Contents of Delivered Data / 30-1 Headers, Keywords, and Group Parameters / 30-9 FOS Paper Products / 30-15 Evaluating Planned Observations / 30-24 Assessing FOS Acquisitions / 30-26 Assessing FOS Science Observations / 30-37 This chapter assumes that you have selected and retrieved some FOS data from the HST Data Archive or that you have your own data. We describe the contents of the delivered data, the structure of the various data files, and how to assess them with particular emphasis on the utility of the FOS paper products. We also provide a few examples of how Phase II proposals were turned into observations and data products. 30.1 Contents of Delivered Data Unless you have read your FOS data from very old data tapes, the data you have on your machine will be in FITS format. These files need to be converted to GEIS format with the STSDAS routine strfits, as is described in Chapter 2 (Volume 1). The resulting GEIS format files will have default suffixes, as described in Table 30.1. You will be most interested in the contents of the .c1*, .c0*, and .c2* files as these contain the flux-calibrated data, wavelengths and errors for an exposure. The easiest way to get a quick glance at your spectra is by using the routine splot or fwplot if you want to plot wavelength vs. flux (see "FOS and GHRS Spectra" on page 3-17). We recommend that you generate the FOS paper products for your data with the STSDAS routine pp_dads and obtain a variety of displays and tables which, as we shall see in "FOS Paper Products" on page 30-15, can facilitate the assessment of your data quality. ___________________________________________________________________________ Table 30.1: FOS File Name Suffixes Suffix File Contents Suffix File Contents Raw Data Files Calibrated Data Files (input for calfos) (output from calfos) .d0h/.d0d Science data image .c0h/.c0d Calibrated wavelengths .q0h/.q0d Science data quality .c1h/.c1d Calibrated fluxes .shh/.shd Standard header packet .c2h/.c2d Propagated statistical error .d1h/.d1d Science trailer line .c3h/.c3d Special statistics .ulh/.uld Unique data log .c4h/.c4d Count rate .x0h/.x0d Science header line .c5h/.c5d Flat-fielded object spectra .xqh/.xqd Science header line data .c6h/.c6d Flat-fielded sky quality spectra .q1h/.q1d Science trailer line data .c7h/.c7d Background spectra quality .c8h/.c8d Flat-fielded object minus smoothed sky spectra .cqh/.cqd Calibrated science data quality Additional Files .jih/.jit Jitter files (.cmh, .cmj, .trl Trailer file .cmi) .pdq/.ocx PODPS/OPUS data quality files _________________________________________________________________________ 30.1.1 Uncalibrated (Raw) Data Files Table 30.1 lists the science files that are used as required input to calfos. These files are described briefly below. Science Data Files (.d0h/.d0d) Science data files contain single-precision floating point values that represent the number of detected counts accumulated in each pixel. The number of data elements in the one-dimensional science data array depends on the observation mode; specifically, the number of diodes, the number of substeps, the number of Y steps, and the number of repeats (sometimes called slices or bins) used in the observation. The maximum number of data elements is 12288. The associated header file also provides information on the different steps to be performed during pipeline calibration processing, and the reference files and tables to be used in the calibration. Science Header Line (.x0h/.x0d) The science header line (SHL) file is a one dimensional array with a length equal to a line of the science data. It contains a partial copy of the unique data log. Science Trailer Line (.d1h/.d1d) The science trailer line (STL) file is also a one dimensional array containing the number of measurements rejected from the various combinations of x substeps, y steps, repeats, etc. The information in these files is used to compute the total effective exposure time per pixel which is later used to convert the counts into count rates. Data Quality Files (.q1h/.q1d) The science data files, science header line files, and the science trailer files have corresponding data quality files that contain the flags for bad or suspect data. These raw data quality files have quality flags as follows: • - Good data has the data quality flag =1. • - Raw data dropouts and filled raw data have the data quality flag =16. • - Data failing a Reed-Solomon error check has the data quality flag =100. • - Fill