Integration Parameters

Integration Parameters is the most frequently used of the three System Parameters menus: it controls the PClock box, Co-Adder, and detector hardware.

Controlling the Detector Clocking and Readout Electronics

The Co-Adder has two main memory buffers, and it always writes new data to one buffer while the PC reads out the other. With SpectroCam, we usually use a technique called sub-buffer chopping, in which the data from each half of the chop cycle are written into different subsections of the active buffer. At the end of an integration (say, 10 seconds long), the PC reads out both sub-buffers and either displays the data or passes them along to ezra. There is also a technique called normal chopping, where each half of the chop is written to one of the memory buffers, but to use this the PC must be able to read out the buffer faster than twice the chop frequency. This only works for relatively slow chopping rates (1 Hz or less with the present PC) or very small ROM tables. Because we have used sub-buffer chopping almost exclusively with SC-10, much of the software has not been fully debugged for use with normal-chopping mode.

As stated above, the hardware drives the detector continuously. When the operator starts a scan, or turns on Continuous Acquistion mode, the PSYS program simply begins listening for a signal from the Co-Adder indicating that an integration has been completed. Once the signal is received, PSYS begins reading out the inactive Co-Adder buffer while data begins accumulating in the other. At the end of a read, the PC begins listening for the completion signal again. If the Co-Adder Reset mode is selected (from the Array parameters menu), the Co-Adder active buffer is cleared at the end of a PC read and the co-adding starts over. This is usually desireable because the readout process generates noise that is picked up by the detector.

Continuous acquisition (or Cont. Acq.) mode operates just like a real scan as far as the detector electronics is concerned, but the data returned to ezra are simply displayed without being saved to disk. A separate integration time is provided because usually a 1 or 2 second integration time is desired for Cont. Acq. while 5 to 20 sec is typical for scans. Whenever you turn on Cont. Acq. or start a scan, the electronics parameters are automatically reset to the proper integration time because in sub-buffer chopping mode the integration time determines how many frames are accumulated in the co-adder. Cont. Acq. is useful for tasks like acquiring sources, quickly checking the signal level when setting the ROM table and Chop Frequency, and focusing.

The Integration Menu allows the operator to specify the ROM table, Frames Per Chop, Chop Frequency, and Cont. Acq. Integration Time. The Integration Time for scans is set in the main SCACQ window. These parameters are programmed into the PClock box and Co-Adder by PSYS after they have been received from SCACQ. The controls are separated between the two menus because we have found that once the appropriate ROM table, FPC, Chop Frequency, and Cont. Acq. Integration Time is determined, changes are infrequently desired. However, changing between short and long integrations is more frequent. Except for the Cont. Acq. Integration Time, these parameters are all Mode Dependent: a separate set of values is saved for each Instrument Mode. Their operation is as follows:

• ROM Table: Click on the ROM Table button to bring up a menu describing each table. Information on normal vs sub-buffer chopping, array size, and frames per chop is contained in this menu. Clicking on one of the lines will put the ROM table number in the text-entry field. Once you become familiar with the available tables you may find it more convenient to simply type the number directly.

• Frames per Chop: If you are using normal chopping ROM tables, the Frames Per Chop (FPC) is user-selectable via this text-entry field. Because sub-buffer chopping tables are set up for a specific FPC, the SCACQ automatically supplies the proper value and will not accept any other.

• Chop Freqency: Enter the desired frequency into the text-entry field.

• Apply Button: To actually send the new ROM, FPC, or Chop Frequency value to the PC, and have the PC reset the PCLOCK and Co-Adder hardware (by transmitting 96 bits over a serial link), click on the Apply button. Because resetting the hardware takes time, you can simultaneously enter new values for ROM Table, Frames Per Chop, and Chop Frequency, then click Apply once to reset all the values. After the Apply button is pressed and the parameters have been successfully set, the indicators to the right of the Apply button will change to reflect the new values.

• Continuous Acq. Integ. Time: Enter a value (units of seconds) into the text-entry field. The Apply button does nothing here; simply press Return to send the value to the PC.

