Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

4.1 The FUV XDL

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4.1.1 XDL Properties

The COS FUV detector is a windowless XDL (cross delay line) device that is similar to detectors used on the Far Ultraviolet Spectroscopic Explorer (FUSE). The XDL is a photon counter with two segments, with a gap of 9 mm between them. The two detector segments are independently operable to provide redundancy. Each segment has an active area of 85 × 10 mm. When the locations of detected photons are digitized, they are placed into an array of 16384 × 1024 pixels; however, the portion of that array used by the active area is less (see Figure 4.2). The long dimension of the array is in the direction of dispersion, and because of the orientation of the detector in COS, increasing pixel number (the detector's x axis) corresponds to decreasing wavelength. The XDL is shown schematically in Figure 4.1.

The locations of detected events are recorded in pixel units. However, the XDL does not have physical pixels in the usual sense, and the location of an event is determined by the analog electronics as they occur.

The FUV XDL is optimized for the 1150 to 1775 Å bandpass, with a cesium iodide photocathode. The front surface of the XDL is curved with a radius of 826 mm so as to match the curvature of the focal plane. When photons strike the photocathode they produce photoelectrons which are then amplified by micro-channel plates (MCPs). There are three curved MCP plates in a stack to go with each XDL segment.

Figure 4.1: The FUV XDL Detector.


 
This is drawn to scale, and the slight curvature at the corners is also present on the masks of the flight detectors. Note that wavelength increases in the direction opposite to the detector coordinate system. The red and blue dots show the approximate locations of the stim pulses. Note that the numbers in parentheses show the pixel coordinates at the corner of the segment's digitized area, and also note that the two digitized areas overlap in the region of the inter-segment gap.
 

The charge cloud that comes out of the micro-channel plates is several millimeters in diameter when it lands on the delay line anode. There is one such anode for each detector segment, and each anode has separate traces for the dispersion (x) and cross-dispersion (y) axes. The location of an event in each axis is determined by measuring the relative arrival times of the collected charge pulse at each end of the anode delay line for that axis.

The electronics that create the digitized time signals also generate pulses which emulate counts located near the edges of the anode, beyond the illuminated regions of the detector. These "stim pulses" (see Section 4.1.6) have several purposes. They provide a first-order means of tracking and correcting distortions. They are also used for determining dead-time corrections. The data reduction pipeline uses the locations of the stim pulses to assign wavelengths to pixels. For this reason, comparisons of COS spectra taken at different times should be made in wavelength space, not in detector pixel coordinates.

The XDL's quantum efficiency is improved with a grid of wires placed above the detector (i.e., in the light path). These wires create shadows in the spectrum that are removed during data reduction. The XDL also includes an ion repeller grid in addition to the DQE grid mentioned above. This reduces the background rate by preventing low-energy thermal ions from entering the open-faced detector. The grid wires cast out-of-focus shadows onto the detector; these are removed by flat-fielding.

4.1.2 XDL Spectrum Response

Initial measurements of the throughputs of the COS optical systems indicate that COS will be considerably more sensitive than STIS and earlier generation HST instruments at comparable spectral resolutions in the far-UV. The point source sensitivities for the COS FUV spectroscopic modes are shown in Figure 5.2.

4.1.3 XDL Background Rates

The XDL detectors have extremely low dark rates, below 10^-6 per pixel per second; see Section 10.3.1 in Chapter 10.

4.1.4 XDL Read-out Format

As noted, the FUV XDL detector actually consists of two separate and independent segments, each of which has an active area of 85 × 10 mm, with the long axis in the direction of dispersion. The physical devices are adjacent, but with a 9 mm gap between the active areas of the two segments. Although this gap prevents the recording of an uninterrupted spectrum, it also makes it possible to position spectra such that significant airglow features - Lyman-, in particular, when G140L is used - fall on the gap. Without this feature, Lyman- emission could sometimes trigger excessive count rates in the detector. For more about the gap, see Section 5.6.

