LOFT

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

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).

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