The European Space Agency (ESA) is studying the ATHENA (Advanced Telescope for High ENergy Astrophysics) X-ray telescope, the second L-class mission in their Cosmic Vision 2015 – 2025 program with a launch spot in 2028. The baseline technology for the X-ray lens is the newly developed high-performance, light-weight and modular Silicon Pore Optics (SPO). As part of the technology preparation, ruggedisation and environmental testing studies are being conducted to ensure mechanical stability and optical performance of the optics during and after launch, respectively. At cosine, a facility with shock, vibration, tensile strength, long time storage and thermal testing equipment has been set up in order to test SPO mirror module (MM) materials for compliance with an Ariane launch vehicle and the mission requirements. In this paper, we report on the progress of our ongoing investigations regarding tests on mechanical and thermal stability of MM components like single SPO stacks with and without multilayer coatings and complete MMs of inner (R = 250 mm), middle (R = 737 mm) and outer (R = 1500 mm) radii.
For more than a decade, cosine has been developing silicon pore optics (SPO), lightweight modular X-ray optics made of stacks of bent and directly bonded silicon mirror plates. This technology, which has been selected by ESA to realize the optics of ATHENA, can also be used to fabricate soft gamma-ray Laue lenses where Bragg diffraction through the bulk silicon is exploited, rather than grazing incidence reflection. Silicon Laue Components (SiLCs) are made of stacks of curved, polished, wedged silicon plates, allowing the concentration of radiation in both radial and azimuthal directions. This greatly increases the focusing properties of a Laue lens since the size of the focal spot is no longer determined by the size of the individual single crystals, but by the accuracy of the applied curvature. After a successful proof of concept in 2013, establishing the huge potential of this technology, a new project has been launched in Spring 2017 at cosine to further develop and test this technique. Here we present the latest advances of the second generation of SiLCs made from even thinner silicon plates stacked by a robot with dedicated tools in a class-100 clean room environment.
Continuing improvement of Silicon Pore Optics (SPO) calls for regular extension and validation of the tools used to model and predict their X-ray performance. In this paper we present an updated geometrical model for the SPO optics and describe how we make use of the surface metrology collected during each of the SPO manufacturing runs. The new geometrical model affords the user a finer degree of control on the mechanical details of the SPO stacks, while a standard interface has been developed to make use of any type of metrology that can return changes in the local surface normal of the reflecting surfaces. Comparisons between the predicted and actual performance of samples optics will be shown and discussed.
While predictions based on the metrology (local slope errors and detailed geometrical details) play an essential role in controlling the development of the manufacturing processes, X-ray characterization remains the ultimate indication of the actual performance of Silicon Pore Optics (SPO). For this reason SPO stacks and mirror modules are routinely characterized at PTB’s X-ray Pencil Beam Facility at BESSY II. Obtaining standard X-ray results quickly, right after the production of X-ray optics is essential to making sure that X-ray results can inform decisions taken in the lab. We describe the data analysis pipeline in operations at cosine, and how it allows us to go from stack production to full X-ray characterization in 24 hours.
Within the ATHENA optics technology plan, activities are on-going for demonstrating the feasibility of the mirror module Assembly Integration and Testing (AIT). Each mirror module has to be accurately attached to the mirror structure by means of three isostatic mounts ensuring minimal distortion under environmental loads. This work reports on the status of one of the two parallel activities initiated by ESA to address this demanding task. In this study awarded to the industrial consortium, the integration relies on opto-mechanical metrology and direct X-ray alignment. For the first or “indirect” method the X-ray alignment results are accurately referenced, by means of a laser tracking system, to optical fiducial targets mounted on the mirror modules and finally linked to the mirror structure coordinate system. With the second or “direct” method the alignment is monitored in the X-ray domain, providing figures of merit directly comparable to the final performance. The integration being designed and here presented, foresees combining the indirect method to the X-ray direct method. The characterization of the single mirror modules is planned at PTB’s X-ray Parallel Beam Facility (XPBF 2.0) at BESSY II, and the integration and testing campaign at Panter. It is foreseen to integrate and test a demonstrator with two real mirror modules manufactured by cosine.
