Fibre Instrument Feed - FIF

A Fibre Instrument Feed (FIF) will be located on the Prime Focus Platform, feeding spectrographs in a room beneath the telescope floor. Whatever form the FIF takes, it must be designed in tandem with the first-light fibre-fed spectrograph, but also have the capability of supporting future generation fibre-fed instruments.
One important issues that needs to be addressed is the telecentric angle variation, leading to changing effective f/ratio of injected beam. For the HET, this tilt amounts to 1.6� at the field edge, resulting in the input beam speeding up from ~f/4.7 to f/3.6. Unless this is accounted for, this will result in non-optimised filling of spectrograph collimator (i.e. over or under filling of it). HET corrected for the telecentric angle by 'zoning' multi-object fibres, i.e. tilting fibres to a constant angle (e.g. 1� for fibre in outer field). Active tilting of fibres was considered too difficult to implement, requiring x, y, q & j motions of the fibres.

For SALT we are investigating a possible implementation of fibre tilting, or employing a field curvature lens to correct for telecentric angle variations. The latter would then require just x, y & z motions to be executed by fibre positioners. Defocus (z) motion could amount to 4 mm displacement from the centre to the edge of the field.