ALMA Band 1 is the lowest frequency band of ALMA receivers, covering the frequency range of 35-50 GHz. From March 2023 in Cycle 10, ALMA started offering Band 1 observation capabilities to the community.

Recently, a technical issue has been identified in Band 1 data, showing a standing-wave-like ripple in both bandpass amplitude and phase across Band 1 observing frequency. Peak-to-peak amplitude is about +/- 1% of averaged flux, and peak-to-peak phase is about +/- 3 degrees. The ripple frequency (Δvrip) is about 25 MHz for the 12-m Array and 40 MHz for the 7-m Array. An example of an amplitude and phase plot from an affected observation is shown in Fig. 1.  The origin of the ripple has not yet been fully identified, but it most likely stems from a reflection of the incoming signal somewhere in the antenna frontend system.

Figure 1. An example of the bandpass ripple for a 7-m antenna. The left panel shows amplitude vs. frequency, and the right panel shows phase vs. frequency.

The ripple cannot be calibrated by standard offline calibration. All Band 1 data are affected by this issue regardless of the array type (12-m or 7-m), spectral resolution, frequency tuning, and observation target. However, the ripple may be smeared out at some level with the TDM spectral resolution. This ripple effectively degrades the spectral dynamic range of science targets.

We encourage users to be aware of the following drawbacks:

• For the target sources with both spectral line and continuum emission, any spectral lines with a flux density smaller than ~1/100 or ~1/(100 x (Δν_line/Δν_rip)^0.5) of the continuum flux cannot be detected if the line width (Δν_line) is comparable/narrower than the ripple frequency or broader than the ripple frequency, respectively.

• Bright lines can create an artificial tail within +/- Δν_rip from the line peak. Any weak line with a peak flux density smaller than 1% of nearby bright spectral lines (located within +/- Δν_rip) cannot be detected.

   

Spectral dynamic range limitations apply to these cases. The sensitivity of dynamic range-limited datasets will not improve with longer integration.. Consequently, the data will be marked as QA2- Semipass and delivered to the PIs without additional executions. Datasets can be identified as spectral dynamic range-limited when:

• For spectral lines against continuum emission:

  • 1% of the peak continuum of the target source exceeds the requested image rms if the "bandwidth used for sensitivity" is comparable or narrower than the ripple frequency.

  • (Δν/Δν_rip)^0.5 % of the peak continuum exceeds the requested image rms if the "bandwidth used for sensitivity (Δν)" is significantly wider than the ripple frequency.

• For weak target spectral lines near a bright spectral line:

  • The nearby bright line lies within +/- 25MHz and +/- 40MHz for the 12-m Array and 7-m Array, respectively

  • 1% of the peak line flux of the nearby bright line exceeds the requested image rms.