[Date Prev][Date Next][Thread Prev][Thread Next][Search]
[Main Index]
[Thread Index]
[HEASARC Archives]
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
http://heasarc.gsfc.nasa.gov/docs/asca/ascagof.html
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
ascanra@athena.gsfc.nasa.gov
Alternatively, all information are available electronically. Visit
our home page at
http://heasarc.gsfc.nasa.gov/docs/asca/ascagof.html
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),
where
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
below.
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
/asca/software
These include a short release note (README_ASCA351) and the installation
instructions (INSTALL_ASCA351) which you should read before installing the
update.
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.
=============================================================================