensitivity.
2. BP observation: there will be one standard bandpass (BP) observation, collected in the FDM setup.
The TDM BP will be determined from the phase calibrator observations and/or the TDM scans of theSPW offset measurement sequence.
(a) Question: do we need a special intent for the TDM phasecals since the TDM BP will also be
obtained from them?
(b) Note on the bandpass solution interval: To minimize unneccessary calibration overheads& reprocessing effort, the needed resolution needs to be examined and uniformly implemented.
Currently the script generator assumes no spectral averaging in the BP solutions; the pipeline
averages down to 240 channels; and likely the online system assumes something else entirely. Fora 60 MHz window, 240 channels gives 250 kHz resolution, which is likely considerably more thanmost ALMA projects require for their BP solutions.

DRA

3

3. SPW phase-offset sequence: As described in § 2.4, there will be a short, 3-scan TDM/FDM/TDM
sequence to determine instrumental phase offsets between the TDM and FDM windows. Questions:
do we want a BWSW-specific intent to identify these scans? What phase accuracy is required for theSPW offset? This is presumably derivable from the standard phase accuracy requirement for the phasecalibrators.
4. Flux Calibration: A primary flux calibrator will be observed in the TDM setup with the usual caveats(avoid spectral lines in solar system objects).
5. Main observing cycle:(a) Phase calibrator (TDM setup)(b) Science source(s) (FDM setup)
6. At end of observation, optionally repeat SPW phase-offset sequence – do this for initial EOC/verification& testing runs at least.

3