ETC Help

The CHEOPS ETC provides the expected photometric precision over several pre-defined time intervals as well as a time interval specified by the user.

It also provides an estimate of the maximum possible observing efficiency (i.e. the fraction of time spent on target per orbit) and the pixel filling factor (in % of the pixel full-well capacity) at the PSF peak. The user may also choose to specify directly the expected flux in the CHEOPS passband in electrons per second as an input.

A .pdf file summarising the results can be created.

Input Parameters

Target G magnitude
The apparent magnitude of the target in the Gaia G band.
Target effective temperature
The stellar effective temperature in Kelvin within the allowed range of 2300 to 40000 K.
Spectral type (stellar granulation)
The spectral type of the target, selectable from a drop-down menu. This value will be used to estimate the amplitude of noise due to stellar granulation.
Defined time interval
The duration (in hours) for which the calculation should be performed, next to pre-defined time intervals of 1, 10, 30, 60, 180 and 360 minutes.
Exposure time
The exposure time of the individual CHEOPS exposures.
Right Ascension
The target right ascension in decimal degrees [dd.d], or hexagesimal format [hh:mm:ss.s].
Declination
The target declination in decimal degrees [dd.d], or hexagesimal format [hh:mm:ss.s].

Additional Parameters

Specify flux in CHEOPS passband
Ticking this box allows to specify the flux in the CHEOPS passband.
If not selected, the flux is computed based on the Gaia magnitude and the effective temperature (recommended).
Flux
The target flux measured by CHEOPS in electrons per second. This value is only used if the option Specify flux in CHEOPS passband is selected.

Specify visit/observation efficiency
Ticking this box allows to specify the observation efficiency. Do not select this option if the efficiency should be interpolated from a table.
Note that the interpolation is approximate and gives an estimate of the maximum efficiency that can be obtained for the specified coordinates.
Efficiency
The user specified observation efficiency in %.
Efficiency refers to the fraction of time spent on-target per orbit. This value is only used if the option Specify visit/observation efficiency is selected.

Calculation Results

Computed or specified efficiency
The efficiency use for the calculations: either the value interpolated from tables, or the value specified by the user.
Saturation level at PSF peak [% FWC]
The ratio of the electrons registered per pixel at the PSF peak and the pixel full-well-capacity.
Values between 10% and 85% are suggested, and values above 100% indicate saturation.

Two tables below indicate the precision obtained when either assuming an efficiency of 100% (no interruptions), or the efficiency given above.
For each, the precision is tabulated for the user-specified time interval and for intervals of 1, 10, and 30 minutes, and 1, 3 and 6 hours.

Stellar granulation noise
Total stellar granulation noise expected for the measurement.
Photon noise
Total photon noise affecting the measurement.
Total noise including stellar granulation
Total noise affecting the measurement, including instrumental and observational (stray light, background, cosmic ray and photon noise), and including the predicted noise due to stellar granulation.
Total noise without stellar granulation
Total noise affecting the measurement, including instrumental and observational (stray light, background, cosmic ray and photon noise), but excluding the predicted noise due to stellar granulation.

Export to PDF

A PDF containing the details of the computation can be created via the button Export results. Optionally, the user can add a comment to this PDF via the Comment on exported PDF field.

Further notes

Aperture Radius

The tool assumes the default aperture radius of 25 pixels.

Exposure Times

The user must specify the exposure time; that is, the time during which photons are collected to record an image (minimum allowed value is 0.001 s, maximum allowed value is 60 s).
The user should always verify that the percentage of the full well capacity of a pixel filled by the highest peak of the PSF is below 100%. Actually, the maximum suggested exposure time is the one where the PSF peak fills 85% of the full well capacity (to provide enough margin to clearly avoid pixel saturation). It is also recommended to select an exposure time close to this maximum suggested exposure time (or 60s if the 85% cannot be achieved), to keep the instrumental noise to a minimum. On the other end, the minimum suggested exposure time is the one corresponding to the PSF peak filling 10% of the full well capacity.
The user is responsible for selecting the correct exposure time and may have reasons not to follow the guidelines given above. For example, the user might be interested in shorter exposure times than recommended to have a faster cadence of images or imagettes (see Observers Manual, Table 2).

Observing Efficiency

Gaps in the light curves exist due to Earth occultations, passages of the satellite through the South Atlantic Anomaly and periods during which the stray light exceeds observationally acceptable limits. Efficiency refers to the fraction of time spent on-target per orbit. Unless the Specify visit/observation efficiency option is activated, the observing efficiency is taken from a precomputed table, which has been constructed from orbit calculations for 12 discrete 48-hour time windows spaced throughout one year. It is assumed that the target will be observed at the time of maximum efficiency. Please note that these values are to be understood as approximate and cannot replace a more thorough efficiency calculation using the Feasibility Checker tool.