LDSS-2 is now a non-common-user
instrument.
Please contact Richard Bower in Durham
(r.g.bower@durham.ac.uk) for info on the availability of
LDSS-2.
Should I use LDSS-2 or WYFFOS/AUTOFIB2 on the WHT ?
Here we list the advantages and limitations of LDSS and WYFFOS/AF2.
Field Coverage and Number of Objects
WYFFOS/AF2 can presently observe up to 110 objects over a field of
1 degree diameter; though it should be noted that there is some
vignetting starting at a diameter of 40 arcminutes.
With LDSS-2, one can observe up to ~100 objects if a blocking filter
is used, over a field of 11.5 arcmin diameter; without a blocking
filter a more typical number is 30-40 objects per mask.
Minimum Spacing
The chief limitation of WYFFOS/AF2 for crowded fields is the
minimum fibre spacing of 25-30 arcsec (in the *best* case), which is
set by the button/prism assembly on the input fibre ends. It should
also be noted that, *unlike* 2dF, at present fibres cannot cross with
WYFFOS/AF2.
LDSS-2 is better in this respect, with object separation
essentially limited by slit length; separations of 5-10 arcsec can be
achieved.
Dispersion/Resolution
WYFFOS/AF2 can achieve dispersions as high as 0.8 A/pixel with a
2400 line grating with the present 2.7 arcsec fibres and a TEK CCD
with 24 micron pixels. When the WYFFOS echelle grating is implemented
in 1998, the dispersion will be ~0.3 A/pixel (R=~9000) with the large
fibres.
The highest dispersion available with LDSS-2 is 2.4 A/pixel (for TEK
or SITe detector) with the 'high res' grism.
Throughput
Because of losses in the 26m-long fibres and the fibre
connectors, the WYFFOS/AF2 throughput is lower than for LDSS-2. Our
best present estimates for the WYFFOS/AF2 sensitivity is 17.0 near
4500 A (this is the AB magnitude giving 1 photon/sec/A). The quoted
WYFFOS sensitivity is for one of the best fibres (#26), and the
throughputs will be correspondingly lower for other fibres (see the Fibre response
tables). See here for more information about the system throughput.
In comparison, the LDSS-2 sensitivity is ~18.5 in B and V with
the SITe CCD. In the B, V and R bands LDSS offers the most efficient setup
for WHT spectroscopy.
Sky subtraction
Another limitation with WYFFOS/AF2 is the large fibre diameter of 2.7
arcsec. Because of the larger sky contribution, for point sources the
S/N will be lower than for LDSS-2, where the slit width is typically
1.5 arcsec. A smaller set of 1.5 arcsec diameter fibres will be
commissioned in 2000, and these will be significantly better for point
sources.
Sky subtraction with fibres is never as good as with a slit
spectrograph, since one is limited by the fibre diameter. At present
the best sky subtraction we have achieved with WYFFOS/AF2 is 3% (based
on the rms residuals on sky dedicated fibres in a twilight sky frame),
but that was based on the system when it displayed the large
scattering caused by the deposit on the field-flattening lens.
Further information on the sky subtraction will be provided when we
have it to hand. Achieving the best possible sky subtraction with
fibres requires careful planning and observing strategies, and
inevitably involves tradeoffs. For instance, one can assign an `sky'
fibre next to each object fibre, which reduces the fibre multiplexing
advantage by a factor of ~2. Another strategy is to `beam-switch'
(move the telescope back and forth between objects and neighboring
sky), but this of course reduces the integration time on source.
LDSS-2 offers standard 'longslit' sky subtraction, if the mask
design permits slits that are significantly longer than the
seeing.
Flux Calibration
True flux calibration can never be done with fibres, since again one
is limited by the fixed fibre diameter. However, with the present
2.7 arcsec fibres and good seeing, one can come close.
With LDSS-2 one can flux calibrate using a wide long slit mask or
using the fiducial holes in a normal fields mask. However, absolute
calibration of the science objects is difficult given the standard slit
width of 1.5 arcsec.
Observational overheads
WYFFOS/AF2: Switching fields during the night involves a ~25 minute overhead
for setting up of the fibers.
LDSS2: no significant overhead is associated with the switching of fields.
Flexibility and Preparing Fields
All multi-object spectrographs are much less flexible than
conventional long-slit spectrographs, since one has to either design
multi-slit masks for LDSS-2, or prepare a Configuration file for
WYFFOS/AF2. In both cases, the relative astrometry has to be
carefully prepared well in advance of the observing run, which is a
significant overhead.
With WYFFOS/AF2, the input Configuration files can be
modified at short notice.
In comparing WYFFOS/AF2 to LDSS-2, with LDSS-2
masks have to be produced well in advance of the observing run, with
the hassle of having to produce a new mask if you wish to make
changes.
Imaging capability
It should also be noted that WYFFOS/AF2 has no imaging capability.
LDSS-2 can be used without grism giving 0.6 arcsec per pixel
resolution for a field of 11.5 arcmin diameter. Standard
100mm-diameter filters are Harris BVR. Other smaller filters can be
mounted but will be less efficient.
Summary
AF2/WYFFOS:
pros: wide field (1 degree diameter), many objects (~110), low to
intermediate wavelength resolution
cons: large minimum spacing of fibers, loss of throughput in
fibers, sky subtraction difficult, 25 minutes overhead when swithing
fields during the night.
LDSS-2:
pros: 30-40 objects within 11.5 arcmin field, normal sky subtraction,
good throughput in BVR, imaging capability
cons: low wavelength resolution