| Long Description | Optics in the beam introduces a telescope focus offset (see table). The 'focus-offset' command checks what is currently in the FASU and ALFOSC filter wheels and applies the corresponding focus offset (using foc-del) when possible. The focus offset introduced by a retarder plate in polarimetry mode is also accounted for. Be very careful when using this command. Focus offset for many optical elements are not known and as a consequence the telescope may in fact not be in focus even after using this command. If the focus offset is not known, 0 will be assumed. If this occurs, a warning message will appear both in the talker window and in the sequencer window. Several warning messages may appear if several optical elements in the beam have unknown focus offsets. The final telescope focus offset applied is the sum of the offsets for the element in each wheel, and may therefore be wrong if any offset is unknown. Do not use this command in scripts unless you are absolutely sure that the focus offset are known for all optical elements to be used.
If you intentionally wish to defocus the telescope (for instance when observing very bright targets), call the script with the [-d] flag, specifying the number of focus units to defocus the telescope with, for instance
focus-offset -d 200
will defocus the telescope with 200 units with respect to optimal focus. Reasonable defocus values ranges from 50-2000 units. |
