Calibration Data

Four types of data for calibration are usually obtained. These are the bias level, the dark level, flat fielding, and wavelength calibration exposures. The abstract commands used to obtain calibration data are given in Table .

The frequency of the acquisition of calibration data is dependent on the quality of the spectra requested. Some comments for the acquisition of calibration data are given here.

• Data for wavelength calibration (Th-Ar)

  The spectrum format moves when the setup of the spectrograph is changed. The spectrum format may slightly shift due to variation of the temperature even though the setup is fixed (see Section ). Therefore the data for wavelength calibration (Th-Ar data) should be obtained during the course of observations throughout the night.

• Data for flat fielding

  The repeatability of the spectrum format is sufficiently good to allow for flatfiled data to be obtained after changes of the setup of gratings (see Section ). There are no significant problems if calibration data for flat fielding (halogen lamp data) are obtained at the beginning or end of each night, or even on a different day during the run.

• Dark current

  As shown in Section , the dark current is not so large that the correction is not required for usual exposures. No significant problems are encountered for usual calibrations if one obtains dark data before or after the observation.

• Bias

  As shown in Section , there is some variation in the bias level during the course of the night. However, this variation can be corrected using the data in the over-scan region which is attached to each CCD data frame. There is no problem if bias data are obtained before or after the observation.

For the Th-Ar lamp, appropriate intensities of the emission lines can be obtained when the data are acquired with the current of the lamp set to 15mA, exposure time of 20 sec, for slit width of 200 μm (no binning in CCD readout).

For the flatfielding, the standard exposure times and setup of the halogen lamp are given in Table , for the case of a slit width of 200 μm and no binning in the CCD readout. The setup may be changed corresponding to the target wavelength. Note that the ``Filter'' in the Table indicates the filters installed not in HDS, but in the calibration source system.

Table 11: Standard setup of parameters for flatfield data

Setup	CCD:	Lamp	Filter	Exposure
StdR	CCD-1:	3A	ND1	10sec
	CCD-2:	3A	B390	8sec
StdB	CCD-1:	4A	ND1	12sec
	CCD-2:	4A	T42A	4sec
StdU	CCD-1:	4A	ND1,B390	24sec
	CCD-2:	4A	U340*	16sec
stdYb	CCD-1:	3A	ND1	15sec
	CCD-2:	4A	ND1	7sec
stdI$_{2}$b	CCD-1:	4A	ND1	4sec
stdNIR	CCD-1:	3A	ND1	10sec
stdHa	CCD-1:	3A	ND1	15sec

*U340 is now installed in HDS, but will also be installed in the CAL system in future.