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.
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.
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.
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.
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:
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.
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.
We used a HST archive model of G191B2B "http://garnet.stsci.edu/STIS/models/tables/g191b2b_mod_002.tab" 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:
See also this Echelle demo image:
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:
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:
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:
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:
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:
Description of the 11 data columns within one binary table extension:
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)
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