The Hubble Space Telescope will provide the opportunity to perform precise astronomical photometry above the disturbing effects of the atmosphere. The High Speed Photometer is designed to provide the observatory with a stable, precise photometer with wide dynamic range, broad wavelenth coverage, time resolution in the microsecond region, and polarimetric capability. Here, the scientific requirements for the instrument are examined, the unique design features of the photometer are explored, and the improvements to be expected over the performance of ground-based instruments are projected

Bless, R. C.

Dolan, J. F.

Elliot, J. L.

Robinson, E. L.

Vancitters, G. W., Jr.

White, R. L.

English

NASA Technical Reports Server

N8 9 - 1331 1
The Hubble Space Telescope High Speed Photometer
G. W. Van Citters, Jr.
Division of Astronomical Sciences
National Science Foundation
Washington, D.C.
and the
HSP Investigation Definition Team*
*R. C. Bless (P. I.), J. F. Dolan, J. L. Elliot, E. L.
Robinson, R. L. White
Short Title: Space Telescope High Speed Photometer
https://ntrs.nasa.gov/search.jsp?R=19890003940 2020-03-20T04:46:10+00:00Z
AbsTract
The atmosphere of the Earth affects the performance of
ground-based astronomical photometers in a variety of ways.
Seeing, scintillation, scattered light, airglow, and
changing extinction all act on various time scales and in
different wavelength regions to make precise photometry
from the ground a challenging task. In addition the
ultraviolet region of the spectrum, important for the study
of hot objects and energetic processes, is not available.
The Hubble Space Telescope will provide the opportunity to
perform precise astronomical photometry above the
disturbing effects of the atmosphere. The High Speed
Photometer is designed to provide the observatory with a
stable, precise photometer with wide dynamic range, broad
wavelength coverage, time resolution in the microsecond
region, and polarlmetric capability. In this paper we
briefly examine the scientific requirements for the
instrument, explore the unique design features of the
photometer, and project the improvements to be expected
over the performance of ground-based instruments.
I. Introduction
An aperture photometer based on one or more
photomultipliers has been an indispensable part of the
equipment complement of ground-based observatories for many
years. The large number of photometric systems that have
been devised and used successfully to determine such
parameters as temperature, surface gravity, metallicity,
interstellar reddening, etc., speak to the widespread
understanding of the basic detector. The photomultiplier
provides a large dynamic range with linear response, broad
wavelength coverage with respectable detected quantum
efficiency, high time resolution, low intrinsic noise, and
an extensive literature that documents both its power and
limitations.
In fact in many spectral and time domains the
limitations on the precision of astronomical photometry
stem not from the basic detector but from the influence of
the Earth's atmosphere. Variable extinction, seeing,
scintillation, airglow, and scattered light all limit the
performance of photometry from the ground. The effects
have been extensively treated [e.g. Young, 1974], so will
not be dealt with in any detail here. Some of the problems
are amenable to variations in basic photometer design, such
as the two-channel approach [e.g. Nather, 1972; Grauer and
Bond, 1981] which can combat transparency variations and
slowly varying sky brightness. The advent of area
detectors in the form of charge coupled devices allows
optimization of signal to noise ratio through synthetic
aperture techniques and simultaneous sky background
measurement [Nather, 1972]. However the photometric
performance of CCDs has not yet been demonstrated to be
equal to that of the well understood photomultiplier.
With a conventional photometer and considerable care
the internal precision of measurements from the ground for
bright (m v = 8) stars can approach 0.001 magnitude [e.g.
Kurtz and Martinez, 1987] for time scales from tens of
seconds to a few hours. At higher frequencies,
scintillation noise becomes increasingly important (see
[Fuentes, et al., 1987] for a recent study in the near IR),
and long term atmospheric variations and instrumental
instabilities make longer period phenomena difficult to
investigate. No technique can provide access to the
ultraviolet wavelengths from the ground.
Launch of the Hubble Space Telescope will provide a
platform from which to perform photometry above the
disturbing effects of the_atmosphere. The High Speed
Photometer InvestigatioL Definition Team set out to design
an instrument that would take advantage of this
environment, both the lack of atmosphere and the relatively
stable ambient conditions, to perform precise photometry
over a wide range of time scales. The instrument was to be
based on the photomultiplier, both because of its
demonstrated performance in space applications and the long
history of use in ground-based photometry. In Section II
