The MAXIM X-ray Interferometry Mission

K. C. Gendreau\*^a^, W.C. Cash ^b^, A. F. Shipley^b^, and N. White ^a^

^a^NASA/Goddard Space Flight Center; ^b^University of Colorado

##### Abstract

The MicroArcsecond Imaging Mission (MAXIM) will resolve the event
horizons of black holes with 0.1 microarcsecond imaging in the X-ray
bandpass. In the NASA "Beyond Einstein" roadmap, MAXIM takes it place as
the "Black hole Imager". In this paper, we will outline the scientific
goals for this mission. We will describe the current state of the
technology- including a discussion of several laboratory demonstrations
of X-ray interferometry. We will describe some engineering studies we
have performed over the past two years.

1.  **INTRODUCTION**

We are now in a golden age for X-ray astronomy with several great
observatories such as XMM-Newton and Chandra in orbit giving us high
quality spectra as well as images comparable to those optical
astronomers have enjoyed for many years. Chandra, in particular, has
provided images nearly as crisp as Hubble Space Telescope (HST). It is
important to note that while HST is diffraction limited at \~0.1
arcseconds, Chandra is not. The diffraction limit for a telescope of
aperture, D, working at a wavelength, λ, is approximately λ/D. A
diffraction limited x-ray (λ∼ 0.1-4 nm) telescope with an aperture
approximately that of Chandra or HST (2 m) would be able to resolve
features sharper than 100 μas- nearly 1000 times finer than HST. As the
apertures approach a few hundred meters, diffraction limited x-ray
telescopes will have angular resolutions nearly a million times sharper
than HST.

The MircoArcsecond Imaging Mission (MAXIM) is currently in the NASA
"Beyond Einstein" roadmap (See figure 1) as the "Black Hole Imager"
(BHI)^1^. With a sparse aperture several hundred meters across, MAXIM
will have better than microarcsecond angular resolution. This capability
allows MAXIM to resolve the event horizon of several black holes.

In the next section, we will discuss science goals for MAXIM. In the
3^rd^ section of this paper, we will describe the baseline design-
including laboratory results from an x-ray interferometry testbed at
Goddard Space Flight Center (GSFC). We will summarize the results of
concurrent engineering studies done for key components of the mission as
well as an end-to-end integrated mission study.

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Figure 1: The \"Beyond Einstein\" Roadmap.

.

2.  **SCIENCE GOALS**

With the observatories we have available today, we can directly resolve
non-solar system objects which have interstellar and larger size scales.
Timing and spectroscopic data allow us to study smaller objects through
modeling. For instance, the temporal variability and spectral structure
of the iron K line (\~6.4 keV) seen in active galactic nuclei (AGN) such
as MGC-6-30-15^2^ tell us part of the story of what is happening to
matter as it makes a final orbit around and plunge into a black hole
event horizon. But direct imaging is needed to confirm the concepts and
further expand our knowledge. When we produce observatories with higher
angular resolution, we will be able to have a fundamentally different
view of the unresolved universe. We will be able to study, in a model
independent way, some of the most interesting objects in the sky. Figure
2 shows some of the power of going beyond arcsecond imaging. To resolve
the structure of AGN jets, stellar coronae and interacting binary stars,
we need at least 1000 times sharper resolution than HST. Another factor
of 1000, for a total improvement of 1 million times finer angular
resolution than HST, will allow us to resolve the event horizons of
super massive black holes.

100 microarcsecond angular resolution will address questions about
stellar astrophysics by directly resolving many of the most famous stars
in the night sky. We will see if coronae of other stars are like our
sun- and then perhaps learn more about our own star. We will measure the
temperature distribution for these coronae. We will see ifplanets affect
these plasmas and measure the sizes and shapes of flare emission
regions. We already know from existing x-ray data that binary stars will
be bright targets for MAXIM. With MAXIM, we will be able to directly
study the dynamo action that creates the plasmas in these systems. In
this way we will use binaries as laboratories for studying the
generation of stellar coronae and winds.

