XMM-Newton Users Handbook

3.5.2.1 Imaging with OM

As mentioned above and shown in Fig. 101, photons entering the OM detector hit a photo-cathode, which is located at the backside of the detector entrance window. An electron liberated from the photo-cathode by the incident photon is amplified by the MCPs, and the emergent electron cloud is converted to a burst of photons by a phosphor. These photons are guided towards the CCD by a fibre taper, creating a photon splash on the OM CCD. The detection of a photon entering the detector is performed by reading out the CCD and determining the centroid position of the photon splash using a centroiding algorithm, which is part of the onboard software. In the process of centroiding a grid of 8$\times$8 “in-memory” pixels is defined, leading to an array of 2048$\times$2048 in-memory pixels with an approximate size of 0”.476 on the sky. In the resulting images at some level there is always a pattern repeating on an 8$\times$8 grid, resulting from a limitation in the algorithm. This spurious pattern can be removed by subsequent processing on ground.

As with all photon-counting detectors, there is a limit to the maximum count rate achievable before saturation sets in. The frame time⁶ of the OM detector is about 11 ms at slowest, so a linearity correction must be applied in the offline data processing for count rates above ca. 10 counts/s for point sources. Both deadtime and coincidence losses contribute to the non-linearity of the OM detector. These effects are corrected by using SAS in the data reduction. Deadtime losses are due to the lack of instrumental response during a frame transfer. Coincidence loss is observed whenever the count rate is such that more than one photon arrive in the same pixel within a given readout frame. Deadtime becomes important for short frametime. On the other hand, longer frametimes are more likely affected by coincidence losses. These effects are quantitatively described in § 3.5.5.

In addition, sources which are too bright can depress the local sensitivity of the photocathode: this is a cumulative effect, so that fainter sources observed for long times have the same effect as brighter sources observed for shorter periods. This places some operational constraints on the instrument. Pointing of bright sources may also yield ghost images in subsequent observations due to fluorescence.

Cosmetically, the OM detectors are good, with few hot or dead pixels, and little global variation in quantum efficiency. Pixel to pixel sensitivity variations on the OM CCD are in some way smoothed by the centroiding mechanism producing the final pixels, as described above.

The OM has experienced wavelength dependent, temporal sensitivity degradation over the mission baseline. This degradation, which affects the whole detector, is attributed to both expected sensitivity loss due to ageing of the photocathode, and contamination, and has resulted in current decreases of throughput of around 12% in the optical bands and up to 24% in the UV filters. The decline is, however, now decreasing very slowly, with further losses of $\le$2% anticipated by 2030.

Local sensitivity gradients are believed to be less than a few percent on scales up to two minutes of arc, over most of the detector. However, it should be noted that due to an accidental observation of Jupiter in July 2017, a region of decreased sensitivity, about 210 x 120 pixels (105 x 60 arcsecs) in size, appeared close to the center of the detector. The highest loss of sensitivity occurs with the V filter (∼ 35%) in the centre of the region. See Sect. 3.5.5 for more details. It should also be noted that the pn boresight location, where target sources are usually placed, lies in the wings of this localised region of depressed OM sensitivity, and objects placed at the pn boresight suffer a further reduction in sensitivity that ranges from 9%-14% in the optical bands and 4%-8% in the UV filters, over and above that of the general OM sensitivity reduction.