an echelle spectrograph for high resolution studies of the solar vacuum ultraviolet spectrum

A N E C H E L L E S P E C T R O G R A P H F O R

H I G H R E S O L U T I O N S T U D I E S O F T H E

S O L A R V A C U U M U L T R A V I O L E T S P E C T R U M

B. C. B O L A N D and B.B. J O N E S

Astrophysics Research Unit, Culham, England

and

S. F .T . E N G S T R O M

Stockholm Observatory, Sweden

(Received 23 October, 1970)

Abstract. An echelle grating spectrograph has been flown in Skylark sounding rockets to investigate the solar ultraviolet spectrum. The instrument can record a range of ~ t000/~, within the limits ,~ 1000 A-23000 A, with a spectral resolving power of 105, and its use in the range 21200-2 2200/~ is described.

Values are quoted for the spectral efficiency of the individual optical components, and for the complete instrument over a wide wavelength range, and a description is given of the method used to estimate and then to discriminate against the in-flight stray light when the solar disc is imaged onto the entrance slit. The mechanical arrangement of the components is described and some of the problems associated with the alignment and operation are discussed.

1. Introduction

Investigations of the solar spectrum above 21200 A have been carried out in recent years by numerous groups (Behring et al., 1958; Tousey et al., 1964; Black et al., 1965) with many experiments at medium spectral resolution. High resolution observations have been more restricted, with spectra above 22100 A published by Tousey et al.

(1967) while below 22100 A a few high resolution spectra have been published by a number of authors (Purcell and Tousey, 1960; Bruner and Rense, 1969; Berger and Bruner, 1969; and Boland et al., 1970, 1971). Much work remains to extend the record of the solar spectrum below ).2000 A at high spectral resolution, and since these observations must of necessity be carried out using either sounding rocket or satellite vehicles the physical size, shape and stability of the high spectral resolution instrument involved is an important factor.

The development of the echelle diffraction grating by Harrison (1949) has provided the means of achieving high spectral resolution whilst maintaining a compact stable mounting suitable for use in a space vehicle. Bruner and Rense (1967) have reporte an echelle spectrograph having a limited spectral range (~ 150 A), used successfully by them in the vacuum ultraviolet region on a number of rocket flights. The present paper describes a simple mounting for an all reflecting echelle system with photographic recording, combining high spectral resolving power (2/A,~ ~ l0 s) with a broad spectral range (~ 1000 A), which has been used successfully on flights of Skylark sounding rockets to observe the solar spectrum between 21200/~-22200 A.

Solar Physics 17 (1971) 333-354. All Rights Reserved Copyright �9 1971 by D. Reidel Publishing Company, Dordreeht-Holland

334 B.c. BOLAND ET AL,

2. Instrumentation

2.1. OPTICAL SYSTEM

The spectrograph utilizes the high dispersive power of an echelle grating crossed with the lower dispersion of a concave grating, the latter acting as an order sorter to separate the overlapping echelle orders. The concave grating is used in a Wadsworth configu- ration and the echelle in an Ebert type mounting. The layout of the optical system as it is mounted in the rocket payload is shown in Figure 1.

Light from the Sun falls initially on to a collector mirror which is external to the spectrograph, and is then focussed to form an image of the solar disc on the entrance slit. Light in the solar image passes through the slit, is made parallel by the collimator mirror and directed to the echelle which is mounted with its grooves parallel to the slit and which is tilted slightly to reflect the beam on to the concave diffraction grating. The rulings of the concave grating are at right angles to those of the echelle (and to the slit) and the spectrum is formed in a two dimensional array in the focal plane of the grating to be recorded on film in the camera, which can be programmed to take four separate exposures during the rocket flight.

The details of the optical components are indicated in Figure 1 and refer to a setting and optimisation for the wavelength range ~ 1200 A-22200 A. The echelle is a replica of part of a larger Bausch and Lomb master grating, and has dimensions of 10 c m x 6 cm x 2 cm with a ruled area of 9.5 c m x 5.5 cm. The concave grating is used with its blaze peak operating at )~ 1200 ~ and with the wavelength 21756 A falling on the normal to the grating about one third from one end of the film. Although the grating has a radius of curvature of 1 m, since it is operated in the Wadsworth mode in parallel light the spectrum is formed at approximately half this distance at a focal length of very nearly 50 cm.

