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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.
Figure B.1 Time-series readout.
The following is a sample time-series window file for use with the Tektronix 1024 × 1024 chip:
CCD-WINS= 'TEK_BINSPEC' / WINDOW NAME
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
ENDUsing this window file, the CCD controller will do the following:
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.