Observing Technical Details

The TESS mission provides the community with an opportunity to make ground-breaking discoveries in the field of exoplanets, astrophysics and planetary science. A summary of observing technical details that proposers should be aware of can be found here.

A description of the overall mission can be found in Ricker et al. 2014. Brief descriptions of the mission operations, including the TESS orbit, field-of-view, time-sampling, and observing strategy, can be found in the Operations page. Details on all aspects of the mission, developed for GI proposers, are in the TESS Observatory Guide.

Photometric performance

Typical noise levels

A noise model for TESS photometry (figure below) shows the expected standard deviation of measurements of relative flux, as a function of apparent magnitude, based on 1 hr of data (Sullivan et al. 2015). For the brightest stars, the precision is limited by the systematic noise floor of 60 ppm. For the faintest stars, the precision is limited by noise from the zodiacal light (shown here for an ecliptic latitude of 30 deg). Over the range of apparent magnitudes 8-13, the photon-counting noise from the star is the dominant source of uncertainty.



The photometric precision for a 10th magnitude star is estimated to be about 200 ppm, so TESS will be sensitive to super-Earths around bright stars.



Image Credit: Zach Berta-Thompson with data from Sullivan at al. (2015)

For fainter stars, such as a 16th magnitude star, the photometric precision drops to about 1%, which is sufficient for many astrophysical studies such as supernovae and stellar variability.



Image Credit: Zach Berta-Thompson with data from Sullivan at al. (2015)

Saturation

Saturation is anticipated in the central pixel at I_C = 7.5. This, however, does not represent the bright limit for precise photometry. Excess charge from saturated pixels is conserved and spread across adjacent pixels in a CCD column until the excess reaches a CCD boundary. This leads to "bleed trails" extending above and below a saturated pixel, similar to what is seen for bright stars in Kepler/K2 photometry. Precision photometry can still be achieved by creating a photometric aperture that is large enough to encompass all excess charge and the TESS bright limit is anticipated to be I_C=4.

Crowding

Because the TESS pixels are large (21 arcsec), the TESS photometry for many targets will be contaminated by nearby objects. One of the goals of the TIC is to provide the information needed to estimate the contamination in the TESS band. This cannot be determined accurately ahead of time because it will depend on the pixels selected for the aperture photometry of each target and the exact position of the target in the aperture. However, it is possible for the TIC to provide some guidance concerning the level of expected contamination, for example by providing the number of known objects and their total brightness in the TESS band for some suitable standard aperture and photometer Pixel Response Function (PRF).

Sky coverage



Fraction of sky coverage for different time baselines that TESS will have over the 2-year prime mission.

Target selection

The TESS Input Catalog (TIC) is a catalog of approximately half a billion objects generated to assist in planning and executing observations and data reduction. The TIC includes detailed information about the characteristics of more than 2 million stars that have been identified as potential targets for the TESS Planet Search. The TIC also includes basic information for optically luminous persistent astronomical objects that may influence the photometry of TESS targets. More information on the TIC can be found here

The Candidate Target List (CTL) is a subset of TIC objects isolated to select the >200,000 targets for 2 min cadence observations in service of the mission's primary science requirements. More information on the CTL can be found here

Proposal tips

Due to the observing strategy of the TESS mission, the TSSC notes that GI proposers should be aware of several details when composing a science cases and target lists.

  • The exact location of the first observing sector (and therefore all subsequent observing sectors) is a sensitive function of the TESS launch date. For GI program Cycle 1, the impact of the uncertainty in the exact sky locations of the TESS FOVs is that there is no guarantee that any given proposed target will not fall into a gap between sectors, camera CCDs, or camera FOVs. The mitigate this uncertainty, Cycle 1 proposers are encouraged to consider target lists that include a number of similar sources distributed across the sky.

  • Adjacent TESS observing sectors have overlapping regions near the ecliptic poles, providing longer-term continuous coverage for stars falling in these regions which in turn provides sensitivity to smaller and longer-period planets. Objects within 12 deg of the ecliptic poles may be observed for ~1 year.

  • Camera 4 is always centered on the ecliptic pole and targets in this camera will be observed continuously for an entire Cycle. This leads to a limitation in the number of 2 min cadence targets that can be selected in the portion of sky covered by camera 4.

  • Camera 1 is always closest to the ecliptic plane and will be contaminated by stray light from the Earth and Moon during some observing sectors. The level of contamination and which sectors will be most affected is dependent on the launch date and the final inclination of the TESS orbit. These parameters will not be known until after commissioning and calibration.

  • Some cameras during some observing sectors will cover portions of the Galactic plane and suffer from increased background contamination. The exact cameras and sectors are dependent on the TESS launch date.

  • A primary science target will nominally be observed nearly continuously for 27.4 days in a given TESS observing sector. However, mission requirements only require a minimum of 20 days of observations out of the 27.4 possible per sector. This requirement accounts for all sources of observing inefficiency, including repointing for data downlink and interruptions due to the Earth and/or Moon in a camera FOV.