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. 2015. 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.

This website as well as the TESS Instrument Handbook and Data Release Notes should be consulted for the latest information regarding observing with TESS.

Photometric performance

Typical noise levels

The figure below shows the 1-hour Combined Differential Photometric Precision (CDPP) from TESS Sector 1. The red points are the RMS CDPP measurements for the 15,889 light curves from Sector 1 plotted as a function of TESS magnitude. The blue x's are the uncertainties, scaled to a 1-hour timescale. The purple curve is a moving 10th percentile of the RMS CDPP measurements, and the gold curve is a moving median of the 1-hour uncertainties. The photometric uncertainties are dominated by pointing jitter, but the best light curves are well below the mission requirements of (1) a systematic error floor at 60 ppm and (2) an achieved CDPP at 10th magnitude of 230 ppm, which is sufficient to detect super-Earths around bright stars. For fainter stars around Tmag = 16, the photometric precision drops to about 1%, which is still sufficient for many astrophysical studies such as supernovae and stellar variability.

The typical noise achieved in each individual TESS sector is described in the Data Release Notes for each sector.



Saturation

The TESS Instrument Handbook and Data Release Notes should be consulted for the latest information regarding observing saturated stars with TESS.

The amount of charge deposited by a star of magnitude m into the peak pixel depends on the fraction of the total charge in the peak pixel: this value generally ranges from 0.2 to 0.4 in the TESS images. The TESS cameras create 15,000 e−/s for a star of m = 10: thus, a star of m = 5 will create 3 × 106 electrons in a two-second exposure. For a flux fraction of 0.3, the charge in the peak pixel is 9 × 105 e−, leading to a bloom length of 5 rows; similarly, a star of m = 2.5 will create a bloom of 50 rows. A key feature of the CCID-80 CCDs used on TESS is their ability to conserve charge even from very saturated stars. Pre-launch ground tests showed that charge will be conserved for stars at least as bright as 4th magnitude. Measurements of charge conservation using flight data are in progress.

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

The fraction of sky coverage for different time baselines that TESS will have over the 2-year prime mission is listed below. Note this does not yet take into account the shift in pointing in Sectors 14, 15, and 16 that are described below.



Pointing shifts

TESS is observing the northern ecliptic hemisphere during the second year of its primary mission. The cameras are oriented along a line of ecliptic longitude (as they were in Year 1), with that longitude determined by the anti-solar longitude at the mid-point of the sector. For most of Year 2, the camera array is oriented such that Camera 4 is centered on the northern ecliptic pole: in this orientation, the southernmost edge of Camera 1 is ~6° from the ecliptic.

However, for Sectors 14 and 15, scattered light from the Earth and Moon is a significant problem in Cameras 1 and 2, reducing the available observing time for exoplanet transits by as much as 75% in those cameras. To reduce the impact of scattered light, the field-of-view of the camera array was shifted north by 31° with respect to its nominal pointing in Sectors 14 and 15.

When the cameras are shifted north, the northern ecliptic pole is located 7° from the center of camera 3, and the southernmost edge of Camera 1 is at an ecliptic latitude of ~37°. In addition, with this shift, the fields-of-view of Cameras 3 and 4 observe “on the other side of the pole”, thereby providing additional observations of parts of the sky that would otherwise only be observed in Sectors 20-22.

Scattered light from the Earth and Moon is also expected to be a problem in Sectors 25 and 26 and, to a lesser extent, in Sectors 16 and 24. The scattered light performance in Sectors 14 and 15 has been reviewed, and it was decided that Sector 16 would also have its pointing shifted north. In addition, it was decided that Sectors 24, 25, and 26 would benefit from having their pointings shifted north as well.

Additional details on TESS observations can be found at the MIT TESS website. The Web TESS Viewing Tool (WTV) has been updated to reflect the change in pointing for Sectors 14, 15, and 16 as well as Sectors 24, 25, and 26.

The sky coverage maps for Sectors 1-21 are given below in the ecliptic and celestial coordinate systems and show the shifted fields for Sectors 14, 15, and 16.

Sectors 14, 15, and 16 shifted north: