2.1

General requirements

The previous mission objectives lead to the following requirements at telescope level (optics and baffle):

Table 1

Telescope requirements

Concerning the last two requirements (Earth straylight rejection and the PSF stability), both of these are considered to be challenging, and for this reason are described in greater detail in the following sections.

2.2-

Straylight requirements

The system must be capable of measuring stellar signal variations with an accuracy better than 10⁻⁶. This corresponds to a sensitivity of a few photons/pixel/second. Straylight flux shall therefore be typically less than 1 ph/pix/s. The following table summarises the current specifications, derived from these requirements :

Table 2

Straylight specifications

With a baffle entrance diameter of around 800 mm, a line of sight (LOS) positioned around 20° from the Earth limb, and an altitude around 850 km, the collected photon rate (in the useful spectral range) originating from Earth is around 10²⁰ photons/s.

This is equivalent, at focal plane level, to 6.10¹² photons/pixel/s, such that the telescope baffle shall have a straylight rejection capability of around 10¹², which is the highest rejection ratio ever to have been required from a space telescope of this class.

Figure 3

Straylight source

2.3-

PSF stability requirements

At the focal plane level, the image is defocused in order to avoid detector saturation. The PSF will thus cover around 250 pixels, the summation of which will give the total energy received by the telescope from the target star.

Due to the non perfect uniformity of the detector’s spatial response (which cannot be calibrated to an accuracy of 10⁻⁶), signal errors can be induced by small displacements of the PSF relative to the detector. The following table gives the resulting PSF stability requirements, in terms of dimension and position.

Table 3

PSF stability requirements

In terms of pointing accuracy, the system requirement is to stabilise the PSF on the detector to within ± 5.4 μm (equivalent to ± 4.5μrad, or ± 0.4 pixel). In order to reduce the severity of these requirements on the thermo-elastic stability of the optics, error signals generated by the telescope detector are used to drive the spacecraft pointing control loop. This approach allows the absolute LOS pointing stability of the telescope to be relaxed to around ± 22.5 μrad over one orbital period. The fine pointing control is thus achieved by the PROTEUS platform, using its reaction wheels. The control loop has a discrete sample and correction rate of 1/32 Hz.

Table 4

PSF displacement requirements

Figure 4

PSF requirements

3.1-

Telescope concept

Firstly, the large diameter of the telescope entrance pupil (around ϕ 300 mm) favours a solution based on reflecting mirrors. Secondly, the large field of view requirement (around 3° x 3°) naturally leads to a TMA (Three-Mirror Anastigmatic) solution. Although such a telescope concept was initially proposed for the COROT mission, it was rapidly found to be unsuitable for reasons of its poor potential for straylight baffling : this would have led to a very long baffle (around 5m), as shown in the following figure.

Figure 5

Baffling concept for a TMA telescope

As such a baffle would be highly incompatible with the weight, volume and cost allocations of the mission, a more compact solution was needed.

In this context, Alcatel Space has proposed an innovative concept, based on an afocal telescope feed and a dioptric imaging camera, in which two real pupils and a field stop are materialised. This design leads to a more compact entrance baffle and even better straylight rejection capacity than the TMA telescope.

Figure 6

Baffling concept for the afocal telescope – imaging camera design

The telescope consists of two co-focal off-axis parabolic mirrors, which act together like a folded beam compressor. The common focal plane of the two mirrors is shared by the entrance and exit pupils of the telescope, and by a square field stop.

A dioptric camera is placed in the collimated exit beam, thereby producing an image of the sky on the detector.

3.1-

Baffle concept

An entrance baffle in front of M₁ is thus an absolute necessity, and the required straylight rejection ratio must be achieved by the elements placed just before M₁. The design of this baffle must therefore adhere to the following rules :

Figure 7

Baffling design rules

The positions of the baffle chicanes, whose principal purpose is to prevent or to limit grazing incidence specular reflections, have been optimised using APART software which takes into account all of the diffraction effects produced by the chicanes. The following figure provides the definition of the optimised baffle.

Figure 8

Optimal locations for baffle chicanes

3.1-

Baffling performance

The performance of a baffle is generally given by its PST (Point Source Transmission) which, for a given incidence angle θ, is the ratio between the incidence irradiance of a point like source at instrument entrance pupil level, and the transmitted irradiance at focal plane level.

Figure 9

PST definition

The following curves give the PST of the previous baffle definition, for two different hypotheses of M₁ contamination level (400 ppm or 2000 ppm).

Figure 10

PST values

The PST function can be used to calculate the detector level irradiance for any given source, by integrating the Source × PST irradiance function over the solid angle represented by the source. It can be seen that between 20° and 90° (corresponding to the angular limits occupied by the Earth), the average PST is around 10^-12, which is the specified rejection ratio for the COROT mission. As a result, the Earth-generated straylight irradiance can be shown to produce a photon rate lower than 1 ph/pixel/s, in the case of a contamination level of around 2000 ppm. The baffle proposed by Alcatel Space is thus compliant with COROT’s straylight requirements.

4.1-

Derivation of requirements

The PSF requirements have been used to generate the optical parameters given in the following table :

Table 5

Derivation of the PSF requirements

The proposed solution to satisfy the above requirements is to use a highly stable material, with a CTE of around 10⁻⁶ K⁻¹ (CFRP type), associated with a thermal control stability of around ± 4°C. The dioptric camera has to be athermalised (by choosing an appropriate association of optics and barrel materials). Alcatel Space has proposed an opto-mechanical design for the complete telescope which is compliant with all of the above requirements.

4.2-

Instrument mechanical concept

It is proposed to employ a mechanical structure comprising two CFRP honeycomb panels linked together with a 6-strut truss. The baffle is isostatically mounted directly onto the upper panel with thermal insulation. The baffle - telescope assembly (called “COROTEL”) is isostatically mounted on the payload equipment module, which houses the payload electronics modules, and provides mechanical support and thermal decoupling of the payload from the spacecraft.

The current instrument design is illustrated in the following figure:

Figure 11

COROT instrument on the Proteus spacecraft