we examine some of the scientific programs and the
resulting instrumental requirements. We then look at the
optical design and electronics in Section III, pointing out
some of the unique features of the Space Telescope
instrument in comparison to its ground-based predecessors.
Finally, Section IV provides a brief description of the
operation of the photometer and some projections of its
performance.
II. Scientific Program and Instrumental Requirements
The scientific interests of the co-investigators are
varied, running from cataclysmic variables to planetary
science. The study of variable compact objects such as
white dwarfs, neutron stars and black holes forms an
important part of our program. The characteristic time
scales of variations in these objects (from seconds or
minutes for white dwarfs down to milliseconds or less for
neutron stars and black holes) occur in a regime where the
atmosphere mimics or masks features in the intrinsic light
curve. The flux maxima of these objects are at ultraviolet
wavelengths inaccessible from the ground. In addition
these objects are intrinsically faint, and the amplitude of
the variations is frequently small, hence reduction of the
sky background is an important goal.
Another class of compact object, the central stars of
planetary nebulae, provides a different observational
challenge and an interesting scientific opportunity.
Evolutionary models [Schonberner, 1981] suggest that the
luminosity decrease of the central star as it joins the
white dwarf cooling sequence should amount to 5 x 10 -4
magnitude per year. With access to the ultraviolet and
with a very precise, stable photometer such a decrease
might be directly measurable on a time scale of several
years. The value of such a measurement to the theory of
advanced stages of stellar evolution is apparent.
Techniques such as those used on the ground to achieve
0.001 magnitude precision, when applied in space, may be
capable of such performance. The HSP, normally thought of
as making observations on millisecond timescales, may
contribute information on the cooling time of central
stars, measured in hundreds of thousands of years.
We also plan to use occultations of stars by the
planets and their associated moons and ring systems to
probe planetary atmospheres and ring composition and
dynamics. These observations will be carried out in the
visual and red regions in addition to the ultraviolet.
Again important structure occurs in the occultation light
curves with frequencies of 1 Hz and higher. The increase
in signal to noise afforded by the-_ck of scintillation,
scattered light and smaller aperture_ will allow many more
occultations to be observed than can be from the ground.
Even with the limitations inherent in ground-based
observations, this technique has added much to our
understanding of the solar system. Observations from Space
Telescope should provide very detailed information on ring
structure, composition and origin and the best temperature
profiles available for the planetary atmospheres.
To provide the observational capabilities for the
programs mentioned above and to provide an overall
photometric capability for the General Observer on the
Hubble Space Telescope, the design characteristics of the
HSP are:
Time Resolution: i0 microseconds, pulse counting
4 millisecond, current mode
Apertures: 0.4, 1.0 arcsec for normal
observations
i0.0 arcsec for acquisition
Wavelength Range: 1200 - 7500 _ngstroms, 23 filters
Polarimetry: 4 ultraviolet filters
Sensitivity: S/N = 10 for 24 m star in B band in
2000 sec
Accuracy: Systematic errors < 0.1% from my=0 to
my=20
III. Instrument Design
Optics
The desire to provide the versatility inherent in the
characteristics presented above and yet to provide a
simple, stable and reliable instrument on orbit resulted in
a rather unconventional design. Four image dissectors and
one conventional photomultiplier provide linear, well
understood detectors. The dissectors are ITT 4012RP
Vidissectors, two with CsTe photocathodes on MgF 2
faceplates (sensitive from 1200 to 3000 Angstroms) and two
with bialkali cathodes on quartz faceplates (sensitive from
1800 to 7000 Angstroms. Each dissector, its voltage
divider, deflection and focus coils are contained in a
double magnetic shield within the housing. The
photomultiplier is a Hamamatsu R666S with a GaAs
photocathode. Three of the image dissectors, the two CsTe
and one of the bialkali tubes, are used for photometry.
The second bialkali dissector is used for polarimetry, and
the photomultiplier along with the bialkali photometry
dissector are used for occultation observations.
Figure 1 shows the arrangement of the detectors and
optics in the HSP. For photometry, light from the Space
Telescope enters the HSP through one of three holes in its
forward bulkhead. Since the focal plane of the HST is
shared by five instruments with the Wide Field/Planetary
Camera on-axis, these entrance holes are of necessity all
centered on an arc about 8 arc minutes off-axis. The beam
from the telescope first encounters the filter-aperture
