CHEOPS Performance and Passband
Photometric Performance of CHEOPS
The photometric stability requirements of CHEOPS is captured in two top-level science requirements (see webpage) which have been the focus of significant work to-date. A comprehensive noise budget has been developed, with a detailed breakdown of the different contributors which are both astronomical (eg. photon noise from the source itself, the zodiacal background, stray-light) as well as instrument/platform (eg. jitter noise due to pointing stability, noise from the analogue electronics chain) in nature: details of this can be found in the presentation on Instrument Performance given at the CHEOPS Open Time Workshop in Schloss Seggau in July 2017
Shown in Figure 1 are plots of the 1-sigma photometric precision (in parts per million) that is foreseen for a 6-hour and 3-hour on-source integration time (left and right panels respectively), as a function of target magnitude. In both cases the dashed horizontal and vertical lines denote the values (precision and magnitude) that are stipulated in the individual requirements. These plots have been generated using Exposure Time Calculators, which are described (presentation) and can be downloaded (files and manuals) from the resources page from the CHEOPS Open Time Workshop.
Figure 1: Plots illustrating the 1-sigma photometric precision (in parts per million) for CHEOPS for two sizing cases: (a) left hand panel: precision foreseen in a total, on-source integration time of 6 hrs, which is equivalent to the transit duration of a planet with an orbital period of 50 days; (b) right hand panel: precision foreseen after a total on-source integration time of 3 hrs, which is equivalent to the transit duration of a planet with an orbital period of 13 days. In both cases the solid black curve indicates the total precision, where the blue,green and red represent contributions from the instrument, stray light and the photon/shot noise respectively. It can be seen that for the brighter targets (V <= 9), the dominant noise source is from the instrument, whereas for the fainter targets (10 <= V <= 12) initially photon noise from the target itself dominates, and then for fainter stars straylight from the Earth dominate.
The CHEOPS payload comprises a single, wide-band photometer covering a wavelength range of 0.33 – 1.1 microns (see overview of the CHEOPS Payload) With no band limiting filters in the instrument, the spectral response of CHEOPS is set by the anti-reflection coating on the CCD at the short-wavelength end and by the response of Silicon at the long-wavelength end. Shown in Figure 2 is the CHEOPS normalised spectral response (band pass) as a function of wavelength: overplotted are the equivalent curves for other exoplanet missions together with the spectral energy distributions (SED) of a Teff~5500K and Teff~4500K (left hand panel) dwarf star, and band passes of the BVRI filter set, together with GAIA (right hand panel).
Figure 2: Plots illustrating the (normailsed) spectral response of CHEOPS as a function of wavelength: (a) left hand panel: CHEOPS = black solid, CoRoT = dashed/dotted; MOST = short/long dash (MOST team/Jason Rowe) , TESS = dotted (eg. Sullivan et al., 2015, ApJ 809, 77), Kepler = long dashed (http://keplergo.arc.nasa.gov/Instrumentation.shtml); cyan = SED of Teff~5500K dwarf; pink = SED of Teff~4500K dwarf (b) right hand panel: CHEOPS = black solid, GAIA = black dotted (https://www.cosmos.esa.int/web/gaia/transmissionwithoriginal); blue,green, red and pink are the band passes of the BVRI filter set (Bessell, M., 1990: PASP, 102, 118, with data taken from http://spiff.rit.edu/classes/phys440/lectures/filters/filters.html )