SWIFT Technical Specifications
View the original specs page.
The Oxford SWIFT spectrograph is an image-slicer
based integral field spectrograph, that is coupled with the PALAO
adaptive optics system on the Hale 200-inch telescope. The instrument
comprises of a pre-optics, image slicer, and identical twin
spectrographs, each consisting of a collimator, VPH grating, camera and
an LBNL thick, fully depleted, 4k × 2k CCD array detector. There is
also an internal calibration unit, with spectral calibration and flat
- Pixel scales: 0.235″/pix, 0.16″/pix and 0.08″/pix, selectable on-the-fly.
- Field of view: 44 × 89 spaxels, corresponding to 10.3″ × 20.9″,
7.0″ × 14.2″, and 3.5″ × 7.1″ at the three spaxel scales.
- Wavelength range: 650 nm to 1000 nm, fixed format, in a single exposure.
- Spectral resolving power: R ≡ λ/Δλ = 3250 at 650 nm,
rising linearly to 4400 at 1000 nm, 2 pixel spectral FWHM.
- Detector: Twin 4k × 2k LBNL thick, fully depleted CCD array detectors with
enhanced red quantum efficiency and negligible fringing.
The following input parameters have been used to estimate the
expected line and continuum sensitivity of the instrument. An exposure
time calculator will shortly be available for detailed sensitivity
calculations. In the meantime, expected performance can be scaled using
the two examples provided below.
- Detector read noise: 5e− per pixel.
- Detector dark current: 8e− per hr per pixel.
- Instrument throughput: The most realistic estimate of the instrument throughput
(excluding telescope, AO and detector) is given by the following table:
|SWIFT Instrument Throughput
- Detector quantum efficiency: The
LBNL thick (250 μm), fully-depleted CCD arrays provide unsurpassed QE
at long wavelengths, providing SWIFT with a sensitivity gain of over a
factor of three at long wavelengths, compared to normal CCDs. The
detector QE at the operating temperature of 140K is given as:
|SWIFT Detector Quantum Efficiency
- Telescope optics throughput: 0.75 (two Al reflections at 0.875 each).
- AO system science throughput: 0.8 (6 reflections at 0.98 (freshly recoated optics),
plus one dichroic beamsplitter at 0.9).
- AO ensquared energy gain: factor
of 1.6 gain in ensquared energy in the central pixel due to sharper
core, no gain if extraction aperture is ≈ seeing disk size.
- Atmospheric transmission: assumed to be unity (detailed atmospheric transmission
curve will be implemented in the ETC).
- Night sky background (average lunar illumination conditions): I band
19.5 mag/arcsec2, z band 18.3 mag/arcsec2.
- Detector exposure times (DIT): 30 minutes (1800 secs) per exposure, longer
exposures are made up of NDIT sub-units DIT seconds long.
- Expected continuum sensitivity: For
a z = 22 mag (Vega) point source, we obtain an SNR of 1.5 in the
continuum (per spectral pixel) in a single 30 min exposure, in 1″
- Expected line sensitivity: For a source with a line flux of
10−17 ergs s−1 cm−2
arcsec−2, with a line width of
150 km s-1, we obtain an SNR of 1 in the line a single 30 min exposure
per spatial pixel. This corresponds to an SNR of 4 per 1″ extraction
aperture in the same time.
Operationally, we expect individual exposure times to
be restricted to 1800 secs due to the number of cosmic ray events. As
the LBNL chips have a thick (250 μm) silicon substrate, we are much
more sensitive to cosmic ray events. Longer exposures will be made up
of 1800 secs sub-units. If allowed by the source size, the rectangular
field of view can be used to nod-on-IFU so that all the exposure
time is spent observing the object.
Overheads: Apart from the overheads
involved in achieving AO loop closure on the desired target, there
should be only a few minute additional overhead for target acquisition
and positioning within the field of view. SWIFT will provide an almost
instantaneous reconstructed image to help with the acquisition.
Detector read-out time is ≈ 40 seconds. SWIFT is equipped with a
shutter to control short exposure times. The only configurable part in
the instrument is the choice of spaxel scale, changes to which will
need less than half a minute.