• Integ. Time (On main SCACQ menu): Enter the integration time to be used during scans. Again, press Return to send the value to the PC.

A field at the bottom of the Integration Menu displays several parameters which depend on the user-settable parameters described above. These are:

• NColsx NRows: The number of columns and rows addressed by the current ROM table.

• Start Col, Row: The starting row and column of the detector subsection addressed by the current ROM table.

• Frame Rate: The number of frames per second being read from the detector. Equal to the Frames Per Chop x Chop Frequency.

• Group Rate: The rate at which a group of four pixels appears on the four detector outputs. This is a function of the number of clock cycles per frame (including overhead required to skip over unwanted rows and columns) and the frame rate.

• Efficiency: Fraction of Integration Time spent on-source. When chopping this can be no higher than 0.5, but is often less because we usually throw out one frame at the begining of each chop to let the chopper settle and avoid phase changes across the array.

• No. of Samples: The number of detector samples per half-chop cycle. Usually (Frames Per Chop / 2) - 1 when the ROM tables skip 1 frame.

• No. of Chops: The number of chops per Integration Time. Equal to Chop Frequency x Integration Time. The hardware requires NChops to be an integer, and so the software finds the closest Chop Frequency and Integration Time that will give an integral NChops. This is why the returned parameters are usually slightly different than what you request. The number of chops per Cont. Acq. Integration Time is displayed just to the right of its text-entry field.

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  Other Integration Menu Controls

  There are several other parameters in the Integration Menu which control hardware and software functions not directly involving the clocking of the detector. The first three of the remaining controls are Instrument Mode-Dependent.

  • Phase: The detector electronics must be phased correctly with the chopping secondary so that detector frames intended for a certain half of the chop cycle don't end up in the wrong memory buffer or subbuffer. A Phase adjustment is provided to account for lags in the electronics and chopping secondary drive system. Enter the desired phase, in units between 0 and 360 degrees, in the Phase text-entry field and press Return. Accurately setting the correct phase is somewhat tricky because of subtle phase-lag effects across the detector (remember, pixels at the top of the detector are read out at a different time, and thus phase, than pixels at the bottom), but we discard the first frame after a chop in most of our ROM tables to minimize this problem. We have found that the excellent performance of the f/70 chopping secondary allows a phase setting within a few degrees of 0 (or 180 depending on which chopping beam should give a positive signal) for all applications.

  • Gain: The PClock box preamplifier gains are software-controllable using these switches. Settings of 10, 40, 80, and 160 are provided. With the current detector (as of July 1994, the device AsM2-1), the detector wells fill up (or, more precisely, reach the end of their linear range), at a detector output voltage of about 1.5V. At a gain of 10, the preamp output signal is 15V, nearly the limit of the preamp outputs and A/D inputs.

    For the software Gain controls to function, the CPU/LOCAL switch on the PClock box must be set to CPU

  • Bandwidth: Like the gains, the preamp bandwidths are software-controllable. The desired bandwidth depends on the rate of the individual signal, reset, and pedestal clock levels driving the detector, which occur at three times the group rate displayed in the Integration Menu. The bandwidth should thus be at least a few times higher than 3 x Group Rate. However, lower bandwidths result in lower noise.

    As with the gain, the PClock box switch must be set to CPU for the software bandwidth controls to function

  The last three controls are Mode-Independent.

  • Chopping: Controls whether the software treats the data as chopped or staring mode. This does not actually turn the chopper drive on and off!

  • Nodding: With Nodding on, a signal is sent from the PC to ezra between beams to request a nod, and ezra transmits the appropriate nod commands to the telescope computer. This also causes the display software to apply the proper sign to the +Beam and - Beam data to always display a positive signal. With Nodding off, no nod signal is sent and the software applies no sign correction.

  • Nod Settle: Sets the delay, in milliseconds, between the Telescope Steady signal from the telescope computer and the commencement of the next integration. The telescope actually takes a few seconds to fully stabilize after the steady signal is received. Using the tip-tilt guider greatly reduces the settling time.