The work on the definition and technological preparation of the ATHENA (Advanced Telescope for High ENergy Astrophysics) mission continues to progress. In parallel to the study of the accommodation of the telescope, many aspects of the X-ray optics are being evolved further. The optics technology chosen for ATHENA is the Silicon Pore Optics (SPO), which hinges on technology spin-in from the semiconductor industry, and uses a modular approach to produce large effective area lightweight telescope optics with a good angular resolution. Both system studies and the technology developments are guided by ESA and implemented in industry, with participation of institutional partners. In this paper an overview of the current status of the telescope optics accommodation and technology development activities is provided.
Silicon Pore Optics (SPO), developed at cosine with the European Space Agency (ESA) and several academic and industrial partners, provides lightweight, yet stiff, high-resolution x-ray optics. This technology enables ATHENA to reach an unprecedentedly large effective area in the 0.2 - 12 keV band with an angular resolution better than 5''. After developing the technology for 50 m and 20 m focal length, this year has witnessed the first 12 m focal length mirror modules being produced. The technology development is also gaining momentum with three different radii under study: mirror modules for the inner radii (Rmin = 250 mm), outer radii (Rmax = 1500 mm) and middle radii (Rmid = 737 mm) are being developed in parallel.
Silicon Pore Optics is a high-energy optics technology, invented to enable the next generation of high-resolution,
large area X-ray telescopes such as the ATHENA observatory, a European large (L) class mission with a launch
date of 2028. The technology development is carried out by a consortium of industrial and academic partners and
focuses on building an optics with a focal length of 12 m that shall achieve an angular resolution better than 5”.
So far we have built optics with a focal length of 50 m and 20 m.
This paper presents details of the work carried out to build silicon stacks for a 12 m optics and to integrate them
into mirror modules. It will also present results of x-ray tests taking place at PTB’s XPBF with synchrotron
radiation and the PANTER test facility.
The Laue lens is a developing technology for focusing soft gamma-rays, that is based on the principle of Bragg diffraction. A suitable arrangement of diffracting crystals is used to concentrate a set of parallel incoming photons onto a common focal spot. In late 2014, the Laue lens assembly station (LLAS) at UC Berkeley was used to construct a prototype lens segment, consisting of 48 5 x 5mm2 crystals - 36 iron and 12 aluminium. The segment is composed of 8 partial rings, each of which is aligned to diffract an energy between 90 and 130 keV. In December 2015 the prototype was tested and calibrated using the LLAS and results are presented here. The crystal mounting speed, accuracy of crystal position and orientation, and crystal reflectivity are addressed.
The Advanced Telescope for High-Energy Astrophysics, Athena, selected as the European Space Agency's second large-mission, is based on the novel Silicon Pore Optics X-ray mirror technology. DTU Space has been working for several years on the development of multilayer coatings on the Silicon Pore Optics in an effort to optimize the throughput of the Athena optics. A linearly graded Ir/B4C multilayer has been deposited on the mirrors, via the direct current magnetron sputtering technique, at DTU Space. This specific multilayer, has through simulations, been demonstrated to produce the highest reflectivity at 6 keV, which is a goal for the scientific objectives of the mission. A critical aspect of the coating process concerns the use of photolithography techniques upon which we will present the most recent developments in particular related to the cleanliness of the plates. Experiments regarding the lift-off and stacking of the mirrors have been performed and the results obtained will be presented. Furthermore, characterization of the deposited thin-films was performed with X-ray reflectometry at DTU Space and in the laboratory of the Physikalisch-Technische Bundesanstalt at the synchrotron radiation facility BESSY II.
ATHENA (Advanced Telescope for High ENergy Astrophysics) is being studied by the European Space Agency (ESA) as the second large science mission, with a launch slot in 2028. System studies and technology preparation activities are on-going. The optics of the telescope is based on the modular Silicon Pore Optics (SPO), a novel X-ray optics technology significantly benefiting from spin-in from the semiconductor industry. Several technology development activities are being implemented by ESA in collaboration with European industry and institutions. The related programmatic background, technology development approach and the associated implementation planning are presented.
A new X-ray parallel beam facility (XPBF 2.0) has been installed in the laboratory of the Physikalisch-Technische Bundesanstalt at the synchrotron radiation facility BESSY II in Berlin to characterize silicon pore optics (SPOs) for the future X-ray observatory ATHENA. As the existing XPBF which is operated since 2005, the new beamline provides a pencil beam of very low divergence, a vacuum chamber with a hexapod system for accurate positioning of the SPO to be investigated, and a vertically movable CCD-based camera system to register the direct and the reflected beam. In contrast to the existing beamline, a multilayer-coated toroidal mirror is used for beam monochromatization at 1.6 keV and collimation, enabling the use of beam sizes between about 100 μm and at least 5 mm. Thus the quality of individual pores as well as the focusing properties of large groups of pores can be investigated. The new beamline also features increased travel ranges for the hexapod to cope with larger SPOs and a sample to detector distance of 12 m corresponding to the envisaged focal length of ATHENA.
