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WEAVE - instrumental overview

Below are summarised the expected observing capabilities of the WHT's new multi-object fibre-fed spectrograph WEAVE, due for commissioning in spring 2020. As a result of the commissioning tests, there may be small changes to some of the numbers given on this page.

WEAVE will be mounted at WHT prime focus, behind a new field corrector / ADC, which will provide good image quality over a 2-degree field of view, at zenith distances up to 65 deg.

Focal-plane modes
The instrument can be used in any of 3 focal-plane modes: MOS (multi-object spectroscopy), mIFU (mini integral-field units) and LIFU (large integral-field unit). The key parameters of each mode are summarised below:

MOS fibresMini-IFUs (20)LIFU
Diameter of field over which deployable (deg) 2.02.0On-axis
Diameter of individual fibres (arcsec)1.31.32.6
IFU field of view (arcsec)-11 x 1290 x 78
Fibres per IFU (and filling factor)- 37 (0.50)547 (0.55)
Minimum separation on sky (arcsec)6060-
Tumbler position (deg) 0 or 180 180 90
Configuration time (minutes)55<20?~ 1

In MOS mode, each of up to ~ 1000 individual fibres can be positioned anywhere within the field of view, with each fibre intercepting a circular area of sky of diameter 1.3 arcsec.

There are actually two sets of MOS fibres, one for each of two focal-plane plates A (960 fibres) and B (940 fibres), and WEAVE is designed so that configuration of one of the two sets fibres (e.g. for plate A) takes place during a 1-hour observation with the other set (e.g. on plate B). Then the tumbler is rotated by 180 deg so that plates A and B swap positions.

In mIFU mode, up to 20 fibre bundles (each 11 x 12 arcsec2 on the sky) can be positioned anywhere within the field of view. For any given observation, the MOS and mIFU modes cannot be mixed.

In LIFU mode, a single IFU (78 x 90 arcsec2) is positioned at the centre of the field of view.

The scale in the WEAVE focal plane is 17.8 arcsec mm-1.

Spectrograph modes
WEAVE's fibres feed a dual-arm (blue + red) spectrograph housed on one Nasmyth platform of the WHT. A 5900-A dichroic splits the light between the blue and red arms. Dispersion is effected by inserting one of three VPH gratings in the blue arm, and one of two in the red arm, giving five spectroscopic modes. In low-resolution mode, WEAVE covers the wavelength range 3660 - 9590 A.

For any given observation, low- and high-resolution modes can't be mixed, e.g. it's not possible to observe at low resolution in the blue arm and high resolution in the red arm.

Low-resolution High-resolution
Blue arm Red arm Blue arm Red arm
Wavelength range (A) 3660-60605790-95904040-46504730-54505950-6850
Inter-CCD gap (A) 5491-55397590-76694525-45365302-53156412-6431
Spec. resolution for MOS, mIFUs 50005000200002000020000
Spec. resolution for LIFU 25002500100001000010000
Scale (A pixel-1) 0.300.480.0760.0900.11
WHT/WEAVE throughput (expected) ~ 0.25 ~ 0.15 to 0.20

Note that in high-resolution mode, observers have a choice of two possible VPH gratings, for wavelength range 4040 - 4650 A ('blue') or 4730 - 5450 A ('green'); they can't be deployed simultaneously.

The inter-CCD gap in the wavelength coverage (tabulated above) arises from the join between the two 6k x 6k low-fringing EEV CCDs comprising the science detector on each arm.

Each spectrum on the CCD occupies about 3 pixels perpendicular to the dispersion direction. The detector readout noise is expected to be about 3 electrons rms per 15-micron pixel. The CCD readout time is expected to be ~ 60 sec.

Signal-to-noise
For an estimate of the signal-to-noise ratio per pixel and per A for each focal-plane and spectrgraph mode, and as a function of sky brightness and seeing, see this table provided by Scott Trager.

Autoguiding
Autoguiding for MOS and mIFU modes is provided by positioning guide-fibre bundles (diameter 3.5 arcsec) at the positions of up to 8 guide stars (mag <~ 15). For LIFU mode, a separate guide camera is provided.

Sky subtraction
The sky background is measured in MOS mode by assigning 5 - 10% of the science fibres to sky, in MIFU mode by assigning one of the mIFUs to sky, and in LIFU mode by sampling sky through 8 dedicated peripheral bundles.

Observations
Observations with WEAVE are specified as observing blocks (OBs), with each OB containing all the information required to make a 1-hour observation, which might include e.g. 3 autoguided science exposures, an arc and a flat. The overheads for target acquisition, CCD readout and calibrations are expected to be <~ 10%.

WEAVE observations are carried out in service mode, by ING staff, with the aid of an OB scheduler. The scheduler tool optimises the use of the night by matching OBs with required observing conditions, and by taking into account the RA, science priority, change overheads etc.

Data-reduction and analysis (and archiving) are carried out automatically by the WEAVE data-processing pipeline. The results of a quick-look data reduction are visible to observers at the telescope.

WEAVE availability
Following on-sky commissioning of WEAVE in spring 2020, an average of 70% of the time on the WHT will be used for the eight designated (pre-planned) surveys (~ 15 million spectra) with WEAVE, while the other 30% is for 'open-time' community observations with WEAVE or with the existing Cassegrain instruments (ISIS, LIRIS, ACAM), or with visiting instruments.

Taking WEAVE on and off the telescope requires several days, so this will not happen frequently. The exact cadence hasn't been decided yet, but might be e.g. 9 months, to allow both WEAVE and non-WEAVE observations to access all RA on reasonable timescales.

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Contact:  (WEAVE instrument scientist)
Last modified: 16 August 2019