HPOL PBO Reticon Halfwave Polarization Data FITS Files
One of the telescopes at the University of Wisconsin's Pine Bluff Observatory (PBO) is a 0.9 m f/13.5 Cassegrain with a dedicated spectropolarimeter (HPOL). From February 1989 to December 1994 this instrument obtained simultaneous spectra and polarization measurements, with a resolution of 25Å, from 3200Å -7600Å for 311 targets. A halfwave plate rotated to 8 distinct angles provide the spectropolarimetric modulation. The detector was a Reticon dual-photocathode array with 1024 diodes in each array. (In 1995 the detector was replaced by a 400 x 1200 CCD with spectral coverage 3200Å -10500Å.) See Wolff, Nordsieck and Nook, 1996, A.J., 111, 856 [pdf] for a detailed description of the instrument. See also the HPOL Web site.
These data have been processed by summing equivalent readouts on a particular object, subtraction of background scans, division by the flat field calibration spectra, flux calibration using spectra of known bright stars, and polarimetric calibration using data obtained of unpolarized and polarized standards. For the halfwave mode data, the modulated scans are differenced and a Fourier analysis, pixel-by-pixel as a function of waveplate angle, is done to give Stokes parameters %Q and %U spectra along with a %Error spectrum, all consisting of 1024 32-bit data points for each array from about 3200Å -7700Å with a 25Å resolution.
Data analysis and software development took place at the University of Wisconsin Space Astronomy Laboratory (SAL). Much of the data reduction was accomplished by a FORTRAN language package (REDUCE) generalized for a collection of Reticon detector/spectropolarimeters developed at SAL to support ground-based (HPOL/PBO), the space shuttle-flown Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) and Rocket UV spectropolarimetry. This software package was initially installed on UNIX based microVAXes for several years and is currently on SiliconGraphics machines running IRIX and machines running Linux.
The above figure illustrates the standard data product of the reduction: a three-part spectrum showing the flux, linear polarization, and linear position angle (angle = 0.5*arctan(U/Q)). The polarization is usually "binned" (neighboring pixels averaged until a constant mean error is achieved) to further improve the signal-to-noise. A standard set of spectropolarimetric analysis tools (vector arithmetic, Q-U vector plots, wavelength dependence fitting) are used for scientific interpretation.
There are 1557 FITS files of 311 objects from 796 nights of observations in this data set. (The HPOL target database includes the Reticon and CCD observations). The Stokes %Q and %U, and %Error spectra and flux calibrated (Flambda- ergs/cm2/sec/Å) data along with pertinent housekeeping data have been written to a FITS file for each observation.
The file name consists of the following:
objname is the name of the object.
The obsdate is the Date (YYYYMMDD) the observation was taken (GMT).
hw is the Halfwave polarimeter mode.
The CFITSIO package, version 2.430 written by Dr. William D. Pence at the Goddard Spaceflight Center, was used to generate the FITS files. They were written on a workstation running Linux. The FITS files are 34560 bytes in length.
An HPOL PBO Reticon Halfwave FITS file looks like the following:
+-------------------+ | | | Primary header | | | +-------------------+ +-------------------+ | Primary data | | Flux (Flambda) | | | +-------------------+ +-------------------+ | | | Extension Header | | | +-------------------+ +-----++----++----++----+ |Wavel|| %Q || %U ||%Err| | || || || | | || || || | | || || || | | || || || | | || || || | | || || || | | || || || | +-----++----++----++----+
The set of basic header information includes the primary array keywords and keywords describing the data arrays, target information, telescope engineering data relative to the observation, and calibration information. The exact header of a sample file is given below. The primary array data consists of the Flux array (1024 data points-32 bits per data point) in units of ergs/cm2/sec/Å (Flambda). The binary tables extension includes headers and vectors for wavelength (Angstroms) and the Stokes parameters %Q & %U, and the %Error (also 1024 32-bit data points per array). For the fluxes, bad data have been zeroed out. For the polarization data, the bad data are when the %Error data have been zeroed out. The %Q and %U data remain (allowing inspection of those data) but should not be used.
