A Fresh Start for the COS FUV Detector

Cristina Oliveira, oliveira@stsci.edu, A. Aloisi, aloisi@stsci.edu, J. Ely, ely@stsci.edu, G. Kriss, gak@stsci.edu, D. Massa, massa@stsci.edu, R. Osten, osten@stsci.edu, S. Osterman, Steven.N.Osterman@Colorado.EDU, S. Penton, penton@stsci.edu, C. Proffitt, profit@stsci.edu, D. Sahnow, sahnow@stsci.edu


To forestall possible loss in recorded flux due to charge depletion, we plan to change the permanent or “lifetime” position (LP) of spectra on the far ultraviolet (FUV) detector of the Cosmic Origins Spectrograph (COS). This change will be accomplished by moving the aperture in the direction across the dispersion. Planned for summer 2012, this fresh start should guarantee the performance of the COS FUV detector for several years. Here, we describe the work undertaken to select the new COS FUV LP.


Gain sag

The COS FUV detector is an open-face microchannel plate (MCP) with a cross delay line (XDL) anode. Incoming photons strike the front surface of the MCP and release photoelectrons, which are accelerated and multiplied in number as they travel down the MCP pores to the back surface. Each photoelectron produces an electron cloud with ~10 million electrons, which are collected by the XDL. The XDL uses timing measurements to locate the X-Y position of the cloud and to measure its total charge (“pulse height” or “gain”). (For details on the COS FUV detector, see McPhate et al. 2010.)

Because the total amount of charge that can be extracted from each location in the MCP over its life is limited, the number of electrons in the cloud decreases with usage. This is the “gain-sag” effect. Eventually, some clouds become too small to be detected by the electronics, causing a local loss in recorded flux. When the local loss in recorded flux reaches 5%, the region is considered to suffer from gain-sag effects.

Figure 1 documents the total usage of the COS FUV detector to date. It is a map of the COS FUV detector, showing total exposure—all the events that have been collected since the instrument was launched. Areas with largest total exposure are more susceptible to gain sag effects.

We have already taken some actions to extend the lifetime of the current, first LP of spectra in COS FUV. We have raised the high voltage across the MCP, which increases the charge available for extraction. However, this step is useful only up to a point, because the instrument is only certified over a limited range of voltage.

To work around localized gain sag due to geocoronal Lyα emission, we can change the grating tilt (FP-POS position) to shift the spectrum along the dispersion. However, this step helps only if a relatively small region in the detector has been affected.