![](media/image2.pct){width="5.659722222222222in"
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MAXIM's resolution will allow us to study supernova remnants in the
Magellenic Clouds and our galaxy in a unique way. We will be able to
make movies of shock waves going through the ISM with frame times of
days. We will watch as the shock wave interact with density and
chemistry variations in the ISM.

MAXIM will allow us to study objects in the Andromeda galaxy as we
currently do within our own galaxy- or better. At M31, our fully
realized angular resolution of better than 1 microarcsecond will
translate to \~1/20^th^ of an AU- clear enough to not only separate
individual stars, but also peer within solar systems. It will provide a
unique vantage on galaxies, and help us to understand our own Milky Way.

MAXIM's resolution will advance our knowledge of Active Galactic Nuclei
(AGN) at many different levels to give us a more complete picture. This
resolution allows us a clear picture of the region immediately around
the event horizon (see the simulation in figure 3). We will be able to
address a key goal of the "Beyond Einstein" effort: "to image directly
matter falling into a black hole". Specifically MAXIM will address 3
issues called out in the Beyond Einstein roadmap^1^:

1)  It will test predictions of general relativity by mapping the motion
    of gas in the vicinity of the event horizon.

2)  It will map the release of energy from black hole accretion disks.

3)  Determine how relativistic jets are formed and see how black hole
    spin affects the process.

This is only a partial list of the discovery space awaiting MAXIM-- as
other leaps in capability have shown us we should "expect the
unexpected". From Galileo's first telescope to HST, imaging made a jump
of only a factor of thirty over about 4 centuries. With that jump, our
understanding of the universe exploded. MAXIM will provide a jump with 6
orders of magnitude improvement- but in only 2 decades. We are bound to
have surprises.

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**3. THE BASELINE DESIGN**

The University of Colorado at Boulder (CU) has designed and tested a
very simple X-ray interferometer^3^ as shown in figure 4. We call this
the "x configuration". The optical components are simple flats. For a
given channel of the interferometer, a primary mirror collects x-rays
from the target and directs them to a secondary mirror. The secondary
mirror acts as a combiner by steering the x-rays to a common focal
plane. At the focal plane, the different channels interfere and fringes
appear. CU built such an interferometer and successfully tested it at
NASA/Marshall Space Flight Center with \~1 nm wavelength light^3,4^.
GSFC has also built such an interferometer and obtained fringes at 23
and 8.35 angstroms this fall (figure 6). This very simple design is the
core of our baseline concept for MAXIM. As we will show later, the
grouping of the mirrors can be optimized to minimize the overall
engineering challenge.

This type of interferometer works for a number of reasons. First, the
individual optical flats are diffraction limited in the way they are
being used^3^. This means that mirror position stability and mirror
figure errors contribute only a fraction of an x-ray wavelength worth of
path length error. For optics used at normal incidence with more
conventional wavelengths (e.g. λ \> 1000 angstroms), this would mean
that the surface figure and placement is good to a fraction of the
wavelength (e.g., 100s of angstroms @ optical wavelengths). However, at
a grazing incidence angle, θ, the required surface quality is loosened
by a factor of 1/(sinθ) (see figure 5). Our x-ray optics work at
incidence angles less than 2 degrees which makes this surface quality
easing factor nearly 2 orders of magnitude. Thus optics which are
diffraction limited for normal incidence UV applications are diffraction
limited for grazing incidence x-ray applications. In the CU/MSFC and
GSFC demonstrations, we used better than "λ/20" (λ=633 nm) optics at
0.25 degree graze angles. For MAXIM, we will want to work at \~ 2 degree
graze angles, thus we will need somewhat better optics (better than
"λ/200" for λ=633 nm), but these are achievable. In fact, these are much
looser figure requirements than those on the mirror for the coronagraph
option for TPF. Second, since the optics are flat with infinite focal
lengths, they are easy to position. Similarly, the depth of focus of the
interferometer is very deep. For the ground demonstrations that have
taken place so far, the depth of focus was 10s of centimeters. For
MAXIM, it will be 10s to 100s of meters.

