The GOLDHELOX Project, NASA payload number G-133, is a robotic soft x ray solar telescope designed and built by an organization of undergraduate students. The telescope is designed to observe the sun at a wavelength of 171 to 181 A. Since we require observations free from atmospheric interference, the telescope will be launched in a NASA Get-Away-Special (GAS) canister with a Motorized Door Assembly (MDA). In this paper we primarily discuss the most important elements of the telescope itself. We also elaborate on some of the technical difficulties associated with doing good science in space on a small budget (about $100,000) and mention ways in which controlling the instrument environment has reduced the complexity of the system and thus saved us money

Campbell, Branton

Moody, J. Ward

Roming, Peter W. A.

Spute, Mark K.

Williams, Memorie K.

English

NASA Technical Reports Server

G-133:
N93-13171
A SOFT X-RAY SOLAR TELESCOPE
Memorie K. Williams, Branton Campbell, Peter W. A. Roming,
Mark K. Spute, J. Ward Moody, Department of Physics & Astronomy
Brigham Young University, Provo, Utah, 84602
Ph#(801)378-4360, Fax:(801)378-2265
ABSTRACT
The GOLDHELOX Project, NASA payload number G-133, is a robotic soft x-ray
solar telescope designed and built by an organization of undergraduate students.
The telescope is designed to observe the sun at a wavelength of 171-181 A. Since we
require observations free from atmospheric interference, the telescope will be
launched in a NASA Get-Away-Special (GAS) canister with a Motorized Door Assembly
(MDA). In this paper we primarily discuss the most important elements of the
telescope itself, we also elaborate On some of the technical difficulties
associated with doing good science in space on a small budget (about $I00,000) and
mention ways in which controlling the instrument environment has reduced the
complexity of the system and thus saved us money.
I . INTRODUCTION
The "GOLDHELOX" Project ("GOLD" for the color of the sun and "HELOX" for
HELiocentric Observations in X-rays) is a near-normal incidence soft x-ray robotic
telescope being built by an undergraduate student research organization. The
organization is composed of a cabinet, a science team, and three systems teams
(optical, electrical, and mechanical), all staffed by undergraduate students. The
project has been financed by two NASA grants, the BYU colleges of Physical &
Mathematical Sciences and Engineering Technology, and by a number of private
contributors. The completed telescope, after installation in a NASA GAS canister
with an MDA, will fly in the bay of a space shuttle for the purpose of photographing
the sun. The telescope will take a sequence of 700 photographs of the solar x-ray
emissions in a wavelength band of 171-181 _. The target date for completion of the
Goldhelox payload is the fall of 1992. We are scheduled to fly our telescope on the
STS-57 Atlantis mission scheduled for launch in April 19931 The telescope will
produce full-disk images of the sun with a spatial resolution of 2-2.5 arc seconds,
and a time resolution of less than 0.I seconds. The photographs will be taken at a
number of different levels of image intensification in order to investigate a wide
range of intensities across the solar disk. Once obtained, the photographs will be
analyzed by graduate students and professors at Brigham Young University and
published.
Our bandpass was chosen to sample Fe IX, Fe X, and Fe XI emission lines. The
solar plasma that emits those wavelengths corresponds to temperatures upwards of 0.8
million degrees Centigrade. These temperatures only occur in the region of the
corona near the upper transition region and are characteristic of a number of solar
features such as active regions, flares, micro flares, and x-ray bright spots. Our
observations are designed to address the following questions:
I. What is the structure of solar plumes, and what is the source of the solar
wind?
2. What mechanism causes the initial energy release in a solar flare2?
3. What is the relationship between the occurrence of coronal mass ejections,
the occurrence of microflares, and the high resolution upper transition region
magnetic structures3?
4. How does the transition re_ion between the corona and chromosphere respond
to the impulse phase of flares_?
The purpose of this paper is to introduce the project from an instrumental and
operational point of view. In section 2 we present an overview of the telescope
design. Section 3 addresses the principles of restricting the instrument
environment to reduce cost and complexity. We discuss the methods this project has
i25
https://ntrs.nasa.gov/search.jsp?R=19930003983 2020-03-17T10:14:45+00:00Z
utilized to produce a complex experimental device capable of making a contribution
to solar physics on a low budget (about $i00,000).
II. THE GOLDHELOX TELESCOPE
It is convenient, for the purpose of discussion, to separate our telescope
into three main elements: i) the optical system which collects, filters, amplifies,
images, and records light from the sun, 2) the mechanical structure which supports
