Numbering of fibres on LIFU detector

There are 603 LIFU fibres = 547 science + 8 x 7 sky fibres, see the page on the geometry of the LIFU array. Three of the science fibres do not transmit light, so the number of spectra on each exposure is 600 (= 603 - 3).

These fibres are grouped along the pseudo-slit (at the entrance of the spectrograph) in 29 slitlets of up to 21 fibres each, and the fibre-numbering accordingly runs 1 - 609 (= 29 x 21). The central fibre of the LIFU array is #305.

Below is a cropped arc image (red-arm, high-res mode, taken in July 2024), showing a small fraction of the full wavelength range (x direction), and the central ~ half of the slit (y direction). The inter-slitlet gaps every 21 fibres are clearly visible, as are occasional gaps due to a few missing / poor-throughput fibres. The thick gap near the centre of the image is due to the horizontal overscan pixels inserted between CCD quadrants.

The fibres are numbered in increasing order of y position on the detector, i.e. with #1 near the bottom of the detector and #609 near the top, both well clear of the upper and lower detector edges in all modes.

As of July 2024, 3 of the fibres (#155, 520 and 577) appear not be transmitting light (see map on LIFU-geometry page) and 6 more of the positions have no associated fibre (#21, 316, 393, 404, 451 and 486), so there are 609 - 9 = 600 spectra on each CCD. The 9 gaps mentioned are useful landmarks, and their approximate y positions are tabulated below.

The tabulated approximate y positions of the fibres (right column) are for red high-res mode and will differ by a few 10s of pixels for other modes. They will also change if the detectors are moved relative to the camera. Fibre #1 lies at about y = 263, and fibre n will lie, roughly, at y = 253 + (9.6 * n).

Central fibre #305
The central lenslet of the LIFU array is #305, which is the 11th (i.e. central) fibre in slitlet 15. The fibres of slitlets 1 - 14 appear below the overscan gap, so #305 is the 11th fibre in the slitlet which is mainly above the overscan (for the observation displayed, the first fibre of slitlet 15 appears below the overscan, and the other 20 are above the overscan). Fibre #305 has y ~ 3200 in R HR mode.

The immediate 6 neighbours of fibre #305 are #283, 279, 300, 327, 353 and 330, all on slitlets 14 - 17, 2920 < y < 3460 on the detector.

Thanks to Javier Mendez for providing much of the information on this page.

Chris Benn, Lilian Domiguez, Cecilia Farina

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