Figure 4.2: Example of a COS FUV Spectrum.


 
Shown is a wavelength calibration spectrum obtained during ground testing. The internal wavelength calibration lamp spectrum is at the top, while the lower spectrum is from a lamp external to COS. Note the size of the active area compared to the overall digitized area. At the bottom is the extracted spectrum of the bottom trace. The bright streak at the bottom is due to an area of enhanced background on the detector segment.
 

Figure 4.2 shows an example of an FUV spectrum obtained during ground testing. Note the difference in x and y axis scales, and note that the image is for only one of the segments. The top portion shows the two-dimensional image, and the bottom shows the extracted wavelength calibration spectrum.

4.1.5 ACCUM and TIME-TAG Modes

As noted, each detected photon is assigned to a pixel. In ACCUM mode, the location in the buffer at those coordinates is then incremented by one. At the end of an ACCUM exposure, the buffer memory is read out and becomes an image of the detected photons.

In TIME-TAG mode, each photon is recorded as a separate event in a long list. Each entry in that list contains the (x, y) coordinates of the photon, together with the relative time it was detected and the pulse height. The time is binned into 32 msec increments, but multiple events can be recorded within a single 32 msec time interval. In almost all cases TIME-TAG is the preferred data-taking mode.

The dead time associated with the detection electronics of the XDL detector is 7.4 µsec. For more on non-linear effects, see Section 5.2.

4.1.6 Stim Pulses

The signals from the XDL anodes are processed by Time-to-Digital converters (TDCs). Each TDC contains a circuit which produces two alternating, periodic, negative polarity pulses which are capacitively coupled to both ends of the delay line anode. When active, these stim pulses emulate counts located near the edges of the anode, beyond the illuminated portions of the detector. While the stim pulses are primarily used as a detector health diagnostic and for calibration, they also provide observers a means to track changes in image shift and stretch during an exposure and provide a first-order check on the dead-time correction. The nominal location of these stim pulses in (x, y) coordinates are: (383, 33) and (15994, 984) but those locations change with temperature.

Four stim pulse rates are available: 0 (i.e., off), 2, 30, and 2000 Hz per segment; this is not user selectable. Exposures longer than 100 sec will use the 2 Hz rate, while exposures from 10 to 100 sec will use 30 Hz. The highest rate is only for calibration.

4.1.7 Pulse-height Distributions

The XDL detector will generate pulse-height distributions (PHDs) along with the science data. The PHD provides important information on the micro-channel plates. The PHD is a histogram of the amplitudes of the charge clouds (pulse heights) associated with all the events detected during an integration. The distribution of the pulse heights of photon events is peaked at the average gain of the MCPs with a width determined by MCP characteristics. Background events, both internal and cosmic-ray-induced, tend to have a falling exponential distribution in pulse height, with most events being at very low pulse heights. On-board charge threshold discriminators are used to preferentially filter out very large and small pulses to improve the achieved signal-to-noise. For the FUV XDL in TIME-TAG mode, the pulse height is recorded with each detected photon event and can be examined during data analysis. For the FUV XDL in ACCUM mode, only the overall pulse-height distribution is recorded.

4.1.8 FUV Detector Lifetime Sensitivity Adjustments

The FUV XDL MCPs are subject to gradual gain degradation due to charge extraction over their lifetimes which reduces their effective sensitivity. The effect is small, but can be important in a localized region where the lifetime fluence is high, e.g. where a strong spectrum feature such as geocoronal Lyman- falls on the detector.

The requirement for COS is for the effect to be no more than a 1% loss in quantum efficiency after 10⁹ events mm^-2 have occurred. Estimates of COS usage show that the total number of events detected in the FUV channel over a seven-year mission would be a few times this value. The net effect is thus likely to be negligible, but nevertheless STScI will monitor any degradation of the XDL detector. There is a provision to move the location of the spectrum imaged onto the XDL detector in the cross-dispersion direction onto a previously-unused portion of the detector by offsetting the aperture mechanism. This can be done up to four times.