What's fixed in Ariel that is moving in other planet-hunting space telescopes?

Question

The BBC News article UK will lead European exoplanet mission says:

Steady platform

And one aspect that would work in Ariel's favour is the absence of any moving parts in its build, commented Plato team-member Dr Don Pollaco from Warwick University, UK.

"The issue with all of these planet experiments is that the signals you are looking for are so incredibly small that any systematics in the instrument itself will dominate the signal," he explained.

"And the systematics are often associated with bits that move. So the great thing about Ariel is that it is fixed-format - nothing changes," he told BBC News.

Question: What's fixed in Ariel that is moving in other planet-hunting space telescopes? Is it only aspects of the satellite's telescope and payload module, or does this difference include aspects of the satellite's attitude control system and other things in the service module as well? See ESA's Ariel.

Answer 1

I think this is a case of selective quoting/editing in the media reporting otherwise something major has changed in the design and plan for the mission that hasn't been released or published officially. The large and comprehensive ARIEL assessment report which was published less than 2 years ago discusses mounting all of the optical components on a large, thermally isolating optical bench to prevent movements of the optics introducing noise into the transit signals. However it also says (page 66) that secondary mirror (M2) will have the normal 3 axis position control to provide focus and tip-tilt. This is also mentioned in a Phase A study report published in 2018. The stability will be improved by the launch of Ariel into the Earth-Sun L2 Lagrange point which is more thermally stable and doesn't suffer from Earth eclipses compared to Kepler or TESS which are closer to the Earth.

This is the normal and sensible accommodation incorporated into satellite optical systems in order to compensate for anything moving slightly during the pretty violent shaking that occurs during launch on a rocket. Locking everything down pre-launch with no adjustment would be a very risky undertaking.

There are no more recent publications listed on the Ariel mission website that would suggest that the optical system has been redesigned to eliminate all moving parts although it is possible a study has been done and the result have not yet been communicated publicly.