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Important updates for ASCA users

ASCA Guest observers,

This is a fairly lengthy message that includes 3 important topics:

	1. Reminder: ASCA AO-5 proposals due Aug 6.
	2. GIS gain map correction.
	3. An ASCA-specific patch to FTOOLS v3.5 has been released.

Items 2 and 3 are also available from the ASCA GOF home page


under 'GIS News' and 'Software News', respectively.

					The ASCA GOF


1. ASCA AO-5 proposals: a reminder

ASCA AO-5 proposals (both electronic submission of the forms and
paper submission of 15 copies of the complete proposals) are due on Aug 6.
If you would like to receive a paper copy of the NRA and appendices, but have
not requested one yet, you should do so immediately by sending an e-mail to


Alternatively, all information are available electronically.  Visit
our home page at


and look under "NASA Research Announcement".


2. GIS gain map correction

E.Idesawa, H.Kubo, Y.Ishisaki, K.Ebisawa, Y.Fukazawa, M. Ishida, 
K.Makishima, M.Tashiro, Y.Ueda and the GIS team

As we reported on April 13, 1996, the GIS-S3 gain apparently
becomes over-estimated towards later epochs of the mission.
This was first noticed through the calibration using ionized iron 
lines from the SNR Cas A. We further investigated this issue over 
the 25 sectors (azimuthal 8 sectors times radial 3 sectors and 
center) of the GIS-S3 detector plane, using the instrumental copper 
line which can be detected over the entire detector plane after 
sufficiently long integration times.

We confirmed that the overestimation is more pronounced in the
central region than in the rim region of the GIS-S3 field of view,
and that the effect increases with time. These effects are thought 
to originate from slight gain decreases in the rim region of GIS-S3, 
where the calibration isotope is attached. This in turn may arise 
either from some decrease in the ultraviolet transmission of the 
quartz window of the gas cell, or to a slight performance drift in the 
imaging phototube. The effects are anyway very small, and corrections 
can be made with a sufficient accuracy to the data from both past and 
future observations. We find no evidence of azimuthal deviation of the 
gain from the original positional gain correction map (gain map).

The deviation from the original gain map is well described with a
parabolic function with respect to the radius from the detector center,
and with a linear function of the ascatime. The coefficients for the 
parabolic function have been determined for each year, using the averaged 
day- and night-earth data; thus we can correct the gain map by interpolating
these coefficients.

A new version of  the 'ascalin' to create the CORRECTED PI is under 
development by ASCA GOF in collaboration with the GIS team. For users' 
convenience we indicate below the analytic form of the gain map correction 
function to be implemented in the new ascalin.

We have applied the same method to the GIS-S2 data as well, and found no 
noticeable deviation from the original gain map within 1%. However for 
consistency, we have also applied the same investigation to the GIS-S2 data 
and derived the nominal correction coefficients (which are consistent with 
"no correction" within the errors).


The gain map compensation function is,

	new-PI = old-PI / (correction factor),
	correction factor = (A2 * r**2 + A0)*ascatime
	                                      + (B2 * r**2 + B0),
	r**2 = (DETX - DET_XCEN)**2 + (DETY - DET_YCEN)**2 ,
	DET_XCEN= 128.5 (PIXELS), DET_YCEN= 128.5 (PIXELS).

Coefficients A0, A2, B0 and B2 for GIS-S2 and GIS-S3 are shown in table 
Note that A2=A0=B2=0 and B0=1 corresponds to "no change to the original
gain map".

sensor	A2		A0		B2		B0
S2	-3.945E-15	+1.090E-11	+2.851E-08	+1.004E+00
S3	-3.871E-14	+2.974E-10	+6.128E-07	+9.970E-01

We have confirmed that this correction compensates the gain change to
an accuracy of 1%, within 90 GIS-pixels from the detector center.


3. Updated ASCA FTOOLS released

New and updated ASCA FTOOLS are now available via anonymous FTP at
legacy.gsfc.nasa.gov.  These should be installed on top of the existing
FTOOLS v3.5 package.  All the necessary files are contained in the directory


These include a short release note (README_ASCA351) and the installation
instructions (INSTALL_ASCA351) which you should read before installing the

The important changes that affect users are listed below:

ascaarf    - uses new XRT calibration files (xrt_ea_v2_0.fits,
             xrt_psf_v2_0.fits) and new "filter" modification
             to the effective area curve that provides power-law
             fit to GIS Crab spectra consistent with Toor & Seward
             (1979; AJ 79, 995). This filter can be switched off using
             the 'filter=no' option. All calibration files can be read
             from either the CALDB or the refdata area. The default is
             the AUTO setting which first checks for CALDB files and if
             they are not found then looks in the refdata area. If CALDB
             is given as the value of the input RMF parameter then ascaarf
             checks the CALDB for an appropriate RMF to use to set the
             energies on which the ARF is calculated.

             Note that, as of 1996 Jul 12, the calibration database at
             Goddard includes both the old (xrt_ea_v1_1.fits) and
             the new (xrt_ea_v2_0.fits) effective area files, and
             both are flagged 'good'.  This results in ascaarf picking
             up the old version.  The flag for v1_1 will be changed to
	     'bad' within a day or so, so that ascaarf will start using
             the new version at Goddard; similar action may be necessary
             for your installation of the calibration database.

             On the other hand, the refdata area users need not take
             any extra action to use the latest calibration.

             Note that the "filter" modification is applied to both GIS
             and SIS data however it has only been calibrated against the
             GIS Crab data.

ascaeffmap - uses new XRT calibration files and AUTO argument for all
             calibration files.

ascalin - now can redetermine GIS/SIS PI values using manually
          input new gain and/or offset numbers.  To use this option,
          use the 'gain fit' command in XSPEC and use the resultant
	  gain and offset numbers on the command line (gainnorm=0.98,
          gainoff=0.03 etc.)  Note, however, the parameter file will
          remember the values supplied; it is recommended to reset these
          to 1.0 and 0.0 respectively, after use.

          Please note that this update is to enable manual adjustment of
          the gain; the update corresponding to the July 10th announcement
          by the GIS team will be available in the next release.

fastwmap - bug fix of the FTOOLS v3.5 release version (this fix was made
           available shortly after the v3.5 release)

ghkcurve - bitrate parameters are checked in the input HK file so that 
           different bitrates are not mixed when specified to use only the
           high bitrate. This version does not allow one to combine High and
	   Medium bitrate data.  The next version will allow one to do that.

ldeadtime - carry out deadtime correction for GIS light curves

sisrmg - several user interface bugs have been fixed.  It should now
         default correctly to grade 0234 for BRIGHT/BRIGHT2 mode data,
         unless the 'grades' hidden parameter is used to override the default.
         The original FTOOLS v3.5 release of sisrmg did not always choose
         the correct grades.

         For fast mode data, it is recommended that you specify the grade
	 explicitly (normally grades=0) on the command line; without this
	 line, sisrmg will still crash on Alpha/OSF machines for fast mode
         data (this version does run on Alpha/OSF machines, provided that
         you supply the grades=0 option on the command line).  However,
         this is believed to be the last (minor) remaining user-interface
         bug: all other known bugs have been fixed.