Observing Modes

  There are currently four observing modes with WHIRCAM. These are:

• STARE - the array is reset, read after the user-defined on-chip exposure times, and reset again.
• ND_STARE - the array is reset and read immediately and read again after the user-defined on-chip exposure time. The difference between the post-exposure and pre-exposure images is taken and stored.
• S+A - In Shift-and-Add mode, images are pre-processed in ALICE. The peak pixel in a user-definable area is found and the image is shifted (by integer pixels) so the peak pixel falls onto a fiducial mark on the array. N images are co-added in this manner. S+A mode works in STARE mode only and is used to sharpen images and reduce seeing/tracking effects. The user is referred to § 8 for more details.
• Atmospheric Monitoring imaging - intended to allow monitoring of such things as seeing and image stability, but not yet implemented.

The two advantages of ND_STARE over STARE is that (1) the direct read-out architecture of the detector means that the integrated charge may be sampled non-destructively and an improvement in read-noise is thus obtained by making multiple reads, and (2) the reset level can vary, so you automatically subtract the latest reset in ND_STARE. However, there is also an extra time overhead in each readout, so for very short exposures (as in S+A) STARE is better. The shortest integration where ND_STARE is likely to be beneficial is about 1 sec.