The CHEOPS Red Book talks about this in a couple ways.
Section 3.2 "Sky Coverage" (in the context of science requirements) says:
SciReq 2.1 Transit search on stars with small planets (L1)
50% of the whole sky shall be accessible for 50 (goal: 60) cumulative (goal: consecutive) days of observations per year and per target with observation duration longer than 50% of the spacecraft orbit duration (>50 min for 100-min spacecraft orbital period).
SciReq 2.2 Follow-up of stars with transiting planets (L1)
25% of the whole sky, with 2/3 in the southern hemisphere, shall be accessible for 13 days (cumulative; goal: 15 days) per year and per target, with observation duration longer than 80% of the spacecraft orbit duration (>80 min for 100-min spacecraft orbit).
(There are paragraphs of detail after each one; SR 2.1 is where the 48-hour observing time is motivated) So that's what they consider important.
They then (Section 3.3) document some visibility requirements:
SciReq 3.2 Earth stray light exclusion angle (L2 SciReq 1.2, SciReq 2.1, SciReq 2.2, SciReq 4.1)
In order to limit stray light contamination, the minimum angle allowed between the line- of-sight and any (visible) illuminated part of the Earth limb, the so-called Earth stray light exclusion angle shall be 35° (goal: 28°).
This angle value is driven by the faint magnitude limit (SciReq 4.1). For brighter targets, this constraints could be relaxed.
This is followed up with a 120 degree "Sun Exclusion angle" and a 5 degree "Moon exclusion angle". (There's also a "Stray light analysis" in Section 4.4.5
The mission design (Chapter 5) motivates the anti-sunward attitude:
So now there are several ways ways to think about answering the Question:
1) (Probably not the intended question, but the way the science team will think about it) 50% of the sky will be accessible for at least 50 cumulative days of observation over the 3.5 year mission. CHEOPS doesn't care when it gets to a particular star, just that it can, for at least 50% of the sky. There's always something interesting to look at, and that sums up to 50%...
2) In any particular 48-hour period, there are constraints from the Sun, Earth and Moon that vary with their position and the intensity of the target star. (Bright targets might be accessible over a slightly wider field) Because CHEOPS is working through a series of target stars, some fraction of those will be accessible within the constraints: The science team (almost certainly) uses those lists of stars visible in each period to construct the observing schedule. But one could still construct a period-by-period calculation of the section of the sky that's accessible during each period.
3) Or you could construct averages based on solid angles.
For that last one: The Sun excludes 120 degrees: cos 120 is -0.5, so that's 3/4 of the sky. Half of the time CHEOPS is over sunlit earth, and the 35 degree exclusion is (from cos 35 = 0.82) 9% of the sky. The other half, there's a slightly less strict 1000km grazing limit: 5% of the sky. And the moon takes away 0.2% when it's present (which is not all that often, given all the other restrictions). These aren't uncorrelated, they dance around is quite coherent ways, but it we assume a lot of averaging:
fraction of sky = 0.25 (0.91+0.95)/2 0.999 = 23%
- or about 2.9 sr
- or about the equivalent of a cone with an half-opening of
- 1.33 radians or
- 76 degrees
(though the shape really isn't a cone; it's more the intersection of two overlapping cones with a hole in it for the moon)