What is CHEOPS ? |An European Space Agency Mission (Part-4)

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Design of CHEOPS

CHEOPS design was driven by the need to precisely measure the ‘signal’ from exoplanet host stars. Precision is the main priority because the transit signal can be extremely tiny for the smallest planets, noise from the instrument itself can obscure the transit. 

The instrument therefore needs to be as stable as possible, both
in terms of keeping jitter (variation in signal's periodicity from it's true frequency) to a minimum while observing the star, & in keeping cool to avoid thermally-induced noise. CHEOPS has a high precision photometer with a 300 mm effective aperture telescope & a single charge-coupled device (CCD) detector covering visible to near-infrared wavelengths. A number of key
elements of the CHEOPS instrument are designed to keep
stray light(usually from the Earth & Moon) from entering
the telescope, we can also understand it easily by taking an example: 
Much as street lights make it harder for us to see the stars, stray light would reduce the observation ability of the telescope.
Instruments also include a large external baffle, a smaller
internal one & vanes inside the telescope.

The baffle has a cover to protect the optics from dust
& contamination on the ground & during launch. This will be opened once it is in Earth orbit with the telescope pointing away from the Sun.

A Sun-shield keeps the instrument shaded, protecting the two instrument radiators that provide cooling to the detector & electronics. The structure also carries three solar panels, which are orientated in such a way as to provide sufficient power to operate the spacecraft in the planned pointing direction. In order to improve pointing stability & minimize misalignment effects, the two spacecraft star trackers are mounted directly on to the instrument. The stability of the spacecraft pointing over time is further improved by feeding back information on the actual position of the target star that is being measured by the CHEOPS instrument to the spacecraft attitude & orbit control system ( system that controls the satellite pointing).

Other vital mission statistics

Dimensions	1.5*1.5*1.5m
Mass	280 Kg including propellant
Solar panel area	2.5 meter square
Science instrument	High precision photometer based on a 300 mm effective aperture Ritchey-Chrétien telescope and a single CCD operating over visible to infrared wavelengths (330 to 1100 NM).
CCD temperature	223 K with stability of 10mK ( -40 degree Celsius with stability of one hundredth of a degree).
Field of view	19*19 arc-minutes ( two third the size of full moon).
Pointing directions	60 degree half cone around the anti-sun directions.
Target stars	Stellar magnitude 6-12 ( stars at the bright end are just visible to the naked eye from the darkest sites).
Precision	20 parts per million(ppm) in 6 hours for the observing time of transit depth of an earth-sized planet orbiting a solar-type star, 85 ppm in 3 hours observing time for detailed characterization of the light curves of the Neptune sized planets transiting smaller cooler stars; to be maintained over 48 hours.
Orbit	Sun-synchronous dusk-dawn at an altitude of 700 Km (low earth-sized orbit with an inclination of 98 degrees (a 100-minute polar orbit that crosses the equator at sunrise & sunset).
Attitude stabilization	3 axis stabilized (telescope always pointing away from the sun), with the science instrument further used to improve the stability (so called payload in the loop).
Data down-link	1.2 G bit/day
Normal mission	3.5 years