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ORFEUS II FITS Data Format

Contents


4. Format of data

The Echelle images as well as the extracted spectra are available in FITS format. The FITS headers, keywords and formats used are described in the following chapters.

A) Images

Echelle images are stored as 1024 x 512 x 16 bit data. This is a sample header of an Echelle image data file:

SIMPLE  =                    T /
BITPIX  =                   16 /
NAXIS   =                    2 /
NAXIS1  =                 1024 /
NAXIS2  =                  512 /
BUNIT   = 'counts/pixel'       / unit of pixel values
TELESCOP= 'ORFEUS-SPAS II'     / mission/satellite name
INSTRUME= 'Tuebingen UV Echelle Spectrometer (TUES)' / instrument name
DATE    = '2000-10-25T11:55:05' / file creation date (YYYY-MM-DDThh:mm:ss UTC)
ORIGIN  = 'IAAT - Institute for Astronomy and Astrophysics Tuebingen'
OBS_CODE= 'TUES2276_2'         / observation reference number
ORFEUSID= '2276    '           / specific ORFEUS target ID number
OBS_NO  = '2       '           / Echelle observation number
OBJECT  = 'HD 93521'           / target name
RA_OBJ  =            162.09750 /[deg] right ascension of object
DEC_OBJ =             37.57028 /[deg] declination of object
EQUINOX =               2000.0 / equinox of coordinate system
DATE-OBS= '1996-11-28T04:56:05' / obs start time (YYY-MM-DDThh:mm:ss UTC)
DATE-END= '1996-11-28T05:14:05' / obs stop time (YYY-MM-DDThh:mm:ss UTC)
CLOCKCOR= 'NO      '           / UTC not guaranteed
TIMEREF = 'LOCAL   '           / Time frame of satellite
TASSIGN = 'SATELLITE'          / Times assigned on satellite
TIMESYS = 'MJD     '           / Modified Julian Date
MJDREF  =                40000 / [days] MJD of reference date
TSTART  =            899873765 / [s] from MJDREF
TSTOP   =            899874845 / [s] from MJDREF
TIERRABS=                    1 / [s] precision of quoted times
EXPTIME =                 1080 / [s] exposure time
FILENAME= 'tues2276_2.fits'    / name of this file
COMMENT
COMMENT  ---  Original ORFEUS Header  ---
COMMENT  Description of keywords (not all of them used):
COMMENT  TARGET: ORFEUS ID / object name
COMMENT  RA/DEC: epoch 2000 coordinates
COMMENT  BEOBNR: number of observation
COMMENT  INTANF: obs.start: day of 1996 / hh:mm:ss
COMMENT  INTEND: obs. end : day of 1996 / hh:mm:ss
COMMENT  INTTIM: integration time in seconds
COMMENT  XTRACT: extraction parameters
COMMENT  ORBITV: orbital velocity towards object [km/s]
COMMENT  VHELIO: heliocentric velocity towards object [km/s]
COMMENT  BLAZEO: blaze correction parameters: offset [mm]
COMMENT  BLAZEW: blaze correction parameters: width
COMMENT  CNTRAT: count rates (maximum, actual, image)
COMMENT  V_KORR: additional radial velocity correction [km/s]
COMMENT  CORRCT: image processing flags
COMMENT  FACTOR: scaling factor of image
COMMENT  UPDATE: last update of ORFEUS Echelle image header
COMMENT  ---  start of ORFEUS header  ---
HISTORY  TARGET: 2276 / HD 93521
HISTORY  RA/DEC: 10 48 23.40 / +37 34 13.00
HISTORY  BEOBNR: 2
HISTORY  INTANF: GMT 333:04:56:05
HISTORY  INTEND: GMT 333/05:14:05
HISTORY  INTTIM: 1080.00
HISTORY  COMMNT:
HISTORY  XTRACT: 40, 61, 1, 0
HISTORY  ORBITV: 0.88, 1.77, 2.63, 3.46, 4.23, 4.93, 5.56, 6.11, 6.56, 6.91, 7.1
HISTORY  VHELIO: -26.46
HISTORY  BLAZEO: 5.00, 5.10, 5.20, 5.30, 5.40, 5.50, 5.60, 5.70
HISTORY  BLAZEW: 0.73, 0.73, 0.73, 0.73, 0.73, 0.73, 0.75, 0.75
HISTORY  CNTRAT: 9500.0, 6227.4, 5169.1
HISTORY
HISTORY
HISTORY
HISTORY
HISTORY
HISTORY
HISTORY  UPDATE: Thu Mar  5 10:14:25 1998
COMMENT  ---  end of ORFEUS header  ---
HISTORY  Version of ORFEUS-ECHELLE-FITS routines:
HISTORY      1.3, 2000-10-25, Juergen Barnstedt
HISTORY  For description of ORFEUS II Echelle spectrometer performance see:
HISTORY      J.Barnstedt, N.Kappelmann, I.Appenzeller, et al.,
HISTORY      Astron. Astrophys. Suppl. Ser. 134, 561-567 (1999)
HISTORY      (astro-ph/0006295)
END

