UVSTAR is an EUV spectral imager intended as a facility instrument devoted to solar system and astronomy studies. It covers the wavelength range of 500 to 1250 A, with sufficient spectral resolution to separate emission lines and to form spectrally resolved images of extended plasma sources. Targets include the Io plasma torus at Jupiter, hot stars, planetary nebulae and bright galaxies. UVSTAR consists of a pair of telescopes and concave grating spectrographs that cover the overlapping spectral ranges of 500-900 and 850-1250 A. The telescopes use two 30 cm diameter off-axis paraboloids having focal length of 1.5 m. An image of the target is formed at the entrance slits of the two concave grating spectrographs. The gratings provide dispersion and re-image the slits at the detectors, intensified CCD's. The readout format of the detectors can be chosen by computer, and three slit widths are selectable to adapt the instrument to specific tasks. UVSTAR has internal gimbals which allow rotation of plus or minus 3 deg about each of two axes. Dedicated finding and tracking telescopes will acquire and track the target after rough pointing is achieved by orienting the Orbiter. Responsibilities for implementation and utilization of UVSTAR are shared by groups in Italy and the U.S. UVSTAR is scheduled for flight in early 1995, timed for an opportunity to observe the Jovian system

Broadfoot, A. Lyle

Sandel, Bill R.

Stalio, Roberto

English

NASA Technical Reports Server

W S T m ,  AN IMAGING SPECTROGRAPH WITH TELESCOPE 
FOR THE SHUTTLE HITCHHIKER-# PLATFORM 
Roberto STALIO 
Department of Astronomy, University of Trieste & Center f o r  Advanced Research i n  
space optics - Area Science Park, Pae ic i ano  99, 34016 - Trieste 
B i l l  R. SANDEL, A. Lyle BROADFOOT 
Lunar and Planetary Laboratory, University of Arizona, Tucson, 
AZ 85721 
WSTAR is an EW spec t ra l  imager intended as a f a c i l i t y  instrument devoted t o  
so l a r  system and astronomy studies .  It covers t h e  wavelength range of 500 t o  1250 A, 
w i t h  su f f i c i en t  spectral resolution t o  separate emission l i n e s  and t o  form spec t ra l ly  
resolved images of extended plasma sources. Targets include the  Io plasma torus  a t  
Jupi ter ,  hot stars, planetary nebulae and bright galaxies. WSTAR consis ts  of a 
p a i r  of telescopes and concave grat ing spectrographs tha t  cover the  overlapping 
spectral ranges of 500 - 900 and 850 - 1250 A. The telescopes use two 30 cm diameter 
off-axis paraboloids having focal  length of 1.5 m. An image of the  target is formed 
a t  the entrance slits of t he  two concave grat ing spectrographs. The grat ings provide 
dispersion and re-image t he  slits a t  t h e  detectors,  in tens i f ied  -8 .  The readout 
format o f  t he  detectors  can be chosen by computer, and three  slit widths are 
selectable t o  adapt the instrument t o  specific tasks. WSTAR has in t e rna l  gimbals 
which allow ro ta t ion  of f3O about each of two  axes. Dedicated finding and tracking 
telescop.8 w i l l  acquire and t rack  the  target after rough pointing is achieved by 
orient ing the O r b i t e r .  Responsabilities f o r  implem8ntation and u t i l i z a t i o n  of WSTAR 
are shared by groups i n  I t a l y  and the  U.S. WSTAR is scheduled f o r  f l i gh t  i n  ea r ly  
1995, t h m d  f o r  an opportunity t o  observe the Jovian syst-. 
W S W ,  Ultraviolet  Spectrograph Telescope fo r  Astronomical Research, is a 
spectrographic telescope f o r  Observations of astronomical and planetary sources; it 
operates i n  t he  500-1250 A w a v e b a n d  a t  = 1 A resolution.  he experiment has 
capabi l i ty  f o r  long slit spectral imaging of extended cosmic sources such as planete, 
planetary nebulae, supernova reumants, H 11 regions, and external galaxies.  WSTAR 
w i l l  f l y  as a R i t c h h i k e r - M  payload on the  Shut t le  (Sigure 1). The  experiment 
complenmnt includes an independent companion instrument t ha t  w i l l  be provided by 
D a r r e l l  Judge (University of Southern California) t o  measure the  absolute so l a r  f l ux  
i n  selected extrema u l t r av io l e t  bands. The two experiments form the IEB 
(Internat ional  Isw/Wnr Hitchhiker) mission. 
