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A N I N S T R U M E N T T O M E A S U R E T H E S O L A R S P E C T R U M F R O M 1 7 0 T O 3 2 0 0 n m O N B O A R D S P A C E L A B t

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A N I N S T R U M E N T T O M E A S U R E T H E S O L A R S P E C T R U M F R O M 1 7 0 T O 3 2 0 0 n m O N B O A R D S P A C E L A B t

G. T H U I L L I E R , P. C. SIMON*, D. LABS**, R. P A S T I E L S * , and H. NECKEL***

Service d'Adronomie du CNRS, B P 3-91370 Verri~res-le-Buisson, France

Abstract. This instrument, at the present time in development, will fly on board Spacelab I in May 1983.

Other flights are foreseen during the following missions. This instrument is composed by three double monochromators covering the range 170 to 3200 nm. The spectrometers have band-passes of 1 nm up to 900 nm and 20 nm from 850 to 3200 nm with an accuracy 10 -2 nm. Calibration lamps are included in the instrument to monitor any change of its sensitivity and wavelength scale.

1. Introduction

T h e E a r t h h a s e x p e r i e n c e d s e v e r a l c l i m a t i c c h a n g e s . It has b e e n o b s e r v e d t h a t t h o s e c h a n g e s h a v e d i f f e r e n t a m p l i t u d e a n d t i m e scales.

T h e c a u s e s of c l i m a t i c c h a n g e s a r e n o t c l e a r l y i d e n t i f i e d as w e l l as t h e d i f f e r e n t p r o c e s s e s ( t h e r m o d y n a m i c , a b s o r p t i o n of s o l a r p h o t o n s . . . ) w h i c h act o n a c o m p l e x s y s t e m c o m p o s e d b y t h e s o l i d E a r t h , o c e a n , a n d a t m o s p h e r e .

I t a p p e a r s t h a t for s u c h a s y s t e m , t h e S u n r e p r e s e n t s t h e m o s t i m p o r t a n t e x t e r n a l s o u r c e of e n e r g y . C o n s e q u e n t l y w h a t e v e r a r e t h e f e e d - b a c k p r o c e s s e s e x i s t i n g in t h e E a r t h - a t m o s p h e r e s y s t e m , a m o d i f i c a t i o n of t h e S u n i n p u t s h o u l d l e a d to c l i m a t e c h a n g e s w i t h a t i m e d e l a y t h a t m o d e l s c o u l d p r e d i c t .

S o m e e v i d e n c e s o f v a r i a t i o n of t h e S u n i r r a d i a n c e h a v e b e e n o b t a i n e d m a i n l y at w a v e l e n g t h s h o r t e r t h a n 2 0 0 n m a n d r e c e n t l y o n t h e s o l a r c o n s t a n t .

T h e n , as s e v e r a l p r o c e s s e s a r e w a v e l e n g t h d e p e n d a n t ( p h o t o d i s s o c i a t i o n . . . . ) t h e scientific a i m s of t h e i n v e s t i g a t i o n a r e :

(a) T h e m e a s u r e m e n t of t h e a b s o l u t e s o l a r s p e c t r a l i r r a d i a n c e in t h e w a v e l e n g t h i n t e r v a l 1 8 0 - 3 2 0 0 nm.

T h e r e q u e s t e d a c c u r a c y in g e n e r a l a d m i t t e d for s u c h m e a s u r e m e n t is 5 % a r o u n d 2 0 0 n m a n d 1 % in t h e v i s i b l e a n d I R r a n g e s .

(b) T h e m e a s u r e m e n t of p o s s i b l e l o n g t e r m v a r i a t i o n of t h e s o l a r s p e c t r a l i r r a d i - a n c e s .

I n this case, t h e r e q u e s t e d p r e c i s i o n is 0 . 3 % in t h e v i s i b l e a n d I R , a n d 1 % in t h e U V .

t Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16-19 September 1980, Scheveningen, The Netherlands.

* Institut d'A6ronomie Spatiale de Belgique, 3, avenue Circulaire-B1180 Bruxelles, Belgique.

** Landessternwarte-Koenigstuhl, D6900 Heidelberg, F.R.G.

*** Hamburger Sternwarte, Gojenbergsweg, D2050 Hamburg 80, F.R.G.

Solar Physics 74 (1981) 531-537. 0038-0938/81/0742-0531 $01.05.

Copyright (~) 1981 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

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532 G. T H U I L L I E R E T A L .

