Permanent Fiber Focus Unit
PEPSI will have have permanently mounted focus stations on each telescope, whereby the spectrograph will be permanently available for non-polarimetric spectroscopy. The observer has a choice of using fibres of three different core diameters, giving resolutions from 32,000, 120,000 and 320,000..
We are using a dedicated 5th Nasmyth focus with its own AGW for on-axis, low-frequency, wave-front sensing and guiding. By rotating the telescope Nasmyth mirror to an angle of 46.5 degree from the optical axis of the central Nasmyth station, the permanent focus unit will see the sky.
No field derotation is required, since the guiding system is using light from the target. Dividing the light between the science focus and the guiding system takes place in an optical module located 400mm ahead of the focal plane. This module consists of an afocal lens, creating a parallel beam, in which an ADC and a dichoric/beamsplitter wheel is located. After this wheel, another pair of identical lenses recreates the F/15 telescope beam. All optics in the parallel beam are immersed in an index-matching liquid, thereby reducing reflection losses by a factor of about 4, to less than 3%. The guiding and wavefront sensing system is fully identical to the off-axis modules used in the LBT AGW units.
Last updated:16:50 13/04 2011, by JB