and protects the optical and electronic elements of the system as well as providing
the telescope with several motional freedoms, and 3) the electrical system which
monitors and controls all functions of the instrument through the use of two
partially redundant computers. This section describes each of these elements.
The Optical System
The Goldhelox telescope is a two mirror optical system consisting of a concave
parabolic primary mirror and a convex hyperbolic secondary mirror in a cassegrain
configuration. Solar x-rays are first collected by the primary mirror, then
reflected towards the secondary mirror which in turn sends them through the one inch
hole in the center of the primary mirror to the detector assembly. The focal plane
is 2.25" behind the primary mirror.
X-ray reflecting mirrors are more complicated than the common visible-light
mirrors. When dealing with visible, infrared, or even near-ultraviolet light, it is
common to find that a mirror reflects as much as 90% of the incident light
intensity. In the X-ray wavelengths, however, most incident light is absorbed or
transmitted by a mirror rather than reflected. To improve the x-ray reflection
efficiency, we use a mirror material with an internal layer structure radiation can
be reflected off the multiple surfaces in the material and then recombined. The
layer thickness is spaced such that x-rays reflecting off of the individual
surfaces combine constructively, resulting in a stronger reflection at certain
angles. It is the thickness of the layers in the mirror material that determines
these angles. These X-ray mirrors are generally one of two types: i) grazing-
incidence or 2) multiiayer coated.
Grazing-incidence telescopes use the natural layer structure of crystalline
materials to obtain multiple reflections. The lattice spacing in most crystals is,
unfortunately, such that constructive reflection occurs only for incident light that
is nearly parallel to the mirror surface. Hence the name grazing'_ncidence
telescopes. Because effective reflection occurs only at grazing angles, these
telescopes typically have to be long and bulky to effectively focus light into any
sort of image.
Because of the size constraints on our telescope, we have instead elected to
use multilayer mirrors, a more recent development in x-ray optics. They are made by
alternately applying thin coatings of two different materials onto a smooth
substrate. Our mirrors use 100 layers, each consisting of 50 A of Silicon above 50
A of Molybdenum . Because the layers are man made, the layer thicknesses can be
chosen to produce good reflections at almost any angle desired, even normal
incidence. Thus multilayer mirrors have made it possible t0 manipulate x-rays _n
almost the same way that visible light is manipulate_.
Our multilayer mirrors are made to have 40% reflectivity at near normal
incidence for soft x-rays in our bandpass (171A to 181A). They act as partial
filters because light outside this range is not effectively reflected through the
telescope. To properly filter out all the radiation outside our bandwidth, we have
also added two additional radiation filters (i000 _ thick sheets of aluminum).
In spite of the fact that multilayer mirrors significantly improve
reflectivity, intensity losses in the telescope remain a problem. The total
efficiency of the optical system is only about 2%, furthermore only a very small
fraction of the energy emitted from the sun falls in the wavelength range in
question 5. This makes it difficult for an optical instrument as small as ours must
be (3" in diameter) to gather enough light for an adequate film exposure.
Therefore, to amplify the image, we use a microchannel plate (MCP) image
intensification device. The maximum factor by which the MCP can boost the incident
image intensity is about 5000 times (the actual image intensification factor will
vary according to a pre-programmed sequence which will be executed by the
telescope's computer). When the MCP is bombarded by x-rays, the x-rays stimulate
cascades of electrons that are accelerated toward a phosphor screen which emits in a
visible wavelength. A visible light image is then conveyed from the phosphor to the
|
i
m
m
s
E
[]
z--
--z
126
35 mm film via a glass cylinder composed of parallel interfused fiber optic cables,
each cable having an internal diameter of 10 microns.
The Mechanical System
The telescope itself (composed of the optical elements just described and the
mounts that hold them in alignment) is mounted between two vertical support walls 7
inches apart. The walls extend up from a rotating plate (see figure I). This plate
rotates upon the barrel-shaped sealed container containing the telescope control
Computers and other electrical components. The sealed container is airtight to
maintain a normal gaseous atmosphere inside. The telescope also rotates about a
pivot axis extending between it's support walls. The pivot is located near both the
center of the GAS canister and the center of the telescope.
