Chandra X-Ray CenterMIT Kavli InstituteMEMORANDUMNovember 24, 2009To:Jonathan McDowell, SDS Group Leader ( )Ken Glotfelty, DS Pipelines/Tools Lead ( )Dale Graessle, DS Calibration Data Coordinator ( )Brad Wargelin, Calibration Scientist ( )Ian Evans, DS End-to-End Scientist ( )From:David P. Huenemoerder, SDS ( initial / date )Subject:Interface Control Document: HRC-S P I Reference Data & AlgorithmRevision:1.4URL:http://space.mit.edu/cxc/docs/docs.html#hrcstgainFile:/nfs/cxc/h3/dph/CXC/HRCS pifilt/Doc/hrcs spi icd-1.4.tex v1.4 changes: small file-nameand header keyword changes tocomply with CalCo’sCALDB-related requirements.1 Overview of HRC-S P I ComputationA new algorithm for computing the HRC-S pulse height (P I, for Pulse InvariantAmplitude) is documented in the memo “A New Gain Map and Pulse-Height Filterfor the Chandra LETG/HRC-S Spectrometer” by B. J. Wargelin, P. W. Ratzlaff,and M. Juda,1henceforth referred to as the Memo. This algorithm can result in areduction of HRC-S background by about a factor of two. There are two new partsto the processing:1. Computation of the P I for every event, which requires a new Cal-ibration Database (CALDB) file and algorithmic support in the tool,hrc process events.1http://cxc.harvard.edu/cal/Letg/GainFilter/gain.pdf, November 25, 20082. Application of a position-dependent filter before binning an LETGS spectrum,which requires a new CALDB region file.This memo will define the new CALDB files, the processing algorithm forhrc process events, and the filter application method.2 P I ComputationThe essence of the the P I computation is given by Equation 5 in the Memo, rewrit-ten slightly here as:Pn=a1mij DnCi(t)Snormij− bij!+ a0. (1)We have changed the notation slightly to make the two-dimensional nature of thecalibration data explicit in detector raw coordinates by subscripts ij, and we haveremoved explicit numerical values which are subject to calibration updates. We usea “row, column” notation, in which row i is a RAWY index, and column j indexesRAWX. For HRC-S, the LETG dispersion is along RAWY. In this equation the termsarePnthe desired value for P I for the nthevent, at detector raw pixel coordinate(RAWX, RAWY), which will be mapped to calibration bin, (i, j).Dnrepresents the uncorrected event pulse-height data value ( “SAMP” in Memoterms), for the nthevent.Other terms are calibration data, two of which are scalars (an), three are two-dimensional maps (m, S, and b), and another is a time-dependent term withone spatial dimension (C).For purposes of efficiency in CALDB representation and event processing, wecan collect terms into a more compact form:Pn= Gij0+Gij1Ci(t)Dn(2)Gijkis the “Gain Map”, with two spatial dimension, i and j, and one polynomialorder dimension, k.Ci(t) is the time-dependent gain correction, with one spatial dimension, i.The CALDB file will hold one 3D image, Gijk, one 2D image, Cin, and co-ordinate grids for each axis. Since the grids may be non-uniform, axes will beenumerated rather than specified by FITS image coordinate terms.22.1 File Structure: HRC-S Gain Coefficients2.1.1 File NamesThe CALDB type for time dependent gain is t gmap. File names will be of theformhrcsD1999-07-22t gmapNnnnn.fitsin which nnnn is a 4-digit version counter (which starts at 0001).2.1.2 HDU ComponentsThe following table describes the file structure by Header-Data Unit number, type,extension name, content, and HDU classes. An asterisk (*) denotes the principalHDU.HDU HDU EXTNAME EXTVER CONTENT HDUCLASS Description0 PRIMARY N/A N/A N/A N/A NULL1 (*) BINTABLE AXAF TGAIN 1 CDB HRCS TGAIN ASCDETCHARTGAINBinary table extensionlisting coefficients forP I gain correction as afunction of time & posi-tion.2.1.3 Columns and Coordinate SystemsColumns for HRC-S gain coefficients are given in the following table. Columnordering in the FITS table is arbitrary. v1.3 update: grids, notationTTYPE TUNIT TFORM TLMIN TLMAX DescriptionRAWXGRID pixel nxE 0 4095 RAWX grid, low bin edges. nxis the number of ele-ments in the RAWX grid.RAWYGRID pixel nyE 0 49151 RAWY grid, low bin edges. nyis the number of ele-ments in the RAWY grid.TIMEGRID MJD ntD N/A N/A Time axis for data stored in column TGAIN. Unitsare MJD. MJDREF is 50814, which corresponds to1998-01-01T00:00:00 (TT), and M JD =JD − 2400000.5. ntis the number of elements in theTIME grid.GAINMAP 1 2nxnyE N/A N/A Coefficients map vs raw coordinate. 2 × nx× nyis thetotal length fo the array.TGAIN 1 ntnyE N/A N/A Normalization coefficient vs time and RAWY. nt× nyis the total length of the array.Grids are required to be monotonic and in ascending order. They are, however,not necessarily uniform. Hence, the grids are tabulated rather than specified bystandard FITS World Coordinate System (WCS) image keywords, CRPX, CDLT,and CRVL.32.1.4 Special Coordinate KeywordsThe image column require several keywords in order to define its coordinate systemand relation to other columns. These follow the conventions given in “Representa-tion of celestial coordinates in FITS”, by Greisen and Calabretta (Sept. 9, 1996)2.iCTYPn: These string-valued keywords define the axes of image columns.For GAINMAP we require:1CTYPn = ’ORDER’2CTYPn = ’RAWX’,3CTYPn = ’RAWY’.Here, n, is the column index for GAINMAP. ORDER refers to the order of thepolynomial coefficient (not spectral diffraction order).For TGAIN, we require:1CTYPn = ’RAWY’,2CTYPn = ’TIME’.with n being replaced by the column number of TGAIN.TDIMn: This string-valued keyword specifies the dimensionality of each axisof image columns. It has the form, ’(Nz, Nx, Ny)’For GAINMAP we require:TDIMn = ’(2, ny, nx)’.For TGAIN we require:TDIMn = ’(ny, nt)’.CREFn: This keyword has a string value which maps the image axes toTTYPEn keywords which specify grids for those axes.For GAINMAP we require:CREFn = ’(ORDER, RAWX, RAWY)’.For TGAIN we require:CREFn = ’(RAWY, TIME)’.For non-enumerated axes (uniform grids), the coordinate strings would be whatthe WCS mapping requires (i.e., 2CRPX, 2CDLT, and 2CRVL).2http://www.cv.nrao.edu/fits/documents/wcs/wcs.all.ps42.1.5 Relevant Header KeywordsRelevant keywords are:TELESCOP: The only allowed value is CHANDRA.INSTRUME: The only allowed value is HRC.DETNAM: The only allowed values are HRC-S.CIPn: For configuration control, this keyword (or list if n is present to enumer-ate several keywords) names the input CIP, or Calibration Interface Productfiles delivered by calibration and formatted