tube. Each tube contains thirteen filters mounted in two
rows 36mm ahead of the ST focal plane. At this location
the converging beam from the ST is 1.5mm in diameter. This
placement of the filters provides insensitivity to small
pointing and guiding errors in the event of filter non-
?
O
uniformities, pin holes or the development of color centers
on-orbit. The superb tracking capability of the HST (0.007
arc sec RMS) allows this departure from conventional filter
placement. Behind the filters and in the focal plane of
the telescope are The field apertures. These 50 apertures
are arranged in rows directly behind the filter strips.
Figure 2 shows a typical arrangement, in this case for the
bialkali dissector filter assembly. The aperture choices
are 0.4 and 1.0 arcsec diameters. Each tube also has one
i0 arcsec diameter aperture available for target
acquisition.
The choice of filter and aperture for a given
observation is thus made by pointing the HST so the object
of interest falls on the correct filter/aperture pair.
After passing through this pair the light is brought to a
focus on the dissector cathode by a relay mirror - a 60 mm
off-axis ellipsoid located some 800mm behind the HST focal
plane. This relay arrangement actually images a complete
filter/aperture set on each dissector cathode allowing a
greater number of detectors to access the focal plane,
providing about 0.25 arc sec diameter images on the
photocathodes. Again The tracking capability of the
telescope allows this approach rather than a conventional
Fabry lens. Within the dissector the photoelectrons are
magnetically focussed and deflected such that the image of
the desired filter/aperture pair is directed through the
180 micron (i arc sec on sky) dissector aperture, and
thence to the 12 stage multiplier section of the tube.
Thus with no moving parts 27 different filter/aperture
combinations are available for each photometry detector.
The reader is referred to the High Speed Photometer
Instrument Handbook available from The Space Telescope
Science Institute for a complete list of the various
filters and their characteristics. Some filters are common
to all three tubes to provide redundancy and to allow the
Tubes to be tied Together photometrically. OTher filters
are similar %o Those flown on previous space missions, to
filters used in ground-based systems and to filters in the
Wide Field and Faint Object Cameras. For occultations, a
beamsplitTer passes red light To the photomultiplier
Through a normal Fabry lens arrangement with the blue light
directed to an image dissector. Thus occultation
measurements can be made simultaneously at 7500 and 3200
Angstroms, or in a single bandpass Through any of The other
filter/aperture combinations.
The polarimetric dissector is located only 4 minutes
off axis and The light proceeds directly %o the cathode
after passing Through the focal plane filter/aperture
assembly. Four ultraviolet filters are provided, each with
four strips of 3M Polacoat rotated 45 ° from strip To strip.
The apertures are 1.0 arc sec in diameter with Two
apertures for each filter/polarizer combination.
Electronics
A full description of the electronics is beyond the
scope of This paper. We concentrate here on those aspects
relevant to the performance of the instrument and its
comparison to other ground-based instruments.
The high voltage power supplies are programmable in
approximately four volt increments from 1500 to 2500 volts
for each of The detectors. Preamplifiers, located near
their respective detectors, provide a voltage gain of 7.
The preamplifier output is received by pulse
amplifier/discriminators with programmable Threshold
settings, allowing optimization of the signal-to-noise
ratio for any high voltage setting. All of the detectors
can be operated in either The pulse counting or current
mode. In The pulse counting mode the integration times can
be as short as i0 microseconds and are commandable in 1
microsecond steps up to about 16 seconds. Similarly each
integration can be separated from the previous by a delay
of from zero to 16 seconds. The pulse pair resolution of
the amplifiers is about 50 nsec, enabling count rates of up
to 2x105 Hz to be accommodated with dead time corrections
of less than one percent. In the analog mode a current-to-
voltage converter measures detector current over a range of
1 nA to i0 pA full scale in 5 gain settings. For each gain
setting the amplifier output is digitized to 12 bits. The
shortest observation times range from 4 milliseconds in the
InA range to 0.4 milliseconds in the i0 pA range.
Initially the absolute time of observations will only
be known to within i0 milliseconds, limited by the
specifications on the HST spacecraft clock. However it is
hoped that observations of "standard" sources such as the
Crab Pulsar will allow better calibration of clock errors
and substantially improved timing after some experience in
orbit.
IV. Operations and Projected Performance
For objects with well known coordinates (or re-vislts)
the HST can be commanded to place the object in the desired
filter/aperture combination. Acquisition of objects with
