Fibers and Image Slicers
The three resolution modes of PEPSI are defined by the three different fiber diameters in combination with three image-slicer dimensions. Each fiber supports its own image slicer (see Spectrograph for Image slicer details). Each fiber head is actually a pair of fibers. A fiber pair consists of an off-axis sky fiber and an on-axis target fiber. One of the fibers is positioned on-axis and is the target fiber, the other one is off-axis and receives light by a sky position 6 arcsec away from the target. So two fibers per telescope are always “active” and receive light. The fiber injection unit couples the beam into the fibers. Three pairs of fibers are available; 100μm, 200μm, and 300μm:
| Fiber | Sky Diameter (“) | # of slices (n) | Slicer Width (μm) | R=λ/Δλ | R Sampl. (pix) |
|---|---|---|---|---|---|
| 100 | 0.74 | 7 | 0.11 | 250,000 | 2.0 |
| 200 | 1.5 | 5 | 0.30 | 130,000 | 4.2 |
| 300 | 2.3 | 3 | 0.77 | 50,000 | 12.1 |
Once the spectral resolution is chosen by the user, a linear stage moves a flat mirror in the position to select either the 200μm or the 300μm fiber head. For the 100μm fiber head, the light continues straight through without an extra reflection. When using the 200 or 300μm fiber, the user may also select between two flat mirrors with either an aluminum coating or a UV-enhanced coating, the latter being optimized for blue light and represents the default.