Sample FITS File: hpolret_pi-aqr_19890808_hw.lis
Main Fits header: SIMPLE = T / file does conform to FITS standard BITPIX = -32 / number of bits per data pixel NAXIS = 1 / number of data axes NAXIS1 = 1024 / length of data axis 1 EXTEND = T / FITS dataset may contain extensions COMMENT FITS (Flexible Image Transport System) format is defined in 'Astronomy COMMENT and Astrophysics', volume 376, page 359; bibcode: 2001A&A...376..359H CRVAL1 = 3000.00 / Wavelength in Angstroms of first pixel CDELT1 = 6.00 / Number of Angstroms/pixel CRPIX1 = 1.00 / Starting pixel number CTYPE1 = 'WAVELENGTH' / Wavelength in Angstroms CUNIT1 = 'angstrom' / Unit of Wavelength BUNIT = 'ERGS/CM**2/SEC/A' / Unit of Flux (FLAMBDA) FILENAME= 'hpolret_pi-aqr_19890808_hw.fits' / FITS file name ORIGIN = 'UW-Space Astronomy Lab' / Creator of FITS file DATE = '2005-07-12T14:28:05' / FITS file creation date DATE_OBS= '1989-08-08' / UT Date of Observation ccyy-mm-dd MJD_OBS = 47746.26 / [days] MJD at start of obs (JD-2,400,000.05) TIME_OBS= 6.3255 / [hours] UT time at start of obs OBSERVAT= 'PBO ' / Pine Bluff Observatory TELESCOP= '36-inch ' / Telescope INSTRUME= 'SPECTROPOLARIMETER' / Instrument Used MODE = 'HPOL ' / Halfwave Polarimeter Mode DETECTOR= 'Reticon ' / Detector OBJECT = 'Pi-Aqr ' / Object Observed NAME2 = 'HD212571' / Alternate Object Name ID = '2235 ' / WUPPE ID number for Object TYPE = 'Be ' / Type of Object SPTYPE = 'B1Ve ' / Spectral Type VMAG = 4.6 / V magnitude EQUINOX = 2000.0 / Equinox for coordinates RA = '2225165 ' / Right Ascension in hhmmsss DEC = '+012238 ' / Declination in sddmmss EXPOSURE= 6451.00 / Useful Exposure Time (seconds) HVPS = 4.40 / High Voltage Power Supply (volts) THERM = -32.30 / Detector temperature (Centigrade) POSANGLE= 90.00 / Observatory Position Angle (Degrees) SLIT = 4 / Slit Number of Entrance Aperture DECKER = 6 / Decker Number of Entrance Aperture COMMENT Note: The most commonly used slit/decker combination COMMENT is slit=4, decker=6 (12" x 12"). COMMENT Slit Widths for this HPOL system are: COMMENT 1: 1.5", 2: 3.0", 3: 6.0", 4: 12.0", 5: 50.0", 0: unknown COMMENT The decker controls the length of the slit being used. COMMENT Deckers for this HPOL system are: COMMENT 1: Occulting 25" with central 6" obscured COMMENT 2: Occulting 50" with central 12" obscured COMMENT 3: 1.5", 4: 3.0", 5: 6.0", 6: 12.0", 7: 25.0" COMMENT 8: 50.0", 9: 100.0", 0: unknown OBSERVER= 'man ' / Observer of the Data REDUCER = 'man ' / Reducer of the Data DATASTAT= 'OK ' / Data Status (DG=Degraded) REDCOMME= ' ' / Data reduction comments COMMENT Useful data generally are from 3180A-7700A with a spectral COMMENT resolution of 25A. COMMENT HPOL reticon flux calibration has a relative flux accuracy COMMENT of 0.05 magnitudes for the 12"/12" slit/decker combination. COMMENT Fluxes are not absolute due to non-photometric skies. Binary extension header: XTENSION= 'BINTABLE' / binary table extension BITPIX = 8 / 8-bit bytes NAXIS = 2 / 2-dimensional binary table NAXIS1 = 16384 / width of table in bytes NAXIS2 = 1 / number of rows in table PCOUNT = 0 / size of special data area GCOUNT = 1 / one data group (required keyword) TFIELDS = 4 / number of fields in each row TTYPE1 = 'WAVELENGTH' / label for field 1 TFORM1 = '1024E ' / data format of the field: 4-byte REAL TUNIT1 = 'ANGSTROM' / physical unit of field TTYPE2 = 'Q ' / label for field 2 TFORM2 = '1024E ' / data format of the field: 4-byte REAL TUNIT2 = 'PERCENT ' / physical unit of field TTYPE3 = 'U ' / label for field 3 TFORM3 = '1024E ' / data format of the field: 4-byte REAL TUNIT3 = 'PERCENT ' / physical unit of field TTYPE4 = 'ERROR ' / label for field 4 TFORM4 = '1024E ' / data format of the field: 4-byte REAL TUNIT4 = 'PERCENT ' / physical unit of field EXTNAME = 'POLARIMETRY' / name of this binary table extension CALIBRAT= 'pbo2b ' / Instrumental polarization calibration used AVE-POL = 1.295 / Mean %Pol over entire wavelength region AVE-ERRO= 0.006 / Mean %Error in %Pol AVE-PA = 156.84 / Mean Position Angle (degrees) U-POL = 0.871 / Synthetic U Filter %Polarization U-ERR = 0.008 / Synthetic U Filter %Error U-PA = 154.38 / Synthetic U Filter Position Angle (Deg) B-POL = 1.429 / Synthetic B Filter %Polarization B-ERR = 0.004 / Synthetic B Filter %Error B-PA = 158.13 / Synthetic B Filter Position Angle (Deg) V-POL = 1.277 / Synthetic V Filter %Polarization V-ERR = 0.005 / Synthetic V Filter %Error V-PA = 156.53 / Synthetic V Filter Position Angle (Deg) R-POL = 1.110 / Synthetic R Filter %Polarization R-ERR = 0.006 / Synthetic R Filter %Error R-PA = 155.07 / Synthetic R Filter Position Angle (Deg) COMMENT The errors given in the error array are the statistical errors COMMENT based on the residuals of the least squared fit to the four COMMENT filter-pair polarimetric data (see section 2.3 of Wolff, COMMENT Nordsieck & Nook 1996 AJ 111, 856). COMMENT The instrumental polarization zero-point stability of the HPOL COMMENT reticon is +/- 0.004% (rms) for data taken on September 28, 1989 COMMENT and later and +/-0.08% (rms) pre-September 28, 1989. COMMENT The polarization efficiency is Pol(%)/100. COMMENT HPOL reticon instrumental polarization has been removed. COMMENT ***IMPORTANT NOTE*** COMMENT **The ERROR array is also being used as a QUALITY array. COMMENT **For bad/contaminated data, the errors have been set to zero COMMENT **but the %Q & %U arrays for those wavelengths remain unchanged COMMENT **to allow inspection of the bad data.
ASCII listings of each of the HPOL Reticon Halfwave FITS files have also been provided. Each ASCII file is named
hpolret_objname_obsdate_hw.lis (e.g. hpolret_pi-aqr_19890808_hw.lis)
with the objname and obsdate as defined above.
These files are the output of the program polfitsread.f, which was written to verify the contents of the HPOL PBO Reticon Halfwave FITS files. It also uses the GSFC CFITSIO software package.