The collector mirror forms a solar image of 4.7 mm diana and is controlled by a servo system (Black and Shenton, 1966) to maintain the image position relative to the slit, which is typically 1 mm long and 0.02 mm wide. The entrance slit dimensions therefore correspond to 6.8 arc rain and 8 arc sec at the Sun, although the resolution in the entrance slit image is limited by astigmatism to 17 arc sec if a spherical mirror is used. Provision is made to programme the servo during flight to preselected positions with a setting accuracy of I arc rain. This feature can be used to investigate active and quiet regions on the Sun or orbtain limb and disk spectra during one flight.

The reflecting surfaces of the collector and collimator mirrors are chosen to suit the experimental requirements, and combinations selected from uncoated silica, G e + Z n S (Hass and Tousey, 1969) and A I + M g F z have been used, the factors influencing the choice being overall spectral efficiency and the elimination of stray light within the spectrograph. These aspects are discussed more fully in later sections.

The disposition of the spectrograph components and the servo controlled collector mirror system when mounted in body-parallel sections of the Skylark rocket is shown in Figure 2.

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Fig. 2. The echelle spectrograph, showing the disposition of the components when mounted in parallel sections of a Skylark rocket.

AN ECHELLE SPECTROGRAPH FOR HIGH RESOLUTION STUDIES 337

2.2. FOCAL SURFACE

The spectrum is formed in the focal plane of the concave grating in an array where the relationship of wavelength to line position on the film is two dimensional with the echelle and concave grating dispersions varying slowly and independently as the wavelength changes. With the direction of the slit at right angles to the concave grating rulings we are here concerned with the vertical focal curve of the grating in the Wadsworth mounting, i.e. the curve which produces focussed images parallel to the slit. This focal curve is symmetrical about the grating normal and is given in expressions developed by Beutler (1945) as

P(vz) = R/cos c~ + cos fl

where P(v~) is the distance from the pole of the grating to the position 2 on the curve for vertical focus, R is the grating radius of curvature and c~ and fi are the angles of incidence and diffraction respectively.

The curve for horizontal focus i.e. focussed images at right angles to the slit, is given by

P(~) = R cos 2 ]3/cos a + cos ]3.

On the grating normal (]3 = 0), P(vz)= P(,~). Figure 3 shows the shape of the verticle focal curve (in the equatorial plane) in the

direction of the concave grating dispersion for the wavelength range 21200 A-2 2200 A, compared with a fiat focal surface placed in a mean position, and the curve for the horizontal focus. The film register surfaces in the camera have been machined to conform to the theoretical vertical focal curve and adjustments are incorporated to enable a focus to be obtained over the whole surface of the film. The curvature in the focal plane at right angles, i.e., in the direction of the echelle dispersion, is small over the width of the film and has been ignored in the construction of the camera. The exposed area of film measures 6.6 cm • 2.2 cm and records approximately 1000 of spectrum, composed of a varying number of echelle cycles depending on the wavelength range under observation. For the present region, 21200 A-22200 A, there are 101 echelle cycles, the length of each cycle in wave numbers being 401 cm-1 which gives a free spectral range of 5.8 A at 21200 A; 9.0 A at 21500 A and 16.1 A at 2 2000 A.

2.3. DISPERSION

The reciprocal linear dispersion (subsequently termed dispersion) of the concave grating in the focal plane is 16.1 A/mm but this serves only to separate the echelle orders and we are principally concerned with the dispersion of the echelle. In this mounting arrangement the echelle dispersion at wavelength s is given by Harrison (1949) as

d2 2

dl 2rP

338 B.C.BOLAND ET AL.

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1400

1600

1756

1800

2000

2200

Ps Pv PH

GRATING NORMAL

I I I I I 50'40 50"30 50"20 50"10 50"00

DISTANCE FROM POLE OF GRATING cm

Fig. 3. The profile of the curve for the vertical focus Pv, in the equatorial plane of the concave grating, compared with the horizontal focus PH and a fiat focal surface I s . The horizontal scale is in cm with 0 cm at the pole of the grating. The vertical scale is in/~, relating wavelength with position

on the film in the frame of the concave grating dispersion.

where r = ratio of groove depth to step height = 2 and P = focal length of the concave grating ~50 cm.

For the present instrument

d2/dl = 0.60A/mm at 21200A

d 2 / d / = 0.75 A/mm at 21500 A

dZ/d /= 1.00A/mm at 22000A

2.4. RESOLUTION

The resolution of the system can be affected by a number of factors, including aberrations of the echelle and concave grating, the slit width, the grain size of the photographic emulsion, and also temperature effects within the instrument.


With the concave grating illuminated by parallel light, it can be shown using the expressions developed by Beutler (1945) that astigmatism is zero on the grating normal and contributes only a small extension to the length of the spectral line at the extremities of the film. Coma is also small in the equatorial plane, increasing towards the ends of the film diagonals, although not to an extent where it becomes the limiting factor in the resolution.

