Available first in AO-10, the RGS Small Window (SW) mode allows the effects of pile-up in very bright sources to be mitigated. In RGS Small Window mode only the central 32 of the 128 CCD rows in the cross-dispersion direction ( of the 5 arcmin aperture) are read. The CCD readout time is therefore decreased by a factor 4 compared to Spectroscopy mode.
The RGS SW mode is relevant if wavelengths important for the scientific success of the observation are likely to be affected by pile-up (see §. 18.104.22.168). Using the RGS SW mode, the point-source flux limits for 2% pile-up given in Table 11 are decreased by a factor 4 if all the eight CCDs are used, and even more if less than 8 CCDs are read or if one CCD is read more often than the others.
This mode is not intended for routine use or, in particular, where simultaneous estimates of the background spectrum are crucial, as is usually the case. It is designed for the benefit of very bright sources, where both pile-up is expected and the background is negligible.
A number of further considerations must be taken into account in order to decide if the mode could be applied with profit for a given observation.
Due to the high telemetry requirements of the RGS SW mode, both EPIC-MOS cameras must be disabled, and the EPIC-pn maximum available telemetry bandwidth is reduced from 40 to 31 kb s. Overall, the impact of the telemetry limitations on EPIC MOS and pn is considered to be minor. On one side the upper flux limit to avoid pile-up on the MOS cameras is 35 mCrab (see Table 3). On the other side, the recommended EPIC-pn modes for bright source observations are Timing and Burst, and only the former has telemetry bandwidth limitations (see Table 3). For very bright point sources for which the RGS SW mode may be considered, such as black-hole binaries in outburst, where fluxes of 80 mCrab or more have been recorded, the EPIC-pn camera is likely to be operated in its Burst mode.
Observers considering use of the RGS SW mode should also make an explicit judgement concerning the background. Naturally, in the absence of data from the cross-dispersion edges of the CCDs, the background cannot be estimated in the normal way from those parts of the detector. While this is of little importance for the brightest parts of a continuum spectrum, interstellar absorption at long wavelengths, for example, often reduces the observed intensity towards values near the expected background value. It is conceivable, therefore, that measurements of a variety of interstellar absorption edges, for example, could require a mixed observational approach, using the RGS SW mode at short wavelengths and the normal mode elsewhere, although with some operational constraints, explained below. In general terms, the detailed expected shape of the spectrum and the corresponding observational objectives must be taken into account. This may be especially relevant for very bright line-rich sources (§. 22.214.171.124.1).
There are some operational restrictions on how normal Spectroscopy and SW modes can be combined. The most important ones are summarised here.