A disadvantage of the flats is that usual magnification techniques do
not work. For this basic design, the magnification is given by the ratio
of the spacing of the primary mirrors to the spacing of the secondary
mirrors. If the secondaries were a meter apart, then fringes from 1 nm
light would be only 10 nm apart from each other on a detector 10 meters
away. Simple detectors not relying on spatial heterodyne methods have a
spatial resolution limit of about 1 micron. The only way to magnify the
fringes is to move the focal plane further away from the optics.
Quantitatively, detectors must be 10s to 1000s of kilometers away from
the in order to get a good matching of detector pixel sizes to fringe
spacing.

![](media/image5.png){width="4.129861111111111in"
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Figure 4: The core concept behind the MAXIM interferometer design.

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Figure 6: Fringes from the GSFC X-ray interferometer testbed at 8.35
angstroms (the aluminum K line).

For this reason and also to allow for larger baselines and sharper
angular resolution, it is necessary to break up our interferometer into
separate formation flying spacecraft. The details of how we implement
our core concept shown in figure 2 determine the formation flying
tolerances. As we will show below, the precision formation flying
requirements are on the order of microns using our baseline
implementation with an optimized grouping of the mirrors. This is loose
compared to some other proposed formation flying missions- but still
beyond our current technological grasp.

In the following sections, we describe how we will implement our core
concept into an x-ray imaging mission. We start by describing how a good
grouping of the optics both makes our tolerances achievable as well as
offers other considerable advantages. We then discuss the detector
requirements. We overview the internal metrology requirements. We
briefly describe how we point this interferometer to acquire and hold a
target. Finally, we present an integrated mission design concept.

**3.1 Periscopes**

Given our basic concept of using flats as shown in figure 2, we have
some latitude in choosing how we group the mirrors physically. For
example, we could fly the primary mirrors in separate spacecraft from
the secondary mirrors. This was one of our original implementations
considered for MAXIM. However, this choice forces us to have a very
difficult attitude control system (ACS) since the mirrors- and therefore
the spacecraft- need to be pointed to the x-ray diffraction limit of the
individual optics- milliarcseconds for X-rays with the size apertures we
are considering. Further more, it works out that a very sensitive
component of our optical path length control is the distance between the
primary and secondary mirror. Thanks to the grazing incidence advantage,
this tolerance is of order nanometers- which translates directly into a
formation flying tolerance for this grouping where the primaries are
flown separately from the secondary mirrors. In this earlier effort on
MAXIM to fly the primaries separately from the secondaries, we had all
the secondaries in one "combiner spacecraft", which became a single
point failure risk for the full mission. Additionally, this design
limited our UV plane coverage to a ring in the sky- which resulted in
considerable wasted collecting area.

A more practical choice of the mirror grouping is to put the primary and
secondary mirrors within the same spacecraft. All the optics for one
channel of the interferometer are now within one spacecraft. The pair of
mirrors behaves like a thin lens- the pair can tip and tilt, but a ray
passing through the system does not change its direction.

It is effectively a periscope. There are limits to the thinness of these
optical systems. First, x-rays reflect at grazing angles of less than 2
degrees. Second, there is some optical path length change induced by
rotations. To keep the OPD due to a rotation less than about 1 angstrom,
our periscopes must pitch no more than 2 mas- similar to the diffraction
limit. This is too fine a pitch control for conventional satellite ACS
systems. The best star trackers will only give you knowledge to about 1
arcsecond On HST, the fine guidance sensor (FGS) gives a reference on
pointing to about 10 mas- but is very massive and expensive.

In order to make these periscope modules "thin" in regards to rotation
insensitivity, we have added two more flat mirrors down the optical path
from the original two mirrors (figure 7). The addition of the additional
pair of mirrors gives the module of 4 mirrors much looser rotational
tolerance- approximately 10 arcseconds- before the optical path length
of the channel is more than 1 angstrom. The cost of the extra four
mirrors is additional weight as well as reduced reflectivity. But the
benefits of going to 4 mirrors greatly outweigh the disadvantages.

