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wavelength calibration is almost always needed in the process of producing useful astronomical spectra .
to calibrate accurately is a non - trivial problem , in particular when working at high or very - high spectral resolution .
fourier transform spectrographs ( fts ) are specially well - suited to this task , but they are not readily applied in conditions requiring high spatial or time resolution , so grating spectrometers are much more commonly used for astronomical observations . in this case , it becomes necessary to set reference positions corresponding to known wavelengths on the detector .
this can be achieved by using very sharp observed telluric lines , but their location in the spectrum can not be chosen by the astronomer .
it is very usual to find spectral calibration lamps available for use with an astronomical spectrograph .
the emission lines produced in the lamps have been previously measured at the laboratory , and this method usually provides a valid reference frame .
however , it is often impractical to expose the calibration lamp simultaneously with the astronomical target and , unless the spectrograph is installed at a very stable focal station , the position of the spectrum on the detector varies depending on the telescope position .
accuracy is then limited by the instrument characteristics and observations of the calibration lamps are required between successive astronomical exposures .
nonetheless , calibrations via arc or hollow cathode spectra are normally accurate enough for most purposes .
ingenious techniques have been used to improve the accuracy of wavelength calibrations , such as placing gas cells at the entrance of the spectrograph ( e.g. , deming & plymate 1994 ) , but it is rare to find such systems available and convenient for regular observations .    on occasion
the available lamps are not very rich in lines in the spectral range of observation . in some circumstances ,
an external check of the final precision in the translation into wavelengths would be desirable .
one method for tackling problems such as these is to use solar spectra as templates .
changes in the wavelengths of the lines in the integrated sunlight spectrum around the solar cycle have been proved to be very small , bellow some 15 m s@xmath1 ( jimnez et al .
1980 ; wallace et al . 1988 ; mcmillan et al . 1993 ; deming & plymate 1994 ) . at 5000
 , this translates into @xmath2 0.3 m , so the solar spectrum does offer a very stable source . in most practical cases ,
the accuracy will be imposed by the spectral resolution achieved . during night - time observations ,
the solar flux spectrum is observable after reflection from the moon .    measurements of solar wavelengths in the integrated solar optical spectrum were published in 1929 by burns and collaborators ( burns 1929 ; burns & kiess 1929 ; burns & meggers 1929 ) , using photographic detectors and a grating spectrograph .
the relatively recent solar flux fts atlases offer a much higher quality spectrum of the sun seen as a star .
as the solar spectrum is so intense , on some solar telescopes no calibration lamps are deemed necessary , and the wavelength scale is set using the solar spectrum itself .
reasonable precision can be reached using the spectrum at the centre of the disc to compare with previously measured disc centre wavelengths , thus avoiding differential shifts due to the limb effect . in this case ,
small scale motions have to be averaged out , integrating in time and/or space , in order to minimize errors .
the _ kitt peak table of photographic solar spectrum wavelengths _ ( pierce & breckinridge 1973 ) has been extensively used by solar observers to set up the wavelength scale on their spectra .
these observations , made on photographic plates , have been superseded in quality by the more recent fts observations at the centre of the disc .
to improve on the various sets of photographically based measurements ( which date back to 1930 in the case of the solar flux spectrum ) , provide them in a homogeneous machine - readable format , use them to test spectral calibrations of very high resolution stellar spectra ( e.g. , allende prieto et al . 1995 ) , and improve the accuracy of our own solar observations , we have determined the position of the central wavelengths of 4947 atomic lines in the optical solar spectrum .
the employed source solar atlases , prepared from fts data , and the fashion in which we performed the measurements is described in the succeeding sections .
among other solar flux fts atlases , the _ solar flux atlas from 296 to 1300 nm _
( kurucz et al .
1984 ) , which is available from the noao ftp site , provided us with a high - quality spectrum of the sun seen as a star .
it was obtained at the mcmath telescope at kitt peak .
the fts disc - centre spectrum included in the newer _ spectral atlas of solar absolute disk - averaged and disk - center intensity from 3290 to 12510  _
( brault & neckel 1987 ; for details see neckel 1994 ) was also obtained at the mcmath telescope .
the wavelengths of selected lines from the table of pierce & breckinridge ( 1973 ) , which was produced from observations with a grating spectrometer and a photographic detector at the same telescope , have been the base for placing its wavelength calibration on an absolute scale .
we have measured line central wavelengths in this atlas , available to us as part of the idl kis library . a flux spectrum is also contained in this fts atlas , which shares the source data obtained by j. brault and collaborators with the atlas prepared by kurucz et al .
