Beagle 2 Microscope Site


  

A microscope for space flight

A microscope can be broken down into 6 key elements. The illumination system and a translation mechanism requires a little explanation.

Firstly, one cannot use the natural ambient light on Mars to illuminate a sample because the microscope itself will cast a shadow over it. Hence, you need an illumination system. Secondly, the depth of field specifies the distance over which an object will appear focussed. The focus of the Beagle 2 microscope will be 12 mm from the front surface. The depth of field, however, is only 40 microns. Any part of the target in front of or behind the focus position by more than this amount will be out of focus. Hence, to ensure you get a sharp image, you need a focussing mechanism which translates the microscope towards or away from the sample.
 

The Beagle 2 instrument has several natural interfaces. The illumination system and the optics are so compact that they naturally form one work package. The detector element forms a second work package. These two elements are joined by a tube which provides a mechanical interface and a light-tight structure of significant strength. The translation stage forms a fourth work package.
 

Optics and illumination system

The optically active elements will comprise a Cook triplet with a spectral corrector which will limit the flux below 400 nm. This is a slight change from the MECA microscope where only a doublet was required. This is because of the smaller pixel size of the Thomson detector (see below) and the larger field of view. The image scale is 4.0 (±0.2) micron px-1. With the 1024 x 1024 CCD, this will give an image size of 4.1(±0.2) x 4.1(±0.2) mm2. The depth of field is around 40 microns. The working distance of the microscope is 12.0 (±1.0) mm. The PAW provides a means of bringing the microscope to within ±3 mm of its target.

The illumination system comprises 12 LEDs arranged symmetrically around the aperture of the microscope. There are four colours - red, green, blue, and UV (375 nm). The UV LEDs have been selected to look for possible fluorescence of materials during imaging. This would be particularly interesting for an exobiology lander in view of the potential fluorescence of lichens etc.

The RGB LEDs run at 10mA each, the UV at 8 mA. The voltage supply is 6 V. The power is 60 milliwatts per LED (48 milliwatts for the UV). Total integrated power is therefore 684 mW when all lights are on. The electrical interface for the illumination system comprises 15 wires - 12 supply lines and 3 ground wires. These wires run under the microscope body to a connector box on the Paw. The connector is a nano-D connector.

The optics and illumination system are provided by the Lunar and Planetary Laboratory of the University of Arizona
(contacts: Peter Smith, Roger Tanner, and Robert Reynolds).

This picture shows the lamp assembly and the optical head. The wires form the electrical interface to the Paw system.
 
 
 

Detector

The baseline requirements for the microscope called for a system similar to that of the microscope for the MECA instrument suite for Mars “01. However, for reasons of mass and cost, it was necessary to change the detector being used. The present baseline is a 14 micron pixel pitch, 1024 x 1024 device with integrated electronics to be provided by CSEM from Neuchatel, Switzerland (contact J.-L. Josset). The system weighs 90 grams and uses an RS 422 interface to communicate with the digital processing unit of the lander.

A technical drawing of the structure of the detector head. The CCD is at the top and light comes from the top onto the detector. The connector is to the left. The whole piece is held together by the vertical bracket.

In designing the optics it needed to be noted that the CCD has a front window. The window is 0.63mm for the thickness and the material is Sapphire. The distance between the top of the window to the CCD plane is 1.12mm.
 

Structure

The optics are connected to the detector head with a carbon-fibre tube assembly (Figure 1.16). The carbon-fibre tube which forms the body of the microscope is not quite circular. It has therefore been machined over a length of 5.9 mm so that the OD (=outer diameter) is 29.5 mm +.05 mm -.00 mm. The tubes slides into a circular aluminium tube provided by the optical head. The inner diameter (ID) of the optical head tube is 29.7 mm. The gluing length of the finished tubes shall be 5.5 mm.

The tubular structure holding the optical head (lower left) which interfaces with the detector. The detector interface plate is shown on the top right.
 

Translation stage

The microscope will possess no focussing mechanism of its own. All operations to focus the microscope must be performed by positioning the whole microscope with respect to the target. The accuracy with which this must be performed is given by the depth of field. Motor type is Arsape AM1020-20, (12V), coupled to a 256:1 planetary gearhead.  This will be linked to a leadscrew which reacts onto a bracket which runs on two slides.

A “Thumb” for the Paw is required to bring the microscope head to a distance equivalent to the working distance from the target thereby reducing the travel required for the stepper motor.

The translation stage is attached to the Paw and is the responsibility of the University of Leicester (contact Shaun Whitehead).
 

Software

The digital processing unit will contain some special software to compress the data from the microscope. The data rates from Beagle 2 to the Earth are strongly constrained and therefore it is necessary to reduce the total data volume as much as possible. The baseline compression algorithm uses wavelet transforms and has been developed by the Technical University of Braunschweig. An image mosaicing algorithm has also been developed by Michael Fernandes to produce a focussed image of a target, the surface roughness of which is much greater than the depth of field of the microscope, by combining several images together.

System details

The optics and detector combined envelope will not exceed 125 mm x 60 mm x 50 mm (111.4 mm x 52.5 mm x 36.0 is the current baseline envelope excluding the illumination system).
 
 
Mass breakdown of the Beagle 2 microscope
Element  Mass [g] Margin [g]
Optics 25 10
Illumination system 25 10
Structure 20 10
Detector module 95 10
Translation stage 80 10
Totals 245 50

 
 
Power breakdown of the Begale 2 microscope
Element Peak Power [W] Average Power [W] Margin [W]
Illumination system  0.684 0.684 0.1
DC motor 0.18 0.18 0.04
Detector module  3.5 2.0 0.3
Totals 4.36 2.86 0.44

 

Team

The team structure is shown here.

Models

Several models of the Beagle 2 microscope will be produced.