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Wide-Field Imaging Surveys from the Ground. S. M. Kahn Stanford U./SLAC and J.A. Tyson University of California, Davis. Wide-Field Imaging from Space.

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Wide-Field Imaging Surveys from the Ground

S. M. Kahn

Stanford U./SLAC


J.A. Tyson

University of California, Davis

Wide-Field Imaging from Space

  • The purpose of this workshop is to explore the scientific opportunities of wide-field optical/IR imaging from space, and the advantages of going to space are obvious:

    • Absence of atmospheric seeing distortions.

    • Broad-band spectral coverage with no gaps.

    • Very faint intrinsic sky background.

    • Absence of weather-related photometric errors.

    • Continuous viewing availability of most of the sky at all times.

    • Availability of a stable environment, both mechnanically and thermally.

    • Lack of degradation of optical surfaces.

      + many more issues too numerous to list!

But Ground-Based Astronomy Still has a role to play!

  • A foremost consideration is cost, of course. Very powerful facilities on the ground still cost less than “small explorers” in the NASA program.

  • There are some other obvious advantages as well:

    • Significantly reduced reliability requirements. Some instruments (not all) can indeed be built fast and cheap.

    • Refurbishability and technology evolution are easily accommodated.

    • Fewer “envelope constraints” - large monolithic apertures and structures are possible.

    • Widely separated telescope designs (e.g. interferometers) do not require station keeping.

    • Fewer limitations on data rates.

    • Potential for multiple funding models - private/public; NSF/DOE versus NASA.

Of course, this field has a long history…

For upcoming surveys, the key consideration is the étendue!

  • The solid angle surveyed per unit time, to some limiting flux, F, at a signal-to-noise ratio, SNR, in exposures of time, t, is given by:

    Here AWis the étendue of the telescope, e is the efficiency of the system, Fskyis the sky intensity, dW is the size of the seeing-limited PSF, and adet is proportional to detector noise and trap depth.

  • Large surveys require high étendue. But with conventional optical designs - high étendue requires large detector area.

  • We are seeing tremendous progress in the field now, primarily because of the relatively recent development of large format CCD arrays!

Some Recent Optical and Near IR Surveys


Near IR






Some Recent Optical and Near IR Surveys


Near IR









Some Recent Optical and Near IR Surveys


Near IR















Some Recent Optical and Near IR Surveys


Near IR






Constant A W t










The CFHT Legacy Survey

  • One of the most powerful surveys currently on-line is the Canada-France-Hawaii Telescope Legacy Survey.

  • CFHT is a 3.6 m equatorial mount telescope on Mauna Kea, operational since 1979.

  • The Legacy Survey is being conducted with the wide-field imager MegaPrime, installed at the prime focus.

  • MegaPrime is equipped with MegaCam, a 36 CCD 1o x 1o fov camera.

The CFHT Legacy Survey

  • Canada and France have made ~ 50% of their dark and gray time on the CFHT available for the Legacy Survey. More that 450 nights over 5 years will be utilized. The survey began in mid-2003.

  • The CFHTLS is actually comprised of three separate surveys:

    • Very Wide: Most of the ecliptic plane inside of +/- 2o, for a total of 1300 square degrees in three colors (g’, r’, and i’). Very good for solar system and galactic structure science.

    • Wide: Will cover 170 square degrees in three patches using the whole filter set (u*, g’, r’, I’, and z’). Will reach down to i’ = 24.5. This is aimed at weak lensing and large-scale structure.

    • Deep: Covers 4 square degrees in four independent fields using the whole filter set, with integration times of 33 to 132 hours. Will reach r’ = 28. Will constrain galaxy evolution and global star formation history.

The CFHT Legacy Survey

The CFHT Legacy Survey

  • MegaPrime is an optical/near IR instrument mounted at the prime focus for 15 to 18 day periods.

  • It consists of the prime focus upper end, a wide field corrector, an image stabilizing unit, the filter assembly, and a focus stage.

  • MegaCam comprises 40 2048 x 4612 CCDs, covering 1 x 1 degree with a plate scale of 0.187”/pixel.

  • The sensors are Marconi (now E2V) CCD42-90 back-illuminated devices, 3-side buttable, optimized in the blue.