The telescope has two motional freedoms: first, rotation in the azimuthal
plane (the plane of the rotating table), and second, rotation in the plane of
declination ( the plane normal to the upper pivot axis.). The rotating table moves
atop a 10 inch diameter turntable gear (see figure 2). The teeth around the outer
edge of the turntable gear are matched to a smaller gear on the shaft of a stepper
motor. This stepper motor which drives the azimuthal telescope motion is mounted on
top of the rotating table, with it's shaft reaching down through the table.
Rotation in the plane of declination is also driven by a stepper motor, but in
this case a worm gear assembly is used. The motor and worm are attached to one of
the support walls, and the worm gear itself is attached to the telescope body. The
telescope body can assume any declination between vertical and 43 ° off vertical and
can rotate through the entire 360 ° in the azimuthal plane.
Within the telescope itself, the mirrors and mirror mounts are attached to
the telescope housing. The MCP flange, which is mounted inside of the camera, and
acts as an image window. The camera is attached to the telescope housing, and the
housing is fastened to the telescope pivot. The telescope has an alignment
tolerance between 1/10,000 and 1/1,000 inches. Because this tolerance is difficult
to meet with ordinary materials, we have chosen to use a composite with very low
coefficients of thermal expansion for the walls of the housing and other parts of
the mirror mounts. This composite is light, strong, resistant to vibration, and
space environment compatible.
The Electrical System
The electrical system centers around two partially redundant computers. The
first computer receives input from the tracking system and controls the positioning
of the telescope. It also warms up the MCP prior to use and performs picture taking
operations. The second computer is the ultimate instrument controller. It puts the
instrument into a low power hibernation mode when the sun in not in view and
controls the distribution of power to every other element of the electrical system.
It monitors and controls the temperature of the environmentally sensitive elements
of the system, and takes over control of the photographic functions of the MCP in
the event that the first computer fails.
Four power supplies are required to operate the entire instrument. One power
supply provides the various DC voltages between 5 and 20 Volts needed by the many
electrical system circuit elements. Another high current supply drives the three
stepper motors used for telescope positioning and film advancing. The last two,
called switching power supplies, provide the 1000 Volt lead and the 5000 Volt lead
required to operate the MCP. Picture taking is actually done by switching the MCP
on and off using exposure times which will vary according to a pre-program_ed
sequence between .1 and .01 seconds.
Another important feature of the electrical system is the tracking circuitry.
The sun-tracking system provides continuous information to the telescope control
computer regarding the location or absence of the sun in the telescope's field of
view There are three tracking sensors. The first we call the "daylight" sensor,
which we place near the top of the telescope allowing it to indicate to the control
computers whether or not the sun is in the instrument field of view. If the sun is
discovered to be in the field of view, the telescope is rotated through the
azimuthal plane until the second or "coarse" sensor indicates that the sun is within
8 ° of the plane of declination of the telescope. When the coarse sensor tests
positive, the controllers check the third or "fine" sensor. The fine sensor has a
2¥2 array of photodiodes. When a light source illuminates the array, this fine
sensor circuitry determines the amount of light incident on each photodiode and
127
0
0
£0
(D
©!
Q)
/
©
0
128
@
.o
@.,4
@
OIP
oo_
14 UO
X_
I.l
o_
OSO
MO •
|
E
T
2--
iE-
=.
E
Secondapy
i ppop
Te tescope
Fi [tep
= / =
Fi [m Fi [m
Spool [
= 1 =
I
L _NeE __
I I
FIGURE 2: A cross-sectional side view of the
GOLDHELOX telescope, with all optical
components indicated. Dimensions are not to
scale.
129
returns two voltages (an 'x' and a 'y' coordinate) indicating how far off the sun is
from the optical axis of the telescope. The computer accordingly repositions the
telescope and again checks the fine sensor. This procedure is repeated until the
telescope is within 1/10 of a degree of being aimed directly at the sun.
III. _DUCING COST AND COMPLEXITY BY RESTRICTING INSTRUMENT
ENVIRONMENT
There are many instances during the design stage of a space bound instrument
in which cost and complexity can be significantly reduced by restricting the
instrument's environment. In much the same way as a decision to have a barbecue in
the back yard as opposed to a picnic in the park eliminates the need to pack half
the kitchen into the trunk of the car, making a few restrictions on the operating
environment of an instrument can eliminate the need for many of the auxiliary
