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ROSAT Status 103

		ROSAT Status Report # 103
		   September 6 1994


MPE has released the detailed and summary timelines for the
first part of AO5.  The files are now in the legacy anonymous FTP area
with the names:



The files cover the time period of ~ October 3, 1994 - April 11, 1994

[                                                                        ]
[                     HINTS & POINTERS for PROS Users                    ]
[  1 September 1994                                                #13   ]
[                                                                        ]


ROSAT PSPC "pi" spectra extracted with the xspectral task QPSPEC (PROS
2.3.1) may be used for XANADU/XSPEC spectral fitting either via
the interface program PROSCON (v. 2.3), or via a newly-implemented FITS
file interface consisting of the IRAF task fitsio.stwfits followed by
ftools.caltools.stw2pha (which is also available within IRAF).  However,
implementation of this FITS-file interface has meant that input PSPC
spectra (for PROSCON as well) must be at full 256-channel resolution,
which differs from the normal PROS practice of working with "down-binned"
spectra in 34 (coarser) energy bins (cf.  "help pspc_fitting" in PROS).
To generate the 256-channel spectra now required, users must set the
QPSPEC boolean parameter "full" to YES, i.e.,

	cl> xray
	xi> xspectral
	xs> qpspec full=yes

XSPEC users are cautioned, however, that in order to avoid cases in
which many channels have too few or zero counts, such spectra should
be rebinned before importing to XSPEC.  Spectra can be prepared for
rebinning in XSPEC by using the GRPPHA from FTOOLS.  (CHANPHA/group in
XANADU also works on the output of PROSCON 2.1, which produced XSPEC
binary files, but CHANPHA will NOT accept the new FITS format files.)

However, such rebinning may significantly overestimate the errors
associated with each bin, leading to anomalously low values of chisq, if
the default error calculations have been performed in QPSPEC (cf., in
PROS, "help explain_errors").  This problem arises because the rebinning
specification column that GRPPHA adds to the FITS file causes XSPEC to
sum errors in quadrature when reading the spectra, normally a correct
procedure.  However, in order to deal with low/zero-count bins, QPSPEC
uses an approximation for the "1-sigma" errors that does NOT sum in
quadrature to a value equivalent to that calculated from the summed

As illustrated in the PROS dialogue below, the problem of low chisq
values can be avoided by changing the POISSERR keyword of the QPOE file
header (see Footnote below) before running the qpspec task in PROS.

The following PROS dialogue (see also Footnote below) illustrates the
generation of PSPC pi spectra for input to PROSCON or the FITS interface
(stwfits + stw2pha):

xs> qpcopy xdata$rp110590n00.qp FIELD arlac.qp ""
xr> ximages
xs> qphedit arlac.qp POISSERR F
	arlac.qp,POISSERR (T -> F): yes
	arlac.qp,POISSERR: T -> F
	continue ? : yes
	arlac.qp updated
xs> imhead arlac.qp long+ | match POISSERR
                  POISSERR=                    F
xs> qpspec full=yes
	source qpoe file : arlac.qp
	source region descriptor : c 6930 9390 240
	background qpoe file : arlac.qp
	bkgd region descriptor : a 6930 9390 300 600
	root name for output files [root_obs.tab, etc.] : arlac
	Output table file(s):  arlac_obs.tab, arlac_soh.tab, arlac_boh.tab
xs> downspecrdf
	list of root names for files to convert [root_obs.tab] : arlac
	Output (pre-RDF) table file:  arlac_prdf_obs.tab

It should be noted, however, that the problem of assigning errors to
few/zero-count spectral bins is an intrinsic one.  Some users may prefer
to use the C-statistic fitting option available in XSPEC (although this
technique does not provide a measure of the "goodness of fit").  While
not rigorous, PROS' use of approximate "1-sigma" errors is an approach
that other users may find adequate.
           The QPOE header keyword POISSERR was added when the RDF
           formats were introduced (PROS release 2.3, 24 DEC 93).
           PROS supports data in pre-RDF and RDF format, but will
           always write output files in RDF format.  The PROS ROSAT
           FITS readers (rarc2pros, rfits2pros) will do any required
           conversions automatically.  However, existing, pre-RDF
           format QPOE files must be converted manually by the user,
           with the task UPQPOERDF:

             xr> xdataio
             xd> upqpoerdf *.qp

As the provider of the PROS software package ("xray" in IRAF), the ROSAT
Science Data Center (RSDC) at SAO distributes HINTS & POINTERS to PROS
Users via email.  When we find answers to often asked questions, or when
we have implemented new software solutions to longstanding problems, we
will send this information directly to our users.  These messages will
also be available via anonymous ftp*, but to receive the email version,
you should register with us. If your username is the one associated with
your site registration, you are already registered for "HINTS &
POINTERS", but we encourage other users (even at the same sites) to
Mailings will be sent as the need arises.  If you would like to suggest
items for future issues of H&P or add your name to our distribution list,
please contact us at the following address:
Internet - rsdc@cfa.harvard.edu       RSDC    MS-3
DECnet   - CFA::RSDC (6699::RSDC)     Center for Astrophysics
UUCP     - ...!harvard!cfa!rsdc       60 Garden St. Cambridge MA 02138 USA
BITNET   - rsdc@cfa                   tel: (617) 495-7134  FAX: 495-7356
* Our anonymous ftp service is available from the PROS home-page on
  WWW (URL: http://hea-www.harvard.edu/pros/pros.html), and can also
  be accessed via conventional ftp:

    o   ftp -i sao-ftp.harvard.edu           # node address:
        [Name: anonymous]
        [Password: <yourusername@yoursite>]
   ftp> cd pub/pros/Hints
N.B.: You can also obtain your own copy of the PROS USERS GUIDE from
WWW or anonftp (from subdir /pub/pros/PUG).