While the OM can access a large part of the field viewed by EPIC, there are limitations on what can be observed in an exposure. These limitations are set in the first place by the amount of on-board memory, and secondly by the telemetry rate allocated to the instrument. Because of constraints on the telemetry rate available, it is not possible to transmit the full data on every photon that the OM detects. Instead, a choice has to be made between image coverage and time resolution. The observer can therefore choose up to 5 ``windows'', of which up to two may be operated in the ``Fast'' mode (see § 220.127.116.11, below). These can be placed where required, with restrictions as identified in § 3.5.10.
In addition to these windows, the instrument assigns itself several small windows for tracking guide stars. The tracking information is included with the other science data.
In general, an observation with the OM will consist of a sequence of exposures in different filters and/or with a sequence of different window setups to cover the full field of view or a defined area around the target.
A set of default OM window configurations is available (§ 18.104.22.168). These will generally be satisfactory for most proposals.
For details on the intricacies of non-default OM window choice and the applicable restrictions, please refer to § 22.214.171.124.