 
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Description of Echelle image keywords:

SIMPLE, BITPIX, NAXIS, NAXISn, BUNIT, TELESCOP, INSTRUME, DATE, ORIGIN :
These are standard keywords used in accordance with the Definition of the Flexible Image Transport System (FITS).
 
OBS_CODE :
This keyword is used as a unique reference to this specific observation. It is composed of the instrument name (TUES = Tuebingen Ultraviolet Echelle Spectrometer), the ORFEUS target ID (4 digits), underline character ("_"), and the observation number for this target.
 
ORFEUSID :
The ORFEUS target ID, see List of all ORFEUS Echelle targets.
 
OBS_NO :
The ORFEUS Echelle observation number for this target. Usually a single number, but in some cases composed from two numbers, separated by an underline character. The second number is the number of the pointing during this observation: Several pointings during a single observation were carried out sometimes to improve the alignment of the telescope ("grid search"). This resulted partially in very short integrations.
 
OBJECT :
The name of the target.
 
RA_OBJ, DEC_OBJ, EQUINOX :
Right ascension and declination of the object in decimal degrees, and equinox (2000.0) of the coordinates.
 
DATE-OBS, DATE-END :
Start date and time and stop date and time of the observation (format yyy-mm-ddThh:mm:ss), Universal Time.
 
CLOCKCOR :
NO = UTC not guaranteed.
 
TIMEREF :
LOCAL = Time frame of satellite.
 
TASSIGN :
SATELLITE = Times assigned on satellite.
 
MJDREF, TSTART, TSTOP :
MJD (Modified Julian Date) of reference date (40000 days), start and stop time of observation in seconds from MJD.
 
TIERRABS :
Precision of quoted times (1 s).
 
EXPTIME :
Exposure time in seconds.
 
FILENAME :
Name of this file.
 

The "COMMENT" lines following these keywords describe the original ORFEUS keywords used with the data supplied to the guest observers. The original ORFEUS header follows as "HISTORY" entries. The original ORFEUS keywords are described here in detail:

TARGET:
ORFEUS ID / name of the target.
 
RA/DEC:
Equinox 2000 coordinates, right ascension and declination. Format: hh mm ss.ss / dd mm ss.ss
 
BEOBNR:
Number of observation (see OBS_NO).
 
INTANF:
Start of observation: day of 1996 / hh:mm:ss
 
INTEND:
End of observation: day of 1996 / hh:mm:ss
 
INTTIM:
Integration time in seconds (see EXPTIME).
 
XTRACT:
Extraction parameters: first and last extracted Echelle order (in most cases all orders: 40 to 61), flag for automatic centering of Echelle orders (im most cases 1), flag for 45 degree tilted extraction (in most cases 0).
 
ORBITV:
Orbital radial velocity component of the satellite towards the object in km/s: 11 equidistant values for the given observation period.
 
VHELIO:
Heliocentric radial velocity component towards object in km/s.
 
