The Neil Gehrels Swift Observatory

Return to: UVOT Digest

Uvotimage and Settling Exposures

Updated February 18, 2011

There is a known issue with the UVOT analysis software uvotimage that causes uvotimage to handle the UVOT settling data incorrectly. Settling data is UVOT EVENT mode data that is collected immediately after the spacecraft slews to point at a transient source, but before the spacecraft has settled to its commanded pointing. The spacecraft pointing and the UVOT voltage may change during the settling exposure. Therefore, the standard UVOT calibrations cannot be applied to settling data, and these data should not be used unless the user is fully aware of the implications and potential pitfalls of using these data.

Photometry of sources in settling exposures may yield incorrect results.

UVOT settling data that are processed by the Swift Data Centre contain the FITS keyword "OBS_MODE = SETTLING". The UVOT analysis software uses this keyword to identify settling data and exclude it when necessary.

There is an issue with uvotimage that prevents it from recognizing settling data, so the SKY files created by uvotimage do not have the correct value of OBS_MODE for settling exposures. This means that the UVOT analysis software cannot recognize settling data in SKY files that are produced by uvotimage. Users need to be aware that settling exposures in these SKY files need to be manually excluded when using the UVOT analysis software.

uvotproduct light curves with settling data excluded

Figure 1: These light curves are produced by uvotproduct using data from the Swift Data Archive. The V-band settling data has been correctly identified and excluded.

uvotproduct light curves with settling data included

Figure 2: These light curves are produced by uvotproduct using data generated by uvotimage. The V-band settling data (the blue point at 110 s) has not been correctly identified and thus has not been excluded. The magnitude of this data point is not correct.

Return to: UVOT Digest