The echelle affects the resolution through the equality of its ruling and an investi- gation (Learner, 1968) of the wavefront curvature from the echelles we are using shows this to be slightly S-shaped resulting in a small comatic type of aberration. The theoretical resolving power 2/A 2 of the echelle for wavelength 2 is given by Harrison (1949) as

2 2Wsin c~

A2 2

where W= the ruled width of the echelle and e = angle of incidence. In this instrument,

2 - - = 1 . 4 4 x 106 at 21200,~ A,~

2 - 1 . 1 6 x 106 at 21500A

A2 and

2 - 0 . 8 7 x 106 at 22000A

AX

However, with the photographic emulsions in current use, Kodak-Path6 S.C. types and Kodak 101-01, the developed grain size is in the region of 0.020 mm, which at 22000 A (d2/dl= 1.0 •/mm) limits 2/A2 to ~ 105, far below the theoretical figure, and this limit applies over the whole wavelength range. The slit width is therefore adjusted to match the emulsion grain size and is operated at the widest setting which does not cause a deterioration in the resolution. Figure 4 shows a laboratory molecular spectrum covering the range 21200 A-)~2200 A, using a microwave source with a filling gas of CO + air at a pressure of 0.40 torr. This type of source is used to give a well populated spectrum over the whole format of the film to facilitate the focus procedures. Figure 5 compares profile widths from this source with a broadened atomic line in the same wavelength region (NI 21494/~), from a laboratory hot cathode source. The widths of the molecular lines are similar to the photographic grain fluctuations on the photometer trace. Over the full wavelength range, Figure 6 indi- cates the quality of profile shape which can be expected from the echelle. The profiles in this figure are all source broadened and do not therefore show the full instrumental resolution.

Effects of temperature changes during a rocket flight were estimated for the structure and for the optical components. Of these the most important are structural


effects, including those which alter the focal separation between the concave grating and the film plane in the camera. To maintain this focal distance within the depth of focus of + 0.12 m m requires that the average temperature change in the structure separating the components should not exceed _+4~ after the final focus setting. To ensure that the experimental exposures are made under the opt imum conditions this necessitates the final focussing procedures to be carried out at the Woomera launch site where temperatures are often far in excess of those in normal laboratory conditions.

ECHELLE SPECTRA [CO & Air 400microns]

Echelle Echelle Dispersion Dispersion 0.6 ~/mm 1.0 ~/mm

Fig. 4.

> Grating Dispers ion

16 "6 ~ /m.

ASTROPHYSICS RESEARCH UNIT CULHAM

The spectrum from a laboratory microwave excited source with a filling gas of CO + air at a pressure of 0.4 mm Hg. Exposure time 2 min. Kodak 101-01 emulsion.

During flight, temperature changes in the instrument are monitored by three thermistors which are positioned (a) on the upper flange adjacent to the collimator and concave grating mounts, (b) on the central flange which carries the slit and camera mountings and (c) at the end of the tubular arm which supports the echelle assembly (Figure 2). Loss of resolution has been noted for long exposures ( ~ 100 sec) during flight and the cause has been traced to thermal effects in the central mounting flange. These effects will be mentioned in more detail in a later section.

2.5 . SPECTRAL EFFICIENCY

With three internal reflexions plus one external at the collector mirror the overall spectral efficiency is a prime consideration. Measurements were made of reflexion efficiency on all of the individual optical components with the exception of the concave grating. Collector and collimator mirrors with different reflecting surfaces were measured over the range 21000-24500 A and the results are shown in Figure 7.

A N E CH E L L E S P E C T R O G R A P H F O R H I G H R E S O L U T I O N S T U D I E S 341

Three echelle gratings were measured for efficiency (Burton and Reay, 1970) over the

same wavelength range and the curves are shown in Figure 8. The curves labelled

A and B in Figure 8 were for echelles from the same master grating Bausch and

Lomb No. 180, both coated with A I + M g F 2. Echelle B however had been cleaned by the makers after an accidental exposure in the laboratory at Culham to silicone vacuum pump oil. The deterioration in efficiency is evident below 13000 A. The

echelle labelled 156 was from a different master, Bausch and Lomb No. 156, and was

also coated with A1 + MgF2. It showed superior efficiency and exhibited a very much lower inter-cycle background scatter, with very low ghost intensity at all wavelengths.

The reflectometer apparatus was not suitable to carry out measurements on the

concave grating, and therefore a measurement of the overall efficiency of the internal

spectrograph optics was made at Ly-~ Z 1216 A, using an ionization chamber with

ECHELLE LINE PROFILES

NI X1494 DENSITY 0.35

Fig. 5.

