A High Resolution Spectrometer for EUV/FUV Wavelengths
Mark Hurwitz and Stuart Bowyer
Space Sciences Laboratory University of California
Berkeley, California 94720
Abstract
We have considered various designs for a high resolution spacebome
spectrometer for point-source astronomy in the 400 - 1200 Å
wavelength
region. Our designs utilize as the primary collecting optic a 1-meter
normal incidence primary mirror of 1" quality and are constrained to
fit within an envelope defined by the size of the advanced Spartan
space platform now under consideration for development by West Germany.
We find the most efficient design to be a multiple Rowland circle
spectrograph, with four toroidal gratings each intercepting a
fraction of the beam from the primary mirror. The advantage of this
design is that each spectrometer can accept a relatively slowly
diverging beam (thus reducing grating aberrations) without the loss
of efficiency caused by an additional reflection or the magnification
of the primary mirror blur and pointing jitter that would be introduced
if a secondary mirror were used to slow the beam. We examine the
detector requirements for the multiple Rowland circle spectrometer and
find that no appreciable loss of resolution occurs if the circular
tangential focal surface is approximated by two flat detectors.
Furthermore, each pair of flat two-dimensional detectors can receive
the spectra from two of the toroidal gratings simultaneously, thus
reducing the number of detectors and associated electronics required.
The specific parameters of the design (line densities, detector size
and pixel size , etc.) are dependent on the drift rate of the space
platform. We present a design optimized for an uncorrected drift of 4"
This design delivers a resolution of 3000 - 4000 and an effective area
(including detector quantum efficiency) of 1 to 6.5 square centimeters
across the bandpass.
Introduction
To combine the advantages
of the observing time available with a free-flying platform, the
reliability of the shuttle, and the large physical dimensions of Aries
sounding rocket payloads, the German Ministry of Sciences is
considering the development of an advanced Spartan platform. The
platform, like existing Spartans, will use the space shuttle as a boost
and recovery vehicle. Unlike existing Spartans it will be developed and
built in Germany and will be designed specifically to accommodate
Aries-class (I meter diameter) astronomical payloads on flights of a
few days duration. We show one firm's concept of this platform in
Figure 1.
A payload under consideration for flight with this carrier would carry
out spectroscopy of astronomical sources in the 400 to 1200 Å
band. The primary optic for this payload is being developed by the
Astronon-dcal Institute of Tubingen and expected to be a 1-meter normal
incidence f/2.7 primary mirror coated with iridium. Light collected by
this optic will be directed by a movable pick-off mirror to one of two
spectrometer systems. The prime spectrometer will be built by Berkeley
and will cover the 400 - 1200 Å bandpass with a resolution of
3000 to 4000. A second spectrometer will be built by researchers at
Heidelberg and will cover 900 - 1200 Å with a resolution of
10,000. In the sections that follow we provide a brief description of
the scientific objectives of the mission and describe the Berkeley
instrument in some detail. The performance of the instrument is
discussed and simulated sample spectra are shown.
Scientific rationale
A substantial number of astrophysical topics can be investigated with
an instrument of this type. Of substantial scientific interest will be
studies of the interstellar medium. Nearly all phases of the ISM can be
probed in this spectral range. Molecular hydrogen, found in the coldest
interstellar clouds, has a large number of transitions between 912 and
1120,&. Measurement of the extinction and reddening of hot stars by
cold interstellar clouds will allow a determination of the size and
composition of interstellar dust grains in these clouds. Absorption
lines of many species and absorption edges of neutral H and He will be
seen against background sources such as hot white dwarfs. Studies of
this type will answer fundamental questions about the cosmic abundance
of elements in the ISM.
In the area of stellar physics, this
instrument will allow determination of the temperature and luminosity
of hot 0 stars by measuring absolute fluxes in the spectral region
where most of their emission occurs. legh spectral resolu- tion
profiles of lines formed in the extended atmospheres of these stars
will provide information about their winds. The coronae of cool stars m
also be studied. The temperatures of these coronae lie somewhere in the
104 to 107 K range. Ions formed under these conditions have many lines
in the bandpass of this instrument. Acereting binary systems should
also prove to be strong sources in the extreme ultraviolet. The
instrument's ability to attach a wavelength and arrival time to each
measured photon will allow the variation of the spectrum during the
orbital period of the binary to be examined. This will bring about a
better understanding of the accretion morphology of these systems.
We plan also to study quasars and active galactic nuclei. Important
lines that can be observed with this instrument include the 0 VI
doublet, which will yield information about the velocity structure and
morphology of hot gas in the system. Observations of the C HI line at
977,X, when combined with existing IUE or future Space Telescope
measurements of the C HI 1909A line, will give us a highly sensitive
temperature probe. Study of bright AGNs might yield information about
the intergalactic medium in the same way that the study of sources
within our own galaxy will yield information about the interstellar
medium.
Instrument design
Our goal has been to design a spectrometer
with a resolution exceeding 2000 across the stated bandpass with the
maximum throughput possible. We have analyzed many designs, both
grazing and normal incidence, both with and without a secondary
telescope mirror (Cassegrain designs). The class of design that best
achieves our goal is a multiple Rowland circle spectrograph placed at
the prime focus of the paraboloidal primary mirror. Light entering the
telescope is brought to a focus by a 1-meter, f/2.7 paraboloidal
primary mirror. A circular aperture is placed at the prime focus to
reduce background contamination. The beam diverges from the prime focus
and strikes an array of four toroidal diffraction gratings (shown with
imaging tracking system in Figure 2). Each grating has a line density
appropriate to its spectral bandpass. Only two detectors are required;
a pair of spectra can be dispersed across each detector. Each detector
consists of two rnicrochannet plate stacks arranged so as to
pproximate the best focal surface (the Rowland circle) as closely as
possible. A far ultraviolet imaging system utilizing a small fraction
of the total beam will locate the target in the slit and provide
updated positions to allow us to correct for spacecraft drift (about I
" per second) post-flight.
