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An instrument for the spectral radiance calibration of light sources in the VUV by means of the synchrotron radiation of the desy machine
NUCLEAR
INSTRUMENTS
AND
METHODS
152 ( 1 9 7 8 )
243-245
,
©
NORTH-HOLLAND
PUBLISHING
CO
AN INSTRUMENT FOR THE SPECTRAL RADIANCE CALIBRATION OF LIGHT SOURCES IN THE VUV BY MEANS OF THE SYNCHROTRON RADIATION OF THE DESY MACHINE E
PITZ and A
SCHULZ
Max-Planck-hlstttut lur Astronomte, 6900 HeMelberg 1, Komgstuhl, B R D
The i n s t r u m e n t consists out of two c o m b i n e d m o n o c h r o m a t o r s with appropriate imaging optics for the s y n c h r o t r o n radiation, for a t u n g s t e n ribbon lamp and for the light source to be calibrated One brach of the i n s t r u m e n t operates in the v~s~ble and determines the electron current of the s y n c h r o t r o n T h e VUV branch is for the cahbrat~on of a hght source by comparison w~th the s y n c h r o t r o n radiation The ~mproved i n s t r u m e n t ~s described and results on the cahbrat~on of d e u t e r m m lamps as transfer standards are g~vcr.
1. Introduction Space astronomy in the vacuum ultraviolet is one of the fields that require absolute radlometrlc cahbratlon of the observmg instruments T he calibration on ground is done by exposing the instrument to hght sources of known output For reasons of flexibility, easy handhng and comfort the light source usually is a " t r a n s f e r standard" which m turn has to be calibrated by one of the fundamental radtometrlc standards hke for instance the hydrogen arc ~) or the synchrotron radtauon 2) The instrument described here is designed for the absolute spectral radiance cahbratton of UV light sources, by comparison with the synchrotron radmtlon of the Deutsches Elektronen Synchrotron, Hamburg (DESY) It operates m the spectral range from 120 ram to about 3 5 0 n m So far the instrument was used tn two calibration periods (1967/68 and 1975) for the calibration of transfer standards as mercury high pressure arcsS), deuterium lamps 34) and xenon discharge lamps ~~) The latest cahbratlon results on deuterium lamps have been lntercompared with the calibration by the National Bureau of Standards, Washington and by the Physlkahsch Techmsche Bundesanstalt, Berhn The results basing on different methods a n d / o r instrumentation agreed within +_3% 4). 2. The instrument The instrument is a combination of two spectroradiometers, one operating m the UV, the other m the v~stble at a fixed wavelength Each consists out of a grating monoc hr om a t or w~th the necessary imaging optics for the synchrotron radiation and for one hght source each The com m on part is the entrance hole to both monochromators (see VII
fig 1) The light sources are imaged by the movable auxlhary mirrors onto the tangential point o f the electron orbit m the synchrotron. Therefore tn each branch - synchrotron and lamp radiation have virtually identical hght paths The visible branch was introduced in order to lmphcttely - determine the number of electrons in the accelerator contributing to the emitted synchrotron radiation This photometric determination of the electron current by the s~mple comparison of the synchrotron radiation with a calibrated tungsten ribbon lamp in the visible seemed to be the most accurate and easy method To overcome any influence of the t~me-dependent fluctuations of the electron current, both branches are used strictly simultaneously, yielding (for each wavelength) a time-independent signal ratio If each branch works hraearlly and has identical sensitivity for synchrotron and lamp radiation, the signal ratios S~v/S~v and SL,,/SSv,, equal the corresponding flux rauos F~v/FSv and b'L,,s/Fv,~s which can be written as
FLv=
F L .FSuv Suv L S,,., S ,.,s FSv~. S ts: v S , ,Ls
In the above equauon the fluxes (F) can replaced by the corresponding intensities (I) since. FSotL At') /SorL UV = "~xa~'UV"UV
and F s°rL --ws
~--
-~
-SoiL
"d'Slv~s/vls
,
where A is the area of the entrance hole and -Quv and .Q,,~ are the respective sohd angles of the two branches Thus the intensity of the lamp to be calIbrated is determined by the intensity of the tungsten ribbon lamp l,t:,s, the ratio lSv/IS,~ gwen by
SPECIFIC
PROPERTIES
OF
SYNCIIROTRON
RADIAlION
244
E
PITZ
AND
SCHULZ
A
grohngs UV ¥1S
~tnchrolt'On beQm
beam stop entrance sift m,rror 2 mirror 3
mirror 1 wffh
d,aphragm
~
UV slit fdter wheel deuterium lamp
/
, v,s sld interference rifler
7 pt~otomultlpher
photomulhpl,er
J
