CHEOPS consistently uses "ICRS, epoch J2000", i.e. the International Celestial Reference System (ICRS) with epoch J2000. This is the one-and-only format you must provide in the PHT1 system, or else your observations might fail. This is emphasised in all documentation, such as the CHEOPS Observers' Manual and the PHT1 system itself.

The CHEOPS Target Checker is the appropriate tool to use. It resolves target names via Simbad and will provide coordinates in the "ICRS, epoch J2000" format. If your target name cannot be resolved via the CHEOPS Target Checker nor Simbad, it is your responsibility to retrieve the coordinates in the correct "ICRS, epoch J2000" format via other means (for example, via MAST).

Do not copy&paste the coordinates you see on ExoFOP. These are taken from the Gaia catalogs, which use the "ICRS, epoch J2015.5". However, ExoFOP does not call these "ICRS, epoch J2015.5", but instead uses the term "J2000 equinox" as a proxy for "ICRS", and typesets them as "J2000, epoch J2015.5". This might lead to confusion among proposers.

What is the meaning of ICRS, ICRF, J2000.0 epoch, and J2000.0 equinox? Gaia DR2 astrometry consistently uses the ICRS reference system and provides stellar coordinates valid for epoch J2015.5 (roughly mid-2015, where J stands for Julian year). Equinox J2000.0 is a currently obsolete concept linked to former, dynamical reference systems such as FK5 which were tied to the celestial equator at a particular time. As of 1 January 1998, the International Celestial Reference System (ICRS) is the standard celestial reference system adopted by the International Astronomical Union (IAU). The ICRS is the set of prescriptions and conventions together with the modelling required to define, at any given time, a triad of orthogonal axes. The ICRS has its origin at the barycentre of the Solar System, with axes that are space-fixed and kinematically non-rotating with respect to the most distant sources in the Universe. In practice, the ICRS is materialised by the International Celestial Reference Frame (ICRF) through the coordinates of a defining set of extra-galactic objects (quasars). Prior to astronomers being able to define and use the ICRS and ICRF, dynamical reference systems were used based on observations of star positions tied in some way to moving objects in the Solar System. These reference systems refer to a mean equator and equinox at a given reference epoch (typically J2000.0), requiring precession/nutation models and corrections to deal with the time-variable fundamental plane. To within ~25 mas, mean J2000.0 equatorial coordinates are the same as ICRS coordinates such that, for "ordinary" applications, they can in practice be considered to be the same. For high-accuracy applications, the appropriate frame conversion shall be used. (Thanks to our friends at Gaia for providing this nice summary here.)


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This website was last updated on 12 March 2024.