# How did TESS scientists choose which stars to observe at a “2 minute cadence” in each field?

The BBC news podcast Science in Action episode New Planet Hunting Mission covers the upcoming TESS mission between 01:00 and 07:15 with Sara Seager, TESS deputy director of science, Massachusetts Institute of Technology, Cambridge, Massachusetts.

After about 05:00 Dr. Seager mentions that the TESS spacecraft will measure a special set of order 20,000 stars in each camera's 24x24 degree field at a two minute cadence.

How were the stars in each field selected for TESS?

The excellent YouTube video Transiting Exoplanet Survey Satellite (TESS) nicely explains how TESS operates and will slowly tile the celestial sphere.

below: From see TESS Operations. Click for full size.

• I think there are 6000 stars too bright for TESS to target. – Mike G Apr 13 '18 at 5:06
• @MikeG ah indeed! "...hat should not be included in TESS Cycle 1..." I'll edit, thank you! – uhoh Apr 13 '18 at 5:43

The candidate target list is designed to meet the priorities of the NASA TESS mission - to find small (rocky) planets in $<10$ day orbits using the transit technique. A subset of targets (near the ecliptic poles) are observed for longer, and there, there is a possibility of finding small planets in longer orbits - up to 120 days. The steps that are taken to prioritise this list are (i) remove giants -- too big for appreciable transits to be detected; (ii) avoid crowded sources -- the TESS point spread function is quite large; (iii) the prioritisation of small stars (which give a large transit signal) and of bright stars (which yield more signal-to-noise in the light curves). The final priorities are artificially boosted for those stars near the ecliptic poles (the continuous viewing zone) and reduced for stars near the galactic plane, where the de-reddening calculations are more uncertain.