The flatfield is created by median-combining all n frames, with detected objects masked out. Useful for objects which are slightly extended, but no bigger than one quadrant of the array. Offset sky frames are taken alternately with object frames. Each object frames is sky-subtracted using the average of the bracketing sky frames. However, optimum performance and minimum user interaction requires the presence of at least two keywords in the image header.
Every observation is considered to be part of a group.
If the observation is a dark frame, then it is in a group by itself. A sequence of observations which are all part of a jitter pattern, or a mosaic, are considered to be part of the same group, and the group number will be the observation number of the first frame in that sequence. A full rundown of the required and optional keywords is contained in orac016, available from the ORAC-DR documentation index . Instead, the existing guide probe arrangement will be used. Although guiding can be commanded on and off, there is no feedback from the system to indicate whether the guide star has been reacquired.