H 2 ~ k 1500 DENSITY 0-36

NO ~ X 1700 DENSITY 0"30

CULHAM LABORATORY

A broadened atomic line of NI 21494 ~ compared with narrow molecular lines from the microwave excited source. The spectral resolution is limited by the grain size of

the photographic emulsion.


nitric oxide filling gas and a lithium fluoride window. The light source was the microwave excited cavity, operated by a 2450 MHz microwave generator, with a filling gas of 75 ~ He + 25 ~ H2. The H 2 molecular spectrum was largely suppressed within the sensitivity range of the counter (21050 A-21340 •) and the source was considered to be monochromatic at Ly-c(. The ionization chamber was first placed immediately behind the entrance slit to monitor the incoming radiation, and was then transferred to the Ly-c~ image position in the focal plane of the concave grating. The measurement gave a figure of 2.5 x 10-3 for the overall efficiency of the internal optics, using echelle No. 180 A, with an A1 + MgF 2 coated collimator mirror and the A1 + MgF2 coated concave grating. Combining this figure with the measured efficiencies for the echelle and collimator mirror yields an efficiency of 37~o for the concave grating at Ly-a 21216 A. The concave grating is blazed for ). 1200 ~ and its efficiency might be expected to fall towards longer wavelengths, but by not more than 50 ~ at 22000 (Burton et aL, 1968), thereafter remaining approximately constant up to 24500 A. Using these measurements it was possible to construct curves showing the overall efficiency of the complete optical system between 21200 A-24500 ~ for alternative coatings of the collector

ECHELLE LINE PROFILES

H6

NI "X 1200.23 DENSITY 0.23

I

NI ~, I199.55 DENSITY 0-30

Fig. 6.

Ly (~X 1216 DENSITY 050

NI ~, 1494 DENSITY 0.35

J

Cu I ~ 2199"75 DENSITY 0.35

Cul X 2199.5B DENSITY O.18

~0"1

Culham Laboratory

Densitometer profiles of atomic lines from laboratory sources. The lines are source broadened.


and collimator mirrors in association with echelle No. 156 and the concave grating, both of which were coated with A1 + MgF v An estimate could now be made of the number of solar emission lines observable at a useable photographic density in exposure times of 100 sec and 200 sec, using published data for the photographic emulsion sensitivity (Burton et al., 1968) and for the solar line intensities (Aboud et al.

1959; Friedman, 1963; Tousey, 1964). These lines are listed in Table I using Kodak- Path6 S.C.-7 as the recording film.

2.6. STRAY LIGHT

Previous investigators of the rocket ultraviolet solar spectrum have been concerned to exclude the much higher intensity unwanted longer wavelength ultraviolet and visible radiations f rom entering the spectrograph slit and being scattered internally. This situation is no easier in an echelle instrument with multiple reflections taking place within the spectrograph. The methods used to overcome the stray light problem have varied according to the experimenters requirements and have included pre- dispersion gratings as collectors, (Tousey et al., 1964; Brunner and Rense, 1967):

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Fig. 7. Measurements of reflexion efficiency for different mirror surfaces in the wavelength range 21200-24500 •. The reflection minimum obtained with the Ge + ZnS coating can be positioned with

respect to wavelength by adjusting the thickness of the Ge coating.

344 B.C. BOLAND ET AL.

zero dispersion double monochromators (Tousey et al., 1967); and servo control of the collector mirror (Black et al., 1965) to offset the solar image with respect to the slit, to achieve limb instead of disc pointing. None of these solutions was appropriate to the present experiment and the method employed relied on a careful arrangement of the internal light baffles combined with discrimination against longer wavelengths by means of selectively reflective surfaces for the collector and collimator mirrors.

The echelle and concave grating, both coated with A1 + MgF 2, are constant factors once they have been efficiently light baffled, and the film area is also screened so that the only radiation reaching the film must come from the final optical surface, which is the concave grating. The choice of surfaces for the mirror components determines firstly the composition of the radiation entering the slit from the collector mirror, and secondly how this is modified by the collimator for onward transmission to the echelle. The other factor to be considered is the film sensitivity in the range 21200 A -22200 A compared with its sensitivity to longer wavelengths particularly in the region 2 3000 ~ - 2 4500 A where the solar flux is high. Above ~ 2 4500 A the emulsions in current use, S.C.-5, S.C.-7 and 101-01, become relatively insensitive.