data have the data quality flag =800. The data quality files are identified by the suffixes .q0h, .q0d, .xqh, .xqd, .q1h, and .q1d corresponding to the science data, science header, and science trailer files. Table 30.2 lists FOS data quality flags. Standard Header Packet (.shh/.shd) The standard header packet (SHP) contains the telemetry values for engineering data and some FOS-unique data. The engineering data include temperatures, currents, and voltages at various points in the instrument. The FOS-unique data varies depending on the onboard processing used for a given observation. The header packet also contains information used in the operation of the spacecraft, such as target name, position and velocity of the telescope, the right ascension and declination of the target, the sun, and the moon, and other proposal information used in the observation which was provided in Phase II of the proposal process. Unique Data Log (.ulh/.uld) The unique data log (UDL) contains the mechanism control blocks used to control the entrance aperture, entrance port, polarizer, and filter grating wheel assembly. This file also contains the discriminator level, disabled diode table, serial engineering data, instrument configuration, and exposure parameters. Trailer File (.trl) The trailer file contains many messages generated by the so-called "generic conversion" of the data from what was onboard the spacecraft into STSDAS images. These messages may contain information on missing or filled data packets. Informational messages produced by calfos during calibration are also stored in the trailer file. The trailer file is identified by the suffix .trl. 30.1.2 Calibrated Data Files Several types of calibrated output files are produced by calfos. These are listed in Table 30.1. More extensive descriptions of each type of file are provided below. Calibrated Wavelength Files (.c0h/.c0d) These files contain single-precision floating point calibrated vacuum wavelengths corresponding to the center of each pixel of the science data. All FOS wavelength solutions assume the first pixel is pixel 0. Calibrated Flux Files (.c1h/.c1d) These files contain single-precision floating point calibrated fluxes (in ergs sec-1 cm-2 Å-1) corresponding to each pixel of the science data. Calibrated Statistical Error Files (.c2h/.c2d) These files contain the statistical errors of the original data. These files are calibrated in lock-step with the science data files. Errors caused by sky and background subtraction, flatfields, and sensitivity are not included in the error estimates. Calibrated Special Mode Data Files (.c3h/.c3d) Data acquired in the rapid-readout, time-resolved, or spectropolarimetry modes require processing steps in addition to (or complementing) those used for standard ACCUM data. The calibrated data are then stored in special mode data files. For RAPID mode, the files contain the total flux, integrated over all pixels, and the associated statistical error for each readout. For TIME RESOLVED mode, the files contain the pixel-by-pixel average of all slices or bins, the difference between each slice or bin and the average, and the average propagated statistical errors. For POLARIMETRY mode, the file contains the Stokes I, Q, U, and V parameters, the linear and circular polarization, and the polarization position angle. The polarimetric quantities and the propagated errors are calculated for each of the separate pass directions, the combined pass direction data, and the combined pass direction corrected for interference and instrumental orientation. Calibrated Data Quality Files (.cqh/.cqd) The quality flags in these files flag the bad pixel values in the calibrated files. The quality flags from the raw data are updated and additional flags are added for problems detected in the calibration process. The data quality flags are defined in Table 30.2. Intermediate Calibrated Output Data Files (.c4* - c8*) At most, six sets of intermediate calibrated output files are produced depending on the observation mode. • - Count Rate Object (and Sky) Spectra: The files containing the count rate spectra are corrected for overscanning, noise rejection, and lost signal caused by disabled diodes. These files are identified by the suffixes .c4h and .c4d. __________________________________________________________________________ Table 30.2: FOS Data Quality Flag Values Flag Description Value Category 1: Data not useful. Data values set to zero. 800 Data filled 700 