SkyMapper and the Southern Sky Survey - a resource for the southern sky
SkyMapper and the Southern Sky Survey
a resource for the southern sky
Stefan C. Keller, Brian P. Schmidt and Michael S. Bessell1
Research School of Astronomy and Astrophysics, Cotter Rd., Canberra, ACT
2611, Australia stefan@mso.anu.edu.au
Summary. SkyMapper is amongst the first of a new generation of dedicated, wide-
field survey telescopes. The 1.3m SkyMapper telescope features a 5.7 square degree
field-of-view Cassegrain imager and will see first light in late 2007. The primary
goal of the facility is to conduct the Southern Sky Survey a six colour, six epoch
survey of the southern sky. The survey will provide photometry for objects between
8th and 23rd magnitude with global photometric accuracy of 0.03 magnitudes and
astrometry to 50 mas. This will represent a valuable scientific resource for the south-
ern sky and in addition provide a basis for photometric and astrometric calibration
of imaging data.
1 The SkyMapper Telescope
The SkyMapper telescope is a 1.3m telescope currently under construction
by the Australian National University’s Research School of Astronomy and
Astrophysics in conjunction with Electro Optic Systems of Canberra, Aus-
tralia. The telescope will reside at Siding Spring Observatory in central New
South Wales, Australia.
The telescope is a modified Cassegrain design with a 1.35m primary and a
0.7m secondary. Corrector optics are of fused silica construction for maximum
UV throughput and a set of six interchangeable filters can be placed in the
optical path. The facility will operate in an automated matter with minimal
operator support. Further details on all aspects of our programme can be
found in [2].
2 Detectors and Filters
The focal plane is comprised of 32 2k×4k CCDs from E2V, UK. Each CCD
has 2048 × 4096, 15 micron square pixels. The devices are deep depleted,
backside illuminated and 3-side buttable. They possess excellent quantum
efficiency from 350nm-950nm (see Figure1), low read noise and near perfect
cosmetics.
http://arxiv.org/abs/0704.1339v1
2 Stefan C. Keller, Brian P. Schmidt and Michael S. Bessell
Wavelength (Angstroms)
Fig. 1. Spectral response of SkyMapper CCDs as measured in the laboratory. The
shaded area encloses the range in response exhibited.
4000 6000 8000
Wavelength (Å)
10000
Fig. 2. The predicted throughput of the Southern Sky Survey filter set, excluding
atmospheric absorption.
The SkyMapper imager will utilise the recently developed STARGRASP
controllers developed for the Pan-STARRS project by Onaka and Tonry et
al. of the University of Hawaii [6]. Twin 16-channel controllers enable us to
read out the array in 12 seconds with ∼4−5 electron read noise.
Figure 2 shows the expected normalised throughput of our system. The
filter set is based upon the Sloan Digital Sky Survey filter set with three
important modifications:- the movement of the red edge of the u filter to
the blue, the blue edge of the g filter to the red, and the introduction of an
intermediate band v filter ( essentially a DDO38 filter ). At this time coloured
glass fabrication of filters of these bandpasses offers the best solution for
spatial uniformity compared to the competing interference film technology.
Our filters are sourced from MacroOptica of Russia.
SkyMapper 3
3 The Southern Sky Survey
Performing the Southern Sky Survey is the primary preoccupation of the
SkyMapper telescope. The survey will cover the 2π steradians of the southern
hemisphere reaching g=23 at a signal-to-noise of 5 sigma. For stars brighter
than g=18 we require global accuracy of 0.03 magnitudes and astrometry to
better than 50 milli arc seconds.
The survey’s six epochs are designed to capture variability on the time
scales of days, weeks, months and years over the five year expected lifetime
of the survey. The 5 sigma limits attained after one 110 second epoch and
after the full six epochs are given in Table 1. In all bands we attain limits
slightly deeper (∼ 0.5mag) than the Sloan Digital Sky Survey.
Table 1. Southern Sky Survey limits (5 sigma) in AB magnitudes from multiple
110 second exposures
u v g r i z
1 epoch 21.5 21.3 21.9 21.6 21.0 20.6
6 epochs 22.9 22.7 22.9 22.6 22.0 21.5
4 Global Photometric Calibration
The greatest impediment to deriving accurate photometry from wide field
imaging cameras is the accurate description of the illumination correction.
The illumination correction corrects for geometry of the optics and inclusion
of scattered light in the system (see Patat and Freudling these proceedings).
During commissioning we will develop an illumination correction for each
filter via dithered observations of a field. We will then rotate the instrument
and repeat the dithered observations to ensure we rigourously understand the
illumination correction for the system. We will establish six such reference
fields at declinations of around -25◦ and spaced in right ascension. Each field
will be 4.6 degrees square following the dither pattern.
During the first year of operation we will perform the Five-Second Survey,
a rapid survey in photometric conditions to provide all-sky standards between
8-16th magnitude. The Five-Second Survey will consist of a set of at least
three images of a field in all filters.
During Five-Second observing we will observe the two highest of our six
reference fields every ninety minutes. This will ensure photometry is obtained
on a highly accurate standard instrumental system. The Five-Second Survey
will provide a network of photometric and astrometric standards to anchor
the deeper main survey images. Furthermore, it enables the main survey to
proceed in non-photometric conditions.
