Minor changes in the fields of Campaigns 11, 12, and 13

News article posted on by Geert Barentsen

K2 users are alerted to a small but necessary adjustment in the position of Campaigns 11, 12, and 13, for which target proposals are currently being solicited as part of the K2 Guest Observer Cycle 4.

The change, explained below, alters ~0.2% of sky area covered and is unlikely to affect the vast majority of proposals. All users are nevertheless encouraged to update the K2fov target selection tool to version 4.0, which was released on 27 Jan 2016 to include the change.

K2fov can be updated from the command line using pip:

pip install K2fov --upgrade

The version number of your K2fov installation may be verified using the following command:

python -c "import pkg_resources; print(pkg_resources.require('K2fov')[0].version)"

This should return "4.0.0" or higher.

Background

Campaigns 11, 12, and 13 are being rotated slightly to accommodate a change in the configuration of the Kepler spacecraft.

During these Campaigns, spacecraft telemetry will be downlinked using a different low-gain antenna than before: LGA2 instead of LGA1. This antenna provides a better orientation to Earth and compensates for the increasing range of the spacecraft to the Earth (now 0.8 AU).

The change of antenna was recently trialled during Campaign 7, where it was found to cause a small increase in the roll drift rate. The reduced pointing performance was understood to be caused by the antenna's radiation pressure, which torques the spacecraft slightly differently when LGA2 is used, increasing the roll rate.

To optimize fuel usage and photometric precision, the K2 team has decided to adjust the roll angle of Campaigns 11, 12, and 13 by a very small amount (0.12--0.16 degrees) to optimally balance the spacecraft against solar pressure and antenna radiation when LGA2 is in use.

As a consequence, there is a minor change in the fields of view covered by K2 during these campaigns. The position of the CCD edges are being shifted by ~0.5 arcsec (~0.1 px) near the center of the focal plane, and by up to 60 arcsec (~15 px) near the corners. The change is illustrated in the figure below, which shows the change in sky coverage for a single CCD channel near the corner of C11, where the shift is the most significant.

Please contact the Guest Observer Office if you have questions.

C11 Roll Change
Figure: Illustration of the effect of the change in sky coverage for one of the CCD detectors near the corner of the focal plane during K2 Campaign 11. The edges of the field are shifted by up to ~60 arcsec.