Then, the p r o p o s e d m e a s u r e m e n t s are i m p o r t a n t in the three following fields:

(i) A e r o n o m y of the middle a t m o s p h e r e mainly concerned by the solar flux up to 400 nm.

(ii) Climatology concerned by the wavelength range a b o v e 400 nm necessary to establish the radiative budget of the a t m o s p h e r e .

(iii) Solar physics concerned by the full solar spectrum.

T h e requested accuracy and precision imply that the m e a s u r e m e n t s are not achievable f r o m the ground. Consequently, the m e a s u r e m e n t must be p e r f o r m e d f r o m space.

T h e vehicle chosen for this investigation is Spacelab launched by the Shuttle on account of:

- T h e mission is short and limits the time to which the instrument is exposed to space e n v i r o n m e n t which has always a degradating action;

- the duration of the Shuttle p r o g r a m is compatible with a study conducted along a solar cycle;

- t h e return of the instrument p e r m i t a

re-calibration after flight

which is quite necessary to reach the accuracy and precision n e e d e d by the investigation.

2 . T h e I n s t r u m e n t

It includes:

- 3 s p e c t r o m e t e r s ;

- 3 detectors;

- electronics;

- 5 in-flight calibration lamps.

A schematic is given on Figure 1.

2.1. SPECTROMETERS

T h e instrument includes three double m o n o c h r o m a t o r s of 10 cm focal length using concave holographic gratings. Their characteristics are:

- T h e six gratings are m o u n t e d on the same mechanical shaft rotating with a screw and nuts system giving a precision of 2 arc sec at any position, the spectral precision is 10 2 nm;

- the U V m o n o c h r o m a t o r range of m e a s u r e m e n t overlaps with the o n e o f the visible m o n o c h r o m a t o r , s a m e as for vibible and I R m o n o c h r o m a t o r s ;

- t r a n s m i t t i n g diffusors (grinds) are placed in front of the three s p e c t r o m e t e r s ; - filters wheels are placed at the exit slits to r e m o v e the second order signal;

- w a v e l e n g t h range and spectral data:

Monochromators Range Band-pass Lines mm -1

UV 160-365 nm 1 nrn 3600

VIS 277-889 nrn 1 nm 1281

IR 805-3160 nm 20 nm 35 ~,

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A N I N S T R U M E N T T O M E A S U R E T I l E S O L A R S P E C T R U M 5 3 3

/ z-

I

i

/ li

I

i ~ V / / ~ i ~

i

! i

9 9 " 't', /

Fig. 1. Schematic of the complete instrument.

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534 G . T I - I U I L L I E R E T A L .

5UN

GRINO

suN.., I

G 2 S~

. ~ ~

GRI NO

-

~-

MI G 1

I ]o_~o

M 3(15115)(level 31 GI'G2I

Oeteetor (level 3)

s2 / /

Level 3

Level 2

Level I

Level 1

!25

Fig. 2. Schematic of one double monochromator. Three similar units are mounted On the same mechanical shaft.

The principle of one spectrometer is given on Figure 2. A complete solar spectrum from 170 to 3200 nm is recorded in 15 min.

2.2. DETECTORS

U V : Photomultiplier tube (EMR 641F);

VIS: Photomultiplier tube (EMR 641E) cooled at +5 ~

IR: PbS cell cooled at - 1 0 ~ with a Chopper working at 512 Hz.

2.3. IN-FLIGHT CALIBRATION LAMPS

T h e u s e d d e t e c t o r s are n o t a b s o l u t e d e t e c t o r s . H e n c e , t h e i n s t r u m e n t has to b e c a l i b r a t e d . B u t , its s e n s i t i v i t y has to r e m a i n u n c h a n g e d all a l o n g t h e g r o u n d

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AN INSTRUMENT TO MEASURE THE SOLAR SPECTRUM 535

operations before flight (which may be 1 or 2 years long for the first) and during the flight. Then, rather to maintain all the instrument stable in the above situations (which is difficult to achieve) it is easier to maintain stable a calibration lamp, the distance filament to slit and the lamp power supply. This philosophy has been adopted for this instrument.

