4.2 The NUV MAMA Detector

4.2.1 MAMA Properties

The COS NUV detector is a Multi-Anode Micro-channel Array (MAMA) identical to that used for the NUV in STIS. (In fact, it is the STIS NUV flight spare.) The COS MAMA has a semi-transparent cesium telluride photocathode on a magnesium fluoride window and is sensitive to photons with wavelengths from 1150 to 3200 Å. The NUV optics focus light through the MgF2 window onto the Cs2Te photocathode. A photoelectron generated by the photocathode then falls onto a curved-channel micro-channel plate (MCP), which generates a cloud of electrons. The active area of the anode array is 25.6 mm square and is divided into 1024 × 1024 pixels on 25 μm centers. The spatial resolution at 2500 Å is 35 μm FWHM. Detector parameters are listed in Table 1.2.

4.2.2 MAMA Spectrum Response

The inherent spectral response of the COS NUV MAMA is essentially identical to that of the STIS NUV MAMA. However, the overall optical train of COS differs from that of STIS, so the throughputs are different (Figure 5.2). The maximum count rates for the NUV detector are listed in Table 10.1.

4.2.3 MAMA Dark Rate

A sum of dark exposures taken away from the SAA shows a relatively featureless background, with slight enhancements at two of the corners (Figure 4.7). Although the early dark rate was lower than had been measured on the ground, the rate has steadily increased since launch, as shown in Figure 4.8. For version 28.2 of the COS ETC, the dark rate adopted for NUV is 1.11 × 10–3 counts/s/pixel, encompassing 95% of observations through March 2020.

Figure 4.7: Spatial Variation in the NUV Dark Rate.

Summed dark exposures show the uniformity of the NUV dark rate across the detector. There is very little variation across the detector, with only a small depression in the dark rate in the lower left corner. The cumulative histograms show the collapsed rows (above) and columns (right).
Figure 4.8: NUV Detector Dark Rate versus Time.

Measured dark rate as a function of time. When the periods affected by the SAA are excluded, the dark rate increased by roughly 200 count/s per year until 2013, when the trend flattened. Neither the increase in the dark rate, nor the increase in the scatter that is seen in the later observations, are understood. The middle panel shows the radio flux of the sun for comparison. The lower panel shows the detector temperature (LNTUBET). This demonstrates the temperature dependence of the dark rate. The two observations of exceptionally low dark rate at 2012.35 are due to a much lower than normal temperature caused by the shut-off of the FUV detector.
Figure 4.9: Example of a COS NUV Spectrum.

A COS NUV spectrum obtained in TIME-TAG mode with FLASH=YES. The stellar spectrum (labeled PSA) is on the bottom, and the wavelength-calibration spectrum (labeled WCA) on the top; each has three stripes. From bottom to top, these stripes are designated A, B, and C, as illustrated. Wavelength increases to the right and toward the top of the detector. The HST +V2 and +V3 axes are also shown. The SHORT, MEDIUM, and LONG designations are used in Phase II with the ACQ/PEAKXD command and the STRIPE optional parameter.

4.2.4 MAMA Read-Out Format

The NUV channel creates six spectral stripes on the MAMA detector, three for the science data and three for the wavelength-calibration data. Stripes are separated by 94 to 143 pixels (2.1 to 3.3 arcsec), center to center, in the cross-dispersion direction. The NUV detector is read out as a 1024 × 1024 array, but in all other respects the data are handled in the same way as for the FUV detector. No pulse-height information is provided for MAMA data. An NUV spectrum obtained in TAGFLASH mode is shown in Figure 4.9.

Figure 4.10: Model LSFs for the COS NUV Channel.

LSFs including the MFWFEs are plotted as solid lines, those without as dotted lines.

4.2.5 MAMA Dead Time

The dead time for the COS NUV MAMA is 280 ns, the same as for the STIS NUV MAMA. The 1% level of non-linearity is reached for C = 36,000 count/s. Corrections for dead-time effects are made in the calcos pipeline, but they are not included in the ETC, which will over-predict count rates for bright targets.

4.2.6 Non-Gaussian Wings in the MAMA PSF

While most NUV observations should be minimally affected by the mid-frequency wavefront errors (MFWFEs) discussed in Section 3.3, they will reflect the point-spread function of the COS MAMA detector, which exhibits faint, extended wings that are unrelated to the telescope optics. While the telescope-induced wings weaken as wavelength increases, the detector wings become stronger with increasing wavelength. Figure 4.10 shows model NUV detector LSFs with and without the MFWFEs at various wavelengths. Beyond 2500 Å, the detector wings dominate.