Updated 1 March 2012

Design and Modelisation of a Straylight Facility for Space Optical Instrument

E. Mazya, Y. Stockmanb, M.L. Hellina

aCentre Spatial of Liege, Avenue du Pre-Aily, 4030 Angleur, Belgium

ABSTRACT

In the framework of instrument calibration, straylight issues are a critical aspect that can deteriorate the optical performances of instrument. To cope with this, a new facility is designed dedicated for in-field and far field straylight characterization: up to 10-8 for in-field and up to 10-10 for far field straylight in the visible to NIR spectral ranges. Moreover, from previous straylight test performed at CSL, vacuum conditions are needed for reaching the 10-10 rejection requirement mainly to avoid air/dust diffusion. The major constrains are to design a straylight facility either for in-field and out-field straylight measurements. That requires high dynamic range at source level and a high radiance point source allowing small diverging collimated beam. Moreover, the straylight facility has to be implemented into a limited envelope and has to be built with vacuum compatible materials and black coating. As checking the facility performance requires an instrument better than the facility itself, that is no easy to find, so that the performances have been estimated through a modelisation into a non sequential optical software. This modelisation is based on CAD importation of mechanical design, on BRDF characteristics of black coating and on statistical averaging of ray tracing at instrument entrance.

Keywords: Straylight facility, straylight modelisation, straylight design, BRDF, black coating scattering

1. INTRODUCTION The Centre Spatial de Liege (CSL) has been involved since twenty years in the characterization and evaluation of stray light for space instruments (ref [1] to [5]). Today CSL is developing a new stray light facility for the stray light characterization of small earth observation satellites. Stray light issues are tackled in different ways. For large payloads deep stray light analysis is carried out, tests at subsystem are performed or partial illuminations are also envisaged. For small EO satellite, it is possible to perform an end to end test to evaluate the stray light characteristic of the instrument.

2. TEST FACILITY REQUIREMENTS For an optical system, the stray light contributions may be summed as In-field Stray light (IFS) and Out-of-field Stray light (OFS). The major stray light contributions are:

- the effects of mirror microroughness,

- the effects of dust or defects on the mirrors,

- the scattering induced by the aperture stop,

- the effects of ghosts,

- the stray light due to Sun and Moon, or any intense light source out of the FOV.

The facility will not be able to directly identify the sources of stray light listed here above, but will verify that the stray light contribution for along angles (taken in absolute values) larger than 10 arc degrees or for across angles larger than 25arc degrees are negligible, and that the dominant contributor to stray light for in field is the pupil stop. The mirror microroughness, dust and defects contributions are indistinguishable in practice and bear a strong angular variation from the incident direction.

The test facility is designed for infield straylight characterization up to 10-8 as close as ± 250 arcsec from the incident and for farfield straylight characterization up to 10-10 as close as ± 10° from the incident.

Updated 1 Mar

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Updated 1 Mar

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Updated 1 March 2012

Figure 11. MAP PU1 paint BRDF profiles

6.3 Strayligth simulation: Nearfield performances

The nearfield contributions are mainly:

• Straylight induced by optical surfaces scattering (particulate contamination, microroughness contribution, dig contribution);

• Straylight induced by the source assembly;

• Straylight induced by backscattering between the payload baffle and the parabolic mirror.

The optical surface particulate contamination impact is based on MIE scattering BRDF with typically CL100 and CL200 level according MIL-1246C. The CL100 surface cleanliness is mandatory to fit with requirements.

Figure 12. Impact of optical surfaces cleanliness on nearfield straylight.

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

-1.5 -1 -0.5 0 0.5 1 1.5

Stra

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ject

ion

FOV [arcdeg]

Requirement

CL100

CL200

Updated 1 March 2012

The optical surface microroughness BRDF is based on Harvey function with typical parameters. Microroughness contributions is driven by the parabolic mirror microroughness which was not specified for this purposes. As shown in Figure 13, the impact of the microroughness straylight contribution is into requirements.

Table 1: Characteristics and Harvey parameters for collimator mirrors.

Parabolic mirror Fold mirror Microroughness (RMS) 1.2 nm 0.4 nm

TIS@805nm 0.035% 0.0038%

Harvey function parameters

b0 = 0.073 l = 0.01 s = -1.8

b0 = 0.0082 l = 0.01 s = -1.8

The scratch and dig of the optical surfaces contribution is mainly driven by the parabolic mirror digs: optical surface is degraded by non negligible digs density (N=5cm-2).The digs induced BRDF is based on lambertian and specular contributions ([8]):

Where λ is the wavelength, Φ is the dig diameter and N is the dig density.

The impact of the optical surfaces digs straylight contribution is above the requirements as shown in Figure 13.

The collimator source assembly straylight contribution is mainly driven by the microroughness on the pyramid. The straylight contribution is negligible.

For nearfield measurements, the payload baffle is implicated in association with the diffusion onto the parabolic mirror (mainly the diffusion from the digs). As shown in Figure 13, this straylight contribution is one order of magnitude below the parabolic mirror dig diffusion.

Figure 13. Impact of optical surfaces microroughness and digs and impact of FPA assembly and payload baffle on nearfield

straylight.

