Appendix B. Observing in Time Series Mode

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Time-series is a special readout mode which allows a rapid sequence of images to be taken without the overheads normally required to read out individual frames. For time-series observing in spectroscopic mode, the CCD dewar must be rotated by 90° so that the spectral dispersion is along the rows rather than the columns. This means that you must have requested time-series mode in your proposal, or well before your run, to ensure that the CCD is correctly aligned. The star is positioned in a predefined region of the chip and a special readout mode is started. The way the chip is read out is specified by the window definition. The readout sequence involves a specified number of integrations, each followed by a shift of the charge by a specified number of rows towards the readout register. During each shift, data in some of the first and last rows can be thrown away and the remaining rows (which are assumed to contain the object) are properly read out and stored in a VAX file.

The rows which are kept can be binned in both the vertical and horizontal directions and windowed in the horizontal direction. The resulting file contains a time series of independent images. The temporal separation of these spectra is just the integration time plus time needed to shift the charge the required amount along the chip. Depending on the size of the shift and the binning and windowing operations accompanying it, the overhead is between 0.01 and 3 seconds.

OBSERVER Commands for Time Series

The relevant OBSERVER commands for running a time-series exposure are:

  • METHOD SERIES- Specifies that a time-series readout will be done. This command must precede any other time-series command. Return to normal operation with the METHOD CCD_NORMAL command.
  • WINDOW your_window - Input the window specifying the readout mode. See below for a description.
  • PERIOD p - Specifies the cycle time for each exposure. This time includes both the integration time and the overheads involved in shifting and readout. The period should not exceed 65.5 seconds. This cycle time is controlled by the hardware and is accurate to better than 1 millisecond. Note that the RGO shutter timing is much less accurate than this; for very short TIMEs (less than a couple of seconds) or when accurate timing is essential, this problem can be bypassed by leaving the RGO shutter open and accepting a small degree of smearing during readout.
  • TIME t - The time for which the shutter is nominally open during each cycle. If this time is too large for the specified period, an error message will be given and either the period or shutter time must be changed.
  • CYCLES n - Specifies the number of cycles. The default is 1 cycle; the value CCD-NSW in the window definition is ignored. This CANNOT be changed during a time-series readout.
  • OBJECT objectname - Identify the object for future reference.
  • RUN - Begins the exposure.
  • PERIOD and TIME can be iterated to achieve the best value; OBSERVER will calculate, from the readout speed and size of window, how long it will take to read out each section; either value can then be adjusted until they fit.
  • Creating a Time Series Window

    In order to operate in time-series mode, a window file which specifies how much of the chip is read, how far the data is shifted between exposures at each operation, and how the data is to be binned, must be supplied. A schematic diagram of a time-series readout operation is given in Figure B.1.

    Figure B.1 Time-series readout.



    The following is a sample time-series window file for use with the Tektronix 1024 × 1024 chip:

    CCD-LTW =                 470     / LINES TO WINDOW
    CCD-NSW =                  40     / NUMBER OF SUB-WINDOWS (CYCLES)
    CCD-PTW =                   0     / PIXELS TO WINDOW
    CCD-NFW =                   1     / NUMBER OF FIELDS IN WINDOW
    CCD-LPB =                  10     / LINES PER BIN IN FIELD
    CCD-PCW =                   5     / PIXELS PER BIN IN CENTRE WINDOW
    CCD-BCW =                 205     / NUMBER OF BINS IN CENTRE WINDOW
    CCD-VSBR=                  24     / VERTICAL SHIFTS BEFORE READOUT
    CCD-VSDR=                  50     / VERTICAL SHIFTS DURING READOUT
    CCD-VSAR=                  76     / VERTICAL SHIFTS AFTER READOUT
    Using this window file, the CCD controller will do the following:
    read 24 lines of data (CCD-VSBR) and discard them
    read the next 50 lines (CCD-VSDR), bin them by 10 (CCD-LPB), and write out 205 × 5 (CCD-BCW × (CCD-VSDR / CCD-LPB)) data points
    read 76 lines of data (CCD-VSAR) and discard them, thus shifting data down the chip.

    Thus at every read operation, 150 lines (CCD-VSBR + CCD-VSDR + CCD-VSAR) are read, with the rest of the chip being shifted down by the same amount. After several cycles, there will be several exposures on the chip, each shifted down by 150 lines. This is illustrated in Figure B.1. The final data will be a cube, with dimensions 205 × 5 × 100, where the dimensions are spectral, spatial, and time, respectively, and the exposure was taken with CYCLES 100. Note that if CYCLES was not specified, there would be 1 exposure, as CCD-NSW is ignored.

    In ordinary window files, the value CCD-LTW describes the number of lines from the start of the chip to the exposed window. In time-series mode, this value is ignored. The readout of the chip commences from the edge of the chip, so the first few exposures in the cube ((CCD-LTW - CCD-VSBR) / (CCD-VSBR + CCD-VSDR + CCD-VSAR); 3 in our example) will be from unexposed regions of the chip and thus will contain no data.

    To avoid re-exposing the shifted lines of the chip, you will probably want to observe through a narrow dekker to reduce the slit length. For instance, dekker 62 gives a 20'' slit (Appendix D). Use the EEV2_FULL, MITLL_FULL or TEK1K_EXACT window to find out where this dekker illuminates the chip; the illuminated width defines the number of lines you want to read out (possibly with binning), and CCD-VSBR can be judiciously adjusted until the dekker is well centred in your readout window. This set-up process can take a fair amount of fiddling, so you are advised to carry it out well before observing starts on your first night.

    Note that if CYCLES was not specified, there would be 1 exposure, as CCD-NSW is ignored.

    Previous: A. OBSERVER Commands | Next: C. Filter Specifications | Back to: The OBSERVER Software | CONTENTS

    Ray Stathakis
    Last Update: 22/3/2002