The Geant4 based ray-tracer used to support the development of Silicon Pore Optics is being extended to take into account more subtle effects that affect the performance of the optics, like thermo-mechanical stresses and detailed surface metrology. Its performance has also been increased to make it possible to simulate rapidly and in detail the optics of Athena so that various possible configurations can be explored and characterized providing important feedback to the development and system teams. In this paper we report on the state of the development.
The ATHENA mission, a European large (L) class X-ray observatory to be launched in 2028, will essentially consist of an X-ray lens and two focal plane instruments. The lens, based on a Wolter-I type double reflection grazing incidence angle design, will be very large (~ 3 m in diameter) to meet the science requirements of large effective area (1-2 m2 at a few keV) at a focal length of 12 m. To meet the high angular resolution (5 arc seconds) requirement the X-ray lens will also need to be very accurate. Silicon Pore Optics (SPO) technology has been invented to enable building such a lens and thus enabling the ATHENA mission. We will report in this paper on the latest status of the development, including details of X-ray test campaigns.
The Laue lens is a technology for gamma-ray astrophysics whereby gamma-rays of particular energies can be focused by a suitable arrangement of crystals. The Laue lens assembly station at UC Berkeley was used to build a technological demonstrator addressing the key issues of crystal mounting speed, crystal position and orientation accuracy, and crystal reflectivity. The new prototype is a lens segment containing a total of 48 5 x 5 mm2 crystals - 36 Iron and 12 Aluminium. The segment is composed of 8 partial rings, each of which is aligned to diffract an energy between 95 and 130 keV from a source at 12:5m with a focal length of 1:5 m.
Cosine has developed the technology to bend and directly bond Si mirror plates in order to produce stiff, lightweight Xray optics which are used for large area space based X-ray telescopes. This technology, Silicon Pore Optics (SPO), also allows us to produce other types of high energy optics. Here we present the latest developments in the design and manufacture of a new generation of soft gamma-ray Laue lenses made using SPO technology named Silicon Laue lens Components: SiLC.
The bending and bonding of 300 μm thin Si single crystals allows us to fabricate a single crystal with radially curved crystal planes, which strongly improves the focusing properties of a Laue lens. The size of the focal spot is no longer determined by the size of the individual single crystals, but by the accuracy of the applied curvature, which is as low as a few seconds of arc. Furthermore, a wedge is incorporated in each individual Si crystal to ensure that all crystals are confocal in the radial direction. A secondary curvature in the axial direction can be used to improve the reflectivity of each crystal, and increase the reflected energy bandwidth.
We present the first SiLC crystals which will be manufactured in the fall of 2013. These are technology demonstrators designed for 125 keV radiation, 3.4m focal length and 600mm2 frontal area. The first measurements at synchrotron radiation facilities are planned for November 2013. With these first prototype lenses we want to demonstrate that the SPO stacking technology can be successfully applied to non-ribbed Si wafer plates and subsequently demonstrate the correct focusing in Laue geometry of both the wedges and radial curvature.
We report on the status of the Laue lens development effort led by UC Berkeley, where a dedicated X-ray beamline and a Laue lens assembly station were built. This allowed the realization of a first lens prototype in June 2012. Based on this achievement, and thanks to a new NASA APRA grant, we are moving forward to enable Laue lenses. Several parallel activities are in progress. Firstly, we are refining the method to glue quickly and accurately crystals on a lens substrate. Secondly, we are conducting a study of high-Z crystals to diffract energies up to 900 keV efficiently. And thirdly, we are exploring new concepts of Si-based lenses that could further improve the focusing capabilities, and thus the sensitivity of Laue lenses.
In this paper we present several novel applications using X-ray mirrors based on Silicon Pore Optics
technology, the present baseline technology for large effective area space based X-ray telescopes. By
cutting, bending and direct bonding of mirrors cut from silicon wafers we can create a variety of
structures in a number of well-defined shapes. One novel application is an X-ray half-mirror for X-ray
interferometry applications based on flat, structured Si mirrors bonded to a glass support structure with
a large open area ratio. A second application is to use bent silicon single crystals as a focusing Laue
lens for soft gamma rays.
