XMM
Optical Monitor
(OM)

Introduction
The OM Telescope
The Filter Wheel
The Detectors
Links to further information

Introduction

XMM, although primarily an X-ray observatory, provides a powerful optical/UV 30 cm telescope which is co-aligned with the X-ray telescopes for simultaneous X-ray and optical observations.

The OM telescope tube is approximately 2 m long. The primary mirror is 30 cm in diameter, hence this is a 30cm telescope. Incoming light falls onto the primary mirror, which reflects it onto the secondary. The secondary mirror then reflects the light onto a third, inclined mirror, which in turn reflects it onto the detector (see above). A filter wheel is mounted in front of the detector, which contains filters and other optical elements, such as two grisms (for low resolution spectroscopy) and a magnifier (i.e. optics for higher resolution on the sky).

The telescope

The actual OM telescope sits inside a baffle which only lets through light that comes directly from its target, stopping any light from other directions from getting to the detector. This is a photo of the outside of the OM, in which you can clearly see the baffle:

This is a view from the front of the telescope tube looking down onto the primary mirror, showing the secondary mirror support and the baffle in the reflection. The secondary mirror is not in place. Note how the brightess decreases as the light is absorbed by the baffle.

Here, we have taken off most of the baffle and left only the the segment closest to the square piece, the one that contains the actual telescope. We're looking down in the same direction as the incoming light and now we can see the primary mirror, waiting to reflect the incoming light, and the mounting structure for the secondary mirror in front of it. In fact, you can see reflected in the primary mirror the secondary mirror, which is itself reflecting the "tube" in the middle of the primary.

This is the mirror mounting structure, which is made of Invar for low thermal expansion. This fits inside the baffle and holds both the primary and the secondary mirrors in place. In this picture, the primary mirror sits at the bottom and the secondary mirror directly above it, facing it. The light comes from the top and gets reflected by the primary mirror onto the secondary mirror. The secondary mirror in turn reflects it through the narrow tube at the bottom onto yet another mirror behind the primary, which then reflects the light by 90 degrees onto the filter wheel and the detector.

The Filter Wheel

Contained within the OM's filter wheel are broad and narrow-band filters, which can be used to obtain spectral information about the target. If the target is bright enough, there are also two grisms for low resolution spectroscopy.

In addition, there is a facility for more detailed imaging of the centre of the field of view through a x4 magnifier contained in the OM's filter wheel.

Here's a photograph of the OM filter wheel, with no filters mounted on it.

The Detectors

Simply by being in space, outside Earth's atmosphere, the OM sensitivity for imaging is comparable to a 4 m telescope on the surface of the Earth. This is due to the elimination sky background caused by our own atmosphere. However, the goal of the OM is to be able to observe very weak sources (24th magnitude). In order to achieve such a high sensitivity, the OM carries special detectors: micro-channel plate (MCP) intensified CCDs (MICs). There are two MCPs behind each other, which amplify the strength of the incoming signal by a factor of about 100,000 before it hits the CCD.

Further Information

How about a Tour of the OM?
Check out the University of California Santa Barbara (UCSB) OM pages.
A description of the (expected) scientific performance of XMM and its instruments is provided in the XMM Users' Handbook.
Have a look at the ESTEC "Building XMM" web pages - they hae many more OM photos for you to look at but be warned - they can take a bit to download! They are worth the wait though.
The ESTEC pages also have the information about the responsibilities of each institute towards the construction of the OM.
The MSSL OM site also has lots of information, photographs and documentation. It can be a bit technical.
The Centre Spatial de Liege (CSL) contribution to the OM.
The Institut d'Astrophysique de Liege has an introduction to XMM in French.

Find out more about the other XMM instruments: Back to the XMM schematic view.

These pages are adapted from various sources, mainly from the ESTEC page "Building XMM" and the Mullard Space Science Laboratory.

Various parties require acknowledgement for the photo material. Particular thanks are due to the Mullard Space Science Laboratory, the XMM Project team at ESTEC, Centre Spatial de Liege, Medialario, Dornier, Max-Planck-Institut für extraterrestrische Physik (MPE) and the Space Research Organization of the Netherlands (SRON), Utrecht.

Last updated: Thu Mar 4th 1999