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F i g . 8. Measurements of echelle efficiency normalized to peak blaze in the wavelength range 2 1 0 0 0 / ~ - 2 4 5 0 0 / ~ . All three echelles were coated with A1 + M g F ~ ; e c h e l l e B was measured after

being cleaned by the manufacturer.


A n estimate of the effect of the residual stray light dur ing the flight experiment can

be made using ground based tests. Util izing the curve showing the dis tr ibut ion of

radia t ion intensity in the solar spectrum between 22000 •-24500 ~ (Allen, 1963)

and mult iplying by the atmospheric t ransmiss ion coefficients and by the reflection

coefficients of the collector and coll imator mirrors over this waveband gives the

spectral intensi ty dis t r ibut ion of the energy reflected from the col l imator to the echelle

for condit ions of clear atmosphere and small solar zenith angle, such as are obtainable

at the Woomera launch site, so that, if

G = a geometric factor for the spectrograph and external optics

I a = solar intensity incident on the Earth 's a tmosphere in erg c m - 2 s e c - 1 / k - 1

z 2 = atmospheric t ransmiss ion coefficient

Rz = collector efficiency

rz = coll imator efficiency

T = exposure t ime in seconds

then the flux reflected f rom the col l imator to contr ibute a fogging componen t on the

TABLE I

Solar emission expected to be observed at a photographic density of 0.3 or greater between 21200/~-22000 A with

the present instrument

Ion 2(/~) Predicted exposure sec

Si 1ii 1206 * 100 H2Ly-a 1216" 100 O I 1304 * 200 OI 1306" 200 CII 1335" 100 CII 1336" 100 Siiv 1394" 100 Sitv 1403 * 200 Sin 1427 200 Si~I 1533 * 200 C IV 1548 * 1 O0 CIV 1551 * 100 CI 1560 100 CI 1561 200 Hen 1640 100 CI 1656" 100 CI 1658 200 AI 1i 1670 200 Sill 1808" 100 Sill 1817" 100 Si In 1892 * 1 O0

* The lines marked with an asterisk have already been observed at reduced photographic density and spectral resolution (see text).


film during a ground based exposure with the instrument pointed at the Sun is

2 4500 F = GT ~ I~z~R~r~ d2.

2000

For the flight exposure similar information is obtained by omitting the atmospheric transmission coefficient -c2 from the above expression. This information, considered with two other factors, (a) the relative sensitivities of the available photographic emulsions in the range 21200 A-22200 A, and 22200 A-24500 A, and (b) the efficiency of a particular mirror coating combination in the region 21200/~-22200 A enables a choice to be made of mirror coatings and emulsion type which will provide the optimum signal to noise ratio for a flight exposure. For the particular combination of mirrors and emulsion chosen a series of exposures can be made on the ground using the instrument in the flight mode with the sun imaged on the entrance slit. The density/exposure curve is constructed for the background fog level on the film and with the assumption that this scales with F an estimate can then be made of the fog level, which will occur for a particular exposure time during flight. Figure 9 shows

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LOG EXPOSURE SECONDS

Fig. 9. The curve shows the predicted background fog level on the film during the flight exposures, and the crosses are the levels actually recorded in exposure times of 10, 25, 50 and 100 see. The collector mirror was uncoated silica and the collimator mirror was coated with A1 + MgF2. The

emulsion was Kodak 101-01.


the predicted curve of background fog density for flight exposures and also the levels

actually recorded for exposure times of 10, 25, 50 and 100 sec during a recent flight, using an uncoated silica collector mirror, an A1 + MgF 2 coated collimator mirror and

Kodak 101-01 emulsion. The observed fog level was uniform over the whole area of the film, and f rom the curve it can be predicted that longer exposures can be achieved with an acceptable level of background fog density.

On another flight coatings of Ge + ZnS were used on both mirrors. These were produced with displaced wavelengths of minimum reflectance (see Figure 7) so as to achieve maximum discrimination against the longer wavelengths. The in-flight fog levels recorded with this combination were similar to those shown in Figure 9.

3. Mechanical Design

The construction of the instrument has been carried out to achieve sufficient stability

to survive the various transport requirements, including the launch phase of the rocket vehicle. Since the instrument is expensive to design, manufacture and adjust, some

thought has also been given to recovering it in a workable condition after flight, given a successful parachute deployment, and a small weight penalty is involved in order to provide the necessary protection.

The complete spectrograph, together with the servo and electronic equipment is enclosed in parallel body sections of a standard Skylark rocket, and since the nose cone is jettisoned in flight, protective hoops are incorporated in the nose cone space to absorb most of the landing shocks at the front end. This system has proved successful, and both spectrographs flown to date have been recovered in full working condition, with the pre-flight resolution intact.

