Dark Squids and Magnetic Field Pickup

Over the last couple of weeks I have been experimenting with some data that seem to exhibit the effects of magnetic field pickup in the detectors. The SCUBA-2 readout system amplifies small changing magnetic fields caused by changing currents in the detectors (that are produced by changing optical and thermal loads on the bolometers) using arrays of SQUIDs. Although there is substantial shielding, some external changing magnetic fields may still leak into the raw data - such as motion of the telescope through the Earth's magnetic field, or fields produced by motors in the dome etc. The readouts are multiplexed, and the signals from columns of detectors pass through a common amplifier chain. For this reason, each column has a bare SQUID with no bolometer, a so-called "dark SQUID" (DKS henceforth), to track this non-thermal/optical noise term. The time-series for these data are stored in raw SCUBA-2 data files in the ".more.scuba2.dksquid" extension. They can be plotted, for example, using KAPPA linplot (this is for the DKS in column 1 of the following raw 30s data file):

$linplot 's4a20100223_00061_0002.more.scuba2.dksquid(1,)'



Just like regular bolometer data, DKS can have spikes (like in this example). Generally speaking the DKS haven't shown a lot of structure so we weren't concentrating on using them to clean the bolometer data.

However, on some of the wider-area data sets the DKS signals can be substantially larger, and seem to introduce a significant noise term. One additional feature of the magnetic field pickup signal that makes it easy to distinguish from other noise sources is that its sign is essentially random, and independent of the detector response to optical/thermal loads, i.e. the signal could be rising in one bolometer, and falling in the next. The following movie shows a data cube from the s8d array after the common-mode signal has been removed. The residual signal oscillates in time, with two clear detector populations that are out of phase with eachother:



These data were taken as part of a 0.5 deg diameter scan, and it is clear comparing the dark squid signals with the change in telescope azimuth that they are highly correlated. Combined with the arbitrary sign of the residual noise, the culprit is almost certainly magnetic field pickup.

A dark squid signal (red dots), and the signal smoothed with 200 sample boxcar (white line) over 100s:


Azimuth offset from the map centre over the same period of time:


The next plot shows the dark squids for each working column over ~25 min. with arbitrary vertical offsets for clarity. Note that while similar, each column has a slightly different response:


The iterative map-maker has a partially-implemented model, called "DKS", for iteratively fitting and removing the DKS signals from working detectors along each column. The relevant settings for the configuration file are:

modelorder = (dks,com,gai,ext,ast,noi)
dks.boxcar = 100

Here we have added DKS cleaning as the first step in the iterative model fitting to the data (note also that we have dropped FLT for now, more on that below), and the second option smooths the DKS data with a 100 sample boxcar prior to fitting and removing it from each bolometer.

The next two images show a map solution using only COM to remove low-frequency noise, followed by a solution that uses both DKS and COM:





While not perfect, it is still clear that the DKS removal has removed a lot of the noise that caused streaking in the first image. Remember, there is no additional high-pass filtering as FLT has been turned off in this example.

If "dks" is specified in "exportndf", the fitted DKS model will be written to a file with a suffix "_dks.sdf". However, this file is not particularly easy to view and make any sense of because the dark squids and the fit coefficients are all bundled together in one large array. For this reason, we've added a new SMURF task called "sc2expandmodel" that will evaluate the model and produce a full 3d data cube with the same dimensions/units as the residual data, e.g.

$sc2expandmodel s4a20100228_00016_0002_con_dks expanded_dks

will produce an easily-viewable file called "expanded_dks". All detectors along a column will have the same shape, but the scale/offset will be different for each detector. Examining this file and comparing to the "_res" output from makemap is useful for getting an idea of how strong this magnetic field pickup is for a given data set.

All of this code is still experimental. It currently has several problems:

• boundaries with padding/apodization will introduce steps, so padstart/padend/apod should probably be set to zero, meaning that it is not optimal for combining with FLT at this stage.
• the DKS are not pre-processed in the same way as the bolometer data to remove spikes / steps etc. This capability will be added shortly.
• Some DKS are clearly dead (e.g. DKS 1 in the multi-DKS plot above). Currently we don't have an automatic system for flagging/removing them. Note also that if they are bad, it will be impossible to DKS-clean working detectors along that column (so we need to decide if we will simply have to ignore them when making a map?)

Once we have worked through some of these issues, we will update the default dimmconfig files in SMURF accordingly.