The Very Large Telescope Survey Telescope (VST)

  • The VST is a 2.6 m alt-az telescope specially designed for high quality wide-field imaging. It is being constructed next to the VLT on Cerro Paranal, and is designed to allow selection of targets for followup with the VLT.

  • The VST is designed for Cassegrain operations, with a 1.5o diameter corrected fov, matched by a 16k x 16k CCD mosaic camera covering 1 x 1 square degree.

  • The telescope and camera are currently being assembled and should be ready for operations in mid-2004.


  • The telescope is a modified Ritchey Chretien f/5.5 2.6 m. The primary is actively controlled with 80 axial active pads and astatic levers, and 32 lateral astatic levers.

  • There are two different Cass configurations:

    • A two lens corrector for the U-I bands at near zenith.

    • A one lens corrector with an Atmospheric Dispersion Corrector for the B-I band, working up to 70 degree zenith angle.

  • Main science goals include:

    • Wide field extragalactic surveys

    • Narrow band imaging surveys

    • Multicolor surveys for stellar populations in the local group

    • Kuiper Belt Objects

    • Galaxy-galaxy lensing

    • Faint galactic haloes

    • Weak lensing as a probe of large scale structure

The VST Detector - OmegaCAM

  • OmegaCAM consists of 32 thinned, low-noise, 3-edge buttable 2k x 4k E2V 44-82 devices with a total geometric area of 26 x 26 cm2.

  • The camera covers a 1 x 1 degree field at 0.21”/pixel.

  • The filter set will be Sloan u’, g’, r’, i’, and z’, and Johnson B and V + possible Ha narrow band.

  • Detector QE is optimized for the blue as in MegaCam.

The Dark Energy Camera

  • The Dark Energy Camera (DEC) is a proposal to build an integrated wide-field imaging camera and wide-field corrector for the prime focus of the Blanco 4-m telescope on Cerro Tololo.

  • The proposed instrument will have an effective fov of 3 square degrees with roughly 500 Mpixels.

The DEC/Dark Energy Survey

  • If selected for implementation, the DEC will be used to carry out the Dark Energy Survey, which will be provided one-third of the observing time on the Blanco over a 5-year period. The survey would be carried out in four optical passbands: g, r, i, and z. It will get to a depth ~ 24th magnitude in r.

  • The main science goal is to constrain dark energy using four complementary measurements:

    • Abundance and clustering evolution of clusters of galaxies

    • Weak gravitational lensing on large scales

    • Evolution of the spatial distribution of galaxies

    • Luminosity distances to Type 1a SNe.

  • The survey will be especially targeted in the Southern galactic cap, an area of sky which will also be surveyed by the South Pole Telescope (SPT). SPT will acquire a census of clusters of galaxies using the Sunyaev-Zeldovitch effect, and the DEC can provide photometric redshifts of all detected clusters out to z ~ 1.

  • A complementary SN survey will revisit 40 square degrees of sky every third night. ~ 1900 type 1a’s are expected out to z ~ 0.8.

The Dark Energy Survey

The Dark Energy Survey

  • The total 5000 square degrees will be divided into several pieces:

    • 4000 squ degs in the southern galactic cap.

    • 700 squ degs in a photometric redshift area well-matched to the suite of followup telescopes in Chile

    • 200 squ degs in an equatorial stripe. This will cover existing survey areas from the SDSS and the VLT, thereby allowing a transfer of photometric standards.

The DEC Sensor Array

  • The DEC team plans to use the LBNL fully depleted 250 micron thick CCDs.

  • These have excellent QE, all the way out to 1000 nm.

  • The reference design includes 60 2k x 4k devices in a close-packed array. The diameter of the camera is ~ 44 cm.

The Panoramic Survey Telescope and Rapid Response System - Pan STARRS

  • Pan STARRS is a novel array of 4 identical small, wide-field telescopes, copointed to achieve very high étendue ~ 54 m2 deg2.

  • It will survey 6000 squ degs per night to R = 24.

  • It will also provide repeated all-sky coverage, with a cadence tailored to the optimal detection of moving and transient objects.

  • The present plan is to install one of the four telescopes (PS-1) first and install it on Haleakala, Maui. It will provide a technology testbed for the project, and make a full-sky static survey for later use.

Pan STARRS Science

  • The primary science goal of Pan STARRS is to discover and characterize Earth-approaching objects, both asteroids and comets.