The instrument is designed so that all of the components are either mounted on or supported f rom a central flange. The slit and camera units are attached directly to the flange, the echelle is mounted at the end of a tubular arm beneath the flange and the collimator and concave grating are positioned side by side on an upper flange. This is supported from the central flange by a tripod arrangement in which three pillars are machined accurately to length to maintain parallelism between the upper and central flanges.

The optical components and the camera unit are supported on similar mountings which include all the lateral and axial adjustments necessary for positioning the components and focussing the spectrograph. Figure 10 shows the type of mounting used, in this case for the concave grating. The grating is restrained in its seat by a frame and lateral pressure pads. At the rear of this assembly and integral with it is machined part of a convex spherical surface, together with a central protruding stem. The convex surface seats into a mating concave member which has a central hole to allow the stem to pass through. By operating on the stem with adjusting screws the convex surface can be made to slide over the concave surface and rotate the grating about the lateral axes, with the centre of rotation at the pole of the grating. It is secured in the correct position by the action of a nut on the threaded end of the stem

348 8. c. BOLAND ET AL.

4

Fig. 10. Section showing the concave diffraction grating mounting. The other components are similarly mounted, with rotational adjustments about the longitudinal and the two lateral axes, and

a translational adjustment along the longitudinal axis.


which pulls the convex surface hard into the concave socket. The pairs of adjusting

screws operate in machined keyways, shown in Section A - A in Figure 10, to eliminate co-operative movement about one axis due to adjustments made about the other axis. Rotation about the optic axis is by a simple screw adjustment on the front housing, and translation along this axis is made by large nuts operating on the threaded barrel which is used to attach the unit to the spectrograph body. Rotation of the assembly is prevented during the translation adjustment by a pin which is engaged in a machined

keyway cut axially along the threaded barrel. All the component mountings are on the same principle and differ only in size.

The four exposure camera shown in Figure 11 has been designed to link the shutter mechanism with the film transport system and to enable comprehensive testing of the camera operation to be carried out after the spectrograph has been installed on the rocket. It is also built to withstand a hard landing in the event of parachute failure. The system is motor operated and requires no manual resetting of the shutter after operation. A geneva mechanism is used to close the shutter, index the film and open the shutter for each of the four exposures in a continuous sequence, which in flight is controlled by signals from a master timer unit. Telemetry monitoring of the exposure in progress and of all intermediate camera positions is included.

The entrance slit unit has one fixed and one moveable blade, the moving blade having V-shaped ends which run on roller guides. The operation of the blade is by a cam mechanism which is spring loaded to avoid any backlash in the adjustment. I f necessary the slit can be opened to a width of ~ 1.5 m m for cleaning and then closed to repeat any original width setting to within _+0.001 mm with parallelism also maintained.

For the flight experiment the position of the solar image relative to the slit is adjusted with the aid of a viewing microscope attachment. The microscope field of view contains a scale in which one division is equal to one arcminute on the solar

image, and the setting can be made to ~�89 division accuracy by adjusting the servo controlling the collector mirror. This setting is maintained by the servo system during the rocket flight within an error limit of _ 3 arc sec peak to peak variation.

The total weight of the experimental payload is 165 lbs and a recent Skylark flight achieved a rocket apogee of 203 km giving a total useful exposure time of about 4 rain after the triaxial stabilisation system (Cope, 1964) had completed solar acquisition.

4. Alignment and Operation

The laboratory alignment and focussing procedures were carried out with the spectrograph in a vertical vacuum tank with a light source at the top of the tank and the collector mirror at its base. The initial approximate focus of the camera was carried out in atmosphere using the long wavelength end of the range ~22000 A. This could be achieved quite quickly since the adjustments were readily made. For the final focus over the complete range it was necessary to make each test exposure with the instrument in vacuum and this made the procedure rather lengthy, with the

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AN ECHELLE SPECTROGRAPH FOR HIGH RESOLUTION STUDIES 3 51

period required from the installation of the optics to final focus being of the order of three months.

Precautions were necessary to exclude dust from the optical system at all stages. With the spectrograph operated in a vertical position, the echelle formed a horizontal surface on which dust would readily accumulate, and to keep the component clean a motor driven flap shutter was in position over the echelle surface at all times except during an exposure. The shutter was removed during the pre-launch preparation sequence.