less certain coordinates (or those in complex fields)
begins by commanding the telescope to place the object in
one of the i0 arcsec acquisition apertures. A 20x20 pixel
scan is then carried out with the dissector to provide an
acquisition image. On-board software can determine the
centroid of the object (including logic to specify the nt h
brightest object) and compute the offset necessary to place
it in the aperture of choice. For more complex fields the
acquisition image can be sent to the ground where the
I0
observer can determine the object location and then have
software compute and uplink the necessary offset.
With multiple apertures associated with each filter,
background ("sky") measurements can be easily interleaved
with object measurements; the dissector is simply
commanded to step between the object and a corresponding
aperture on the same filter strip between integrations.
However, since the telescope must be moved to change
filters, multi-filter observations involve a delay of 30
(if the filters on the same dissector) to 60 (if the
filters are on different dissectors) seconds between
observations in different filters. To allow high time
resolution measurements at widely separated wavelengths,
three pairs of filters have been linked with beam-splitting
prisms. These provide two pairs in the ultraviolet
separated by approximately 1200 Angstroms, and one pair on
the bialkali tube that spans from below the atmospheric
cutoff to 5500 Angstroms. The filter pair for occultations
can, of course, also be used for high time resolution
observations.
Radiometric calibration of the HSP will be accomplished
through observations of stars with known spectral energy
distributions. Figure 3 has been adopted from the
Speed Photometer Instrument Handbook and gives estimated
integration times necessary to reach a signal-to-noise
ratio of i00 for a mv=15 source as a function of source
temperature for a subset of filters. The throughput of the
entire HST-HSP system has been taken into account in this
calculation.
Final figures on both systematic and random error
sources will have to await on-orbit experience. However
the photometer has been designed to minimize both types of
error sources and hopefully both will contribute at a level
less than 0.1% of the signal. Such performance will
provide an excellent photometric capability for the Hubble
11
Space Telescope, both for the scientific programs of the
Investigation Definition Team and for those of the many
General Observers using the instrument for years after
launch.
Acknowledgements
In preparing this paper, the author has relied heavily
on (and borrowed liberally from) material presented by
Bless [1982], White [1985], and material in the observing
proposals of members of the IDT. Their contributions are
gratefully acknowledged.
References
Bless, R.C., 1982, The High Speed Photometer for Space
Telescope, in The Space Telescope Observatory, ed. D.
Hall, NASA.
Fuentes, F. J., Fuensalida, J. J., Sanchez-Magro, C., 1987,
M.N.R.A.S., 226, 769.
Grauer, A. D. and Bond, H. E., 1981, P.A.S.P., 93, 388.
Kurtz, D. W. and Martinez, P., 1987, M.N.R.A.S., 226, 187.
Nather, R. E., 1972, P.A.S.P., 84, 149.
Nather, R. E., 1972, Rev. Sci. Instruments, 43, 1012.
Schonberner, D., 1981, Astron. Astrophys., 103, 119.
White, R. L., 1985, "High Speed Photometer Instrument
Handbook", NASA Space Telescope Science Institute.
Young, A. T., 1974, in Methods of Experimental Physics,
Vol. 12A, (N. Carleton, ed.; Academic Press, New York).
12
G. W. Van Citters, Jr. : Division of Astronomical Sciences,
National Science Foundation, Washington, D. C. 20550
R. C. Bless : Washburn Observatory, University of
Wisconsin, 475 N. Charter St., Madison, WI 53706
J. F. Dolan : NASA/Goddard Space Flight Center, Code 681,
Greenbelt, MD 20771
J. L. Elliot : Dept. of Phys. & Earth and Planetary Sci.,
MIT, Bldg. 54-422A, Cambridge, MA 02139
E. L Robinson : Department of Astronomy, University of
Texas at Austin, Austin, TX 78712
R. L. White : Space Telescope Science Institute, Homewood
Campus, Baltimore, MD 21218
13
,ORIGINAL PAGg IS
DE POOR QUALITY
- IDT-POLARIMETRY
/
/
APERTURE PLATES/-_ ___ -- _z_'-
____.._/_ -_-_-c_--_-_--_ Lz__._ LJ_--x:-_-_j ELUPSO,DS
PHOTONS . _'- 3 IDT's-PHOT_.._
HSP ENCLOSURE __TIONS
Figure i. Schematic Diagram of HSP Optics and Detectors
(Figure courtesy of Space Telescope Science
Institute)
Aperture & Fl}ter Layout for \,'1._ IDT
(As seen from behind focal plane)
Pl I_m [tc_,m_phttel ,/._
F355M /.450_
F419N \ F5,52 W
\\ // t b2OW
\ ' ; /
• _ "i' b 400LP
...j ]_--"
F 160LP {PMT)
1'
Figure 2. Visible Wavelength Image Dissector Apertures and
Filters (Figure courtesy of Space Telescope
Science Institute)
14
•9!__IGINAL PAGE IS
D_ lqDOR QUALITY
Time to Reach S/N:-IO0 for V_-15
3.0
2.0
o
40E oq 3 oeo_ _.oE.o_. _.oe.o_ o.oe.co
Temperature (K)
Figure 3. Time to Reach V=I5 with S/N=100 for various
filters (Figure courtesy of Space Telescope
Science Institute)
15



The Hubble Space Telescope high speed photometer

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