Moving On Up—A New Lifetime Position for COS FUV Spectra

R. A. Osten, osten@stsci.edu, A. Aloisi, K. A. Bostroem, J. Debes, J. Ely, G. Kriss, D. Massa, C. Oliveira, S. Osterman, C. Proffitt, S. Penton, D. Sahnow, P. Sonnentrucker,  and J. Roman-Duval

Realtors will tell you that location is everything when buying or selling a house. Location is also paramount in the operation of the Cosmic Origins Spectrograph’s (COS) Far Ultraviolet (FUV) cross-delay line detector. After three years of on-orbit operations, the default location of science spectra taken with the COS FUV channel was changed on July 23, 2012. Since that date, the positions of all external target spectra are shifted upward by about 3.5”, or 41 pixels, on the detector, in a direction orthogonal to the dispersion direction. This move ensures that the spectra obtained with COS will continue to be of the highest quality possible (see Figure 1). Here, we describe the activities that occurred in order to enable and calibrate this new position.

 

The road to a new lifetime position

From its inception, the design of the COS FUV instrument included a plan for regular moves of the default location of spectra, or “lifetime positions.” This is due to the fact that the cross-delay line detector has a limited amount of charge that can be extracted from any position on the detector.  As the cumulative number of photons collected at a given location increases, the total number of electrons generated from the incident photons decreases, leading to a localized loss in sensitivity. This is the so-called “gain sag” effect. When the number of electrons in the charge cloud (the pulse height amplitude, or PHA) drops below a critical threshold, the valid photon events can no longer be recovered. Locations on the detector particularly susceptible to this effect include regions where geocoronal Lyman α airglow lines are located. Sagging in these regions was seen as early as fall 2010, only 1.5 years after the commencement of on-orbit operations. Further investigation revealed that the accumulation of photons from regular usage in external science targets was affecting larger regions of the detector. This motivated a series of exploratory programs designed to specify the next lifetime position.

The final decision on the second lifetime position—a location offset by +3.5” in the cross dispersion direction from the original position—was made in December 2011.  Factors influencing the location included the need to maximize spectral resolution, to maintain spectral quality by minimizing the effect of gain sag from previously exposed regions or other detector artifacts, and to extend the amount of time available for operations at the second lifetime position. In the 2012 volume 29, issue 01 of the Institute Newsletter, Oliveira et al. called the exploration for this selection “A Fresh Start for the COS FUV Detector.”

The first “lifetime move,” or change of the default location of spectra on the COS FUV detector, was akin to the activities during the Servicing Mission Observatory Verification (SMOV) in summer/fall 2009, as the performance at the new lifetime position needed to be confirmed. Based on experience with operating the detector on orbit for 3.5 years, a “spot-check” approach was utilized, and other changes to these “mini-SMOV” activities were made based on prior on-orbit experience. The process started before the move by mapping out requirements for programs that would enable science at the new location, and for programs that would provide calibrations to support science at the new location.  These activities will be required for every subsequent lifetime move.