ngsten ribbon lamp Fig I Schematic vtew of the mstrumem It ~s at a distance of about 40 m from the tangenttal point of the electron orbit m the synchrotron The dtaphragm m Front of m t r r o r ] chooses two portions of the synchrotron radmtton beam, symmetncally dmplaced above and below the synchrotron plane One is for the UV, the other m for the vmtb]e spectroradmmeter The b u n d l ~ are Focused into the common entrance hole and htt the,r respecuve graungs The interference filter between exit sht and photomult|pher of the v,slble branch reduces strayhght to pract,cally zero The filter wheel m the UV branch serves to determine the strayhght Alter having reg,stered the synchrotron radtatfon stmultaneously m both channels, mtrrors 2 and 3 are moved into the beam and image virtually the,r respecuve lamps onto the tangential point of the electron orbit, thts way yleldm 8 ~dent~cal beams For lamp and synchrotron radmtlon The beam stop is to keep out of the apparatus most of the hard X-ray and gamma-ray rad~auon wh,ch ,s concentrated m the synchrotron plane
the Schwmger theory 6) and by the four stgnals produced by the mstrument
This mtenstty needs to be corrected for in the followmg respects. - The reflecttvity of the auxtliary mirrors (mirror 2 and 3), whtch affect only the lamp, not the synchrotron radiauon (error ~ 2 % ) , - t h e transmtssion of the vacuum wmdow m front of the tungsten ribbon lamp (error
5%), -the polarizauon characterisUcs of the monochromators whtch cause different transmission for the h~ghly polarized synchrotron radtation g w e n by the Schwlnger theory - as compared to
that for the unpolanzed lamp radiatton (error ~ 0 5%), - strayhght The major improvement of th,s instrument and tts calibration results - as compared to the state of ref 3 - is the better straylight reducUon and cor-
recuon for it These efforts became necessary after it had been shown 7) that previous calibration re, I , I t I , I a I • I a
Intensity of -
lamp [3 15 NO6
/
";E -;c4.5"7
~.-
3.5-
1go
2o'0
22'0
2/0
26'0
28'0 "~[oml
F~g 2 Absolute spectral radtance, of the 0 8 mm d,ameter center spot of the hght dlstnbutaon of deuterium lamp D 15 no 6
SPECTRAL RADIANCE CALIBRATION OF LIGHT SOURCES IN THE VUV
suits 3) suffered from unsufficiently corrected straylight in the spectral range from 250 nm to 270 nm For the 1975 calibration period all nonoptical surfaces of the spectroradlometers and of the vacuum tank have been black painted with 3M Nextel Velvet coating. Additional light baffles have been installed, partially with honeycomb structures A set of longwave-pass filters in the filter wheel behind the exit slit of the UV monochromator was used for the suppression of light of higher orders and for the straylight measurement The straylight correction was largest for wavelengths below 170 nm but did not exceed 1.5%. For more instrumental details see ref. 8 The very good agreement of the new calibration
VII
245
results with those of other laboratories ~) ~mply that the straylight problem is solved Fig. 2 shows as a typical example the calibration result on the deuterium lamp D 15 no 6 References t) W R Ott, P Fleffc-Prevost, W L W~ese, Appl Opt 12. (1973) 1618 :) D Lemke, D Labs, Appl Opt 6 (1967) 1043 3) E P~tz, Appl Opt 8 (1969) 255 4) j M Bridges, W R Ott, E Pttz, A Schulz. D Emfeld, D Stuck, Appl Opt 16 (1977) 1788 5) D Stuck, B Wende, JOSA 62 (1972) 96 6) j Schwmger, Phys Rev 75 (1949)1912 7) W R Ott. J D Bartoe, Annual Meeting Optical Society of America (1972) Th F 13 8) A Schulz, D,plomarbc,t Heidelberg (1976)
SPECIFIC PROPERTIES OF SYNCHROTRON RADIATION
INSTRUMENTS
AND
METHODS
152 ( 1 9 7 8 )
243-245
,
©
NORTH-HOLLAND
PUBLISHING
CO
AN INSTRUMENT FOR THE SPECTRAL RADIANCE CALIBRATION OF LIGHT SOURCES IN THE VUV BY MEANS OF THE SYNCHROTRON RADIATION OF THE DESY MACHINE E
PITZ and A
SCHULZ
Max-Planck-hlstttut lur Astronomte, 6900 HeMelberg 1, Komgstuhl, B R D
The i n s t r u m e n t consists out of two c o m b i n e d m o n o c h r o m a t o r s with appropriate imaging optics for the s y n c h r o t r o n radiation, for a t u n g s t e n ribbon lamp and for the light source to be calibrated One brach of the i n s t r u m e n t operates in the v~s~ble and determines the electron current of the s y n c h r o t r o n T h e VUV branch is for the cahbrat~on of a hght source by comparison w~th the s y n c h r o t r o n radiation The ~mproved i n s t r u m e n t ~s described and results on the cahbrat~on of d e u t e r m m lamps as transfer standards are g~vcr.
