Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

4.2 The NUV MAMA

---

4.2.1 MAMA Properties

The COS NUV detector is a MAMA (Multi-Anode Micro-channel Array) that is essentially identical to that used for the NUV in STIS (it is, in fact, the STIS NUV flight spare). The COS MAMA has a semi-transparent cesium telluride photocathode on a magnesium fluoride window; this allows detection of photons with wavelengths from 1150 to 3200 Å. The background achieved with this MAMA is about 25% of the level seen with the STIS NUV MAMA.

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) and the MCP then generates a cloud of about 700,000 electrons. A single MCP manufactured by Litton Electro-Optical Systems is used to multiply photoelectrons generated by the photocathode into this charge pulse. The active area of the coded anode array is 25.6 mm square and is divided into 1024 × 1024 pixels on 25 µm centers.

The window is stepped since the photocathode must protrude into the tube body to within 0.25 mm of the MCP. At this spacing and with a photocathode-to-MCP gap potential of 800 volts, the spatial resolution at 2500 Å is 35 µm FWHM.

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 STIS, so that the COS throughputs are different.

4.2.3 MAMA Non-linearity

As noted in Section 5.2, the MAMA detector is expected to be essentially linear over the count rate range permissible. For count rate limits, see Section 11.5.

4.2.4 Detector Format

As noted in the instrument description, the NUV channel creates three spectrum stripes on the MAMA detector, and there are three separate stripes for the science data and three for the wavelength calibration data. This is shown schematically in Figure 4.4. Note that each stripe is separated by 2.80 mm center-to-center from its neighbor, and there is a gap of 3.70 mm between the reddest science stripe and the bluest calibration stripe.

Shown in Figure 4.3 is an example of an NUV spectrum (the two-dimensional image) obtained during ground testing in TAGFLASH mode.

4.2.5 Pulse-height Distributions

For the MAMA detector no pulse-height information is available.

Figure 4.3: Example of a COS NUV Spectrum.


 
Shown is a wavelength calibration spectrum, with both the WCA and the PSA illuminated by separate lamps in this set-up. Note the "science" spectrum on the right and the wavelength calibration spectrum (on the left); each have three stripes. These stripes are designated A, B, and C, in going from right to left in this illustration. Wavelength increases going down, opposite to the sense of the y axis.
 
Figure 4.4: Schematic spectrum layout for the COS MAMA.


 
The blue, and red stripes correspond to the shortest- and longest wavelengths, with green being intermediate. The stripes on the left are the wavelength calibration spectra and those on the right are the science spectra. The sense of x and y is the same as in Figure 4.3.
 

4.2.6 Read-out Format, A-to-D Conversion, etc.

The COS NUV MAMA 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. As noted, no pulse-height information is provided with MAMA data.