Guiding and WFS (AGw)
PFU Guiding and WFS
For the PEPSI-PFUs, guiding is always on-axis; there is no mechanism to select a guide star in the focal plane, and the only degree of freedom is along the optical axis to adjust focus. The acquisition field of view is 27.7″ square. The 512 x 512 guide camera images can be binned and there is an option to use one of several ND filters.
PFU tracking is in parallactic mode. With parallactic tracking, the Az-El axis will remain fixed but the field will rotate about the back-reflected image of the pinhole, so the directions N-E-S-W will depend on the parallactic angle. Parallactic angle is defined as the angle from N through E to the positive elevation axis. The positive elevation axis points to the lower left for the SX images and to the lower right for the DX images.
Figure 1: Depiction of PEPSI PFU Guider CCD Orientation and size
Figure 2: Acquisition Image example
The GCS subsystem uses the direct image of the star to guide and the light that is diverted to the WFS to collimate.
The PEPSI guider (SX and DX PFU GUIs) maintains the back-reflected star image in the pinhole of the entrance fiber by adjusting the guider hotspot to compensate for systematic drift as the telescope tracks. The guider hotspot refers to the position of the direct star image on the guide camera which centers the light in the 3″ diameter WFS pinhole (the Shack spots are well centered in the lenslets). It is written in the bottom right section of the PEPSI@LBT GUI and it is adjusted slightly as the PEPSI guider tries to keep the back-reflected star image on the pinhole. When the PEPSI guider adjust the guider hotspot, the star moves slightly within the WFS pinhole.
The hotspot depends on the beamsplitter in use. For BS4 in particular, the pinhole is close to the left edge of guide image where there is a bright band; this can cause problems. For BS5, the pinhole is also pretty close to the left edge.