Data filled due to GIM correction 400 Disabled channel 300 Severe saturation (uncertainty greater than 50%) 200 Inverse sensitivity invalid (l < 1100 A or l > 7000 A) Category 2: Data Uncertain. Uncertainty not Indicated in Error Calc. 190 Large saturation correction (uncertainty greater than 20%) 170 Intermittent noisy channel 160 Intermittent dead channel 130 Moderate saturation correction (uncertainty greater than 5%) 120 Sky or background fixed or extrapolated 100 Reed-Solomon decoding error Category 3: Data uncertain. Uncertainty in propagated error file 50 Sampling less than 50% of nominal ___________________________________________________________________________ • - Flatfielded Object Count Rate Spectrum: These files contain .c4 object-only data further corrected for paired pulses, detector background, flatfield structure and, if applicable, pipeline corrected for GIM effects. The flatfielded object spectrum files are identified by the suffixes .c5h and .c5d. • - Flatfielded Sky Count Rate Spectrum: These files contain .c4 sky-only data further corrected for flatfield structure and, if applicable, also pipeline corrected for GIM effects. The flatfielded sky spectrum files are produced only if a sky observation was obtained. These files are identified by the suffixes .c6h and .c6d. • - Background Count Rate Spectrum: The scaled background spectrum that has been subtracted from the .c4 product is identified by the suffixes .c7h and .c7d. • - Flatfielded and sky-subtracted Object Count Rate Spectrum: If the sky is observed, then a smoothed sky-subtracted object spectrum prior to flux calibration is produced. The files containing the smoothed sky-subtracted object spectrum are identified by the suffixes .c8h and .c8d. Trailer File (.trl) We list the trailer file again as any log comments written by the pipeline calfos procedures are appended to spacecraft and generic conversion information already in the file. The trailer file is identified by the suffix .trl. 30.1.3 Paired-Aperture File Structures The FOS paired apertures were sampled in two different ways which lead to very different internal file structures and data products: • - Case 1: RPS2 (RPSS) specification of STEP-PATT=SINGLE which leads to group parameter APER_POS=SINGLE and header keyword YSTEP1=OBJ. • - Case 2: RPS2 (RPSS) specification of STEP-PATT=OBJ-OBJ or OBJ-SKY (in early cycles referred to as STAR-SKY) which leads to group parameter APER_POS=UPPER or LOWER and header keywords YSTEP1 and YSTEP2 being populated with either OBJ (STAR) or SKY as appropriate. Anomalies exist in some archived datasets that will result in incorrect calfos processing of paired aperture data (see "Paired Aperture Calibration Anomaly" on page 31-25). For case 1, the files are identical to those for single aperture data and as described in the foregoing data product sections. For case 2, measures were taken from the upper and lower sections of the aperture (always starting in the lower aperture section which has the more negative Y-base value) in approximate 10-second alternating segments, readout to memory at the interval appropriate to the data-taking mode (either ACCUM or RAPID). In the .d0* files the data are stored in groups that are twice as long as single aperture groups, e.g., the default paired aperture ACCUM group size is 4128 pixels. The first 2064 pixels correspond to data readout at the Y-base for data taken in YSTEP1 and the second 2064 pixels are from YSTEP2. YSTEP1, which can be either OBJ (STAR) or SKY, always corresponds to the lower aperture. (The mapping of OBJ (STAR) and SKY to upper and lower halves of the aperture was dependent upon user-specification.) Case 2 .c4* files contain twice as many groups as in case 1 with the odd-numbered group containing to the count rate spectrum groups for YSTEP1 and the even-numbered groups containing the count rate spectrum groups for YSTEP2. 