Prior to launch the films were loaded into the camera, the optical covers were removed and the assembled payload was placed in the vacuum tank and pumped over an extended period to out-gas the complete structure, and remove all traces of water vapour. After flushing with dry nitrogen followed by further pumping, the instrument was raised to slightly above atmospheric pressure in dry nitrogen and the gas flow was maintained during transport from the preparation laboratory to the launcher and was continued until a few minutes before firing.

Two flights of the experiment have been carried out, and on each occasion four exposures were made, in the sequence 10 sec, 25 sec, 100 sec and 50 sec, commencing at a height of ,-, 140 km and continuing through apogee to ,-, 140 km on the descent. Solar spectra were recorded on all exposures with a maximum background density of D 0.18 on the 100 sec exposures.

For the first flight (April, 1969) the collector mirror was coated with Ge. ZnS and during the launch operation the mirror was accidentally contaminated so as to reduce its efficiency below N22000 •, limiting the recorded spectrum to the range 22000~- 22200 N (Boland et al., 1970). For the second flight (April, 1970) the collector mirror was uncoated silica and the collimator was coated with A1 + MgF2. The spectra covered a much more extensive wavelength range, the Fraunhofer spectrum being extended below 2 2000 A and a number of emission lines observed, including hydrogen Ly-c~, 21216 A. However examination of the data showed changes in the spectral resolution during flight. The 10 sec and 25 sec exposures maintained the pre-flight performance of 0.025 A but the following 100 sec exposure had degraded to approximately 0.10 A, whereas the final 50 sec exposure had improved to 0.033 A. The reason for the de-focus of the later exposures was mentioned earlier as being due to thermal effects in the central flange from which all of the components are mounted. This flange also serves to connect the instrument mechanically to the rocket body and is the only part of the spectrograph in contact with the body, and can therefore be heated by conduction from the rocket skin which undergoes frictional heating during passage through the atmosphere. The thermistor information shows that the temperature of the flange rises slowly but continuously throughout the experimental period of the flight. Two effects stem from this heating; firstly the flange itself expands and since the slit is directly mounted to it the position of the slit with respect to the other optical components is changing continuously, resulting in a smeared image which is evident on the longer exposures. Secondly, the three pillars supporting the upper flange carrying the collimator and concave grating are heated by conduction from the central flange,


and because of the irregular shape and mass distribution of the central flange the pillars are not heated evenly and introduce a progressive tilt between the upper and central flanges which effectively moves the position of the spectral line image formed by the concave grating, which is the focussing component, and contributes a further smearing of the image. These effects have been reproduced in the laboratory in the recovered flight instrument by applying similar temperature levels and gradients to those experienced during the flight exposures.

During the first two exposures of 10 and 25 sec the temperature rise in the flange was small and within the short exposure times the resolution was maintained at the pre-flight value. Through the 100 sec exposure the flange temperature increased by ,,~7,0~ and the resolution was degraded. In the latter stages of the flight the rate of temperature rise decreased significantly with an improvement in the resolution during the 50 sec exposure.

In Table I the lines marked with an asterisk * were observed on the 100 sec exposure on the second flight at reduced photographic density due to the defocussed condition. The background fog level in this exposure enables the prediction that a stable 200 sec exposure would have an acceptable fog level, with almost all of the lines in Table I being recorded at useful densities. Valuable information was obtained from the 10 sec, 25 sec and 50 sec exposures on this flight and the analysis is proceeding and will be published separately. Figure 12 illustrates part of the 25 sec exposure, showing the photospheric spectrum below 22100A, through the intensity discontinuity at

Fig. 12.

SKYLARK SL803 APRIL 7th 1970

ASTROPHYSICS RESEARCH UNIT

Part of the 25 sec exposure recorded on a recent flight of the echelle instrument. The spectral resolution is 0 .025 /~ . The emulsion is Kodak 101-01.


~22087A, with absorption lines clearly visible on the film down to ~21900A. The figure also shows the Sin 3s 2 3p-3s 3p 2 emission line triplet at 2.1808 A_ and

21817 A which is being used with other emission lines recorded during the experiment to determine the thermal and turbulence components contributing to the observed ion energies (Boland et al., 1971). The data reduction and analysis of the photospheric spectrum is complete (Boland et al., 1970) for the first flight and covers the range 2 2000 A-2 2200 A. This is being repeated over the extended wavelength range available from the second flight.

5. Future Work

High resolution solar spectral data below ,~2000 A are being accumulated slowly, at a much lower rate than the initial low resolution observations, and a great deal of experimental work is still required even to produce reliable unambiguous data on the twenty or so strong emission lines within the range 21200-22000 ~.

With the present instrument the immediate task is to complete the thermal insulation modifications to remedy the mid-flight defocus observed on the later exposures. A longer exposure is then possible with acceptable levels of stray light background to record the majority of the strong lines in the range, and a further rocket flight to achieve this is being planned.