The Nuclear Compton Telescope (NCT) is a balloon-borne soft γ-ray (0.2-10 MeV) telescope designed to perform
wide-field imaging, high-resolution spectroscopy, and novel polarization analysis of astrophysical sources. NCT
employs a novel Compton telescope design, utilizing 12 high spectral resolution germanium detectors, with the
ability to localize photon interaction in three dimensions. NCT underwent its first science flight from Fort
Sumner, NM in Spring 2009, and was partially destroyed during a second launch attempt from Alice Spring,
Australia in Spring 2010. We have begun the rebuilding process and are using this as an opportunity to update
and optimize various aspects of NCT. The cryostat which houses the 12 germanium detectors is being redesigned
so as to accommodate the detectors in a new configuration, which will increase the effective area and improve the
on-axis performance as well as polarization sensitivity of NCT. We will be replacing the liquid nitrogen detector
cooling system with a cryocooler system which will allow for long duration flights. Various structural changes
to NCT, such as the use of an all new gondola, will affect the physical layout of the electronics and instrument
subsystems. We expect to return to flight readiness by Fall 2013, at which point we will recommence science
flights. We will discuss science goals for the rebuilt NCT as well as proposed flight campaigns.
Laue lenses are an emerging technology that will enhance gamma-ray telescope sensitivity by one to two orders
of magnitude in selected energy bands of the ~100 keV to ~1.5 MeV range. This optic would be particularly
well adapted to the observation of faint gamma ray lines, as required for the study of Supernovae and Galactic
positron annihilation. It could also prove very useful for the study of hard X-ray tails from a variety of compact
objects, especially making a difference by providing sufficient sensitivity for polarization to be measured by
the focal plane detector. Our group has been addressing the two key issues relevant to improve performance
with respect to the first generation of Laue lens prototypes: obtaining large numbers of efficient crystals and
developing a method to fix them with accurate orientation and dense packing factor onto a substrate. We present
preliminary results of an on-going study aiming to enable a large number of crystals suitable for diffraction at
energies above 500 keV. In addition, we show the first results of the Laue lens prototype assembled using our
beamline at SSL/UC Berkeley, which demonstrates our ability to orient and glue crystals with accuracy of a few
arcsec, as required for an efficient Laue lens telescope.
We present an experimental study on the method of surface grooving for bending crystals for the realization of a
hard x-ray Laue lens. Bent Si and Ge crystalline plates were analyzed by x-ray diffraction of their (111) planes
at the European Synchrotron Radiation Facility. Crystals diffracted photons from 150 to 700 keV with efficiency
peaking 95% at 150 keV for Si. Measured rocking curves of the samples showed flat-topped profiles with their
FWHM equal to the crystal bending, i.e., the method of surface grooving proved to evenly bend the crystals,
their energy passband being very well controlled. Surface grooving technique has been found to offer both high
reproducibility and easy control of diffraction properties of the crystals. Besides, this method is cheap, simple
and compatible with mass production, making it a reliable technique for fabrication of a Laue lens, where serial
production of crystals should be envisaged. A Laue lens made of crystals bent by surface grooves can lead to
significant detection improvement in astrophysical applications.
NuSTAR is a hard X-ray satellite experiment to be launched in 2012. Two optics with 10.15 m focal length focus Xrays
with energies between 5 and 80 keV onto CdZnTe detectors located at the end of a deployable mast. The FM1 and
FM2 flight optics were built at the same time based on the same design and with very similar components, and thus the
performance of both is expected to be very similar. We provide an overview of calibration data that is being used to
build an optics response model for each optic and describe initial results for energies above 10 keV from the ground
calibration of the flight optics. From a preliminary analysis of the data, our current best determination of the overall
HPD of both the FM1 and FM2 flight optics is 52", and nearly independent of energy. The statistical error is negligible,
and a preliminary estimate of the systematic error is of order 4". The as-measured effective area and HPD meet the toplevel
NuSTAR mission sensitivity requirements.
DUAL will study the origin and evolution of the elements and explores new frontiers of physics: extreme energies that
drive powerful stellar explosions and accelerate particles to macroscopic energies; extreme densities that modify the laws
of physics around the most compact objects known; and extreme fields that influence matter in a way that is unknown on
Earth. The variability of these extreme objects requires continuous all-sky coverage, while detailed study demands an
improvement in sensitivity over previous technologies by at least an order of magnitude.
