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Telescope optical concept

The camera is based on a compact Cassegrain-type telescope and a four-lenses field corrector

Figure 5:

Telescope optical concept

Figure 6:

Optical Sub-assembly characteristics

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Silicon carbide for mirrors and structure

The RSI design is based on an all-SiC opto-mechanical architecture (telescope structure, mirrors, and focal plane structural elements).

This monolithic design approach, combined with the intrinsic SiC100 properties (high stiffness, low density, low thermal expansion, high thermal conductivity) allows to combine a high level of stability together with a low mass.

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Telescope structure

The telescope structure is only featuring three main parts: the main plate (supporting the primary mirror), the secondary mirror support, and the rod connecting those two parts.

Figure 7:

Telescope structure

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Telescope mirrors

SiC mirrors can be light-weighted and polished with a high accuracy. Both mirrors were SiC CVD¹ coated before polishing in order to minimize the roughness The Wave-front Error (WFE) was measured below 20 nm rms for each mirror, with a roughness lower than 1.0 nm rms.

Figure 8:

Primary & Secondary blank mirror

Figure 9:

Mirrors main features

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Refocusing capability

The secondary mirror is fixed on the structure by its interface flange. The primary mirror is fixed on to the structure through three iso-static invar mounts and thus thermally decoupled from the structure. Its temperature is controlled by a heater plate located between the mirror and the mounting plate. Setting different thermal control set points between the telescope structure and the primary mirror leads to a variation of the focal plane position, thanks to the low - but nonnull – thermal expansion coefficient of silicon carbide. The refocusing capability is +/- 200μm for a +/- 5°C thermal set point variation.

Figure 10:

Re-focus principle

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Focal Plane Assembly (FPA)

The focal plane assembly features only two CCD for the 5 required spectral bands.

One CCD is dealing with the Panchromatic band and the other one is dealing with the multi-spectral bands. The separation of the entrance optical bean is ensured by an optical field separator.

Figure 11:

FPA optical design

3.2A

4-line CCD for Multi-spectral bands

ROCSAT2 took benefit of the pre-development performed by Atmel, under a CNES R&D contract. The TH31547 multi-spectral CCD consists of 4 photodetector lines, each line being made of 6000 photodiodes with 13μm step. The detector is operated at 5 Mpixel/s per video output.

Figure 12:

The quad-linear CCD

Each CCD line is coupled with a spectral band filter. The four slit filters are coated on the same glass substrate glued on the CCD.

Figure 13:

MS filter

Figure 14:

PAN and MS transmission

High speed video processing for the panchro- matic channel

The panchromatic detection chain is based on the wellknown TH7834B detector (12000 useful 6.5 x 6.5 μm² pixels). The challenge was to operate the four serial read-out registers at a 10 MHz pixel rate for satisfying the 308μs integration time required to achieve the 2- meter resolution.

Figure 15:

The 12000 pixels Panchromatic CCD

Front end electronics

Each CCD is connected to a dedicated front-end-electronic board which ensures the clock driver distribution and the video signal pre-amplification.

Figure 16:

The PAN Electronics Board

Integrated FPA

Figure 17:

integrated FPA

Figure 18:

FPA main feature

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Supporting programs

Two main areas of concern were identified:

Detection:

Opto-mechanical aspects:

Therefore, two support programs were conducted :

Mechanical support campaign

This support program consisted in verifying the telescope stability by testing a Structural Model (SM) of the Instrument. This SM was built of the telescope structure flight model equipped with mirror blanks and the FPA DM

The stability of the structure was verified using a 3-D machine.

Figure 24:

support programs

Figure 25:

SM test campaign

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RSI Assembly and integration

The RSI FM AIT started at the completion of the support programs and delivery of the telescope mirrors.

Figure 26:

FM camera integrated

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Proto-qualification tests

All the environmental tests were performed at the same place (Centre Spatial de Liège). The qualification sequence was the following:

Vibration tests

The Instrument was submitted to low level sine vibration and quasi-static load tests (15g axial, 9g lateral). The compatibility with the specified acoustic levels was in a first step demonstrated by analysis and tested at spacecraft level.

Figure 27:

Sine vibration tests

EMC tests

The Instrument was submitted to EMC conducted susceptibility and emission tests. The compatibility with the EMC radiated levels was in a first step verified at IPU unit level and then switched to Spacecraft level with a full representative environment.

Figure 28:

EMC test configuration

Thermal vacuum tests

The test sequence consisted in a thermal cycling phase followed by the operational cold case / hot case performance tests.

Figure 29:

T/V test configuration

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CTF measurements

The following curve gives, for the panchromatic band, the CTF values obtained for the nominal operation thermal case.

Besides this nominal case, it has been verified that the performances can be maintained in the +/- 200μm focussing adjustment range.

Figure 30:

CTF around best focus

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SNR measurements

The following figure summarizes the SNR results (the min values are based on the less performing pixels)

Figure 31:

SNR performance results