For shorter wavelengths the situation is rather different. There seems to be no technical reason to prevent the use of an echelle, as an optical component, to obtain a spectral resolving power 2/A2 approaching l0 s below 21200 A, but major problems of efficiency arise due to the reduced reflectivities of available materials. As an example we consider the present instrument with A1 + LiF coatings on all optical surfaces, in which case it is estimated that an exposure of ~ 1.6 x 103 sec would be required to record hydrogen Ly-/~ 21026 A at a useable photographic density ( ~ DO. 3 to DO'5) . For OvI 21032 ~ the required exposure would be ~0.5 x 103 sec and for Cm 2977 A, ~ 5 x 103 sec. The experimental time available during a rocket flight will not be sufficient in the forseeable future to satisfy these requirements, and methods of solving the attendant stray light problem will worsen the situation. For even shorter wavelengths the best reflecting materials (e.g. Pt or Au) also result in prohibitive exposure times. Future high resolution solar experiments in the region below ~)~ 1200 A will therefore require non-photographic primary recording, although the final data could still be recorded on film. Possible forms of detector could be one of the many types of image tubes, channel array plates or photoelectric scanning using the more conventional photoelectric detectors. These devices have the advantages of high system gain and insensitivity to the longer wavelength radiation. They are also suitable for use in satellites as well as in rockets and are therefore able to take full advantage of the extended observation times available in the former vehicles.

Acknowledgements

We are grateful to the members of A.R.U. Projects Group at Culham for their

354 B.C. BOLAND ET AL.

assistance in the engineering and manufacture of the instruments, and to Mr. P. Monk for experimental support at all stages. We are also indebted to Mr. W. M. Burton and Dr. N. K. Reay for permission to use Figure 8, and finally we wish to express our gratitude to Dr. R. Wilson for his continued interest and guidance throughout the project.

References

Aboud, A., Behring, W. E., and Rense, W. A.: 1959, Astrophys. J. 130, 381. Allen, C. W. : 1963, Astrophys. Quantities, Athlone Press, London. Behring, W. E., McAllister, H., and Rense, W. A.: 1958, Astrophys. J. 127, 676. Berger, R. A. and Brunet, E. C.: 1969, Astrophys. J. 155, Ll15. Beutler, H. G.: 1945, J. Opt. Soc. Am. 35, 311. Black, W. S., Booker, D., Burton, W. M., Jones, B. B., Shenton, D. B., and Wilson, R. : 1965, Nature

206, 654. Black, W. S. and Shenton, D. B. : 1966, Peaceful Uses of Automation in Outer Space, Plenum Press,

New York, p. 152. Boland, B. C., Engstrom, S. F. T., Jones, B. B., Noci, G., and Wilson, R.: 1970, Roy. Soc. Symposium

on Solar Studies, London. Boland, B. C., Engstrom, S. F. T., Jones, B. B., and Wilson, R.: 1971, to be published. Bruner, E. C. and Rense, W. A. : 1967, J. Opt. Soc. Am. 57, 709. Bruner, E. C. and Rense, W. A.: 1969, Astrophys. J. 157, 417. Burton, W. M., Hatter, A. T., and Ridgeley, A.: 1968, ESRO SP-33. Burton, W. M. and Reay, N. K.: 1970, Appl. Opt. 9, 1227. Cope, P. E. G. : 1964, J. Brit. lnterplanet. Soc. 19, 285. Friedman, H. : 1963, Ann. Rev. Astron. Astrophys. 1, 66. Harrison, G. R.: 1949, J. Opt. Soc. Am. 39, 522. Harrison, G. R., Davies, S. P., and Robertson, H. J.: 1953, J. Opt. Soc. Am. 43, 853. Hass, G. and Tousey, R.: 1959, J. Opt. Soc. Am. 49, 593. Hatter, A. T. and Ridgeley, A. : 1970, Internal ARU Memorandum. Learner, R. C. M. : 1968, Private communication. Purcell, J. D. and Tousey, R. : 1960, J. Geophys. Res. 65, 370. Tousey, R., Purcell, J. D., Austin, W. E., Garrett, D. C., and Widing, K. G. : 1964, Space Res. IV, 703. Tousey, R.: 1964, Quart. J. Roy. Astron. Soc. 5, 123. Tousey, R., Purcell, J. D., and Garrett, D. C.: 1967, Appl. Opt. 6, 365.

an echelle spectrograph for high resolution studies of the solar vacuum ultraviolet spectrum

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