30.1.4 Other Important Observation-related Files Observation Log Files (.jih/.jit) As of February 1995, the observation log (or jitter) files (.jih, .jit ,etc), which are provided by the Observatory Monitoring System (OMS), became routinely available (some files from the October 1994 through February 1995 time period are also available). These can give useful information, such as telescope performance during the observation and the position of the target within the aperture. A so-called jitter ball is a routine part of all FOS paper products for which jitter files exist (see Figure 30.8). This plot shows (V2,V3) motions of the FGSs needed to keep the guide stars centered, recorded every six seconds throughout the exposure. See Appendix C for a description of the observation logs and jitter files. PODPS Data Quality File (.pdq) STScI staff performed a quick Data Quality Assessment (DQA) of the target acquisitions and the science observations. This assessment identified any target acquisition failures, guide star acquisition failures, other spacecraft anomalies, or instrumental problems; the assessment was recorded in the data quality keywords of the PDQ (Procedural Data Quality) file. PDQ files are archived to data class PDQ and can be retrieved from the HST Archive using StarView. The FOS paper products list each relevant PDQ file comment for every exposure. OCX files (Observatory Support System (OSS) Observer Comment files) contain information about the success or failure of target acquisitions and science observations. Before April 17, 1992, OCX files were not always archived separately, and, in some cases, were prepended to the RSDP pipeline history file, the .trl trailer file. After February 1995, OCX files were produced only to document real-time activity (e.g., interactive target acquisition) in support of an observation. After October 1996, OCX files may contain information about observation execution problems. The PDQ files and the comments they contain should not be over-interpreted. The remarks were based upon a visual examination of the data and the experience of the OPUS staff. Technical remarks are usually sound, but some comments about the "quality" of the observations can arise from an incomplete knowledge of the science goals of the original proposer. Unless the object was significantly miscentered, for example, the achieved signal-to-noise is probably close to what was expected, even if it may appear low in a single exposure. To assess whether or not a low or varying flux is meaningful, you should examine the jitter file or other records of spacecraft performance. The lack of a comment is also not a guarantee that there were not some problems with the data. 30.1.5 Reference Files and Tables The calibration pipeline used reference files and tables to flux calibrate FOS data. Data in the archive and the data that were sent to GOs were calibrated with the reference files that were available at the time of the observation. These are generally not the "best" files to use to calibrate these data today. StarView allows you to retrieve either the "used" or the "best" reference files. The "best" reference files reflect our accumulated experience of FOS calibrations and derive from the late 1997 FOS closeout calibration analyses. Naming and Structure of FOS Reference Files and Tables The reference files and tables are typically referred to by the name of the Calibration Data Base System (CDBS) reference relation that holds their names. The suffixes of the reference tables and files are of the form .cyn, .rnh and .rnd where n represents a value from 0 to 9 and A to D (see Table 30.3). These files are maintained in the CDBS. The STScI web site also includes a catalog of these tables and files. A thorough description of the internal header and data formats of FOS reference files and tables is beyond the scope of this document, but all this information can be found in rigorous detail in STScI document ICD-47 which is available from the FOS WWW page and in paper format from help@stsci.edu. A description of what each FOS reference file and table is used for is found in "Details of the FOS Pipeline Process" on page 31-13. Except for some spectropolarimetric reference files, which are twice this length (for two pass directions), all reference files contain a vector of length: (Nchan + Nover - 1) X Nx Where • Nchan - is the number of channels observed (keyword NCHNLS). • Nover - is the number of channels multiplexed (keyword OVERSCAN). • Nx - is the number of substeps (keyword NXSTEPS). Although the reference files can be generated for any combination of NXSTEPS, FCHNL (first channel), NCHNLS, and OVERSCAN, the routine calibration reference files have a length of 2064 pixels, corresponding to the standard keyword values: • - NXSTEPS = 4 • - FCHNL = 0 • - NCHNLS = 512 • - OVERSCAN = 5 For other values of FCHNL, NCHNLS, and NXSTEPS calfos interpolates from or resamples the standard reference files. Only OVERSCAN = 5 is supported by FOS calibration. ___________________________________________________________________________ Table 30.3: FOS Reference Files and Tables Header Filename File Contents Keyword Suffix CCS0 .cy0 Aperture areas CCS1 .cy1 Aperture positions CCS2 .cy2 Sky emission line positions CCS3 .cy3 Sky and background filter widths CCS4 .cy4 Polarimetry parameters CCS5 .cy5 Sky shift parameters CCS6 .cy6 Wavelength dispersion coefficients CCS7 .cy7 GIM correction scale factors CCS8 .cy8 Predicted background (count rate) CCS9 .cy9 Un-illuminated diodes for scattered light correction CCSA .cya OTA focus positions for aperture throughputs CCSB .cyb Aperture throughput coefficients CCSC .cyc Throughput corrections versus focus CCSD .cyd Instrument sensitivity throughput correction factors CCG2 .cmg Paired-pulse coefficients BACHFILE .r0h & .r0d Default background file (count rate) FLnHFILE .r1h & .r1d Flatfield file IVnHFILE .r2h & .r2d Inverse sensitivity file (ergs cm-2 A-1 count-1 diode-1 ) RETHFILE .r3h & .r3d Retardation file for polarimetry data DDTHFILE .r4h & .r4d Disabled diode file DQnHFILE .r5h & .r5d Data quality initialization file AISHFILE .r8h & .r8d Average inverse sensitivity file ____________________________________________________________________________ 30.2 Headers, Keywords, and Group Parameters Header files provide much of the information needed to reduce FOS data. A description of each keyword is generally provided in the header itself. Some important header parameters, such as exposure timing information, background correction, or scattered light correction, have different values for each group of data within the file. Such parameters are termed group parameters. Table 30.4 is a description of the different topics covered in the various header files. The header files used most often are the standard header packet (.shh), the science data header file (.d0h), and the calibrated science data header file (.c1h). Most of the information needed to understand the data is found among the header keyword types that describe general information (Table 30.4a) and among the processing and calibration information (Table 30.4c) sections of the headers. Table 30.5 lists many important header keywords used to interpret FOS data. IRAF tasks such as imhead or hselect can be used to list the various header keywords. Values of group parameters, on the other hand, can be obtained only with imhead. Table 30.6 provides a listing of all FOS group parameters, with typical values and explanatory comments for each. ___________________________________________________________________________ Table 30.4: Information in FOS Header Keywords Keyword Type Information in Keywords Source =========================================================================== a) General Information =========================================================================== General Data General structure of information for All headers data file in standard FITS style Group Parameters: Acquisition data description, All headers OSS including time of acquisition (modified Julian date), maximum and minimum data values, and axes information Group Parameters: Observation type and ground-based GIM Calibrated PODPS correction values from GIMP_CORR header files Generic Existence of science trailer line and .d0h Conversion reject array Keywords: FOS Descriptor Description of FOS file and GIM All headers Keywords: correction COSTAR Keywords: Positions of the COSTAR FOS M1 mirror .shh ========================================================================== b) Engineering Information ========================================================================== Time conversion Spacecraft and Universal time at .shh Keywords: start of observation CDBS Keywords in Engineering data regarding .shh SHP: temperatures, currents, and voltages at various points in the instrument CDBS Keywords in Data acquisition details, such as .ulh UDL: number of channels used, value of magnetic field deflections used ========================================================================== c) Processing and Calibration Information ========================================================================== Statistical Processing information .shh, .d0h, Keywords: and calibrated data headers Calibration Flags Type of observation and configuration .d0h and and Indicators: grating, and detector calibrated data headers Calibration Reference files and tables for calfos .d0h and Reference Files & processing (either used or to be used) calibrated Tables: data headers Calibration