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Calibration Status of 4- and 2-CCD mode data

Calibration Status of 4-CCD Mode Data

The SIS team is continuing to improve the spectral calibration of the
instruments, and recently discovered several important effects that users
of 4-CCD mode should be aware of.

(1) Secular gain change

SIS team has provided a preliminary calibration for the secular gain
change, which is used by sispi, an FTOOL to populate the PI columns
of the SIS event files.  A study of the Ni line (in the detector
background) shows, however, that the secular gain change is dependent
on CCD mode.  The current calibration file (sisph2pi.fits) is applicable
to 1-CCD mode data, while for the 4-CCD mode data, no secular change
is apparent at this stage, at least in the Ni line data.

Guest observers who have 4-CCD mode data are invited to try running:

example% sispi launch=yes
Input ASCA SIS science file name[@ft.list] test_s0.evt
Input SIS gain/CTI history file name[/ftools/SUN/release/refdata/sisph2pi.fits]

This "launch=yes" option tells the software that the gain values at launch
should be assumed, rather than those `appropriate' for the time of observation.
Sispi will then adjust the relative gains of the chips and produce
a uniform PI scale but will not apply secular gain correction.  This
does not require any additional software or calibration files; existing
ones already have the capability.

HOWEVER, please be warned that the apparent lack of secular gain change in
4-CCD mode is probably the result of 2 competing effects cancelling each other,
when Ni line data are collected from all over the chips.  You may well see
position dependence in line energies after the above procedure.
Moreover, see point (2) below.

(2) Chip-to-chip variation of gain

The current calibration of relative gains among the 8 CCD chips
is based on observations of the SNR W49B, which were unfortunately taken
in a mixture of 1, 2 and 4-CCD modes.  As a result, the relative
gain calibration was not as accurate as we hoped.  We believe
the S0C1 gain to be approximately correct; S1C3 is ~0.5% off.
The difference between the chip with the smallest and largest
`gain' is 2% at present.

(3) Secular degradation in spectral resolution

The spectral resolution is degrading in 4-CCD mode, presumably due to
radiation damage (although the details are being studied).  It may be
possible to recover (close to) the original resolution, if a large
amount of time is spent on calibration observations and software
development necessary to carry out the correction.  The SIS team is
studying various options.  Please note that current versions of software
(sispi and the response builder) DO NOT correct for any changes in spectral

Calibration Status of 2-CCD Mode Data

Calibration of 2-CCD mode data is in progress; SIS data in this
mode are expected to display secular changes between those of
1-CCD and 4-CCD mode, but this has not been quantified yet.
The result of the on-going effort will be reported later.

	[A version of this report will become available via our
	WWW service shortly; that version will include several
	plots that illustrate the current status of gain calibration]

			Koji Mukai and the ASCA GOF
			on behalf of the SIS Team