Detector tools

Tap subtraction

COO detectors read out in taps, and each tap carries a signal half and a reset half side by side. The tap_subtraction tool rebuilds a corrected frame by gathering the reset halves of one frame, gathering the signal halves of another, and subtracting one from the other.

This is instrument-specific, so it stays off unless you ask for it:

tools:
  tap_subtraction:
    enabled: true
    tap_width: 128        # pixels per tap, must be even
    num_taps: 32

Or for a single session:

atlas signal.fits reset.fits --enable tap_subtraction

Using it

Tools → Subtract Signal/Reset Taps operates on the current frame and the one before it: the earlier frame supplies the signal halves, the current one supplies the reset halves. So you choose the pair with Frame → Next and Previous rather than from a dialog, and the result arrives as a new frame.

The difference is computed in int64, which matters: reset minus signal is genuinely signed, and an unsigned accumulator would wrap negative pixels around to full brightness.

Geometry, and the errors it produces

The frame has to match the configured geometry. atlas expects a width of tap_width * (num_taps + 1), the extra tap being the reference tap, and it says so plainly when the numbers do not line up:

Tap subtraction failed: frame is 2048 px wide but 32 taps of 128 px needs 4224

Two other cases you may meet in the status bar:

Message	Means
Tap subtraction needs two frames; open a second image first.	Only one frame is open
expected a 2D frame, got shape ...	The frame is a cube or a colour image

tap_width must be even, since each tap splits into two halves; a configuration with an odd tap_width is rejected at startup rather than at the moment you click the menu item.

The detector profile

The bundled detector profile assembles the pieces a COO detector session usually wants:

atlas --profile detector

display:
  mode: tile
  tile_columns: 1         # signal above reset, not side by side
tools:
  header: true
  histogram: true
  tap_subtraction:
    enabled: true
    tap_width: 128
    num_taps: 32
  zmq:
    enabled: true
    address: tcp://localhost:5555
    socket_type: SUB
  shm:
    enabled: false        # off until verified against a real segment
    segment_name: hispec_tracking_camera

Note that shm is present but disabled in the shipped profile. Turn it on for a session with atlas --profile detector --enable shm, or copy the profile into your own --config file and set it there along with the right segment_name. See Shared memory.