The On-Orbit Anomaly of the Cosmic Origins Spectrograph of April 2012

Alessandra Aloisi, aloisi@stsci.edu, John Bacinski, bacinski@stsci.edu, Justin Ely, ely@stsci.edu, Cristina Oliveira, oliveira@stsci.edu, Steve Penton, penton@stsci.edu, Charles Proffitt, proffitt@stsci.edu, David Sahnow, sahnow@stsci.edu, Alan Welty, welty@stsci.edu, & Tom Wheeler, wheeler@stsci.edu

On April 30, 2012, observations with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope were anomalously suspended when one of the two segments (segment A) of the COS far-ultraviolet (FUV) cross-delay line (XDL) microchannel plate detector experienced an excessively high count rate, which caused the autonomous count-rate protection procedures to safely lower the detector high voltage and suspend instrument activities.

The anomaly occurred during the execution of a routine observation of the COS FUV sensitivity-monitoring program (visit 11 of Hubble Cycle 19 program 12715, PI: Osten). The exposure executing at the time, using G130M at the central wavelength of 1291 Å, was similar to others taken many times before.

Within hours of the anomaly, engineering and science data collected up to the suspend event were captured and transferred to the ground. COS was then transitioned to its standard safe mode, and science observations with Hubble’s other instruments continued as planned.

In the following days, an examination of the data taken before and during the anomaly showed that the excessive counts were not due to unexpected light coming from an external source (i.e., from the target or any other source outside Hubble) nor to any unexpected change in the intensity of the calibration lamp. It appeared instead to be the result of field emission within the detector segment, from an area that is not usually illuminated by external or internal light (see Figure 1).