System notes

One of the biggest impacts on the final quality of images is the presence of direction dependent errors, which can be attributed to antenna elements within the PAF for specific telescopes which are malfunctioning or missing from the signal chain. Another cause of direction dependent errors is due to pointing offsets of one or more telescopes.
Generally, the emphasis of system changes has been on improving the quality of the formed compound beams and pointing stability.
The most significant upgrades to the system are listed below in chronological order:

Synch-optics boards on RTC and RTD:

On 3 September 2019 and 5 August 2019, synch-optics boards were installed on dishes RTC and RTD. These two dishes are furthest from the control building. They were suffering from delay issues; due to jumps in interpreting the clock signal, the residual delays for these dishes would change by up to hundreds of nanoseconds, happening at random times during observations. The synch-optics boards stabilized the clock signal, preventing the very large residual delay that would lead to the loss of usable data from the dishes.

Targeted maintenance of individual elements:

The Apertif PAF is relatively sparsely filled. Thus, any given compound beam relies predominantly on 1-2 elements. If those elements are missing from the signal chain, that compound beam will have a non-ideal shape. When in a compound beam of on one dish, the signal of a key element is missing, this presents an odd response compared to that compound beam on all other dishes. The resulting images for that beam have strong direction dependent errors. Since October 2019, the performance and gains of individual elements are closely monitored so that they can be repaired in the most expedient manner possible.

Attenuation tuning of individual elements:

After a major maintenance, the single antenna elements need to be tuned to the nominal receiving power. If elements lie outside the accepted range of parameters, they are ignored during the calculation of the beamweights. The attenuation tuning algorithm was significantly improved starting 1 October 2019. 

Improved beam weights calculation:

The beam weight measurements used to calibrate the PAF are single-dish measurements and hence heavily affected by RFI. An improved method for identifying subbands affected by RFI was implemented on 10 December 2019. This allows beam weight values that are affected by RFI to be interpolated from nearby subbands that are RFI-free, resulting in higher quality beamweights (especially at lower frequencies) which leads to an improved sensitivity of the system.

Updated pointing model:

On 11 December 2019 and on 11 June 2020, the pointing model for the dishes was updated. This corrects for pointing offsets caused by changes in the mechanical structure and physical pointing of the dishes.

Other general caveats:

Ghosts:

Channels 16 and 48 of each subband have a “ghost”; bad signal in these channels cause a false source to appear at the center of images. Thus, any source identified at the exact center of a pointing should be treated with extreme caution.

Aliasing:

The coarse channelization of the data into subbands uses a filter that does not have a perfectly sharp frequency response. This results in some overlap of response between adjacent subbands.  This effect is strongest for channels near a subband edge and also results in a sharp drop in overall response for channels at the subband edges, namely channels 0, 1 and 63 of every subband.  Currently, no correction is done for the aliasing. A brute force approach is used to deal with the suppressed signal at the edges of the subbands — the low signal channels are flagged.  An offline anti-aliasing filter is under development; when it is available, aliased signals will be removed, and the sharp drop in subband response will be evened out. Until then, we note that aliased signal may occur in the presence of strong HI emission and that 3/64 channels are flagged at full spectral resolution. The impact of this flagging on the spectrally-averaged line cubes is described in the “External Comparison” section of the HI validation.

Telescope specific issues:

Due to operational needs (exceptional maintenance) or because of failures (the above mentioned high residual delay issue, tracking issues, extreme RFIs, etc), one or more telescopes could be missing from an observation. Information about specific observations can be found in “Notes on specific observations“.