ORFEUS II Echelle-Datenreduktion und FITS-Datenformat
Contents
- 1. Steps of data processing
- 2. Artefacts
- 3. Publications
- 4. Format of data
1. Steps of data processing
A) Extraction
All orders were extracted automatically from the images, i.e. during extraction a centering in y-direction (cross dispersion direction) was applied according to the center of gravity for each individual echelle order. The extraction is done in x-direction (main dispersion direction) by summing up a well defined number of pixels in y-direction. The number of pixels used for extraction in y-direction varies with the echelle order and is indicated in the header of the data file for each echelle order ("cut width"). The center for the extraction in y-direction follows a straight line along each order and is located on whole pixel numbers.
Some echelle images show tilted absorption lines within the strip of the echelle orders. In such cases the extraction was done summing up the pixels tilted by 45 degrees, which results in a significant increase in resolution.
B) Background
Between the echelle orders a line of 3 pixels width was used to estimate the background. With exception of orders 40, 41 and 42 the background was calculated as average of the strip above and below the corresponding order. For the first three orders only the background values below each order were calculated. The so calculated background was smoothed twice with a width of 21 pixels: First a median filter was applied and as a second step a boxcar smoothing was used. This smoothing works fine if the counts within the background pixels are not too low. For very low counts within the background field other smoothing methods might be more satisfying.
Within very broad absorption lines (e.g. Ly-alpha) the background subtracted is in general overestimated. The reason might be that by far the strongest contribution to the background comes from stray light of the echelle grating:
This stray light is scattered exactly in horizontal direction on the detector while the echelle orders run slightly tilted across the detector. So within very broad absorption lines stray light is reduced within the echelle orders, and almost normal in the background extraction fields.
C) Pixels 511/512
Due to arithmetic rounding errors at the calculation of the photon position within the echelle electronics an artefact is observed, especially in the middle of the detector image, which leads to a higher intensity in one of these two pixels while the other shows a corresponding loss of intensity. This means, that the pixel border between these two pixels seems to be somewhat shifted. This effect is eliminated by applying an averaging to these two pixels.
D) Correction of the blaze function
The efficiency of echelle gratings has a maximum for one direction of diffraction (blaze angle), while the efficiency is reduced as a function of the deviation from this angle of diffraction (blaze function). The optimized diffraction direction was pre flight adjusted to point to the center of the echelle detector. What we find now is that the center of maximum efficiency is different for each echelle order. Furthermore we found that both the position and the width of the blaze function differ from observation to observation.
Therefore it was neccessary to introduce an individual blaze correction for each observation. Often only the overlap region between two adjacent orders could be used as a criterion for a good blaze correction.
E) Wavelength calibration
The wavelength calibration was calculated from the positions of 814 interstellar absorption lines from 12 different echelle images. Using these data we determined the parameters for the dispersion function. Radial velocity values were not used for the calibration, the zero position of the dispersion function was determined seperately. The accuracy is better than ±0.005nm, i.e. better than the optical resolution of the instrument.
We used the position of the Ly-alpha geocoronal emission line as a reference for the absolute wavelength zero position. We found that for different observation blocks the position of the Ly-alpha line differed up to 0.006 nm. For this a wavelength correction for each observation block was applied.
Further wavelength corrections are:
- Heliocentric radial velocity correction
- Radial velocity correction due to the satellite's orbital velocity.
The given extractions use the on board integrated images, for these images only the average of the orbital velocity component during the observation was used for correction. In future we will extract the spectra of the individually registered photons for which the orbital radial velocity correction will be applied to each individual photon.
An additional wavelength error might occur, if the target was not exactly centered within the 20" diaphragm. The maximum resulting uncertainty is ±1.2 10-4 as a relative wavelength error. This error was not corrected up to now.