BLAZEO:
Offset of blaze function from center of detector [mm], given for the Echelle orders 40, 43, 46, 49, 52, 55, 58 and 61. The values for the orders in between are linearly interpolated.
 
BLAZEW:
Relative width of blaze function, given for the same Echelle orders as the BLAZEO values.
 
CNTRAT:
Three count rate values in counts/s:
  1. Maximum count rate of lower electronic threshold, taken from this or any other observation of this object.
  2. Actual average count rate of lower electronic threshold.
  3. Registered count rate in this Echelle image.
     
V_KORR:
Additional radial velocity correction in km/s (in most cases 0.0).
 
CORRCT:
Used only in rebinned images, not available in orignal Echelle observations. 5 flags indicating the corrections applied to this image for rebinning:
  1. Averaging of pixel 511/512.
  2. Observing block wavelength correction.
  3. Orbital radial velocity correction.
  4. Heliocentric radial velocity correction.
  5. Additional radial velocity correction.
     
FACTOR:
Used only in rebinned images, not available in orignal Echelle observations. Scaling factor of image, needed to minimize rounding errors during rebinning of the image.
 
UPDATE:
Time and date of last update of ORFEUS Echelle image header.
 

 
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B) Extracted data of individual echelle orders

The Echelle spectra are extracted order by order, there are overlap regions between adjacent orders. The extracted orders are stored in FITS format as binary table extensions. The spectra are available as each order in one file or as all orders contained in one file. The primary header contains all information concerning this certain observation or this file, the binary table extension header contains all information concerning the extracted data of the respective order.

Sample spectrum primary header:

SIMPLE  =                    T / Written by IDL:  Wed Oct 25 10:21:15 2000
BITPIX  =                    8
NAXIS   =                    0
EXTEND  =                    T
FILENAME= 'tues2276_2_ord61.fits'/ name of this file
ORD_FRST=                   61 / first echelle order in this file
ORD_LAST=                   61 / last echelle order in this file
WVL_FRST=              904.673 / [A] first wavelength in this file
WVL_LAST=              926.478 / [A] last wavelength in this file
TELESCOP= 'ORFEUS-SPAS II'     / mission/satellite name
INSTRUME= 'Tuebingen UV Echelle Spectrometer (TUES)' / instrument name
DATE    = '2000-10-25T09:21:14' / file creation date (YYYY-MM-DDThh:mm:ss UTC)
ORIGIN  = 'IAAT - Institute for Astronomy and Astrophysics Tuebingen'
OBS_CODE= 'TUES2276_2'         / observation reference number
ORFEUSID= '2276    '           / specific ORFEUS target ID number
OBS_NO  = '2       '           / Echelle observation number
OBJECT  = 'HD 93521'           / target name
RA_OBJ  =            162.09750 /[deg] right ascension of object
DEC_OBJ =             37.57028 /[deg] declination of object
EQUINOX =               2000.0 / equinox of coordinate system
DATE-OBS= '1996-11-28T04:56:05' / obs start time (YYY-MM-DDThh:mm:ss UTC)
DATE-END= '1996-11-28T05:14:05' / obs stop time (YYY-MM-DDThh:mm:ss UTC)
CLOCKCOR= 'NO      '           / UTC not guaranteed
TIMEREF = 'LOCAL   '           / Time frame of satellite
TASSIGN = 'SATELLITE'          / Times assigned on satellite
TIMESYS = 'MJD     '           / Modified Julian Date
MJDREF  =                40000 / [days] MJD of reference date
TSTART  =            899873765 / [s] from MJDREF
TSTOP   =            899874845 / [s] from MJDREF
TIERRABS=                    1 / [s] precision of quoted times
EXPTIME =                 1080 / [s] exposure time
COMMENT  *********************
COMMENT  The following data are displayed in the
COMMENT  original ORFEUS header format.
COMMENT  ---  ORFEUS header  ---
COMMENT
COMMENT  RA / DEC (2000) [hh mm ss.ss / +dd mm ss.ss] :
RA_DEC  = '10 48 23.40 / +37 34 13.00'
COMMENT  integration start time [UT, day of 1996 /hh:mm:ss] :
INTSTART= 'GMT 333:04:56:05'
COMMENT  integration stop time [UT, day of 1996 /hh:mm:ss] :
INTSTOP = 'GMT 333/05:14:05'
COMMENT  original image file :
IMGFILE = '/users_12/ORFII/BILDER/E2276/2276_2.bil'
TILTEDEX= 'no'                 / tilted extraction (yes/no)
COMMENT  ORFEUS comment :
ORFCOMM = ''
CORBLAZE= 'yes'                / blaze correction (yes/no)
COMMENT  blaze offset values [mm]:
OFSBLAZE= ' 5.00, 5.10, 5.20, 5.30, 5.40, 5.50, 5.60, 5.70'
COMMENT  blaze width values :
WIDBLAZE= ' 0.73, 0.73, 0.73, 0.73, 0.73, 0.73, 0.75, 0.75'
COREFFAR= 'yes'                / effective area correction (yes/no)
COMMENT  count rates (maximum, actual, image) [counts/sec]:
CNTRATES= '9500.0, 6227.4, 5169.1'
CORBACKG= 'yes'                / background subtraction (yes/no)
AVERG512= 'yes'                / averaging of pixels 511/512 (yes/no)
EDGECORR= 'yes'                / edge correction (yes/no)
OBSBLKCO= 'yes'                / observation block correction (yes/no)
BLKCORR =            1.0000321 / observation block wavelength correction factor
ORBRVCOR= 'yes'                / orbital relative velocity correction (yes/no)
ORBITRV =                  4.6 / [km/s] average orbital relative velocity
VHELCORR= 'yes'                / heliocentric correction (yes/no)
VHELIOCE=                -26.5 / [km/s] heliocentric velocity
AVELCORR= 'yes'                / additional radial velocity correction (yes/no)
ADDRADV =                  0.0 / [km/s] additional radial velocity
COMMENT  wavelength calibration parameters :
WLCALPAR= '41.5990, 475.945, 3.52602, -0.480310, -0.335360, -0.0108750, 1.06074'
COMMENT  ---  end of ORFEUS header  ---
COMMENT  *********************
HISTORY  Version of ORFEUS-ECHELLE-FITS routines:
HISTORY    1.2, 2000-10-13, Juergen Barnstedt
HISTORY  For description of ORFEUS II Echelle spectrometer performance see:
HISTORY    J.Barnstedt, N.Kappelmann, I.Appenzeller, et al.,
HISTORY    Astron. Astrophys. Suppl. Ser. 134, 561-567 (1999)
END

 
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Description of spectrum primary header keywords:

SIMPLE, BITPIX, NAXIS, EXTEND, FILENAME :
These are standard keywords used in accordance with the Definition of the Flexible Image Transport System (FITS).
 
ORD_FRST, ORD_LAST :
First and last extracted echelle order in this file (range 40 - 61).
 
WVL_FRST, WVL_LAST :
First and last wavelength in this file (in Å).
 
TELESCOP, INSTRUME, DATE, ORIGIN, OBS_CODE, ORFEUSID, OBS_NO, OBJECT, RA_OBJ, DEC_OBJ, EQUINOX, DATE-OBS, DATE-END, CLOCKCOR, TIMEREF, TASSIGN, TIMESYS, MJDREF, TSTART, TSTOP, TIERRABS, EXPTIME :
See description of image header keywords.
 

The following keywords show the original ORFEUS header data used with the files supplied to the guest observers. The keywords are described here in detail:

RA_DEC :
Equinox 2000 coordinates, right ascension and declination. Format: hh mm ss.ss / dd mm ss.ss
 
INTSTART :
Start of observation: day of 1996 / hh:mm:ss
 
INTSTOP :
End of observation: day of 1996 / hh:mm:ss
 
IMGFILE :
Original image file at IAAT.
 
TILTEDEX :
Extraction flag: tilted extraction (yes/no).
 
ORFCOMM :
Comment on this observation.
 