The DUAL payload is composed of an All-Sky Compton Imager (ASCI), and two optical modules, the Laue-Lens Optic
(LLO) and the Coded-Mask Optic (CMO). The ASCI serves dual roles simultaneously, both as an optimal focal-plane
sensor for deep observations with the optical modules and as a sensitive true all-sky telescope in its own right for all-sky
surveys and monitoring. While the optical modules are located on the main satellite, the All-Sky Compton Imager is
situated on a deployable structure at a distance of 30 m from the satellite. This configuration not only permits to maintain
the less massive payload at the focal distance, it also greatly reduces the spacecraft-induced detector background, and,
above all it provides ASCI with a continuous all-sky exposure.
Laue lenses are an emerging technology allowing the concentration of soft gamma rays in the ~ 100 keV -
1.5 MeV energy range. Two lens designs based on recently measured crystals are presented in this paper. A
lens dedicated to the understanding of the progenitors and explosion physics of Type Ia supernovae through
the observation of the 847 keV line produced by the decay chain of the radionuclide 56Co. With a Compton
camera at the focus (as proposed for the DUAL mission), we find that a space-borne telescope could reach a 3-σ
sensitivity of 1.5×10-6 ph/s/cm2 for a 3% broadened line in 105 s, enabling the detection of several events per
year with enough significance to strongly constrain the models. On the other hand, a second generation prototype
is proposed. Made to realize a balloon-borne telescope focusing around the electron-positron annihilation line
(511 keV), this lens would primarily be a technological demonstrator. However with an estimated sensitivity of
5×10-6 ph/s/cm2 in 104 s observation time, this Laue lens telescope could bring new hints in the search of the
origin of the Galactic positrons. To build this prototype, a dedicated X-ray beamline has been built at the Space
Laue lenses are an emerging technology based on diffraction in crystals that allows the concentration of soft
gamma rays. This kind of optics that works in the 100 keV - 1.5 MeV band can be used to realize an highsensitivity
and high-angular resolution telescope (in a narrow field of view). This paper reviews the recent
progresses that have been done in the development of efficient crystals, in the design study and in the modelisation
of the answer of Laue lenses. Through the example of a new concept of 20 m focal length lens focusing in the 100
keV - 600 keV band, the performance of a telescope based on a Laue lens is presented. This lens, uses the most
efficient mosaic crystals in each sub-energy range in order to yield the maximum reflectivity. Imaging capabilities
are investigated and shows promising results.
In a Laue lens a large number of crystals are disposed on concentric rings such as they diffract via Braggdiffraction
the incident gamma-rays onto a common focal spot. Compact structured high-Z mosaic-crystals are
among the most efficient diffraction media for the domain of nuclear astrophysics (i.e. 300 keV ≤ E ≤ 1.5 MeV).
We have studied the potential of various high-Z crystals such as Ir, W, Au, Ag, Pt, Rh and AsGa for a Laue
lens application. The diffraction performance of gold, silver and platinum crystals have been measured during
runs at the European Synchrotron Radiation Facility and in a reactor-beamline at the Institut Laue Langevin,
Grenoble in France. Several of the tested high-Z materials show outstanding performances with reflectivities
reaching the theoretical limits for mosaic-crystals, and hence open the way towards efficient focusing optics at
The science drivers for a new generation soft gamma-ray mission are naturally focused on the detailed study of
the acceleration mechanisms in a variety of cosmic sources. Through the development of high energy optics in the
energy energy range 0.05-1 MeV it will be possible to achieve a sensitivity about two orders of magnitude better
than the currently operating gamma-ray telescopes. This will open a window for deep studies of many classes of
sources: from Galactic X-ray binaries to magnetars, from supernova remnants to Galaxy clusters, from AGNs
(Seyfert, blazars, QSO) to the determination of the origin of the hard X-/gamma-ray cosmic background, from
the study of antimatter to that of the dark matter. In order to achieve the needed performance, a detector with
mm spatial resolution and very high peak efficiency is needed. The instrumental characteristics of this device
could eventually allow to detect polarization in a number of objects including pulsars, GRBs and bright AGNs. In
this work we focus on the characteristics of the focal plane detector, based on CZT or CdTe semiconductor sensors
arranged in multiple planes and viewed by a side detector to enhance gamma-ray absorption in the Compton
regime. We report the preliminary results of an optimization study based on simulations and laboratory tests,
as prosecution of the former design studies of the GRI mission which constitute the heritage of this activity.