To overcome this problem we will try to get a hint about the image position within the diaphragm from the count rates. Rapidly changing count rates indicate that the image was close to the edge of the diaphragm. In these cases, we can correlate the ASTRO-SPAS pointing data with the count rate fluctuations and thus find out the x/y-coordinates of the image in the diaphragm. Anyway this correction procedure will be complicated and tedious.
F) Correction of loss of sensitivity in the detector edges
The corners of the detector image and the left edge show a loss of sensitivity which is probably due to a reduced efficiency of the repeller grid in front of the detector. The electrical field in front of the MCPs (about 50V/mm) is used to force those photo electrons back into the MCP channels, which are released from the areas in between the channels. This improves the quantum efficiency by about 30% (causing also a loss of 10% due to shading of the grid in front of the detector). Probably due to an inhomogenious field at the borders of the detector the efficiency of the repeller field is reduced, and there is a rather sharp step visible between lower and normal sensitivity. This is visible in some images as a circular shaped area. We estimated a loss of about 25% and corrected this by applying a "smooth" step function. The position, width and height of the step was estimated for each order from the sum of all echelle measurements. The actually used correction values are listed in the column "EDGE_CORR" (see below).
A detailed flat field correction was not applied. The reason is, that the optical light path of the spectrometer cannot be reproduced in our laboratory. This however would be essential for an exact estimation of the flat field behavior of the detector. Any other correction methods are too uncertain to be useful.
G) Absolute flux calibration
We used an HST archive model of G191B2B (downloaded from garnet.stsci.edu/STIS/models/tables/g191b2b_mod_002.tab, now available from ftp://ftp.stsci.edu/cdbs/oldcalspec/g191b2b_mod_002.fits) as a reference for the absolute flux calibration. The calibration was additionally checked with a model of BD +28°4211 (R.Napiwotzki). We guess an accuracy of ±10% for the flux calibration. This is valid, if the object was fully centered within the diaphragm. There are, however, some observations for which the object was not completely centered. The reasons are probably some temperature drifts of the telescope causing a shift of the alignment. In some cases also slightly wrong coordinates of the target might have led to a decentralization.
Observations with badly centered targets are identified by their strongly varying count rates. The flux calibration was calculated for the maximum observed count rate for the corresponding target (also from other observations of this target, if neccessary). This count rate was scaled with the registered count rate in the integrated image. The corresponding count rates are documented within the file headers.
The file headers contain a maximum count rate and an actual (average) count rate of the lower electronic threshold. The third value is the registered count rate in the actual image. The actual count rate of the lower threshold and the registered count rate differ for the following reasons:
- The electronic upper threshold suppresses pulses too large to be processed.
- Electronic dead time effects and dead time effects from the on board processor reduce the electronic efficiency.
2. Artefacts
- Testpulse peak
- Some spectra show a peak at 1317 Å in order 42, which is due to a not completely removed testpulse peak of the echelle electronics.
- Airglow und geocorona
- Airglow: OI lines at 1302, 1305 and 1306 Å, also at 989 Å
- Geocorona: Ly-alpha (very strong) at 1216 Å, Ly-beta at 1026 Å
See also this Echelle demo image:
- GIF format (50 KB)
- JPEG format (90 KB)
- JAVA applet for JPEG image
3. Publications
- J.Barnstedt, N.Kappelmann, I.Appenzeller et al.,
The ORFEUS II Echelle Spectrometer: Instrument description, performance and data reduction
Astronomy & Astrophysics Suppl. Ser. 134, 1998, 561-568
[astro-ph/0006295]
- See also: ORFEUS publications
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
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:
- Maximum count rate of lower electronic threshold, taken from this or any other observation of this object.
- Actual average count rate of lower electronic threshold.
- 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:
- Averaging of pixel 511/512.
- Observing block wavelength correction.
- Orbital radial velocity correction.
- Heliocentric radial velocity correction.
- 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.
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
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:
- Maximum count rate of lower electronic threshold, taken from this or any other observation of this object.
- Actual average count rate of lower electronic threshold.
- 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).
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
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.
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)