Calibration steps for calfos .d0h and Switches: processing (either used or to be used) calibrated data headers Pattern Keywords: Magnetic field deflection pattern used .d0h and all in acquiring the data calibrated data header files Calibration Observing tme, user-supplied GIM offset .d0h and Keywords: table name, LIVETIME, DEADTIME, calibrated position angle of aperture, burst noise data headers rejection limit Aperture Aperture position in RA and Dec .d0h and Position: calibrated data headers Exposure Exposure information and commanded FGS .d0h and Information: lock calibrated data headers =========================================================================== d) Observer-Supplied Observing Information from Phase II Proposal =========================================================================== Support Schedule: Information on cover page of proposal .shh Program Info and type of output data requested by GO Support Schedule: Type of observation requested by GO, .shh Flags and for example, the aperture, the Indicators detector, the number of channels, etc. Proposal Info: Observing strategy, e.g., instrument .shh configuration, target description, and information on flux, exposure moving target, spatial scan etc. Target and Target and PEP information .shh Proposal ID: ========================================================================== e) Observing Information Produced in TRANS Stage ========================================================================== Support Schedule: Telescope pointing and instrument .shh Data Group II configuration on the sky, i.e., target RA and Dec and offset objects, position angle of diode array, OFFSET information, spacecraft velocity, guide stars, etc. Onboard Ephemeris Spacecraft ephemeris .shh Model: __________________________________________________________________________ __________________________________________________________________________ Table 30.5: FOS Header Keywords Keyword Description and Comments ========================================================================== General Information from Header File - usually in .d0h or .c1h ========================================================================== GCOUNT Number of groups in data file YTYPE Nature of observation, important for paired aperture observations. (Not real values in .d0h file). Files are OBJ, BKG, or SKY YPOSn Location of diode center in Y-base units, of the nth group, useful for interpreting ACQ/BIN data. If there is only one group then YPOS is the Y-base of that one group. Not populated with real values in the .d0h file YBASE YPOS of group #1 XBASE XDAC units needed to center aperture on the diode array for group #1 DEFDDTBL UDL disabled diode table to be used (value: T) or DDTHFILE table to be used (value: F) BUNIT Flux units of the data. Values: COUNTS, COUNTS/SEC, ERGS/SEC/CM^2/A, or ANGSTROM FILLCNT Number of sequences of filled data ERRCNT Number of sequences with bad data ROOTNAME Rootname of the observation set. Will start with the letter "y" FILETYPE Type of data in the file: SHP is science header packet, UDL is unique data log, SDQ is raw science data quality, WAV is wavelength, FLX is calibrated flux, ERR is calibrated flux error, MOD is CALFOS special mode processed data, SCI is object,sky, or background science data, OBJ is object data, BKG is background data, CDQ is calibration data quality, SKY is sky data, NET is sky-subtracted object data. GRNDMODE Ground software mode of FOS. Can be SPECTROSCOPY, TARGET ACQUISITION, IMAGE, RAPID-READOUT, SPECTRO-POLARIMETRY, or TIME-RESOLVED DETECTOR Detector in use for the observation. AMBER or BLUE APER_ID Aperture used for the observation; A-1 corresponds to the 4.3", A-2 to the 0.5" pair (square), A-3 to the 0.25 pair (square), A-4 to the 1.0" pair (square), B-1 to the 0.5" (round), B-2 to the 0.3" (round), B-3 to the 1.0" (round) B-4 is blank, C-1 to the 1.0" pair (square), C-2 to the 0.25"x2.0" slit, C-3 to the 0.7"x2.0" bar, and C-4 to the 2.0" bar apertures respectively POLAR_ID Polarization waveplate used for the observation. A is the waveplate A, B is the waveplate B and C is no polarizer used (clear) FGWA_ID Filter and grating used for the observation. Hxx means Gxx0H filter, L15 means G160L, L65 means G650L, PRI means prism, and CAM means camera (mirror). POLANG Initial angular position of the polarizer in degrees FCHNL First diode used in observation (first diode in array designated as zero) NCHNLS Number of diodes used in