ROSAT Status Report # 103:

September 6, 1994

Timelines

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:

ao5pt1.details

ao5pt1.timeline

The files cover the time period between about October 3, 1994 and April 11, 1994.

HINTS & POINTERS for PROS Users

ON USING QPSPEC SPECTRA WITH XSPEC

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 overestimate the errors associated with each bin significantly, 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 counts.

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.

Footnote: 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

ABOUT HINTS & POINTERS

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 register.

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: 128.103.42.3
        [Name: anonymous]
        [Password: ]
 
   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).

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