1.E-191.E-181.E-171.E-161.E-151.E-141.E-131.E-121.E-111.E-101.E-091.E-081.E-071.E-061.E-05

-1.5 -1 -0.5 0 0.5 1 1.5

Stra

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th re

ject

ion

FOV [arcdeg]

Requirement

Microroughness

S&D(N=5)

FPA

Instrument/mirror+Acktar

Updated 1 March 2012

Table 2 summarizes the nearfield performances at 4.1 arcmin from the nominal field of view. Excepted the dig straylight contribution, all other contributions fit with the requirement (< 10-8).

Table 2: Straylight contribution for nearfield measurements at FOV = 4.1 arcmin (250 arcsec).

Optical surfaces microroughness: 1.247 10-8 ± 11% @1σ Optical surface cleanliness (CL100): 7.55 10-9 ± 7% @1σ

Parabolic mirror digs (N=5 cm-2) 1.52 10-7 ± 7% @1σ Source assembly: 5.5 10-14 ± 28% @1σ Payload baffle: 2.48 10-8 Requirements: 10-8

6.4 Straylight simulation: Farfield performances

The farfield contributions are mainly:

• Straylight induced backscattering between the payload baffle and the black tent;

• Straylight induced backscattering between the payload baffle and the parabolic mirror;

• Straylight induced by the air dust.

The strayligth induced by the black tent is lower than 10-12 with MAP PU1 and one order of magnitude below by using Acktar® black coating on the payload baffle (Figure 14). The straylight induced by the air dust is limited or suppressed by using a small vacuum into the chamber.

Figure 14. Impact of payload baffle scattering on farfield strayligth.

1.E-15

1.E-14

1.E-13

1.E-12

1.E-11

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

-20 -10 0 10 20

Stra

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ion

FOV [arcdeg]

Instrument/mirror + Acktar

Shroud

Shroud+Acktar

Updated 1 March 2012

7. FACILITY STATUS The facility is building up:

• Collimator source assembly is at black coating level;

• Light source, baffling, black tent, MGSE, monitoring and collimator are available at CSL.

Straylight test on PROBAV QM are foreseen in December 2012. Straylight test on SOLOHI are foreseen in January 2013.

8. CONCLUSIONS Stray light characterization of earth Observation satellites has become a growing necessity to guarantee the mission success. To fulfill this, a new stray light test facility is under development at CSL. In this paper we have demonstrated that the ability to measure PSFs at below 1E-10 should be possible from the visible till the NIR. The facility will be able to test payload of several 100 of kg with FOV of +/- 300 arc degrees across track and +/- 15° along track. The acquire data will not allow to identify each the stray light source mechanisms; it provides the integrated PSF taking into account all the contributors. Nevertheless the big advantage of this stray light test facility is its ability to measure nearfield stray light. The measurement will not only be used for the final acceptance of the instruments but also for removing the stray light contribution from the in flight data images. Simulations gives confidence on the facility performances.

9. ACKNOWLEGMENTS This development would not have been possible without the help of the Belgium GSTP program.

REFERENCE

[1] Y. Stockman, I. Domken, H. Hansen, JP. Tock, T.A. Decker, A. Rasmussen, A.F.J. den Boggende, J.W. den G-Herder, F. Paerels, D. de Chambure, P. Gondoin, “XMM Flight Modules with Reflection Grating Assembly and X-ray baffle testing”, Proc. SPIE 3444, (1998).

[2] J.Y. Plesseria, E. Mazy, J.M. Defise, P. Rochus, E. Lemmens, D. Vrancken, “Optical and mechanical design of a straylight rejection baffle for COROT”, Proc. SPIE 5170, (2003)

[3] JY.Plesseria, E. Mazy, JM. Defise, P. Rochus, A. Magnan, V. Costes, “Straylight analyses of the external baffle of COROT”, International Conference on Space Optics – ICSO, (2000)

[4] J.M. Defise, J.P. Halain, E. Mazy, P. Rochus, R.A. Howard, J.D. Moses, D.G. Socker, R. Harrison, G.M. Simnett, “Design and Tests for the Heliospheric Imager of the STEREO Mission”, Proc SPIE 4853, (2003)

[5] E. Mazy, J.P. Halain, J.M. Defise, P. Rochus, R.A. Howard, J.D. Moses, C. Eyles, R. Harrison, “Design and Performances of the Heliospheric Imager for the STEREO Mission”, Proc. SPIE 5962, (2005)

[6] A. Zuccaro Marchi, M. Taccola, Jorg Versluys, Didier Beghuin, Yvan Stockman, Ronald Kassel, M. François, Ignacio Torralba Elipe, “PROBA V Multispectral Imager: Status”, International Conference on Space Optics – ICSO, (2012)

[7] Yvan Stockman, ML Hellin, A. Zuccaro Marchi, M. Taccola, Jorg Versluys, M. François, E. Mazy, “Conceptual design of a straylight facility for earth observation satellites” International Conference on Space Optics – ICSO, (2012)

[8] G.L. Peterson, “Strayligth analysis with ASAP”, Breault Research Organization, (2010)