This design allows each grating to see a relatively slow beam, thus
reducing grating aberrations. As compared to the use of a secondary
mirror to slow the beam, our design is equal in throughput and results
in higher resolution. Tle equality in efficiency is due to the
relatively low reflectivity (about 20%) of iridium and other inert
coatings at EUV wavelengths. Our design allocates about 20% of the area
of the primary mirror to each grating, hence the rough equivalency in
terms of throughput. If a secondary mirror were used to slow the beam,
it would enlarge the image blur, drift, and jitter through the
unavoidable magnification of the effective focal length. Our design
does not suffer this effect. Furthermore, our design takes advantage of
the annular nature of the beam (the inner .5 meter of the aperture is
blocked by the spectrometer) and splits the f/2.7 beam into four
sections each slower than f/5.4.
The resolution of the system will be determined by how precisely we can
remove the spacecraft pointing drift from the spectra in the data
analysis. The image size of the primary mirror is expected to have a
half energy width less tnan one arcsecond and will not limit the
resolution. We have chosen to optimize a design based on an uncorrected
drift of 4" which will require an update from the imaging system every
4 seconds. This value of the uncorrected drift was chosen based on
detector limitations. When multiplied by the focal length 4'
corresponds to a spot size of 52 microns on the detectors. To
calculate the spectral resolution this value is summed in quadrature
with the grating aberrations to produce a total optics/drift blur of
about 60 microns. A detector pixel size of 30 microns is required to
sample this blur fully. This represents a challenging but achievable
goal for the quasi 1-dimensional detectors 30 mm long (each MCP stack)
we plan to use. To sample an optics/drift blur much smaller than 60
microns would present formidable problems in detector design; hence we
do not consider drift update periods less than 4 seconds. We optimized
the parameters of the design (line densities, blaze angles, etc.) given
the 4" uncorrected drift discussed above subject to the constraints
imposed by the dimensions of the platform and the limits imposed by
diffraction grating manufacturing techniques. The parameters of the
finalized design are summarized in Table 1.
Table 1
Diameter of Rowland Circle
1300 mm
Second Radius of Curvature of Toroid
1282 mm
Incidence Angle
9.5 degrees
Blaze Angle
4.7 degrees
Line Densities and Wavelength Coverage
3658 lines/mm
389- 523 Å
2774 lines/mm
512 - 690 Å
2104 lines/mm
676 - 910 Å
1595 lines/mm
910 - 1200 Å
Instrument performance
The spectral resolution of the design
increases with wavelength from 3000 to 4000 within each sub-bandpass.
This variation with wavelength is due to the fact that the resolution
is primarily opticsidrift blur limited. The resolution element
measured in Angstroms is roughly constant across each sub-bandpass,
resulting in a resolution which increases with wavelength. We have
performed geometrical raytraces of the design and verified that it
delivers the stated spectral resolution. Off-axis aberrations are
negligible given the absolute pointing accuracy ± 10"
expected for the
platform. To calculate the effective area of the instrument we
include the reflectance of the iridium-coated primary and grating, the
quantum efficiency of the detector, and a diffraction efficiency 50% of
the value theoretically attainable (nearly 100% at all wavelengths). We
show the effective area in Figure 3.
The minimum detectable flux in
each spectral pixel depends not only on the effective area but on the
background rate as well. Diffuse airglow lines will contribute to the
background in wavelength intervals determined by the width of the
entrance slit, and scattered Lyman alpha radiation and detector dark
counts will contribute to an overall background rate. We assume a full
slit width of 30" for our calculations. Scatterin@f of 3.25 Uorayleighs
of Lyman alpha at a level of 10 /,& is included, as well as a detector
dark count rate of I count/cm2/sec. We show the 3-sigma minimum flux
detectable in I hour of observation in Figure 4.
Sample spectra
In Figure 5 we show a sample spectrum of the hot white dwarf HZ 43
assuming an integration time of 1 hour. Our model is that of Basri
(personal communication) viewed through a column density of hydrogen
and helium determined by Auer and Shipman1. Poissonian
noise due to subtraction of the background discussed above has been
added. Major interstellar absorption lines of H and He are shown. Other
lines are detectable as well;
in Figure 6 we show the minimum
equivalent width of an absorption line detectable in 1 hour at the
3-sigma level against the background continuum of HZ 43. Many lines
of several species are expected to lie above this level.
In Figure 7 we show the far ultraviolet spectrum of a main sequence
O9 star2
511 pc distant located behind 5.6 magnitudes of UV extinction assuming
1 hour observing time. The need for brevity, not a dearth of
interesting targets limits the number of sample spectra included in
this summary.
Conclusions
We have designed an efficient spectrometer for high- resolution studies
of point sources in the 400 - 1200 Å bandpass. This spectrometer
fits within the envelope available in the advanced Spartan platform
under consideration for development by West Germany. The maiden flight
of this instrument will provide a wealth of data in this unexplored
region of the spectrum.
Acknowledgments
This work was supported by
NASA grant NGR-05- 003-450 and NSF grant INT-8116729.
References
1. Auer, L.H. and Shipman, H.L., Ap. J. Letters, 211, L103, 1977.
2. Bradley and Morton, Ap. J., 156, 687, 1969.
This paper is was originally published as Hurwitz, M., & Bowyer, S."A
High Resolution Spectrometer for EUV/FUV Wavelengths",Instrumentation
in Astronomy VI, David L. Crawford, Editory, Proc. SPIE, 627, 375-378
(1986). Provided here with permission of the authors.