4 Stefan C. Keller, Brian P. Schmidt and Michael S. Bessell
We will establish the six reference fields to include stars with photometry
in the Walraven system [4]. As demonstrated by Pel & Lub (ibid), the Wal-
raven system zeropoint is highly accurate: the closure solution over 2π in right
ascension has rms of less than 1 millimag. In addition, the Walraven stars we
have selected are spectrophotometric standards from the work of Gregg et al.
[5]. The use of these standards will provide absolute flux calibration for our
system.
5 A Filter Set for Stellar Astrophysics
The majority of science goals identified for SkyMapper are based on the iden-
tification of stellar populations. It was therefore fundamental to the science
output of the telescope that we choose a filter set that offers optimal diagnos-
tic power for the important stellar characteristics of effective temperature,
surface gravity and metallicity. Below I will discuss some specific examples.
Through an exploration of colour parameter space derived from model
stellar atmospheres and filter bandpasses we arrived at the filter set shown
in Figure 2. The filter set possesses two filters, u and v, distinctly either side
of the Balmer Jump feature at 3646Å.
Fig. 3. Precision of determined surface gravity from our filter set as a function of
temperature and surface gravity (error bars show estimated uncertainties at each
point for photometric uncertainties of 0.03mag in each filter).
SkyMapper 5
5.1 Blue Horizontal Branch Stars
In Figure 3 we show the uncertainty in the derived stellar surface gravity as
a function of temperature for a range of surface gravities with photometric
uncertainties of 0.03mag. per filter. In the case of A-type stars we expect
to determine surface gravity to ∼ 10%. The sensitivity to surface gravity
arises from the u−v colour which measure the Balmer Jump and the effect
of H− opacity, both of which increase with surface gravity. It is at these
temperatures that we find blue horizontal branch stars (BHBs). Due to their
characteristic absolute magnitude BHBs are standard candles for the Galactic
halo.
A line of sight through the halo inevitably contains a mixture of local
main-sequence A-type and blue straggler stars. However as is shown in Figure
3 the SkyMapper filter set enables us to clearly distinguish the BHBs of
interest on the basis of their lower surface gravity. Simulations show that
we will be able to derive a sample of BHBs to 130kpc with less than 5%
contamination.
5.2 Extremely Metal-Poor Stars
In the case of cooler stars (F0 and cooler) the u and v filters indicate the level
of metal line blanketing blueward of ∼ 4000Å. Figure 4 shows the v−g, g−i
colour-colour diagram for a range of metallicities and surface gravities. The
v−g colour has a strong dependency on the metallicity and little dependency
on the surface gravity hotter than K0 (g−i ∼ 1.7).
log[Fe/H]=-1
Solar, logg=2
-5.4 Frebel et al. 2006
Fig. 4. v−g vs. g−i for stars of solar metallicity (dashed lines) and for a range of
surface gravity (solid lines). Open circles are stars from the sample of [1] and the
star symbol is HE1327-2326 from [3].
6 Stefan C. Keller, Brian P. Schmidt and Michael S. Bessell
This enables us to cleanly separate the extremely metal-poor stars in the
halo from the vast bulk of the halo at [Fe/H]<-2. Our simulations show we
should find of order 100 stars with [Fe/H]<-5.
6 Data Products and Their Possible Application to ESO
Calibration
The first SkyMapper data product will be the Five-Second Survey of 8-16th
magnitude stars in the southern hemisphere. Two main survey data releases
will follow. The first data release will occur when three images in each filter
have been reduced for a field and the second (reaching 23rd magnitude in g)
when the full set of six have been obtained and undergone quality control.
The survey will provide sufficient density and spectral sampling of stan-
dard stars to enable photometric calibration of any field imaged in any broad-
band filter in the southern hemisphere. The largest source of dispersion in
transformations between photometric systems is due to the lack of knowl-
edge of the surface gravity and metallicity of the sample. The SkyMapper
photometric system provides a prior on both these points of uncertainty.
Consequently we will be able to provide improved transformations from our
photometric system to any other system. Scheduled observations on, for in-
stance VLT or VST, may then dispense with photometric standards and also
proceed under non-photometric conditions.
References
1. Cayrel, R. et al. A&A 416, 1117 (2004)
2. Keller, S. et al. PASA in press, astro-ph/0702511 (2007)
3. Frebel, A. et al. Nature 434, 871 (2005)
4. J.W. Pel & J. Lub: The Walraven System. In: The Future of Photometric,
Spectrophotometric and Polarmetric Standardization, PASP in press
5. M. Gregg: Next Generation Spectral Library. In: http://lifshitz.ucdavis.edu/-
mgregg/gregg/ngsl/ngsl.html
6. P. Onaka & J. Tonry: StarGrasp - Detector Controllers for Science and As-
tronomy. In: http://www.stargrasp.org/
	SkyMapper and the Southern Sky Survey  a resource for the southern sky
	Stefan C. Keller, Brian P. Schmidt and Michael S. Bessell
