HPOL PBO Reticon Halfwave Polarization Data FITS Files


Description of the Instrument

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.

HPOL PBO Reticon Halfwave Data Set

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.

Pi Aqr Triplot example

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.

FITS File Naming Convention

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.

(e.g. hpolret_pi-aqr_19890808_hw.fits)

Description of FITS file

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.

FITS File Header

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 HPOL PBO Reticon Halfwave FITS files

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.

HPOL Publications

A complete list of HPOL Publications is given here.