1. Introduction Space astronomy in the vacuum ultraviolet is one of the fields that require absolute radlometrlc cahbratlon of the observmg instruments T he calibration on ground is done by exposing the instrument to hght sources of known output For reasons of flexibility, easy handhng and comfort the light source usually is a " t r a n s f e r standard" which m turn has to be calibrated by one of the fundamental radtometrlc standards hke for instance the hydrogen arc ~) or the synchrotron radtauon 2) The instrument described here is designed for the absolute spectral radiance cahbratton of UV light sources, by comparison with the synchrotron radmtlon of the Deutsches Elektronen Synchrotron, Hamburg (DESY) It operates m the spectral range from 120 ram to about 3 5 0 n m So far the instrument was used tn two calibration periods (1967/68 and 1975) for the calibration of transfer standards as mercury high pressure arcsS), deuterium lamps 34) and xenon discharge lamps ~~) The latest cahbratlon results on deuterium lamps have been lntercompared with the calibration by the National Bureau of Standards, Washington and by the Physlkahsch Techmsche Bundesanstalt, Berhn The results basing on different methods a n d / o r instrumentation agreed within +_3% 4). 2. The instrument The instrument is a combination of two spectroradiometers, one operating m the UV, the other m the v~stble at a fixed wavelength Each consists out of a grating monoc hr om a t or w~th the necessary imaging optics for the synchrotron radiation and for one hght source each The com m on part is the entrance hole to both monochromators (see VII
fig 1) The light sources are imaged by the movable auxlhary mirrors onto the tangential point o f the electron orbit m the synchrotron. Therefore tn each branch - synchrotron and lamp radiation have virtually identical hght paths The visible branch was introduced in order to lmphcttely - determine the number of electrons in the accelerator contributing to the emitted synchrotron radiation This photometric determination of the electron current by the s~mple comparison of the synchrotron radiation with a calibrated tungsten ribbon lamp in the visible seemed to be the most accurate and easy method To overcome any influence of the t~me-dependent fluctuations of the electron current, both branches are used strictly simultaneously, yielding (for each wavelength) a time-independent signal ratio If each branch works hraearlly and has identical sensitivity for synchrotron and lamp radiation, the signal ratios S~v/S~v and SL,,/SSv,, equal the corresponding flux rauos F~v/FSv and b'L,,s/Fv,~s which can be written as
FLv=
F L .FSuv Suv L S,,., S ,.,s FSv~. S ts: v S , ,Ls
In the above equauon the fluxes (F) can replaced by the corresponding intensities (I) since. FSotL At') /SorL UV = "~xa~'UV"UV
and F s°rL --ws
~--
-~
-SoiL
"d'Slv~s/vls
,
where A is the area of the entrance hole and -Quv and .Q,,~ are the respective sohd angles of the two branches Thus the intensity of the lamp to be calIbrated is determined by the intensity of the tungsten ribbon lamp l,t:,s, the ratio lSv/IS,~ gwen by
SPECIFIC
PROPERTIES
OF
SYNCIIROTRON
RADIAlION
244
E
PITZ
AND
SCHULZ
A
grohngs UV ¥1S
~tnchrolt'On beQm
beam stop entrance sift m,rror 2 mirror 3
mirror 1 wffh
d,aphragm
~
UV slit fdter wheel deuterium lamp
/
, v,s sld interference rifler
7 pt~otomultlpher
photomulhpl,er
J
