In-Orbit updates for Observers

Technical news, in-orbit performance, data systematics, and tips from the CHEOPS Science Team

 

1) Technical news

Default window location

To optimize the limited telemetry of CHEOPS, only a portion of the full frame images, known as "window images" are downlinked for nominal science observations. These window images are 200x200 pixels in size and feature the target star at the centre. Additionally, to further reduce data usage, the spacecraft's onboard software crops the corners of these window images to create circular images of a diameter of 200 pixels that are then transmitted to the ground. The position of the target, and therefore the window in the CCD, is evaluated weekly as part of the Monitoring and Characterization Program. The goal is to minimize noise caused by bad pixels by moving the target to different regions of the CCD if necessary. The table below shows the log of the default window locations over time. The coordinates correspond to the CCD-pixel where the bottom-left corner of a 200x200 window is located.

Date

Default Window Location

18.04.2020 (BJD_TDB 2458957)

(157, 759)

07.06.2020 (BJD_TDB 2459007)

(163, 742)

20.09.2020 (BJD_TDB 2459112)

(180, 728)

18.07.2021 (BJD_TDB 2459413)

(191, 730)

29.01.2023 (BJD_TDB 2459973)

(193, 731)

 

Change in the read-out mode allocation

The instrument team has decided to change the readout mode for long exposures (Texp = 11.27 s to 60 s) from FAINT to FAINT FAST. This change was implemented to be less sensitive to hot pixels in the covered section. The FAINT read-out mode reads the full-frame image and the window image (which is the product downlinked to the ground) is cropped afterwards by the In-Flight Software. The ULTRABRIGHT, BRIGHT and FAINT-FAST read-out modes, on the other hand, only read out the stripe of the CCD containing the window, avoiding any traces of hot pixels in the cover section placed above Y=200 px. Both read-out modes, FAINT_FAST and FAINT, have the same read-out frequency (100 KHz) and can be used interchangeably when the exposure time is longer than 11.27 seconds. It was therefore decided to replace the FAINT read-out mode with the FAINT-FAST in the science visits.

To summarise, from launch until 29.01.2023 any OR submitted using the PHT2 was allocated readout modes as listed in the following table:

READ-OUT MODE texp min texp max
ULTRABRIGHT 0.001 1.05
BRIGHT 1.05 2.326
FAINT FAST 2.326 11.27
FAINT 11.27 60

 

As of 30.01.2023, any new Observation Request submitted using the PHT2 will be allocated the following read-out modes as a function of the exposure time:

READ-OUT MODE texp min texp max
ULTRABRIGHT 0.001 1.05
BRIGHT 1.05 2.326
FAINT FAST 2.326 60.0
FAINT -- --

 

 

2) In-orbit performance

The CHEOPS Mission Consortium was responsible for the in-orbit commissioning of the instrument and is now responsible for an on-going monitoring and characterisation programme. Details of the performances that been measured over the course of the mission are summarised in the presentations listed below:

  • Measured photometric performance - overview (updated January 2022)
  • Observing outside of nominal magnitude range of CHEOPS (May 2021)
  • Illustration of photometric performances (includes lightcurves)  (January 2022)

 

3) Data systematics

All CHEOPS science observations are processed at the Science Operations Centre at Geneva Observatory, using the CHEOPS data reduction pipeline (DRP). The DRP (Hoyer et al. (2020) https://doi.org/10.1051/0004-6361/201936325)) is designed to calibrate and correct CHEOPS observations, and to extract photometric light curves from the corrected image series. What the DRP does not do is to take out time-dependent trends in the data which are related to, for example, instrument temperature, position of the target on the detector etc. These systematics effects are removed from the data by decorrelation or detrending. Identifying and correcting for systematics has been one of the major tasks of the two technical working groups within the CHEOPS Science Team

In the list below are papers in which the treatment of specific systematics seen in CHEOPS data have been rreated.

  • Bonfanti et al. (2021) "CHEOPS observations of the HD 108236 planetary system: a fifth planet, improved ephemerides, and planetary radii" (https://doi.org/10.1051/0004-6361/202039608)   --      - detailed elaboration of the roll angle, its impact on CHEOPS data and how to decorrelate for it; discussion on smearing patterns (due to bright stars in the field,) and an example of internal reflection and its manifestaion in CHEOPS images.
  • Wilson et al.(2022) "A pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 characterized with CHEOPS", (https://doi.org/10.1093/mnras/stab3799) - includes treatment of the "ramp" and various roll angle-relatred issues, Descirbes the PSF PCA (Principal Compnent Analysis)method that has been developed to remove these systematics.

 

4) Tips from the CHEOPS Science Team

The presentations below capture the understanding/experience of CHEOPS and its data that has been built up by technical working groups in the Science Team.

  • Tips and guidelines for determining the duration of individual visits (Phase 1) , and tips on how to define your individual observations reqeusts (Phase 2) at this link (December 2022)
  • Heading off-piste -  how well multi-epoch observations can be combined (e.g. for stellar monitoring) and the impact of transit stacking (February 2022)

 

Questions about CHEOPS or the GO Programme? Please email cheops-support at cosmos.esa.int and we will be happy to help!
This website was last updated on 26 February 2024.