With a 4 mirror periscope module for each channel of the interferometer,
we can realize:

1)  Very loose module to module alignment tolerances- in the case of our
    full system design, we need only hold the modules relative to each
    other at the level of 10 microns in directions perpendicular to our
    optical axis and \~1 foot along the optical axis before OPDs are
    introduced at the level of 1 angstrom.

2)  Easy attitude control requirements for individual modules in our
    array. We can use conventional star trackers available today to
    point one of these modules to better than 10 arcseconds. This is the
    specific reason we have gone to a 4 mirror design- but it allows us
    to benefit from the general advantages of having periscope
    components for our individual channels.

3)  Because each periscope can also act as an adjustable delay line, we
    can place apertures anywhere in the UV plane. We can have fully
    nonredundant baselines, which will result in the concentration of
    more power within our central beam.

4)  High redundancy. If one mirror module fails, it is not a loss of the
    whole mission.

5)  Affordability. In our initial GSFC engineering studies, we have
    concluded that we would benefit greatly from mass production savings
    using "Price H" -the same cost estimating engine used to predict US
    Department of Defense system costs. Specifically, we have estimated
    that to make enough modules to get 1000 cm^2^ of collecting area-
    including all the structural, thermal-mechanical and electrical
    control systems required less than \$60 Million. This is a fraction
    of what the mirror on Chandra cost for the same collecting area. We
    still need satellite infrastructure, but, again, mass production of
    components will save us money.

We have conducted an initial concurrent engineering study of a 4 mirror
periscope module. In this study, we designed a structure which would
hold 4 mirrors in position, allow us to introduce path length and mirror
angle control, and maintain better than lambda/400 mirror surface
figure. The design includes shutters as well as mounts for holding the
module to a satellite bus. Figures 7 and 8 show the basic design as well
as a visualization of modules with a total of 1000 cm^2^ of area mounted
on simple satellite buses mounted around a detector space craft inside a
Delta 4 shroud. The most tantalizing advantage of the periscope
implementation is how it loosens our formation flying tolerances. As
discussed in Shipley, et al^5^, we only have to hold periscopes in
position with a stability of \~\<10 microns. While difficult, this is
much easier than maintaining nanometer positions- particularly in the
case of formation flying. Table 1 gives a breakdown of the tolerance
requirements for mirrors relative to each other within the periscope
module as well as the periscope to periscope alignment tolerance.

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+-----------------------+-----------------------+-----------------------+
| **DOF**               | **Mirror**            | **Periscope**         |
|                       |                       |                       |
|                       | **Tolerance**         | **Tolerance**         |
+-----------------------+-----------------------+-----------------------+
| ###### X              | ±1.7 nm               | ±4 μm                 |
+-----------------------+-----------------------+-----------------------+
| **Y**                 | ±0.3 mm               | ± 0.5 mm              |
+-----------------------+-----------------------+-----------------------+
| **Z**                 | ±94.7 nm              | ±0.32 m               |
+-----------------------+-----------------------+-----------------------+
| **X-rot (yaw)**       | ±6.9 (arcmin)         | ± 7.8 arcmin          |
+-----------------------+-----------------------+-----------------------+
| **Y-rot (pitch)**     | ±2.3 marcsec          | ± 10 arcsec           |
+-----------------------+-----------------------+-----------------------+
| **Z-rot (roll)**      | ±0.13 arcsec          | ±18.5 arcsec          |
+-----------------------+-----------------------+-----------------------+

Table 1: Tolerances for mirrors within a periscope module as well as the
periscope to periscope alignment tolerance. Note the huge difference for
tolerances of mirrors relative to each other within the periscope and
the periscope to periscope tolerances