CORBLAZE :
Extraction flag: blaze correction (yes/no).
 
OFSBLAZE :
Offset of blaze function from center of detector [mm], given for the Echelle orders 40, 43, 46, 49, 52, 55, 58 and 61. The values for the orders in between are linearly interpolated.
 
WIDBLAZE :
Relative width of blaze function, given for the same Echelle orders as the OFSBLAZE values.
 
COREFFAR :
Extraction flag: effective area correction (yes/no).
 
CNTRATES :
Three count rate values in counts/s:
  1. Maximum count rate of lower electronic threshold, taken from this or any other observation of this object.
  2. Actual average count rate of lower electronic threshold.
  3. Registered count rate in this Echelle image.
     
CORBACKG :
Extraction flag: background subtraction (yes/no).
 
AVERG512 :
Extraction flag: averaging of pixels 511/512 (yes/no).
 
EDGECORR :
Extraction flag: edge correction (yes/no).
 
OBSBLKCO :
Extraction flag: observation block correction (yes/no)
 
BLKCORR :
Observation block wavelength correction factor.
 
ORBRVCOR :
Extraction flag: orbital relative velocity correction (yes/no).
 
ORBITRV :
Average orbital relative velocity in km/s.
 
VHELCORR :
Extraction flag: heliocentric correction (yes/no).
 
VHELIOCE :
Heliocentric velocity in km/s.
 
AVELCORR :
Extraction flag: additional radial velocity correction (yes/no).
 
ADDRADV :
Additional radial velocity in km/s.
 
WLCALPAR :
Seven wavelength calibration parameters (for internal documentation only).
 

 
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Sample spectrum binary table extension header:

XTENSION= 'BINTABLE'           /Binary table written by MWRFITS
BITPIX  =                    8 /Required value
NAXIS   =                    2 /Required value
NAXIS1  =                   42 /Number of bytes per row
NAXIS2  =                  837 /Number of rows
PCOUNT  =                    0 /Normally 0 (no varying arrays)
GCOUNT  =                    1 /Required value
TFIELDS =                   11 /Number of columns in table
INHERIT =                    T / primary header applies to this extension
EXTNAME = 'ORDER_61'           / name of this binary table extension
ECHORDER=                   61 / echelle order (range 40 - 61)
WVLSTART=              904.673 / [A] start wavelength in this extension
WVLEND  =              926.478 / [A] end wavelength in this extension
CUTWIDTH=                    9 / [pixel] extraction cut width
Y_SHIFT =                   -3 / [pixel] extraction y-position shift
TFORM1  = 'E       '           /
TFORM2  = 'I       '           /
TFORM3  = 'E       '           /
TFORM4  = 'E       '           /
TFORM5  = 'E       '           /
TFORM6  = 'E       '           /
TFORM7  = 'E       '           /
TFORM8  = 'E       '           /
TFORM9  = 'E       '           /
TFORM10 = 'E       '           /
TFORM11 = 'E       '           /
COMMENT                               /
TTYPE1  = 'WAVELENGTH'                /
TUNIT1  = 'ANGSTROM'                  /
TDISP1  = 'F9.3    '                  /
COMMENT                               /
TTYPE2  = 'PIXEL   '                  /
TUNIT2  = 'PIXEL NO.'                 /
TDISP2  = 'I6      '                  /
COMMENT                               /
TTYPE3  = 'COUNTS_RAW'                /
TUNIT3  = 'COUNTS/PIXEL'              /
TDISP3  = 'F10.1   '                  /
COMMENT                               /
TTYPE4  = 'COUNTS_BLAZE_CORRECTED'    /
TUNIT4  = 'COUNTS/PIXEL'              /
TDISP4  = 'F10.1   '                  /
COMMENT                               /
TTYPE5  = 'BACKGROUND_RAW'            /
TUNIT5  = 'COUNTS/PIXEL'              /
TDISP5  = 'F10.1   '                  /
COMMENT                               /
TTYPE6  = 'BACKGROUND_SMOOTHED'       /
TUNIT6  = 'COUNTS/PIXEL'              /
TDISP6  = 'F10.1   '                  /
COMMENT                               /
TTYPE7  = 'PHOTON_FLUX'               /
TUNIT7  = 'PHOTONS/CM^2/SEC/ANGSTROM' /
TDISP7  = 'G15.5E3 '                  /
COMMENT                               /
TTYPE8  = 'ENERGY_FLUX'               /
TUNIT8  = 'ERG/CM^2/SEC/ANGSTROM'     /
TDISP8  = 'G15.5   '                  /
COMMENT                               /
TTYPE9  = 'RELATIVE_ERROR'            /
TUNIT9  = '        '                  /
TDISP9  = 'F8.3    '                  /
COMMENT                               /
TTYPE10 = 'EDGE_CORR'                 /
TUNIT10 = 'SCALING FACTOR'            /
TDISP10 = 'F8.3    '                  /
COMMENT                               /
TTYPE11 = 'BLAZE_CORR'                /
TUNIT11 = 'SCALING FACTOR'            /
TDISP11 = 'F8.4    '                  /
COMMENT                               /
END