the observation, useful for interpreting ACQ/BIN data and exposure time. Usually 512 (except target acquisition and other specific modes, see below) OVERSCAN Number of overscans used in the observation, useful for interpreting ACQ/BIN data and exposure time. Usually 5. FOS reference files and calibration support only OVERSCAN=5 NXSTEPS Number of X substeps used in the observation, useful for interpreting ACQ/BIN data and exposure time. Usually 4 MINWAVE Minmum wavelength in A. (Not populated in .c0h file) MAXWAVE Maximum wavelength in angstroms. (Not populated in .c0h file) YFGIMPEN Onboard GIM correction enabled. T or F KYDEPLOY COSTAR mirror deployment for the FOS. T or F =========================================================================== Exposure Time Information - usually in .d0h or .c1h =========================================================================== FPKTTIME Time of first data packet sent to the SDF, i.e., approximate time of the end of the group exposure. The units are modified Julian date. Each group has its own unique FPKTTIME, accurate to about 1/8 second. LPKTTIME Approximate time of the last data packet sent to the SDF. The units are modified Julian date. DATE-OBS FPKTTIME of group 1 converted to standard notation for date TIME-OBS FPKTTIME of group 1 converted to standard notation for time, truncated to integer value; thus these are only accurate to 1/8 of a second EXPSTART Exposure start time in modified Julian date EXPOSURE Exact exposure time per pixel in seconds for each group. Note that this keyword is not populated with real values in the d0h file ========================================================================== Pattern Keywords for Exposure Times - usually in .c0h or .c1h ========================================================================== LIVETIME Time, in units of 7.8125 microseconds, during which accumulator is open DEADTIME Time, in units of 7.8125 microseconds, in which accumulator is closed INTS Number of repetitions of the live time/dead time cycle YSTEPS Number of Y substeps used in the observation. Usually 1 NPAT Number of patterns used per readout SLICES Number of repeats of the magnetic field deflection sequence. Usually 1 NREAD Number of readouts per memory clear. For the ACCUM mode this is usually the number of groups. For RAPID mode this is 1 NMCLEARS Number of memory clears per obs. 1 for ACCUM, number of groups for RAPID mode when NMCLEARS > 1 ========================================================================= Aperture Orientation Information - usually .shh and .d0h of acquisition image ========================================================================= OPMODE Operation mode of the FOS for the observation. Can be: ACQ, ACQ/BIN, ACQ/PEAK, ACQ/FIRMWARE, IMAGE, ACCUM, RAPID or PERIOD PA_APER Position angle of the aperture in degrees RA_APER1 RA of aperture center in degrees DECAPER1 Dec of aperture center in degrees _________________________________________________________________________ _________________________________________________________________________ Table 30.6: FOS Group Parameter Keywords Keyword Typical Value Alternate Name Description in Header ========================================================================== CRVAL1 1. PTYPE1 Reference pixel value CRPIX1 1. PTYPE2 Pixel number of reference pixel CD1_1 1. PTYPE3 Pixel increment DATAMIN 1.946443E-14 PTYPE4 Minimum value in this data DATAMAX 1.118037E-13 PTYPE5 Maximum value in this data RA_APER 297.4443291667 PTYPE6 Right ascension of aperture (deg) DEC_APER 48.961 PTYPE7 Declination of aperture (deg) FILLCNT 0 PTYPE8 Number of segments containing fill ERRCNT 0 PTYPE9 Error count of the data FPKTTIME 50441.0428552372 PTYPE10 Time of the first packet (Modified Julian Date) LPKTTIME 50441.042853645 PTYPE11 Time of the last packet (Modified Julian Date) CTYPE1 PIXEL PTYPE12 First coordinate type APER_POS SINGLE PTYPE13 Aperture used PASS_DIR 0 PTYPE14 Polarization pass direction YPOS -1597 PTYPE15 y-position on photocathode (Y-bases) YTYPE OBJ PTYPE16 Observation type: OBJ, SKY, BCK EXPOSURE 62.5 PTYPE17 Exposure time per pixel (seconds) X_OFFSET 0. PTYPE18 x_offset for GIMP correction (diodes) Y_OFFSET 0. PTYPE19 y_offset for GIMP correction (defl.units) SCT_VAL 0.1351963 PTYPE20 Scattered light correction (cts/sec/pixel) SCT_ERR 0.01424327 PTYPE21 Scattered light correction error (cts/sec/pixel) ___________________________________________________________________________