**3.2 Detector**

To minimize the distance between the detector and the optics, we want
the smallest pixels possible. X-ray microcalorimeters offer high energy
resolution, but it is difficult to get the individual pixel sizes below
100 microns. The best hope for small pixel microcalorimeters may be the
PoST position sensitive transition edge sensors of
Figuerora-Feliciano^12^ which will be able to resolve 10s of microns at
the expense of some energy resolution. In addition, the lateral control
requirement of our detector space craft is driven by the size of the
detector array. A bigger detector array leads to a loose control
requirement. At this point, microcalorimeter arrays are very small.
Perhaps an array a few centimeters across will be available in the
future, but we are far from that now. X-ray CCDs have been made with
pixel dimensions as small as 4 microns and do not require the
complicated cooling systems which calorimeters need. Physical processes
within CCDs such as the range of the photoelectron and diffusion after
the photoelectric interaction limit the spatial resolution of these
devices for X-ray work to a couple microns. Furthermore, CCD focal
planes can be made with dimensions the size of dinner plates. For these
reasons and others, MAXIM has CCDs as the baseline detector.

For MAXIM, we are baselining CCDs with 15 micron pixels to oversample
fringes from our interferometer by a factor of 5-10. For 100
microarcsecond imaging, this puts our detector about 200 km away from
the optics. To get to 1 microarcsecond, our detector needs to be about
20,000 km away. We want to minimize this distance as slewing costs grow
somewhat more than linearly with spacecraft distance. This same
dimension also defines the formation flying tolerances on the
positioning of the periscope modules. Thus, we see that the choice of
the CCD pixel size has important ramifications on the formation flying.
We have made our choice to both minimize the distance between optics and
detector spacecrafts and also to minimize the tolerance on formation
flying control. These reasons lead to contradictory requirements on the
pixel size- but at this point, 15 microns seems optimal.

CCDs can be assembled into large focal planes. Several ground based
telescopes have CCD cameras with focal planes nearly a foot across. The
SNAP mission is considering a CCD array nearly 30 cm across^8^. We will
use a similarly large focal plane- even though we expect our targets to
fill up FOVs of only a centimeter or two across (10 mas @ 200 km, 100
microarcseconds @ 20,000 km). The larger focal plane will help us in 3
different ways: 1) it will help in acquisition of the target (to be
discussed below), 2) it loosens the formation flying control
requirements for the detector spacecraft, 3) it reduces risk if a CCD
fails. During acquisition, we would quickly readout each part of our
large focal plane to find where the target is landing. Once we have
centered on the target, we will only readout the region of interest,
reducing power and readout times. A 30 cm focal plane gives a
detector-limited FOV of nearly half an arcsecond which which is similar
to a single resolution pixel of Chandra. The CCD readout of the target
region will be accomplished in less than 100 milliseconds which also has
implications on the formation flying control. With these fast readouts,
control can be replaced with knowledge for the formation flying
situation. We will discuss this in the "Line of Sight" section below.

If the periscopes are placed to minimize redundancies in baselines, then
we need less energy resolution in the detector to distinguish power in
our core image from power in outside ghost fringes. Blurring due to
differing plate scales from different wavelength light is also small. A
CCD with 2% energy resolution will produce energy resolved images with
negligible plate scale smearing across our targets of interest..

**3.3 Metrology**

There are several levels of metrology in MAXIM. Pointing individual flat
mirrors within a module to the diffraction limit requires approximately
30 nm of linear control on one edge of the mirror. Once we have pointed
one mirror so that x-rays from it hit the secondary mirror, we will want
to adjust the spacing between the mirrors to give us some path length
control. Our periscope module engineering study has baselined
capacitance gauges for this. Next, we want to be able to control the
relative positions of periscope modules with respect to each other to 15
microns. JPL is developing the MSTAR system^9^ for separated spacecraft
interferometry missions including Terrestrial Planet Finder (TPF) and
Planet Imager (PI). This system should provide sub micron positions over
kilometers of range using an RF modulated laser system.

**3.4 Line of Sight Alignment and Target Acquisition**

Perhaps the most difficult issue for MAXIM is the target acquisition and
maintenance of the tremendously tight line of sight (LOS) alignment.
Notice that this is different from pointing any individual component of
our interferometer to the target. For example, the huge depth of focus
affords the pointing of the detector a looseness of degrees . As
discussed above, the periscope modules act like thin lenses and can
possibly wobble by many arc seconds without image distortion. The hard
task is that we need to determine the relative alignment of the
positions of all the periscopes with respects to a line between the
detector and our target to the level of microarcseconds.