( 1984 ) .
the atlases cited achieve signal - to - noise ratios of about 2500 and a resolving power @xmath3 400000 .
a quantitative basis for confidence in these atlases has been established by comparison between central wavelengths of 1446 fei lines in the solar spectrum and at rest , performed by allende prieto & garca lpez ( 1998 ) .
briefly , they found :    * the atlases have been corrected for all doppler shifts between the centre of mass of the sun and earth , and the maximum shift to the red exhibited by the lines studied is the gravitational shift : 636 m s@xmath1 , corresponding to a null convective blue shift .
* there is no stronger than expected trend of the line shifts with wavelength . *
a clean correlation exists between the equivalent width and the line shift , reaching a plateau near the gravitational redshift for lines stronger than 200 m .
this work enlarges the measurements of allende prieto & garca lpez ( 1998 ) to many other species , with the aim of optimizing its use for wavelength calibration . the line list from thvenin ( 1989 , 1990 ) , including 6606 lines classified by moore et al .
( 1966 ) as singly blended or unblended , has been chosen as a guide to select the features to be measured .
a fourth - order polynomial was fitted to the 50 m  wavelength interval around the line minimum to find the line centre as precisely as possible . in the electronic version of the atlases
, this corresponds to 11 points for the solar flux spectrum and 25 points for the centre - of - the - disc spectrum .
errors in the wavelength determination were estimated by translating the standard deviation of the fit into the corresponding units of the wavelength axis , neglecting the extremely low photometric noise .
table 1 , available only in electronic form from the cds , lists the central wavelengths in the flux and disc - centre solar spectra , the errors of the measurements , and the element identification , excitation potential and solar oscillator strengths ( @xmath4 ; from thvenin 1989 , 1990 ) for 4947 lines between 3944 and 7960   included in thvenin s list .
more than one line corresponding to the same feature is listed when the identification is not clear from the wavelengths published by thvenin .    a comparison between a limited sample of wavelengths ( 42 lines ) listed by pierce & breckinridge ( 1973 ) and those we have measured in the atlas of the disc - centre shows that the absolute scales agree ( difference : 79 @xmath5 6 @xmath6 10@xmath7  ) and that the relative differences are fully accounted for by a straightforward consideration of the expected errors in the photographic atlas ( @xmath2 2.5 m )
.    errors of the wavelengths measured in the flux spectrum can be as large as 100 m s@xmath1 , as quoted by kurucz et al .
for the wavelengths in the spectrum of the disc centre , systematic errors are given from the comparison with the photographic atlas of pierce & breckinridge ( 1973 ) , which has been used as reference .
they estimate the absolute accuracy from comparison with the interferometrically determined wavelengths of adam ( 1952 , 1958 ) and nichols & clube ( 1958 ) , arriving at 0.3 m .
our systematic errors are then of the order of @xmath8 0.9 m , while fts intrinsic errors are about 10 m s@xmath1 ( neckel & labs 1990 ) , yielding a final precision of about 50 m s@xmath1 at @xmath9 5000  .
these numbers allow us to claim that errors quoted in table 1 ( typically @xmath0 m s@xmath1 ) are conservative .    the present database updates others which were previously available based on photographic spectra , and the catalogue is presented in an homogeneous machine - readable format available via internet from the cds .
the wavelengths measured conform a reference frame which can be used to calibrate or field test calibrations of astronomical spectra by comparing with the solar spectrum , and to set up an accurate wavelength scale for solar spectra , among other applications .
we thank h. neckel for his help in dealing with his solar atlas .
we wish to extend our gratitude to f. thvenin , who has kindly lent a digital copy of his line list , m. collados for fruitful discussions on the wavelength calibration of solar spectra , j. e. beckman for careful reading of the draft and h. h. r. kroll for performing the installation and maintenance of the kis computer libraries at the iac .
nso / kitt peak fts data used here were produced by nsf / noao .
allende prieto , c. , garca lpez , r. j. , lambert , d. l. , gustafsson , b. 1995 , in stellar surface structure , iau symp .
176 : poster proceedings , ed .
k. g. strassmeier ( vienna : institut fr astronomie der universitt wien ) , 107
we present accurate measurements of the central wavelengths of 4947 atomic absorption lines in the solar optical spectrum . 
 the wavelengths , precise to a level @xmath0 m s@xmath1 , are given for both flux and disc - centre spectra , as measured in relatively recent fts solar atlases . 
 this catalogue modernizes existing sources based on photographic measurements and provides a benchmark to test and perform wavelength calibrations of astronomical spectra . 
 it will also permit observers to improve the absolute wavelength calibration of solar optical spectra when lamps are not available at the telescope .