functions, fail-safes, and precautions that burden the instrument without increasing
the value of the data that it obtai_ _ _ _
When we first began to plan the design of the Goldhelox telescope, we learned
that much could be done on a small budget if we didn't try to do everything. For
example we looked for several months for an extremely sensitive, high resolution
camera film that was vacuum compatible (won't become brittle) over a wide range of
temperatures, and obtained cost estimates from a few companies to custom process or
even develop the film we required at a price of around $1000/yard, with a minimum
purchase requirement of $10,000. Such an expenditure was well out of our reach.
Then a team member suggested that we Just use regular 35 mm film and simply not let
the film be exposed to the space environment. He pointed out that this would be
possible if our MCP were mounted in a standard vacuum flange and sealed onto the
front of the camera with the atmosphere inside. Although the inside of the camera
is now less accessible, this simple design modification saved thousands of dollars
and greatly simplified our film processing requirements.
The most difficult technical problems that our project has had to deal with
has been i) temperature control and 2) tracking our target, the sun. It is
difficult to find affordable optical and electronic components that won't
malfunction or even fail completely at the hot and cold temperature extremes.
Tracking sensors for example, can be purchased from stock, unless you want them
guaranteed at temperatures ranging from -100°C to 100°C (a temperature range which
is standard in a space environment). The tracking system also had three problems of
it's own, in addition to the temperature problem. First, the instrument must be
able to determine when the sun is in view and when it isn't, properly discriminating
against all other sources including the brightly illuminated earth which frequently
fills the entire telescope field of view. Second, the telescope has no advance
information regarding when the sun will be in view. And third, high resolution
pictures require that the telescope maintain it's orientation with respect to the
sun without deviating more than two seconds of arc during picture taking. This is a
challenge even for state of the art tracking systems when the many different types
of orbital motion assumed by the shuttle on a typical mission are all considered.
NASA provided a way for the con_lexity of the electrical system to be greatly
reduced through the availability of standard options, which not only solved our
tracking problem but also eliminated the extreme temperatures previously
anticipated, thus permitting us to purchase less expensive electrical componentS.
By allowing for as many as six actions to be performed for a GAS payload by the
shuttle crew during flight. This way the crew will be activating our instrument
only during a few specific time,intervals in which the shuttle bay is directly
facing the sun. Hence the telescope will only have to cope will minor amounts of
rotation, eliminating the need for complicated tracking subroutines. Also the MDA
on our canister can now be closed during nearly all of the mission, protecting our
instrument from the more extreme temperatures common to open lid flights, and saving
us tens of thousands of dollars of additional expenditure on military specification
electrical components. This illustrates the general principle (which many with
more experience than we have already know) that for cost-effective space research,
regularly consider the possibilities of trading instrument freedom for more
simplicity.
130
SUMMARY
The GOLDHELOX Project is an unusual effort in that it is both managed and
staffed by undergraduate students. The project, if successful, will prove that a
real contribution to space research is possible in a university setting, and on a
low budget. The current trend in space exploration is towards 100% reliability,
requiring researchers to purchase only the highest quality components, and at great
cost. If the GOLDHELOX telescope satisfies the hopes and wishes of it's makers, it
will not only help us gain a greater understanding of the sun, but will set a
precedence for others who are interested in space research, but believe it to be out
of their financial reach.
ACKNOWLEDGMENT
This work is supported by grant #NAGW-2612 from the Education Division and the
Space Physics Division of NASA Headquarters.
REFERENCES
1 "Shuttle Manifest", Final Frontier, Oct. '91, p25
2 "Solar Max 1991 Proposal", NASA publication Pg. 1-7
3 "Solar Soft X-ray Pulsations", Harrison, Astron. Astrophys. 182, p337-347 (1987)
4 "Fast Fluctuations of Soft X-rays from Active. Regions", Simnett & Dennis, NASA
Conf. Pub. 2449, pg123-124
5 "The Importance of Spectroscopy in the 80-800 A Region for Plasma Diagnostics in
the Solar Atmosphere", Feldman & Doschek, Space Science Reviews, 22, 1978
131
i|
Z
_L
• i
Z
J_
_r
mm
L_
F_


G-133: A soft x ray solar telescope

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server