We are considering using high precision gyroscopes and laser beacons in
the following innovative way^6^:

1)  Use conventional star trackers to point both the hub and the
    detector spacecraft to within 1 arcsecond of the target.

2)  Position the detector spacecraft behind the optics spacecraft. Put a
    \~1 Watt laser with \~ arcsecond divergence on the back end of the
    optics hub pointing toward the detector spacecraft. This laser
    beacon would be similar in design to the high power laser system
    being developed for LISA^10^. With the conventional star tracker on
    the detector spacecraft, this laser will appear as a very bright
    star. The detector spacecraft will position itself so that this
    artificial star or beacon appears to be within 1 arcsecond of where
    we expect to see the target. Radio ranging will provide enough
    information to put the detector spacecraft at the correct distance
    (at focus) behind the optics spacecraft.

3)  The size of our CCD focal plane is very large, nearly one arcsecond
    across at the detectors position behind the optics hub. We can
    relatively quickly scan through all the individual CCDs of this
    focal plane looking for the image from the periscopes. If we do not
    see this image, we shift the detector spacecraft laterally until we
    do. Once we find the target, we readout just the region of interest
    with high speed.

4)  At this point, we make use of the high precision gyroscopes, which
    sit on the detector spacecraft.. First we fine point the detector
    spacecraft so that the optics hub beacon is centered on a high
    quality telescope with a quad cell detector- like the GP-B quartz
    telescope. This telescope will be approximately f/40 with a 120 mm
    aperture. At its focal plane will be a quad cell detector consisting
    of roof prisms, beam splitters and high speed photomultipliers. The
    telescope will be able to detect if the beacon moves at the 0.3
    microarcsecond level with 10^15^ photons, which is achievable with 1
    second of integration and a 1 Watt / arcsecond diversion laser at
    distances \~20,000 km.

5)  There are several ways that the beacon will appear to move in the
    detector craft "quartz" telescope. First, the detector spacecraft
    might pitch and yaw. Second, the detector spacecraft could move
    laterally off the line of sight determined by the target and the
    optics hub. Third, it could do both. We are only concerned about the
    lateral motion components. Since our depth of focus huge, the
    detector can pitch and yaw without loss of performance. To break the
    symmetry, we make use of the gyroscope. The GP-B gyroscope will
    probably be the most stable gyroscope made by man in the next
    decade. Its drift will be less than 1/3 of a microarcsecond per day.
    Some changes in the readout scheme of GP-B's gyroscope could allow
    us to measure pure rotation. Depending on the readouts of both the
    gyroscope and telescope, the detector spacecraft will either
    translate or rotate to maintain the alignment of the LOS.

Currently we consider this to be the most viable method for the line of
sight alignment. A fuller discussion of how we came to this as well as
other options considered is given elsewhere^6^. We are looking at
alternatives to the GP-B gyroscope. These include atomic interferometric
gyroscopes as well as super fluid gyroscopes. This is the most
technically challenging issue for MAXIM, and more development is needed
.

**3.5 Mission Implementation**

In May of 2002, we took our current concepts for a scaled down version
of MAXIM (MAXIM Pathfinder-MP) to the Integrated Mission Design Center
(IMDC) at NASA/GSFC^11^. This scaled down version used a simple
approximation to the periscopes and only accounted for about 75 cm^2^ of
collecting area. While we still need to run the full MAXIM mission
through this design center, this pathfinder study is useful for
estimating the needs of a full MAXIM mission. The IMDC brings together
engineers from all aspects of a NASA space mission, , integration and
testing, launch, mission operations, communications, orbital dynamics,
and more, to perform a concentrated 1 week study for a customer. They
try to understand the feasibility of putting the payload into space and
running it. The IMDC estimates total mission costs, determines the
optimal launch vehicle, points out long poles, and more.

