1. Point Source Transmission Simulations on the COROT baffle
by JY. Plesseria - E.Mazy
4th December 2002
Institut d ’Astrophysique - Liège - Belgique
Liège, 4th December 2002
COROT WEEK n°3

2. Presentation plan - Context - Objectif of the COROT baffle
- Definition of the baffle
- Geometrical analysis (first order)
- Modelling with ASAPTM
- Walls scattering analysis
- Edges scattering analysis
- Uniformity in the focal plane
- Sensitivity to hypotheses
- Work to be done
- Conclusions
Liège, 4th December 2002
COROT WEEK n°3

3. Context Belgium supports COROT with hardware contributions:
CSL + industry: - Optical design of the baffle
- Mechanical design and procurement of the Baffle
- design and qualification of a cover mechanism
- procurement of the cover
- flight acceptance of the baffle and the cover
Sonaca : - Corotcase
Liège, 4th December 2002
COROT WEEK n°3

4. Objectif of the baffle
Scientific objectifs: astroseismology and planet transit detection
Method: observation and interpretation of light variation of stars during long duration
Consequence: any other periodic light variation must be minimised
Driving straylight source: the Earth around which COROT orbits
Objectif of the baffle: reduce periodic straylight, reflected by the Earth, that enters the baffle and reach the focal plane to a level lower than the observable useful variations
Liège, 4th December 2002
COROT WEEK n°3

5. Definition of the baffle (1)
Early definition of the baffle was done in France by LAM and Alcatel using ApartTM software
Model was proposed to CSL as well as new geometrical constraint (due to mechanical design of the telescope). CSL made a first model in ASAPTM and point out some problems in the design
A updated design was proposed by LAM and Alcatel and has been deeply analysed by CSL.
Rem: ApartTM works by computing view factors while ASAPTM uses ray-tracing as basic working principle.
Liège, 4th December 2002
COROT WEEK n°3

6. Definition of the baffle (2)
Liège, 4th December 2002
COROT WEEK n°3

7. Definition of the baffle (3)
Liège, 4th December 2002
COROT WEEK n°3

8. Geometrical analysis (First order) (1)
Search for critical surfaces in the baffle i.e. surfaces that are seen by the baffle aperture and the primary mirror.
-> development of a MatLab script to identify the percentage of the primary mirror area seen by any point in the baffle.
Liège, 4th December 2002
COROT WEEK n°3

9. Geometrical analysis (First order) (2)
Vane S3
Vane S3Bis
Liège, 4th December 2002
COROT WEEK n°3

10. Geometrical analysis (First order) (3)
Vane 1
Vane S2
Liège, 4th December 2002
COROT WEEK n°3

11. Geometrical analysis (First order) (4)
Liège, 4th December 2002
COROT WEEK n°3

12. Modelling with ASAPTM (1)
CSL has build a geometrical model with ASAP including:
- Baffle (internal surfaces)
- Entrance pupil of the telescope
- Primary mirror
- Secondary mirror
- Intermediate focal plane (primary mirror focal plane)
- Entrance pupil of the dioptric objective
- Primary and secondary mirrors envelopes
Required parameters for ASAP are geometry, surface scattering properties and computation parameters
Liège, 4th December 2002
COROT WEEK n°3

13. Modelling with ASAPTM (2)
Liège, 4th December 2002
COROT WEEK n°3

14. Modelling with ASAPTM (3)
Liège, 4th December 2002
COROT WEEK n°3

15. Modelling with ASAPTM (4)
Exit pupil design. Cutting by blue dotted line not performed in model
Liège, 4th December 2002
COROT WEEK n°3

16. Modelling with ASAPTM (5)
Coatings
Chemglaze Z306 paint
Mirror roughness (1 nm)
Liège, 4th December 2002
COROT WEEK n°3

17. Modelling with ASAPTM (6)
Coatings
Mirror ppm
Mirror ppm
Liège, 4th December 2002
COROT WEEK n°3

18. Modelling with ASAPTM (7)
Surface scattering with ASAP
- Each ray has its own power
- Created rays can be directed towards user defined preferred surfaces
- Energy in rays is computed taking into account surface BRDF and solid angles
- Numbers of rays is user defined
- Additional random rays can also be created
Incoming ray
Preferred surface #1
Preferred surface #2
Liège, 4th December 2002
COROT WEEK n°3