The concept of a gamma-ray telescope based on a Laue lens offers the possibility to increase the sensitivity by more
than an order of magnitude with respect to existing instruments. Laue lenses have been developed by our
collaboration for several years : the main achievement of this R&D program was the CLAIRE lens prototype, which
has successfully demonstrated the feasibility of the concept in astrophysical conditions. Since then, the endeavour
has been oriented towards the development of efficient diffracting elements (crystal slabs) in order to increase both
the effective area and the width of the energy bandpass focused, the aim being to step from a technological Laue lens
to a scientifically exploitable lens. The latest mission concept featuring a gamma-ray lens is the European Gamma-
Ray Imager (GRI) which intends to make use of the Laue lens to cover energies from 200 keV to 1300 keV.
Investigations of two promising materials, low mosaicity copper and gradient concentration silicongermanium
are presented in this paper. The measurements have been performed during three runs: 6 + 4 days at the
European Synchrotron Radiation Facility (Grenoble, France), on beamline ID15A, using a 500 keV monochromatic
beam, and 14 days on the GAMS 4 instrument of the Institute Laue Langevin (Grenoble, France) featuring a highly
monochromatic beam of 517 keV. Despite it was not perfectly homogeneous, the presented copper crystal has
exhibited peak reflectivity of 25 % in accordance with theoretical predictions, and a mosaicity around 26 arcsec, the
ideal range for the realization of a Laue lens such as GRI. Silicon-germanium featuring a constant gradient have
been measured for the very first time at 500 keV. Two samples showed a quite homogeneous reflectivity reaching
26%, which is far from the 48 % already observed in experimental crystals but a very encouraging beginning. The
measured results have been used to estimate the performance of the GRI Laue lens design.
A Laue lens gamma-ray telescope represents an exciting concept for a future high-energy mission. The
feasibility of such a lens has been demonstrated by the CLAIRE lens prototype; since then various mission concepts
featuring a Laue lens are being developed. The latest, which is also the most ambitious, is the European Gamma-Ray
Imager (GRI). However, advancing from the CLAIRE prototype to a scientifically exploitable Laue lens requires still
substantial research and development. First and foremost, diffracting elements (crystals) that constitute the Laue lens
have to be optimized to offer the best efficiency and imaging capabilities for the resulting telescope. The characteristics
of selected candidate crystals were measured at the European Synchrotron Radiation Facility on the high-energy
beamline ID 15A using a beam tuned at 292 keV. The studied low mosaicity copper crystals have shown absolute
reflectivity reaching 30%. These crystals are promising for the realization of a Laue lens, despite the fact that they
produce a diffracted beam featuring a Gaussian intensity profile, which contributes to the spread of the focal spot. A
composition gradient Si1-x-Gex crystal has been investigated as well, which showed a diffraction efficiency reaching
50% (disregarding absorption) - half of the theoretical maximum - that represents an absolute reflectivity around 39 %,
the best that we measured at this energy to date. This gradient crystal also showed a square-shaped rocking curve that is
almost the best case to minimize the spread of the focal spot. We also show that bending a gradient crystal could still
enhance the focusing. Thanks to the better focusing, a factor of 2 in sensitivity improvement may be achieved.
With focusing of gamma rays in the nuclear-line energy regime establishing itself as a feasible and very promising approach for high-sensitivity gamma-ray studies of individual sources, optimizing the focal plane instrumentation for gamma-lens telescopes is a prime objective. The detector of choice for a focusing nuclear-line spectroscopy mission would be the one with the best energy resolution available over the energy range of interest: Germanium. Using a Compton detector focal plane has three advantages over monolithic detectors: additional knowledge about (Compton) events enhances background rejection capabilities, the inherently finely pixellated detector naturally allows the selection of events according to the focal spot size and position and could enable source imaging, and Compton detectors are inherently sensitive to gamma-ray polarization. Suitable Ge-strip detectors that could be assembled into a sensitive high-resolution focal plane for a gamma-ray lens are available today. They have been extensively tested in the laboratory and flown on the Nuclear Compton Telescope balloon from Ft. Sumner in 2005. In the course of the ACT vision mission study, an extensive simulation and analysis package for Compton telescopes has been assembled. We leverage off this work to explore achievable sensitivities for different Ge Compton focal plane configurations - and compare them to sensitivities achievable with less complex detectors - as a step towards determining an optimum configuration.