In the old days, we used to take a focus plate by taking a multiple exposure in which the telescope focus and position were incremented about 5-10 times. With CCDs, we can do the same thing, but we don't have to run downstairs to the darkroom. Furthermore, with ICE, we can eliminate another process in the chain. We no longer need to increment the telescope position: the software commands the detector to ``reverse vertical shift" its charge between successive exposures, effectively simulating a stepping of the telescope. The last exposure in the sequence is given a double shift for easy identification.
You can initiate a focus sequence the same as any other observing command, simply by using observe and selecting ``focus" as the image type. An example is shown in Figure 8. In this example, the astronomer has chosen to make seven exposures, shifting the charge on the CCD between each exposure (``Shift type=detector"). S/he has chosen to manually set the focus between each exposure, rather than relying on the telescope computer to automatically set the focus to the next appropriate value. The shifts were each of 30 rows, except that a double shift (60 rows) will be found before the last exposure. The focus was stepped by 20 units from a focus reading of 1200 to 1320. (These numbers are telescope dependent; see the table below.) Before each exposure the astronomer was prompted with the focus number and the routine waited until the astronomer hit [CR]. Note that in order to carry out this focus sequence, it was not necessary to alter a single parameter in any of the parameter files; all the necessary items were requested. The setfilter flag in obspars was set to yes, and for that reason it was possible to change the filter to position 3 before the exposure began.
After the multiple exposure series is complete, look at the images on the display to select the optimum focus setting of the telescope. There is a newly implemented routine kpnofocus specifically tailored for analyzing focus frames obtained with ICE; simply run kpnofocus and use the ``g" key to mark the top-most image for a few well-exposed stars in the focus sequence (see Section J.1). In addition, the general-purpose image examination routine imexamine is described in Section G.