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Optical Monitor (OM)


The first ever X-ray space observatory with an optical and ultraviolet capacity


The entire OM telescope

The entire OM telescope at MSSL

A 15-strong team at the Mullard Space Science Laboratory (MSSL), led by principal investigator Keith Mason, started working on the XMM-Newton project in 1989. As leaders of the Optical Monitor multinational consortium of research institutes, they have been responsible for providing the multi-wavelength capacity of the XMM-Newton space observatory, which is unique in its ability to view simultaneously regions of the sky in the visible, ultraviolet and X-ray ranges.

"XMM-Newton was conceived from the start with this optical telescope. Previous missions had demonstrated the need. EXOSAT, for instance, had no such instrument and attempting to get simultaneous data in the visible range using ground based observatories had a very small success rate," explains Keith Mason.

The telescope, detectors and canister tube were largely produced in-house in the laboratory's engineering workshops, partners in Belgium and the United States providing, respectively, the power supplies and the data processing units.

How does the Optical Monitor work?

The Optical Monitor telescope is mounted on the mirror support platform of XMM-Newton alongside the X-ray mirror modules. Designed and developed at MSSL, the Optical Monitor is an improved Ritchey-Chrétien telescope (a telescope design that gives a high-quality image over a relatively wide field of view) with a 30 cm aperture, and has a sensitivity for imaging comparable to a 4-metre instrument on the Earth's surface. It provides coverage between 170 nm and 650 nm of the central 17 arcmin square region of the X-ray field of view, permitting routine multiwavelength observations of XMM-Newton targets simultaneously in the X-ray and ultraviolet/optical bands.


Schematic plan of the OM Telescope

One of the two filterwheels

One of the two filter wheels positioned in front of the OM's CCD detectors.


After being focused by two mirrors, the light is directed towards one of two fully redundant identical filter wheels and detector chains. In each, an ultra-compact electronic image intensifier amplifies the light signal a million times before it falls on a silicon detector chip (CCD) capable of registering 100 frames per second. Detailed imaging on the central region of a view is possible using a ×4 magnifier situated on the filter wheels, which also incorporate two grisms for low-resolution spectroscopy. Data is then processed and compressed before being sent back to Earth as part of the spacecraft telemetry.

OM main characteristics

  • 2 m long telescope tube
  • 30 cm Ritchey-Chrétien telescope
  • Focal ratio of f/12.7 and focal length of 3.8 m
  • Total coverage between 170 nm and 650 nm of a 17 arcmin square field of view
  • A primary mirror of 0.3 m and a hyperboloid secondary mirror
  • Two (redundant) filter wheels with 11 apertures: one blanked off, six broad band filters (U, B, V, UVW1, UVM2 and UVW2), one white, one magnifier and two grisms (UV and optical)
  • Two (redundant) detectors: micro-channel plate intensified CCD with 384 × 288 physical pixels (of which 256×256 are used for science observations). Photon events are centroided, subsampling each of the 256×256 CCD pixels into 8×8 pixels
  • Two (redundant) Digital Electronics Modules

The ability to observe X-ray targets simultaneously in the visible and UV is contributing to a vast increase of our knowledge of the Universe.

More OM information is available at the XMM-Newton Optical Monitor Home Page.

Last Update: 18 August 2015

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