To achieve the best possible efficiency an integration time of at least about 1 ks is recommended for each exposure. With the EPIC/RGS Image Mode configuration of 5 exposures per filter, this leads to 5 ks duration per composite exposure. Note that longer integration times are allowed (§ 3.5.3.1). We remind, however, that the maximum allowed exposure time per exposure are: 2.2 ks for the Fast Mode, if two Fast Mode windows are in use; 4.4 ks for the Fast Mode, if one Fast Mode window is in use; 5 ks for the Image Mode. Shorter exposure times are limited by the OM telemetry allocation. For the modes EPIC/RGS Image and Science User Def. the images are downloaded during the subsequent exposure, therefore the windows must be small or the subsequent exposure integration time long enough in order to allow the instrument memory to be emptied. This can be critical for crowded fields mainly with the optical filters. The current minimum exposure times are: 800 s for the Image Mode and 1200 s for the Fast Mode.
There are in principle no constraints on the exposure time in Full Frame Low/High Resolution Modes. However, it is forbidden to exceed 5000 seconds, to avoid potential onboard memory corruption after interaction of cosmic rays with the camera, or counter wraparound. Tab. 16 summarizes the integration time constraints:
| Science window configuration | Minimum (s) | Maximum (s) |
| Only Image Mode windows | 800 | 5000 |
| Image Mode + 1 Fast Mode windows | 1200 | 4400 |
| Image Mode + 2 Fast Mode windows | 1200 | 2200 |
| Full frame (low or high resolution) | 800 | 5000 |