Once the target is acquired on the guide TV, the telescope is offset to move the target from its off-axis position on the guide TV to the on-axis science CCD. Any suitable off-axis star that then moves into the f.o.v. of the guide TV can be used for guiding.

Though targets are usually first acquired on the Guide TV to confirm their identity as described in the preceding paragraphs, in some cases observers may choose to forego that step and place the target directly on the science CCD. The diagonal must still remain in the "In" position to bring guide stars to the guide TV, but POCO can be made to send the target directly to the on-axis science CCD. See CCD/Guider on Axis below.

See Nickel Lightpath for a diagram of the lightpaths and diagonal configurations for spectroscopy and direct imaging. Use of the diagonal for spectroscopy is described in the spectroscopy version of the Main Window.

For direct imaging, this option is set usually set to "Guider on Axis" so that POCO attempts to place the object on the guide TV. However, with POCO's accuracy and the large direct imaging field, some observers may prefer to place the target directly on the science CCD without first checking acquisition on the guide TV. For this target placement, select "CCD on Axis".

The advantage of using "CCD on Axis" is the time saved in not having to offset the target from the guide TV to the science CCD. The disadvantage is that one is shooting blind, and if the target is missed, the observer will not discover it until after the first integration. We generally recommend using "Guider on Axis" to first confirm target acquisition, but if working in a relatively small area of the sky (e.g. within a cluster) and if one's coordinates are accurate, using "CCD on Axis" to place the target directly on the science CCD may work well.

Because the guide TV and spectrograph have essentially the same axis, the Axis option does not appear in the spectrograph version of the Main Window.