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 mid-2021. 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.
(Photo by Émilie Lhomé, September 2020. Click to see full size.)
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 fibres | Mini-IFUs (20) | LIFU |
| Diameter of field over which deployable (deg) |
2.0 | 2.0 | On-axis |
| Diameter of individual fibres (arcsec) | 1.3 | 1.3 | 2.6 |
| IFU field of view (arcsec) | - | 11 x 12 | 90 x 78 |
| Fibres per IFU (and filling factor) | - |
37 (0.50) | 547 (0.55) |
| Minimum separation on sky (arcsec) | 60 | 60 | - |
| 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) |
3660-6060 | 5790-9590 | 4040-4650 | 4730-5450 | 5950-6850 |
| Inter-CCD gap (A) |
5491-5539 | 7590-7669 | 4525-4536 | 5302-5315 | 6412-6431 |
| Spec. resolution for MOS, mIFUs |
5000 | 5000 | 20000 | 20000 | 20000 |
| Spec. resolution for LIFU |
2500 | 2500 | 10000 | 10000 | 10000 |
| Scale (A pixel-1) |
0.30 | 0.48 | 0.076 | 0.090 | 0.11 |
| 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 1 hour includes any observing overheads.
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.
Availability of time on the WHT during the WEAVE years
This is described in the
document linked here.
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.
The above-linked document also discusses the frequency with which WEAVE will be taken on and
off the telescope. The cadence is currently planned to be ~ 9 months, but this may evolve
on the basis of experience.