The IMDC prepared a mission profile which had 4 phases as shown in
figure 9. A Delta-IV 4240 will take the entire multi-spacecraft
observatory to an orbit around the Earth-Sun L-2 position. The transit
phase from Earth to the L-2 station will take 6 months. Once at station,
MP will operate in science phase 1: a detector spacecraft will fly in
formation 200 km from a single optics spacecraft. After a year of
observing about 1 target per week at \~ 100 microarcsecond angular
resolution, MP will enter its second science phase. In science phase 2,
MP's optic spacecraft will split into a hub module containing 9
periscope modules and 6 free flyer spacecraft containing 11 periscopes
each (figure 10). The periscopes here are much more simple
approximations to what we have designed in later engineering studies as
shown in figures 7 and 8. However, the total mass and power requirements
for the number of periscopes used in each free flyer are nearly
comparable to that of our more refined model for the periscopes (within
a factor of 4). The hub spacecraft will contain a 1 Watt laser with a 30
cm beam expander similar to that used for the LISA mission. This
powerful laser will be used for the gyroscope/"quartz telescope" based
LOS alignment with the detector spacecraft 20,000 km away. A laser
ranging system such as MSTAR will be used to range at the micron level
the various free flyer spacecraft. Science phase 2 was designed to have
a 3 year life observing targets for about 3 weeks at a time- which is
approximately how we expect to operate the full mission.

We chose L-2 for the orbit in order to minimize gravity gradient forces.
We could have also chosen a heliocentric orbit which would drift away at
\~ 0.1 AU per year. At either orbit, solar radiation pressures of about
1 microPascal are the dominant environmental force. Orbital
considerations for the pathfinder mission of this study are no different
than for the full MAXIM mission

In figure 10, we show the optics spacecraft in both the science phase 1
and science phase 2 configurations. It consists of 7 hexagon shaped
spacecraft. The central unit is the hub while the 6 outer units will
eventually become the free flyers. Each unit is about 1 meter across.
The hub holds laser alignment beacons for the Line-Of-Sight alignment
described above. In science phase 1, we use a milliwatt laser to get
enough photons onto our LOS telescope on the detector spacecraft. In
science phase 2, we use a LISA like 1-watt narrow divergence laser to
generate more photons for the LOS telescope in order to achieve
microarcsecond knowledge. The hub also acts as an intermediate
communication port between the freeflyer crafts and the detector
spacecraft. The freeflyers use star trackers to look at alignment
beacons on the hub spacecraft to determine their longitudinal and
azimuthal positions. They also make use of an MSTAR like laser ranging
system to get the radial ranges from corner cubes on the hub spacecraft.

The thermal stability requirements for the optics spacecraft is of order
+/- 1 degree in order to control the placement of the periscopes to \~15
microns. The periscopes themselves are maintained at 20C+/- 0.1 degrees
in order to maintain the high mirror figure quality. During phase 1, the
thermal management is

![](media/image9.jpeg){width="6.0in" height="4.5in"}

Figure 9: Launch, transfer, science phase 1, and science phase 2 of the
MAXIM Pathfinder mission.

handled by the radiators of each hexagon unit being louvered shut with
the exception of one on the outside, which is coupled to the hub hexagon
via a removable heat pipe.

The positions of the radiators as well as the positions of the solar
panels and laser ranging systems forces the clocking of the free flyers
in science phase 2 to be fixed with respect to the Sun. Thus, the free
flyers are not interchangeable.

In figure 11, we show the detector spacecraft for this pathfinder study.
For the full MAXIM mission the detector spacecraft would be no
different. This spacecraft will communicate with Earth on a daily basis
using DSN. There are two key science instruments on the detector
spacecraft.. First, we have a large 30cm on a side CCD focal plane. Its
view is collimated to about 0.5 degrees to reduce the cosmic x-ray
background. The full array is used for acquisition of the target. Once
the target is acquired, we readout small regions of interest. The large
size also allows us some comfort in the lateral control of the
spacecraft. We still need extremely fine lateral displacement knowledge-
to 15 microns. The second science instrument on the detector spacecraft
handles this responsibility. It is the gyroscope/"Quartz telescope"
instrument described above and elsewhere^6^.