The following set of lamps is included in the instrument:

Lamp type Spectrometer Purpose Power Number

Tungsten ribbon VIS and I R \ Instrument f 5 W 2

Deuterium U V .( sensititvity ~15 W 2

Hollow cathod UV and VIS Wavelength scale 2 W 1

(filled with He) and band-pass

and has the general characteristics:

- T h e radiating region (ribbon and anode) are imaged by lenses on the input grinds limited bypreslits;

- t h e diameter and magnification of the lenses are chosen so that the signal due to the lamps is of the order of the one due to the Sun;

- l a m p s and lenses are mechanically stable with respect to the entrances slits;

- t h e lamp currents are regulated a t ]0 .4 tO provide 0.1% stability;

- the lamp currents, voltage and temperature are telemetred;

- other characteristics are given in Finkenzeller and Labs (1979).

Consequently, we measure in space and at ground all possible change of the instrument sensitivity. The wavelength scale and band-passes is also determined by use of selected lines emitted by the hollow cathod lamp. In this mode the rotation of the grating is performed by elementary step of 15 arc sec corresponding to 10 -1 nm.

3. Calibration of the I n s t r u m e n t 3 . 1 . A B S O L U T E CALIBRATION

The black body of the Landessternwarte Laboratory is used (3000 K). A linear pyrometer monitors the temperature. This device uses a set a interferential filters and a silicon cell which are temperature stabilized (0.1 ~

The accuracy of the calibration is estimated to be:

A < 2 5 0 n m 5%

2 5 0 < A < 3 0 0 nm 3%

300 < A < 400 nm 2%

A < 400 nm 1%

3.2. W A V E L E N G T H C A L I B R A T I O N

Its purpose is the measurement of the instrument band-passes and the calibration of the wavelength scale.

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536 G. THUILLIER ET AL.

Hollow cathod lamps filled with Argon or Neon are used. Other elements such as Ni, Se, Ru, Hg, Ce, Sn, As, Ba, Te, Si, Bi, Cd permit to cover the range from 170 to 3200 nm.

A tunable laser is also used.

3.3. LONG TERM STANDARD (LTS)

The scientific aim (b) needs a stable reference during some years typically an half solar cycle. This requirement can be achieved by use of lamps since they have the needed stability.

Fig. 3. L o n g T e r m S t a n d a r d box showing the five d e u t e r i u m a n d five t u n g s t e n ribbon lamps. T h e lamps can be m o v e d to light the three e n t r a n c e slits of the m o n o c h r o m a t o r s .

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AN INSTRUMENT TO MEASURE THE SOLAR SPECTRUM 537

The LTS consists of: 10 Tungsten ribbon lamps (W17G from Osram) for h >

250 nm and 10 Deuterium lamps (D60 from Hanau) for h < 250 nm. Five of each are carefully kept in laboratory while the remaining set are on special equipment names LTS box, able to follow the instrument at different sites for integration, preparation to launch and after mission. The LTS box can be placed on the top of the main instrument (Figure 3).

The precision of the LTS is:

(i) 0.2% for 100 hr burning time with alternate DC power supply for tungsten lamps as it appears from measurements from 1960's in the Landessternwarte laboratory of Heidelberg.

(ii) 3% for 100 hr burning time with deuterium lamps (Bridges et al., 1977).

But any change of their irradiance can be monitored by comparison with the tungsten ribbon lamp is the range 280-350 nm.

The set kept in laboratory will not be used more than 10 hr per year.

The sequencing of calibration and LTS use is given on Figure 4, where it appears that the LTS box will be calibrated against the black body using the space instrument itself.

CALJl '/

BB J1

//'

LTS .J1 F t ~ L

ORBIT

"

",,3 wgeks

.~I0 days

n rl

~ . ,, fl,

." 2 day_s - 1 - i m e

Fig. 4. Sequence of calibration and use of LTS box. h Cal: Band-pass and wavelength calibration.

BB: Black body calibration. LTS: Long Term Standard. ICL: Use of In-flight Calibration Lamps.

3 . 4 . C O M P A R I S O N S

Comparisons before flight with the two absolute radiometers also on board Spacelab I is foreseen.

Other comparisons with calibrated source as the ones existing at the National Bureau of Standard are desired.

3.5. A C C U R A C Y AND PRECISION Accuracy has been given in Section 3.1.

The final precision of the instrument is estimated to be 0.3% taking into account the counting rate, the stability of the in-flight calibration and LTS lamps.

R e f e r e n c e s

Bridges, J. M., Ott, W. R., Pitz, E., Schultz, A., Einfeld, D., and Stuck, D.: 1977 AppI. Opt. 16, 1788.

Finkenzeller, U. and Labs, D.: 1979, Appl. Opt. 18, 3938.

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