IBIS Coded Mask

IBIS (Imager on Board INTEGRAL Satellite) is one of the two main instruments on board the INTEGRAL mission. IBIS will observe simultaneously with the other instruments on board, celestial objects of all classes ranging from the most compact galactic systems to extragalactic objects.
IBIS is a gamma-ray telescope with imaging and spectroscopic capabilities. It will cover the energy range from 20keV up to 10MeV, with 12 arc min angular resolution.
The imaging characteristics of the telescope are achieved by using a Coded Mask Aperture System. The Coded Mask projects a shadowgram onto the detector plane. Images of the sky will be reconstructed by decoding the shadowgram with the mask pattern.
Design, manufacturing, testing at subsystem level and delivery of IBIS Mask are responsibility of the Astronomy and Space Science Group (GACE) in the University of Valencia, with the support of INTA for thermal design and tests. Main Contractor for the Mask development is the Spanish company SENER. About another 10 companies have taken part in the different Mask activities.

IBIS Mask Description

The IBIS Mask is a Modified Uniformly Redundant Array (MURA, see also attached file) code 53x53 four times repeated. Pixel size and shape have been selected according to ISGRI and PICSIT detector geometries. Pixel size is 11.2x11.2mm and 16mm thickness. The total coded area of the IBIS Mask is 1064x1064mm.
Approximately half of the pixels are opaque to photons in the range of 20keV-10MeV (opacity requirement 70% at 1.5MeV), and the remainder pixels should result as transparent as possible (transparency requirement on axis 75% and off axis 60% both at 20keV).

 

IBIS mask pattern, flight model
IBIS FM Coded MaskIBIS Mask FM code on the Assembly Tool before code integration at Mecanizados Ginés workshop

 

The Code is composed of four Tungsten alloy (Densimet 18) plates of 16mm thickness. The selected cutting technique for the manufacturing of the W code is electrodischarge wire cutting (EDWM).
In order to meet the required strength and stiffness to survive launch and space environment operations, the Code plates have been integrated in a suitable support structure. Code, support structure and I/F frame define the Mask Assembly System.
The four Coded plates are fixed by Ti bolts to a carbon fibre panel. This panel is formed by a carbon honeycomb core covered by two skins of Carbon Fibre laminates. An external Carbon Fibre frame provide the required interfaces between the mask assembly and the Satellite PLM. A peripheral W Shield limits the field of view to the active.

The Mask is provided with two optical reference mirror cubes located on the +Y and +Z axis of the Mask for instrument alignment.

 

Into the Thermal ChamberVibration test
IBIS FM Coded Mask ready to be introduced in the`Thermal Chamber for the Thermal Vacuum Test at INTA facilitiesIBIS FM Mask during vibration Tests at INTA

 

The use of composite materials guarantees the required stiffness with a minimum loss of transparency. 16 Kg of carbon fibre supports the total weight of the IBIS Mask around 200Kg under launch and operational conditions.


Another essential function of the Mask is to act as a structural element in the INTEGRAL Satellite, providing structural continuity to the PLM. The Mask is located on the top PLM, 3.4m above the PLM base.


Three models of the IBIS Mask have been developed between 1997 and 2000: Development Model (DM), for technology and manufacturing demonstration; STM/QM, built in flight standards and tested at full Qualification Levels at INTA and Alenia; and the Flight Model (FM). IBIS STM/QM is the Flight Spare.

The test campaign was carried out at Qualification levels on the STM/QM Mask, and at Acceptance Levels on the FM during August and September 2000 at INTA facilities.


During Thermal Vacuum Test, FM IBIS Mask was submitted to 4 temperature cycles within ( –50ºC, +35ºC), in a high vacuum conditions (less than 0.0013Pa).
IBIS Mask is also within electrical continuity specifications.

 

 

Dimensional controlTransparency test
IBIS FM Mask during Dimensional Control at INTAIBIS Mask. Transparency Test Set-Up in UV Clean Room.

Qualification levels for the Vibration Test are 9g Axial and 12g in Lateral. The Axial fundamental frequency of the IBIS FM Mask was found in 86 Hz (requirement: >75Hz) and the lateral fundamental frequencies were found in the range between 350 and 360 Hz (requirement > 120Hz). These values are very closed to STM/QM Model frequencies.
For the FM Mask Acceptance Vibration levels were applied according to the same notching philosophy agreed for the QM.

The pixels position/size uncertainty and the transparency/opacity values of the Mask define the optical performances of the IBIS Mask, required to perform accurate imaging of celestial gamma-ray sources.
Several alignment checks have been performed on the Mask. The results of the controls show that pixel positions, pixel sizes and I/F holes are within the scientific and manufacturing requirements.
The final uncertainty of the FM IBIS pixel position can be quoted in 0,03mm in the Y and Z axes (scientific requirement 0,15mm) and the pixel size uncertainty in 0,01mm for both axes (requirement 0,05mm).

A dedicated facility has been developed at the University of Valencia to perform transparency measurements. Procedures and equipment were tested during STM/QM campaign carried out in 1999.
The number of pixels tested to determine the transparency of the IBIS FM Panel was 72. W Code opacity was measured in 6 cells. The main parameters of the transparency measurements are:

  • Ge detector (resolution ~0.8@60keV)
  • Mask rotation angles: 0º, 1º, 2º, 4º & 10º.
  • Collimator angle: a=1.28º
  • Area:100mm2 (equivalenct to pixel size)
  • Energies: 13, 17, 31, 35, 60, 81, 356, 511keV (Radioactive sources: Am241, Ba133, Na22).
  • Integration time: 1hour.

Measurements were performed at UV Clean Room (Class 1000)

The Flight Model of the Mask was delivered to ESA and integrated into the FM Satellite at Alenia in October 2000.

See the IBIS Mask image gallery.

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