19. Modelling with ASAPTM (8)
Rays number limitation
Let N be the numbers of rays at the entrance of the baffle for one direction
We need to measure an attenuation of at least Assuming an average TIS of 10%, the required number of diffuse reflections is 13.
If, at each scattering we create 6 rays, that leads to a final number of N x113 billions rays created ! What is far beyond human/computers capabilities.
Solution: decrease periodically the number of rays by suppressing the « less useful » rays.
For this, the baffle will be separated in several sequencial sub elements.
Liège, 4th December 2002
COROT WEEK n°3

20. Modelling with ASAPTM (9)
Rays number limitation
11
10.1
10.01
10.001
Liège, 4th December 2002
COROT WEEK n°3

21. Modelling with ASAPTM (10)
Rays number limitation
NB: a 9th step is added extracting from step 8 rays (reaching the exit pupil) the rays inside the lens barrel FOV
Liège, 4th December 2002
COROT WEEK n°3

22. Modelling with ASAPTM (11) Rays number limitation
Second possibility: suppressing rays with low energy
Expected rays in exit pupil: ~104.
Expected power in exit pupil: ~10-13
Expected energy per ray: 10-17
Before primary mirror, minimum significant ray (assuming mirror TIS at 0.1%): 10-14
Computation hypothesis: cutoff before primary: 10-17
cutoff after primary: 10-18
Liège, 4th December 2002
COROT WEEK n°3

23. Modelling with ASAPTM (12) Rays number limitation
Liège, 4th December 2002
COROT WEEK n°3

24. Wall scattering analysis
Liège, 4th December 2002
COROT WEEK n°3

25. Edges scattering analysis (1) Details size limitation
A grid of 171x171 rays is the source. Meaning that there is 1 ray for 20 mm².
This is sufficient for most of the baffle except the edges (40 µm)
Specific computations are performed:
- Rays a specifically sent towards one edge (half ring)
- Entrance energy is proportional to the projected area of the edges with respect to baffle aperture
- Resulting energy is measured in the exit pupil
Liège, 4th December 2002
COROT WEEK n°3

26. Edges scattering analysis (2)
Liège, 4th December 2002
COROT WEEK n°3

27. Edges scattering analysis (3)
Liège, 4th December 2002
COROT WEEK n°3

28. Uniformity in the focal plane (1)
Energy in the focal plane is assumed uniformly spread on the CCD. That has to be verified.
CCD and dioptric objective are not modelled so we verify the angular distribution of energy in the exit pupil of the telescope, what allows interpretation of the results on CCD.
Field 20°
Azimuth 0°
Azimuth 90°
Azimuth 180°
Liège, 4th December 2002
COROT WEEK n°3

29. Uniformity in the focal plane (2)
Field 25°
Field 40°
Azimuth 0°
Azimuth 90°
Azimuth 180°
Liège, 4th December 2002
COROT WEEK n°3

30. Sensitivity to hypotheses (1)
Entrance grid size:
Liège, 4th December 2002
COROT WEEK n°3

31. Sensitivity to hypotheses (2)
Edges thickness:
Liège, 4th December 2002
COROT WEEK n°3

32. Sensitivity to hypotheses (3)
Computation stability:
Liège, 4th December 2002
COROT WEEK n°3

33. Work to be done
Preliminary computations are completed. The work to be done now is:
- adjustment of the model to manufacturing design
- refinement of coating hypotheses by real BRDF measurements
- evaluation of design modification required for manufacturing constraints.
Liège, 4th December 2002
COROT WEEK n°3

34. Conclusions
- The work performed shows that the optical design of the baffle satisfies the actual requirements
- The baffle do not present variable asymetry (on straylight point of view)
- A reliable model is available to be updated with baffle manufacturer informations/modifications
- BRDF measurements have to be done to refine the model
- Remaining point is … to manufacture this baffle.
Liège, 4th December 2002
COROT WEEK n°3