![](media/image10.jpeg){width="4.825in" height="3.609027777777778in"}

Figure 10: The optic modules: grouped together for science phase 1 and
separated for phase 2.

![](media/image11.jpeg){width="5.3125in" height="3.984027777777778in"}

Figure 11: The detector spacecraft

The propulsion systems for this mission span a dynamic range of 5 orders
of magnitude of thrust. In order to slew the observatory from one target
to the next in a reasonable time (\~1day for science phase 1 and \~ 1
week for science phase 2), we need large thrusts of order 0.02 Newtons.
But to compensate for environmental forces such as solar pressure, we
need \~0.3 microNewton control. At the high end, hydrazine thrusters
will be used for orbit stabilization. For fine formation control, pulsed
plasma thrusters (PPTs) will be used.

Two independent costs estimates from the IMDC agree to within 2% that
the total mission costs would be about \$550M (\$650M with contingency)
for this pathfinder mission. This includes the instrument costs, total
integration and testing, launch, and mission operations for 4 years.
While cost estimates of the CCD and the optic modules are relatively
easy, estimating the price of the LOS system is more difficult. We have
estimated that it would cost \$100M to reproduce the line of sight
alignment gyroscope. The GSFC cryogenics branch feels that the use of
cryocoolers should result in a negligible cost for cooling compared to
the dewar now used on GP-B.

To extend this study toward the full mission, we need to make the
following changes:

1)  Include the details of our more refined periscope module design.
    This will lead to minor packaging changes within the freeflyer space
    craft as well as some rescaling of the free flyer dimensions and
    thruster requirements.

2)  Include more free flyers to carry the total collecting area of MAXIM
    (1000 cm^2^). In our periscope engineering study, we decided to put
    two sets of modules containing a total of 4 periscopes on each
    freeflyer. The total collecting area per freeflyer would then be
    nearly 40 cm^2^. To get the 1000 cm^2^ of area, we need 25 free
    flyers, while our pathfinder study included only 7. We did a simple
    packaging analysis and concluded that the Delta 4 Heavy would have
    enough capacity to handle the extra space craft. Furthermore, we
    benefit from mass production savings.

We plan on conducting this enhanced full mission design study this
coming year.

**4. SUMMARY**

We have presented the latest concepts for the MAXIM mission. MAXIM will
resolve the event horizon of super massive black holes with enough
collecting area in the right bandpass to reveal the mysteries in the
regions near the Schwarzschild radius.

We have described how a good grouping of the various interferometer
components can have a significant impact on the mission implementation.
The periscope configuration allows us to consider micron as opposed to
nanometers levels of formation flying control. Periscopes offer many
other advantages which also help to make the mission easier.

Our biggest challenge pole is the Line-of-Sight alignment for the
various interferometer components. Currently, we are considering using
high precision gyroscopes, possibly modified versions of those on GP-B-
coupled with a beacon alignment system. This is a completely new
approach which has many advantages but still needs some development.

We anticipate a launch circa 2020 for MAXIM. With this launch, we will
sharpen our pictures of the sky by 6 orders of magnitude and vastly
expand our knowledge of the universe.

**ACKNOWLEDGEMENTS**

The GSFC IMDC and ISAL teams played invaluable roles in making this
study possible. Jesse Leitner and Landis Markley worked hard to keep our
feet on the ground. We also thank the members of the MAXIM science study
team- particularly Chris Reynolds. We also had many useful discussions
with the GSFC Interferometry working group. Zaven Arzoumanian and Steve
Queen at GSFC have played key roles in getting the GSFC X-ray
interferometer testbed working.

**REFERENCES**

1\. http://universe.gsfc.nasa.gov/be/roadmap.html

2\. Tanaka, *et al.*, Nature 375, 659, 1995.

3\. Cash, W. *et al.* Nature 407, 14 September 2000.

4\. Joy, M. Shipley, A., Cash, W. Carter, J. "Results from a Grazing
Incidence X-ray Interferometer", Proc. Soc. Photo-Opt Instr. Eng.
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> 5\. Shipley, A., Cash, W., Gendreau, K. "MAXIM interferometer
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