XMM Users' Handbook

   
EPIC filters

The next factor influencing the EPIC effective area, specifically in the low energy part of the passband, are its optical blocking filters. These are used, because the EPIC CCDs are not only sensitive to X-ray photons, but also to IR, visible and UV light. Therefore, if an astronomical target has a high optical to X-ray flux ratio, there is a possibility that the X-ray signal becomes contaminated by those photons. The resulting analysis of data would be impeded in three ways:

1.

Shot noise on the optically-generated photo electrons will increase the overall system noise, and hence lower the energy resolution. Spectral fitting will be inaccurate, because the calibration files will assume narrower spectral lines than observed.

2.

The energy scale will be incorrectly registered, because a nominally zero signal pixel will have a finite offset. For each optically generated photo electron which is registered, the energy scale shifts by about 3.6 eV. This is comparable with the accuracy with which we expect brightest emission line features can be centroided. Consequently, contamination by visible light plays a crucial role in defining the proper energy scale.

3.

Excess signal and noise fluctuations can affect the detection efficiency as well, by disguising single pixel X-ray events as events split between pixels.

To prevent this, the EPIC cameras include aluminised optical blocking filters, and also an internal ``offset table'' which is calculated before each exposure to subtract the constant level of (optical) light or other systematic shifts of the zero level of charge measurements. This is the reason why there is always a calibration exposure before the start of a science observation.

If these measures work perfectly, the above problems are minimised. However, the use of a thick blocking filter capable of minimising the optical light contamination for all scenarios would necessarily limit the softest X-ray energy response. Each EPIC camera is therefore equipped with a set of 4 separate filters, named thick, medium, thin and open. It is necessary for the observer to select the filter which maximises the scientific return, by choosing the optimum optical blocking required for the target of interest.

The following guidelines apply to point sources of optical light. Extended objects are not expected to be a significant problem. The calculations have been performed for a worst case, i.e., for the brightest pixel within the core of the PSF. Therefore, averaging the brightness of an extended object over a scale of one PSF (say, 20'') should provide a corresponding estimate with a significant margin of safety.

  

Figure 20: The EPIC MOS effective area for each of the optical blocking filters and the ``open'' (no filter) position.

  

Figure 21: The EPIC pn effective area for each of the optical blocking filters and the ``open'' (no filter) position.

• Thick filter

  This filter should be used if the expected visible brightness of the target would degrade the energy scale and resolution of EPIC. It should be able to suppress efficiently the optical contamination for all point source targets up to m_V of 1-4 (MOS) or m_V of 0-3 (pn). The range depends on the spectral type, with only extremely red or blue colours (M stars for example) causing the change to 3 magnitudes fainter level.

  Note that these data apply to full window modes only, and that a change to a partial window mode with an order of magnitude faster readout rate can allow suppression of optical contamination at 2-3 visible magnitudes brighter for ALL filters. The GO can make an estimate on optical contamination improvement based on the mode time resolution compared with full window mode (Table 4).

• Medium filter

  The optical blocking is expected to be about 10³ less efficient than the thick filter, so it is expected that this filter will be useful for preventing optical contamination from point sources as bright as m_V = 8-10.

• Thin filter

  The optical blocking is expected to be about 10⁵ less efficient than the thick filter, so the use of this filter will be limited to point sources with optical magnitudes about 14 magnitudes fainter than the corresponding thick filter limitations.

• Open filter

  This should only be used when trying to detect the very softest photons in the bandpass. Even the diffuse zodiacal light will produce measureable optical light contamination, so the energy response will be compromised every time the ``open'' position is employed. Furthermore, present calculations show that the CCDs could become contaminated with ice and hydrocarbons on a timescale of about one day, leading to loss of calibration and a necessity to initiate a bakeout and re-calibration sequence. Therefore observations in the open position are not recommended by the SOC as a routine operation. To request open filter position observations, the GO must supply strong scientific arguments that the response to the softest photons around 0.1 keV is crucial for the proposed investigation and, moreover, that the expected visible light contamination is expected to be minimal.

The default filter, which is the most sensible choice for most observations, is the thin filter.

Figs. 20 and 21 display the impact of the different filters on the soft X-ray response of both types of EPIC cameras.