WSTAR consis ts  of a movable platform and an opt ica l  system. The platform w i l l  
provide fine pointing ( f 5  arcsec) w i t h i n  f3' from t he  nominal v i e w  direct ion,  which 
145 
https://ntrs.nasa.gov/search.jsp?R=19940014705 2020-06-16T17:12:42+00:00Z
is  near the  Shuttle +Y a d s ,  i.e. perpendicular t o  the  long axis of the  Shut t le  and 
in the  plane of the  w i n g s .  The optical system consis ts  of a set of two telescopes and 
Rowland concave-grating spectrographs w i t h  in tens i f ied  CCD detectors.  The  first 
spectrographic telescope (PW) operates i n  the  850-1250 A spectral range; the  second 
(Ew) covers the 500-900 A region. A number of pointed observations w i l l  be made of 
ca l ibra t ion  targets, planets, u l t r av io l e t  stellar sources and extended objects.  A t  
any t i m e  t he  WST24R is pointed toward the  hemisphere outward fram the Earth and the  
Shut t le  is on celestial lock, it is possible t h a t  a source of i n t e r e s t  f o r  w 
astronomy w i l l  be within the pointing range. In t h i s  case, WSTAR w i l l  operate i n  
a llserendipitous/ta~t-of-opportunity'l mode; in t h i s  mode targets t h a t  w i l l  have 
been defined i n  advance w i l l  be acquired and observed. 
WSTAR has two nearly ident ica l  spectrograph channels (EW and FW) t h a t  w i l l  
observe the same target. Each channel consis ts  of a telescope mirror, 
and a concave grating spectrograph w i t h  its own in tens i f ied  CCD detector.  The 
telescopes fonn images of the target a t  the  entrance s l i t  of the  spectrographs. The 
concave grat ings of the Spect~og~aphS both disperse and re-image the  l i g h t  from the  
target onto the  2-D detectors.  Spatial resolution along the  slit is preserved, so 
that i n  the  cross-dispersion direct ion t h e  detectors records many spectra 
corresponding t o  d i f fe ren t  pa r t  of the source simultaneously. The opt ica l  design is 
driven by the  facts that a t  the EW wavelengths of i n t e re s t  
only reflective opt ics  CBP be Used, 
0 reflectivities are poor, which requires minbiZing the  number of re f lec t ions  if 
weak sources are t o  be detected i n  a short  period of time. 
Bigure 1: Sketch of the opto-mechnical configuration of WSTAR 
146 
The telescope mirrors a re  off-axis paraboloids of 30 cm diameter and 1 .5  m focal  
length. They are Sic coated fo r  maxhmm ref lec t iv i ty ;  are made by Zerodur t o  prevent 
defocussing due t o  thermal changes; a re  joined t o  the spectrographs by rods of a 
carbon composite material of about zero thermal expansion coeff ic ient  along its 
length. 
Each spectrograph employs a s ingle  concave, holographic grat ing t o  disperse the  
radiation and focus it on the detector.  Holographic ruled gratings can achieve 
aberration correction and a f l a t  focal plane over a length of 8 nun i n  the  cross- 
dispersion, and 15.4 nrm i n  the  dispersion direct ions.  The 70 X 100 mm gratings have 
a radius of curvature of 275 mm. To maintain good focus over a range of 
temperatures, the  gratings w i l l  be fabricated on Zerodur blanks. The  grat ing cell is 
mounted t o  permit small motions re la t ive  t o  the  spectrograph housing. The posit ion of 
the  grat ing r e l a t ive  t o  the  entrance sl i t  and detector is  established by invar rods 
tha t  extend from the  grat ing cell t o  fixed posit ions near the  entrance sl i t  and 
detector .  
The two spectrographs cover overlapping spectral  ranges, 500-900 A and 850-1250 
A. In  each case t h e  spectrum of about 400 A is  dispersed along 1152 pixels,  
giving a dispersion of = 0.35 A per pixel .  The  array is  298 pixels  w i d e .  In the  
image mode t he  spec t ra l / spa t ia l  images a re  comprised of 2 x1152 X 298 tha t  
is, nearly 7 x l o 5  pixels  o r  1 . 4  X l o 6  bytes i f  each pixel  is  d ig i t ized  t o  16-b i t  
accuracy. The read out format is  controlled by computer so t h a t  it can be matched t o  
t h e  needs of a par t icu lar  experiment. 
pixels,  
Each of the  spectrographs is  equipped w i t h  a mechanism f o r  select ing among three 
entrance slits. A l l  slits have the same length of 0.25' but differ i n  w i d t h .  The 
slits w i d t h s  and t h e i r  uses are: 
0 Narrow slit (3 pixels,  9 arcsec) . T h i s  s l i t  w i l l  provide about 1 A resolution and 
w i l l  a l so  have s p a t i a l  resolution it. 
