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Functional chains
P. L.PLATO kick-off meeting– CNES, 09 Nov. 2010
Main photometric chain• photometry of faint stars (mv > 8), in the visible• made by 32 normal cameras• continuous observation, detector readout every 25.0 s• 1 Data Processing Unit (DPU) for 2 cameras• output: light curve and spot barycentre time series (up to 1 sample / 50s)
2 “fast” photometric chains• pointing error information for the satellite (AOCS)• high rate and low delay for delivery to AOCS• imperative redundancy of the information• photometry of bright stars (mv < 8), • with a chromatic information: 1 “red” camera and 1 “blue” camera• continuous observation, detector readout every 2.5 s, • outputs:
pointing error at a rate of 1 sample / 2.5 s light curve and spot barycentre time series (1 sample / 50s)
The Camera
P. L.PLATO KO meeting – CNES, 09 Nov. 2010
Optics• fully centred refractive concept, 6 lenses + a front window,• bandwidth : 500 – 1050 nm• pupil diameter 120.0 mm, f/2.1 • PSF size: 90% of photo-electrons in a diameter of 36 µm (30 arcsec on the sky), • very wide field of view ~1100 dg²
Focal Plane Assembly• 4 detectors assembled in a square area of ~16 x 16 cm²• each detector:
4510² x 18 µm square pixels (~20 Mpx) back thinned, back illuminated 2 outputs 2 different modes: FF (Normal camera) or FT (Fast camera)
Video Electronics: N-FEE et F-FEE• readout @ 4 Mpx/s,• conversion on 16 bits• 25.0 s fixed cycle time for Normal cameras• 2.5 s fixed cycle time for Fast cameras
Powered and synchronised by: N-AEU or F-AEU
Accommodation of the cameras
P. L.
The overlapping field of view• the 32 cameras are organised in 4 sub-groups, mounted on an optical bench• each sub-group has then 8 cameras with the same line of sight (LoS)• the 4 LoS are tilted by 9.2° from the satellite axis, in four perpendicular directions
offers:• an overall FoV of 2200 deg² in 4 different zones, with an equivalent pupil diameter going
• from 675 mm (32 cameras) on 300 deg²• … to 340 mm (8 cameras) on 950 deg²
• ( fast camera LoS are aligned on ZPLM axis )
PLATO KO meeting – CNES, 09 Nov. 2010
and then optimizes simultaneously the number and the brightness of cool dwarfs and subgiants.In addition, allows us to re-observe during the Step&Stare phase, some stars for which interesting planets where detected during the previous long monitoring phases.
Accommodation on the optical bench
P. L.
Snapshot of the payload presented at the end of the assessment phase, very similar to the current accommodation presented by the satellite contractors (except 32 + 2 cameras now)
PLATO KO meeting – CNES, 09 Nov. 2010
Thermal of the cameras
P. L.
Thermal needs• focal plane temperature lower than -65°C (dark current, radiations)
on a design where:• the front window sees the sky and then, is cold,• the last lens is close to the focal plane and then is cold,• axial gradients shall be minimized for optical performance
then: a cold opticsMore: in this concept with several cameras the only way to evacuate the power dissipated inside the detectors is the direction of the line of sight
Thermal design of the camera is then based on a FPA power evacuation through the telescope structure and then by the baffle, leading to:
• a highly conductive telescope structure• a FPA thermally connected to it• a baffle, used as a radiator
This sub-system is isolated from its environment by use of: low conductivity bipods for the telescope low conductivity flexi-cables between detectors and their video electronics use of MLI on each critical surfaces
PLATO KO meeting – CNES, 09 Nov. 2010
Thermal stability
P. L.
Photometric performances are highly linked to thermal stability of the camera. This stability is ensured by:
• a highly isolation from the environment (as said before), • a stable thermal environment given by the orbit (light variation during the 3 months exposure)• a constant power dissipation in the detectors on timescales higher than 25 s• a power dissipation by the baffle on a stable source (sky)
Added to these favourable conditions, a temperature control ensured by the satellite service module allows :
• a compensation of the thermal environment difference between various camera locations• an adjustment of the structure mean temperature in a small range around the nominal temperature for a slight re-focus of each camera in flight
And finally, a temperature monitoring on several points of the camera allows possible corrections of the photometry (on ground)
PLATO KO meeting – CNES, 09 Nov. 2010
Mechanical design
P. L.
Need a design able to• accommodate the dimensional changes between integration (~20°C) and operation (~ -80°C) without stress in the materials (in particular lenses)• preserve the optics centring in this large temperature change• fulfil the thermal requirements
• telescope structure in Albemet (CTE + highly conductive)• 3 bipods in titanium (good stiffness + poor thermal conductivity)• lenses barrels (Albemet or titanium) under form of several flexible blades• Focal Plane Assembly: positioning adjustment by 3 shims
At any interface (lens/barrel, baffle/structure…): • flexible or quasi-isostatic mounts, •three-points attachment, spherical washers …
PLATO KO meeting – CNES, 09 Nov. 2010
Data Processing System
P. L.PLATO KO meeting – CNES, 09 Nov. 2010
Electrical design
P. L.PLATO KO meeting – CNES, 09 Nov. 2010
P. L.
More information in the
PLATO Payload Description Document (PPDD)
Ref.: PLATO.LAM.INS.REP.1065
Thank youPLATO KO meeting – CNES, 09 Nov. 2010
