While the OM can access a large part of the field viewed by EPIC, there are limitations on what can be observed in any one 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. The observer can therefore choose up to 5 ``windows'', of which up to two may be operated in the ``fast'' mode (see § 3.5.5, below). These can be placed where required, with restrictions as identified in § 3.5.9.
In addition to these windows, the instrument assigns itself several small windows for tracking guide stars (§ 3.5.9). 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.
A set of default OM window configurations is available: these will generally be satisfactory for most proposals. SOC support for non-default OM observations will be limited. For AO-1, a specific scientific justification will be required to use a non-default configuration. It is therefore strongly recommended to use the OM default configurations. The default setups are as follows.
For details on the intricacies of non-default OM window choice and the applicable restrictions, please refer to § 3.5.9.2.