0 Medium s l i t  (13 pixels,  39 arcsec) . This  s l i t  w i l l  give global images of extended 
sources (planetary nebulae, SN remnants, Jup i t e r ' s  disk) a t  the wavelengths of most 
intense emissions. Resolution is  4.5 A. 
0 W i d e  slit (33 pixels ,  100 arcsec) .  T h i s  aperture w i l l  allow the  whole image of the 
Io  plasma torus  t o  be dispersed in to  its major emissions and re-imaged w i t h  a 
resolution of 12 A.  
The in tens i f ied  CCD (ICCD) detectors consist  of standard proximity-focused image 
i n t ens i f i e r  tubes t h a t  are f iber-opt ical ly  coupled t o  CCDs. They have successfully 
flown i n  previous missions. Both image i n t ens i f i e r s  are windowless. Charge from 
photoevents is amplified by the  microchannel p l a t e  and e l ec t ros t a t i ca l ly  accelerated 
147 
to a phosphor screen on a fiberoptic window. The visible-wavelength image from the 
phosphor is transferred to the CCD by fiber optics for readout. A special position 
of the slit changing mechanisms seals the spectrograph enclosures to protect the 
detectors and gratings from contamination. Each spectrograph is equipped with a 
-11 ion pump to assure a satisfactory vacuum environment for operating the 
windowless intensifiers on orbit. 
The ICCD detectors of WSTAR are 2-D detectors with photon-counting capability. 
Thus the overall instrument performance is related to the rate at which photoevents 
are generated at the photocathode of the detector. For the imaging spectrographs, 
Table I gives the sensitivity for two kinds of source, namely a point source of 
continuum emission, and an extended source of monochromatic emission. The Wavelength8 
are in A, the area of the telescope mirror (A) is in an2, n is the solid angle, 
measured in sr, corresponding to the field of view of a single pixel, 6 (A/pixel) is 
the spectral dispersion, 0 is the 
E the quantum efficiency of the photocathode, p the reflective efficiency of the 
telescope R is the response for a point source in units counts/pixel/s per 
Rayleighs; the units are counts/pixel/s per Rayleigh. 
reflective-dispersive efficiency of the grating, 
photon/- 1 /s/A, s is the response for monochromatic emission of brightness I 
WSTAR includes capabilities for independent target acquisition and tracking. 
The spectrograph package has internal gimbals that allow angular movement of f 3 O  from 
the central position. Rotations about the azimuth axis (parallel to the Shuttle Z 
axis) and elevation axis (parallel to the Shuttle X axis) will actively position the 
field of view to center the target of interest in the fields of the spectrographs. 
Two dedicated visible imagers (Finder and Tracker) having different fields of 
view, and a quadrature diode, provide guiding information.The Finder has 60 nun focal 
length and f/1.4 optics giving a field of view of 6O X 8O. The system, fully planned 
and constructed at CARSO, works in two modes: 
From a coarse knowledge of the pointing direction (* So), the syste~r. recognizes 
autonomously the acquired stellar field and point to the chosen direction within f 
5 " .  
After having reached the wanted direction, it operates with three different 
techniques : 
1) s- as the : at every acquisition, the system repeats the star field 
identification routine. In this mode the algorithm requires, on the average, a 
processing time of 300 ms (on a 386/387 at 25 MHz). 
2)  - the system chooses three stars out of the stars selected in 
the and by means of a triangulation procedure, calculates the position of 
the center of the sensor which concides with the chosen pointing direction. In this 
case the algorithm need8 a few ms of processing time. 
3) - the target star is at the center of the sensor. we correct 
its position at every acquisition keeping it at the center. The algorithm takes only 
few ms of processing time. 
148 
As an a l te rna t ive  t o  the  above mentioned routines we have developed another 
procedure (- ) w h i c h  doesn't use any ident i f ica t ion  procedure, but simply 
searches f o r  the brightest object i n  the FoV, brings it a t  the  sensor center and 
tracks it w i t h  the  algorithm 3. 