ngsten ribbon lamp Fig I Schematic vtew of the mstrumem It ~s at a distance of about 40 m from the tangenttal point of the electron orbit m the synchrotron The dtaphragm m Front of m t r r o r ] chooses two portions of the synchrotron radmtton beam, symmetncally dmplaced above and below the synchrotron plane One is for the UV, the other m for the vmtb]e spectroradmmeter The b u n d l ~ are Focused into the common entrance hole and htt the,r respecuve graungs The interference filter between exit sht and photomult|pher of the v,slble branch reduces strayhght to pract,cally zero The filter wheel m the UV branch serves to determine the strayhght Alter having reg,stered the synchrotron radtatfon stmultaneously m both channels, mtrrors 2 and 3 are moved into the beam and image virtually the,r respecuve lamps onto the tangential point of the electron orbit, thts way yleldm 8 ~dent~cal beams For lamp and synchrotron radmtlon The beam stop is to keep out of the apparatus most of the hard X-ray and gamma-ray rad~auon wh,ch ,s concentrated m the synchrotron plane
the Schwmger theory 6) and by the four stgnals produced by the mstrument
This mtenstty needs to be corrected for in the followmg respects. - The reflecttvity of the auxtliary mirrors (mirror 2 and 3), whtch affect only the lamp, not the synchrotron radiauon (error ~ 2 % ) , - t h e transmtssion of the vacuum wmdow m front of the tungsten ribbon lamp (error
5%), -the polarizauon characterisUcs of the monochromators whtch cause different transmission for the h~ghly polarized synchrotron radtation g w e n by the Schwlnger theory - as compared to
that for the unpolanzed lamp radiatton (error ~ 0 5%), - strayhght The major improvement of th,s instrument and tts calibration results - as compared to the state of ref 3 - is the better straylight reducUon and cor-
recuon for it These efforts became necessary after it had been shown 7) that previous calibration re, I , I t I , I a I • I a
Intensity of -
lamp [3 15 NO6
/
";E -;c4.5"7
~.-
3.5-
1go
2o'0
22'0
2/0
26'0
28'0 "~[oml
F~g 2 Absolute spectral radtance, of the 0 8 mm d,ameter center spot of the hght dlstnbutaon of deuterium lamp D 15 no 6
SPECTRAL RADIANCE CALIBRATION OF LIGHT SOURCES IN THE VUV
suits 3) suffered from unsufficiently corrected straylight in the spectral range from 250 nm to 270 nm For the 1975 calibration period all nonoptical surfaces of the spectroradlometers and of the vacuum tank have been black painted with 3M Nextel Velvet coating. Additional light baffles have been installed, partially with honeycomb structures A set of longwave-pass filters in the filter wheel behind the exit slit of the UV monochromator was used for the suppression of light of higher orders and for the straylight measurement The straylight correction was largest for wavelengths below 170 nm but did not exceed 1.5%. For more instrumental details see ref. 8 The very good agreement of the new calibration
VII
245
results with those of other laboratories ~) ~mply that the straylight problem is solved Fig. 2 shows as a typical example the calibration result on the deuterium lamp D 15 no 6 References t) W R Ott, P Fleffc-Prevost, W L W~ese, Appl Opt 12. (1973) 1618 :) D Lemke, D Labs, Appl Opt 6 (1967) 1043 3) E P~tz, Appl Opt 8 (1969) 255 4) j M Bridges, W R Ott, E Pttz, A Schulz. D Emfeld, D Stuck, Appl Opt 16 (1977) 1788 5) D Stuck, B Wende, JOSA 62 (1972) 96 6) j Schwmger, Phys Rev 75 (1949)1912 7) W R Ott. J D Bartoe, Annual Meeting Optical Society of America (1972) Th F 13 8) A Schulz, D,plomarbc,t Heidelberg (1976)
SPECIFIC PROPERTIES OF SYNCHROTRON RADIATION