Characteristics of the Data

• FITS data structure

  The HDS data unit includes the output of one CCD with 2048 (slit direction) by 4100 (dispersion direction) pixels (in the case without binning) and the over-scan region. Over-scan indicates the additional readout to the CCD pixels exposed. The data in the over-scan region provides the bias level for the frame itself.

  Since there are two output points for each CCD, the unit of one output is originally composed of 1024 × 4100 pixels. The over-scan region (50 × 4100 pixels) is added to this data unit. One file is composed of the two units, after the addition of the over-scan region. Then the two files, corresponding to the two CCDs (as shown in Figure ) are obtained by one exposure. Note that the above explanation also applies to data obtained with binning of slit direction in the readout, i.e., the over-scan region of the same size (50 × 4100 pixels) is added to one data unit for the data with binning. In the case of the 2 × 2 binning, the over-scan region of 50 × 2050 pixels is added.

  Figure 9: Schematic figure of the HDS CCD data

  

  As mentioned in Section , it is desirable to correct the variation of the bias level using the data in the over-scan region.

• Frame ID

  A sequential number (frame ID) is assigned to the data obtained with each exposure. Since two files are produced by one exposure, corresponding to the two CCDs, two frame IDs are given for each exposure. The frame IDs are the 8 figures following ``HDSA,'' beginning with 'HDSA00000001'. The number does not go backwards, and the number will be missing if the data acquisition is canceled after frame IDs have been assigned. The file name is produced by attaching the extension '.fits' to the frame ID (e.g., HDSA00000001.fits).

• Characteristics of FITS Data

  The FITS file obtained for each exposure consists of usual header/data unit, an ASCII extension table, and its header. In the extension table, the spectrum format (order, wavelength and position of the order projected on the detector) is described based on the calculation using the setup parameters of the gratings. Examples of a header unit and an ASCII extension table are given in Section .

  When the light monitor is used, a second ASCII extension table is attached to the above data format. In the extension table, the data obtained by the light monitor are recorded.