The Tracker is a 75 man diameter cassegrain telescope w i t h  a focal  length of 1.5 
m and a field of v i e w  of 0.24O X 0.32O. Finder and Tracker have t h e i r  own ICCD 
detector,  384 x 288 pixe l  arrays.  The t racker  ICCD w i l l  have the  same p la t e  scale 
as the  spectrograph. In  addition the tracker telescope w i l l  a l so  be equippea 
w i t h  an in tens i f ied  quadrature diode w h i c h  w i l l  track a 8-9 magnitude s t a r  w i t h  
negl igible  processing tima. The diameter of the  photodiode is 3 nnn and the  s t a r  image 
w i l l  be 1 nun. The tracking accuracy w i l l  be better than f5". 
1 ASTRONOblY 
The "pointedv1 observations from WSTAR w i l l  be of 25f5 a s t r o n d c a l  sources per 
fl ight.  A similar  number of pointed ObSer~atiOn8 w i l l  be available t o  the planetary 
physics program. A t  the  end of t he  program we w i l l  have observed 100 t o  150 
astronomry sources. About 40% of them w i l l  be 1) cal ibrat ion,  2) EW, and 3) 
extended sources. 
Predicted sens i t i v i ty  curves f o r  point sources and exanples of source 
observabili ty a r e  displayed i n  Figures 2, 3 and 4. The s ens i t i v i ty  curves are 
presented under the form of fluxes w h i c h  can be observed i n  25 minutes w i t h  S/N = 6 
i n  the  two  channels, EW and F W .  Two observing modes, f u l l  resolution and reduced 
resolution (about 6 A, IUE-like), are considered. In  Figure 2 are a l so  displayed the  
energy dis t r ibu t ion  of two important W sources: HZ 43, a nearby subdwarf, w h i c h  is 
among the f e w  stars obsemable i n  the  500 - 912 band and the  bright quasar 3C273. 
Both are w e l l  observable from WSTAR, even i f ,  a t  the  shorter  FUV wavelengths, the  
s ignal  t o  noise r a t i o  of 3C273 w i l l  be s l igh t ly  less than 6 using the  "standard" 
exposure time of 25 minutes. Figure 3 gives compares the  $w sens i t i v i ty  curves w i t h  
energy fluxes of two representative cataclysmic variables; Figure 4 does the  same f o r  
two important Seyfert  galaxies.  In  t h i s  las t  case both continuum fluxes and l i n e  
fluxes are reported. 
A 
The effects of i n t e r s t e l l a r  ext inct ion on exposure times a r e  presented i n  Figure 5 
f o r  f u l l  resolution spectra. W i t h  the  IUE-like resolution one gain a l i t t l e  b i t  less 
than 2 magnitudes. T h e  effects a180 include the absorption f o r  molecular hydrogen. 
The FW i n t e r s t e l l a r  reddening curve and t h e  H2 absorption have been discussed i n  a 
previous study of low resolution data from the  Voyager W Spectrometers (ref. 1). 
The E W  program w i l l  be fundamentally d i f fe ren t  from tha t  a t  longer wavelengths 
because of t he  opacity of neutral  hydrogen i n  t h e  i n t e r s t e l l a r  medium. In  general we 
w i l l  be able t o  observe only within a f e w  tens  of parsecs from the  Sun; however, in 
those direct ions where the ISM has low density, we w i l l  go farther and porsibly 
observe some stars. In  general only a f e w  hot w h i t e  d w a r f s  are listed as candidates. 
However, the recent r e su l t s  by ROSAT and E W E  indicate  tha t  a subs tan t ia l  nzmPber of 
sources would be Observable. As an example, among the  E W  sources detected by ROSAT 
Owc (at 100 and 137 A) i n  a 60 x 60 degree field centered a t  a = 14h and 6 = 10' are 
t h e  low X-ray binaries Sco X-1 and Her X-1, t he  FW 80urce Spica (B1V star) ,  3C273 ( 
149 
-10 
-10.5 
-11 
-11.5 
c e -12 
I T 
- 
-- - -- \ -- 
I -12.2 1 
-13.8 ! I 
850 900 950 1000 1050 1100 1150 1200 1250 
wavo longth  (A)  
~ 
2 
t . -12.6 
e 
G 
5 
n" 
-13.4 
-12.4 -- 
2 -12.6 
1 - 1 2 . 8  ' -13 
8 -13.2 
-13.4 
I 
'c 
ss C Y 0  
- -  
-- 
- -  
-* 
- -  
O V I  I - 1 2 . 2 - r  
& 
c 111 
- 1  8.6 I 
0 6 0  900 960 1000 1060 1100 1160 1200 1260 
wavo longth  ( A )  
F i g u r e  4:  Same as Figure 2 for the Seyfert ga lax ie s  NGC 1068 and NGC4151. The symbol + refers to line i n t e n s i t i e s  of NGC 4151; t h e  symbol * t o  NGC 1068 
150 
16 
14 
12 
P l: 
' b  
4 
2 
0 
-2 + I 
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 
E(6-V) 
Figure 5: The diagram shows the limiting magnitude reachable for a S/N = 6 at full 
resolution in 25 minutes for different spectral types and different E ( B - V ) .  The 
effect of molecular hydrogen absorption is also included 
(quasar), NGC 4152 and MRK 478 (Seyfert galaxies) and the E W  source HZ43. ROSAT list 
includes many other sources in the same general area: spiral galaxies, white dwarfs 
and late type stars. EUVE has recently observed intense E W  emission from the B2 I1 
star E 8da in the 500-740 A and 345-605 A bands (Ref. 2). 
2 P M T A R Y  PHYSICS 
Io's plasma torus is a natural laboratory for investigations of plasma physics. 
It is our best-studied exanple of an astrophysical plasma, having been probed by both 
in-situ and renote sensing techniques. The full implication of early measurements 
revealing neutral sodium and ionized sulfur near Io's orbit was not realized until 
the discovery, by the Voyager WS, of an intense hot plasma (Te =lo5 K), f o d n g  a 
complete toroidal cloud about Jupiter, near Io's orbit. As a consequence of the high 
electron temperature, the dominant emissions from S 11, S 111, S IV, and 0 11 are in 
the E W  and FW spectral regions, the brightest feature being near 685 A. About 2X 
l0l2 W is radiated in the EW; this is the dominant mechanism for energy loss from 
the torus. Although ground-based studies of the visual wavelength torus emissions 
have addedto our knowledge of this plasma, measurements in the E W  are a more direct 
mathod of studying processes in the torus. To date, studies of the plasma torus have 
demonstrated that measurement in the E W  spectral region is essential, full spectral 
coverage in this region is required, and morphological studies, i.e. imaging of the 
whole system are necessary. 
WSTAR will complement and extend observations of Jupiter and Io's plasma torus 
made by the G a l i l e o  spacecraft orbiting around Jupiter. Galileo's orbital geometry 
limits its opportunities for viewing the plasma torus with the W instruments, and 
especially with the E W  spectrograph, which is most diagnostic of torus conditions. 
WSTRR will provide observations of the Jupiter system at improved spectral 
resolution. The imaging spectrographs will measure the intensity of several important 
emission lines of S 11, S 111, S IV, and 0 I1 as a function of position in the torus, 
measure the Hp band emissions of Jupiter'sdayglow and aurora, and clarify the 
coupling of Jupiter's aurora and the torus. The H Ly-a dayglow intensity will be 
151 
correlated with the absolute solar flux measurements taken with the companion 
instrument of the University of Southern California. Spectral dispersion with 
simultaneous imaging makes the xnost efficient use of obseming time and photon 
collecting ability. No photons within the bandpass of the instrument are wasted. 
These capabilities open the way for a far more thorough investigation of torus WN 
morphology and dynamics than is possible with other existing or planned 
instrumentation. 
I 
The spectrographs, when operating in the wide-slit configuration, form two- 
dhensional images of a target within the slit. Images at different emission linea 
are displaced in the dispersion direction. The spectral dispersion is great enough to 
separate monchromatic images of the torus in several ravelengths. Several of these 
will be strong enough to be analyzed as a snapshot of emission xnorphology. The 
spatial resolution over the disk of the planet will separate polar and equatorial 
emissions of H and Hp. 
1. R. Longo, R. Stalio, R.S. Polidan, and L. Rossi, " I n t r i n s i c  W Spectral 
Energy Dis tr ibut ions  of OB Stars", Astrophysical J. 339 (1989) 474-80. 
2. J.V. Vallerga, P.W. Vedder, and B.Y. Welsh, " I n t r i n s i c  B W  Emission from 
the B S t a r  E ma",  Center for Fw Astrophysics (1993), Preprint No.  532. 
I600 700 2 .35  .ll . 4  .35 3 . 7  .000017 I 
I1000 700 2 .35 .15 . 2  .45 3 .3  .000015 I 
152 


UVSTAR: An imaging spectrograph with telescope for the Shuttle Hitchhiker-M platform

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server