Institute for Astronomy and Astrophysics


LOFT (Large Observatory For X-ray Timing) is a planned X-ray observatory that was included in the ESA Cosmic Vision Program's shortlist for the M3 and M4 launch slots, but was ultimately not selected. With a very large effective detector area (8.5 m2) and an extremely high time resolution of 10μs, LOFT is able to study the state of matter on neutron stars and near black holes. Furthermore, a wide field monitor allows continuous observation of a large area of the sky and can measure variations in the intensity of individual X-ray sources on both long (~ months) and subsecond time scales and transmit the information without long delay.

Originally planned launch: 2024

Energy range:

  • Large Area Detector: ~ 2-80 keV
  • Wide Field Monitor: ~ 2-50 keV

Scientific goals: Observation of bright variable X-ray sources, discovery of hitherto unknown variable X-ray sources, obtaining information on the equation of state of matter in neutron stars, verification of predictions of general relativity


The two instruments on LOFT are the Large Area Detector (LAD), which can specifically observe individual objects with extremely high time resolution, and the Wide Field Monitor (WFM), which observes a large area of the sky and can thus simultaneously monitor a large number of sources.

Large Area Detector (LAD):

With an effective detector area of approximately 8.5 m² at 6 keV, the LAD will be one of the largest detectors ever placed in orbit. The instrument consists of six individual panels, each consisting of 21 modules. The entire structure is modular and failures of individual modules have almost no influence on the scientific performance. The individual modules each consist of 16 silicon drift detectors (SDDs), very fast semiconductor detectors, which have already been used in the ALICE experiment at CERN, and which have a large number of anodes on two opposite sides. One of the biggest challenges with this instrument is the large number of individual detectors and electrical components required for selection. For each single SDD, 14 ASICs are required for readout, resulting in a total of 28 224 electronic readout units.

In order to limit the field of view of the detectors, collimators are used which are mounted in front of the actual detector. Due to the extremely large detector surface, the use of conventional lead or tungsten collimators is not possible due to the weight. The development of microcapillary collimators in the size of the detectors makes it possible to cover the entire surface. For LOFT, a 3 mm thick lead glass plate with ~20 μm large pores is produced, which are chemically etched. The collimator can then absorb photons up to 50 keV and has a field of view of about 1°.

Wide Field Monitor (WFM):

The task of the WFM is to monitor a large part of the sky in order to observe variations and activities of variable ("transient") sources and to pass them on to the astronomical community as quickly as possible. The WFM consists of four pairs of cameras looking at neighboring regions of the sky. One pair consists of two so-called "1.5D" cameras, Coded Mask cameras, which have a very good spatial resolution in one dimension, and a moderate spatial resolution in the other dimension.

The detectors of the WFM are also SDDs, where the number of individual anodes is about a factor four higher and thus a very good spatial resolution in one dimension is possible. The generated charge is also distributed over several adjacent anodes. This allows a precise reconstruction of the position at which a photon has hit, both in the direction of the anodes and in the drift direction of the charges. Both detectors and masks in a camera pair are orthogonal to each other, so that each pair has the same field of view. The data from both detectors can then be used to reconstruct the positions and brightness of the individual sources with high spatial resolution.

Table: Main characteristics of LAD and WFM (expected performance).

Energy range 2 - 80 keV 2 - 50 keV
Field of View 0.95° FWHM 5.5 steradian
Energy resolution 180 eV @ 6 keV < 300 eV @ 6 keV


The detectors for LAD and WFM are large area silicon drift detectors (SDDs). The most important advantages of silicon drift detectors are their high energy and time resolution as well as their low weight (~ 1 kg/m2). The interaction of an X-ray photon with the detector material generates an electron cloud. A constant electric field with negative voltage decreasing from the center to the sides of the detector provides a potential gradient to the readout anodes at the edge. The electron cloud now drifts from the point of impact to the anodes within a few microseconds and expands slightly. The measured charge distribution at the anodes then ultimately depends on where the photons arrived in the detector and what energy the photon had.

Scientific goals

LOFT is used to study compact objects, especially galactic and extragalactic neutron stars and black holes, with high-resolution X-ray observations. With these observations two essential questions shall be investigated:

  • "What is the equation of state of matter in neutron stars? and
  • "Do the predictions of General Theory of Relativity apply to matter near the event horizon?

The scientific objectives are thus divided into two categories:

  • EOS: The interactions between dense matter can be described by the so-called equation of state (EOS). The equation of state indicates the pressure-density-temperature ratio of matter. Among other things, the equation of state of matter with supra-nuclear density shall be determined by measuring mass and radius of different neutron stars. In addition, the inner structure of isolated neutron stars will be investigated by observing seismic oscillations.
  • SFG: LOFT will use time-resolved spectroscopy to directly measure the motion of matter in strong-field gravity (SFG) near the event horizon of accreting black holes. For this purpose, strong-field effects of general relativity will be demonstrated and comparative studies with neutron stars will be performed.

IAAT Participation

The Institute for Astronomy and Astrophysics in Tübingen (IAAT) is mainly involved in the development and production of back-end electronics for both instruments:

  • The back-end electronics in the LAD are divided into two levels: On the one hand, each detector module has its own Back-End Electronics (MBEE) module, which processes the data directly from the detector and makes offset and gain corrections. On the other hand, each panel has a Panel Back-End Electronics (PBEE) that coordinates the individual MBEEs, collects the data and forwards it to the on-board computer for transmission to a ground station.
  • In the WFM, there is also a back-end electronics with the same tasks for each camera. While with the LAD the impact position of the photons is not determined (there is no imaging), with the WFM the measured charge distribution at the anodes is further processed to determine the position. Thus, the shadow cast by the mask can be used to create an image of the sky.


At the University of Tübingen, several studies have been carried out on the structure of the individual components of the back-end electronics.

Diploma, Master and Bachelor theses:

  • February 2013
    Uter, Pascal
    Development of the Module Back End Electronics for the Large Observatory For X-ray Timing
  • July 2014
    Jetter, Florian
    Development of the Back End Electronics for the Wide Field Monitor on-board LOFT
  • July 2014
    Gschwender, Michael
    Hardware Implementation and Testing of the Module Back-End Electronics for the LOFT Mission

PhD Theses 

  • July 2015
    Henning Wende
    Next Generation Data Processing for Future X-ray Observatories

National and international collaboration

  • Germany: Dr. Remeis Sternwarte Bamberg, Prof. Jörn Wilms
  • Netherlands: SRON, Jan-Willem den Herder; University of Amsterdam, M. Van der Klis, A. Watts
  • Italy: INAF Roma, M. Feroci, A. Argan; Osservatorio Monte Porzio, L. Stella
  • Switzerland: ISDC Geneve, E. Bozzo
  • UK: MSSL, S. Zane, D. Walton
  • France: CNRS Toulouse, D. Barret; CEA Saclay, A. Goldwurm, F. Lebrun

Last Update 10/2018: Inga Saathoff, Chris Tenzer