 
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Description of spectrum binary table extension header keywords:

XTENSION, BITPIX, NAXIS, NAXISn, PCOUNT, GCOUNT, TFIELDS :
These are standard binary extension keywords used in accordance with the Definition of the Flexible Image Transport System (FITS): Binary Table Extension.
 
INHERIT :
Always = 'T' : Primary header applies to this extension.
 
EXTNAME :
Name of this binary table extension ('ORDER_' + number of Echelle order).
 
ECHORDER :
Number of this Echelle order (range 40 - 61).
 
WVLSTART, WVLEND :
Start wavlength and end wavelength (in Å) in this binary extension table.
 
CUTWIDTH :
Vertical width of extraction strip (in pixels).
 
Y_SHIFT :
Y-position shift of extraction strip with respect to default position (in pixels).
 
TFORMn, TTYPEn, TUNITn, TDISP :
These are standard binary extension keywords used in accordance with the Definition of the Flexible Image Transport System (FITS): Binary Table Extension.
 

 
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Description of the 11 data columns within one binary table extension:

1: WAVELENGTH (Single Precision Floating Point)
Wavelength in Å.
 
2: PIXEL (16-Bit Integer)
X-pixel number within Echelle image
 
3: COUNTS_RAW (Single Precision Floating Point)
Number of counts after extraction, only averaging of pixel 511/512 applied (in counts/pixel).
 
4: COUNTS_BLAZE_CORRECTED (Single Precision Floating Point)
Number of counts after all processing steps (in counts/pixel).
 
5: BACKGROUND_RAW (Single Precision Floating Point)
Background before smoothing, scaled with the extraction widths of background and Echelle order (in counts/pixel).
 
6: BACKGROUND_SMOOTHED (Single Precision Floating Point)
Background smoothed, as used for subtraction (in counts/pixel).
 
7: PHOTON_FLUX (Single Precision Floating Point)
Flux in Photons/cm2/s/Å.
 
8: ENERGY_FLUX (Single Precision Floating Point)
Flux in Ergs/cm2/s/Å.
 
9: RELATIVE_ERROR (Single Precision Floating Point)
Relative error (poisson statistics of COUNTS_RAW and BACKGROUND_SMOOTHED).
 
10: EDGE_CORR (Single Precision Floating Point)
Factor for correction of sensitivity loss at the detector edges.
 
11: BLAZE_CORR (Single Precision Floating Point)
Correction factor of blaze function.
 

Formula of calculation:

COUNTS_BLAZE_CORRECTED =
(COUNTS_RAW - BACKGROUND_SMOOTHED) / BLAZE_CORR / EDGE_CORR
RELATIVE_ERROR =
SQRT(COUNTS_RAW + BACKGROUND_SMOOTHED*width_factor) /
(COUNTS_RAW - BACKGROUND_SMOOTHED)


The above page was written by Jürgen Barnstedt from the ORFEUS Group at Tübingen and is reproduced here for the MAST archive with his permission.