Acquisition, guiding and dithering in LIFU mode

Science targets can currently be acquired onto the central fibre (fibre 305) of the LIFU with rms accuracy ~ 0.3 arcsec, i.e. much less than the radius of a fibre (1.3 arcsec).

Acquisition is normally carried out by pointing the WHT to the required position (the telescope pointing rms is a few arcsec) and then centring the guide star at a reference position on the autoguider. This ensures that the science target is centred on the central fibre of the LIFU array.

The software that achieves this (the LIFU acquisition tool) automatically identifies the guide star (by comparing the autoguider image with a finding chart) and takes into account the coordinates of the guide star and science target, the relative positions of the autoguider and science array in the focal plane, and the effects of differential refraction across the focal plane. It also calculates any required rotational tweak implied by comparison of the autoguider image and the finding chart.

If required (not usually), a final check of the acquisition can be made by inspecting a LIFU 'image' reconstructed from the spectra.

The rms guiding accuracy is ~ 0.3 arcsec. Autoguiding is currently in x and y, not rotation (although that may also, at some point, be implemented).

The LIFU autoguider is a standard ING autoguider head, AG2, an E2V47-20 CCD. There are no pre-autoguider optics apart from the CCD window. The CCD provides a square field of view 3.8 arcmin across, i.e. an area on-sky ~ 7 times larger than that of the LIFU itself. The pixel size is 13 micron.

Guide stars are selected automatically during phase 2 of the application process. Open-time PIs don't need to identify suitable guide stars beforehand. The limiting magnitude in g band was determined during commissioning and depends on seeing:

Seeing (arcsec)	Limiting mag (g)
0.7	18.1
1.0	17.9
1.5	17.5
2.0	17.1
2.5	16.6
3.0	16.2

The limiting magnitudes are not much affected by sky brightness (which is typically > 18 mag arcsec^-2). To be on the safe side, it's recommended that guide star magnitudes be in the range 14.5 < g < 16. In addition, the guide star needs to be isolated (to avoid confusion) and should not lie near the edge of the autoguider field of view.

In most cases, a guide star can be found at the requested position angle, and in almost all cases, a guide star can be found at some position angle. If a small change of sky PA is required, the open-time PI will be informed.

The LIFU lies off-axis at a radius of 20 mm (6 arcmin at the focal-plane scale of 17.8 arcsec mm^-1). The autoguider is off-axis by 74 mm (= 22 arcmin) in the opposite direction. The separation (94 mm = 28 arcmin) is set by the physical sizes of the two components. The relative amounts off-axis are chosen to keep any optical aberrations at the position of the LIFU within acceptable bounds. Because of fibre-cladding etc, the filling factor of the LIFU array is only 0.55, so most observers will want dithering. The image below left (from WEAVE internal document FIB-003, ter Horst et al 2020) illustrates how in LIFU mode the light from a star, observed in reasonable seeing, may (1) all be intercepted by a given LIFU fibre, (2) be partially intercepted or (3) be lost completely, hence the need for a dither of at least 3 points (below right) for full spatial coverage.

The standard dither patterns for mIFU and LIFU observations (table taken from version 8.0 of WEAVE internal document ICD-030, Worley et al 2022, section 5.4.3.8) are subsets of a hexagonal array (e.g. 3 points, 4 points, 5 points), click on table for enlargement:

Observations will be processed correctly by the data-reduction pipeline if one of these dither patterns is used.

PIs should consider at phase 1 (of the time-application process) what dithering they will need, since this also affects the exposure times. A final decision about dithering can be made at phase 2.

Chris Benn, Lilian Domínguez, Cecilia Fariña

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