  • However, given the high étendue, many other science topics will be enabled:

    • Study of both near Earth objects and outer solar system objects.

    • Proper motions for a large number of stars in thee galaxy.

    • Weak lensing.

    • Large scale structure.

    • Formation and evolution of AGNs

    • Supernovae

    • GRB afterglows

    • Planet searches via transit across stellar photospheres

Pan STARRS Design

  • The basic Pan STARRS design incorporates 4 1.8 m Richey-Chretien telescopes. Light from the primary bounces off a concave secondary to a Cassegrain focus. There are 3 refractive corrector lenses before the camera. The design is f/4 with 38.5 microns/arcsec at the focal plane.

  • The fov is 3 degrees. Each focal plane is tiled with an 8 x 8 array of 4k x 4k Orthogonal Transfer Array CCDs, each of consists of an 8 x 8 array of individual 512 x 512 OT devices.

  • The OTCCD can electronically shift charge in arbitrary directions to compensate for image motion.

The Large Survey Synoptic Telescope (LSST)

  • The LSST will be a large, wide-field ground-based telescope designed to provide time-lapse digital imaging of faint astronomical objects across the entire visible sky every few nights.

  • LSST will enable a wide variety of complementary scientific investigations, utilizing a common database. These range from searches for small bodies in the solar system to precision astrometry of the outer regions of the galaxy to systematic monitoring for transient phenomena in the optical sky.

Concept Heritage

  • The LSST concept has been identified as a national scientific priority by diverse national panels, including three separate NAS committees!

    • “The Committee supports the Large Synoptic Survey Telescope project, which has significant promise for shedding light on the dark energy.” Connecting Quarks with the Cosmos.

    • “The SSE [Solar System Exploration] Survey recommends [the construction of] a survey facility, such as the Large-Aperture Synoptic Survey Telescope (LSST)… to determine the contents and nature of the Kuiper Belt to provide scientific context for the targeting of spacecraft missions to explore this new region of the solar system…” New Frontiers in the Solar System.

    • “The Large-aperture Synoptic Survey Telescope (LSST) will catalog 90% of the near-Earth objects larger than 300-m and assess the threat they pose to life on Earth. It will find some 10,000 primitive objects in the Kuiper Belt, which contains a fossil record of the formation of the solar system. It will also contribute to the study of the structure of the universe by observing thousands of supernovae, both nearby and at large redshift, and by measuring the distribution of dark matter through gravitational lensing.” Astronomy and Astrophysics in the New Millennium.

LSST Survey

5-band Survey: 400 – 1000 nm

Sky area covered:18,000 deg2

Limiting magnitude:26.5 AB mag

Source density:60 galaxies/sq.arcmin

3 billion source galaxies with color redshifts

The Essence of LSST is Deep, Wide, Fast!

  • Dark matter/dark energy via weak lensing

  • Dark matter/dark energy via supernovae

  • Galactic Structure encompassing local group

  • Dense astrometry over 30,000 sq.deg: rare moving objects

  • Gamma Ray Bursts and transients to high redshift

  • Gravitational micro-lensing

  • Strong galaxy & cluster lensing: physics of dark matter

  • Multi-image lensed SN time delays: separate test of cosmology

  • Variable stars/galaxies: black hole accretion

  • QSO time delays vs z: independent test of dark energy

  • Optical bursters to 25 mag: the unknown

  • 5-band 27 mag photometric survey: unprecedented volume

  • Solar System Probes: Earth-crossing asteroids, Comets, TNOs

LSST Project timeline

Dev. Engineering: 2003-06

Construction: 2006-10

First light: 2011


LSST Optical Design

  • The optical design is based on a concept by Angel et al. (2000), which modifies the Paul-Baker 3-mirror telescope to work at large apertures.

  • Seppala (2002) further developed this approach, simplifying the aspheric surfaces and achieving a flat focal plane.

  • There are three aspheric mirrors feeding three refractive elements in the camera. These yield a 3 degree circular field of view, covering a 55-cm focal plane array.

LSST Optical Design

Crisp Images Over Entire Field

L. Seppala, LLNL

LSST Telescope Mount

Two Possible Configurations

LSST Camera

Camera Components

  • Focal plane array

    • 10 mm pixels  0.2 arcsecond/pixel (~1/3 seeing-limited PSF)

    • 60 cm diameter  8.6 square degree FOV

       2.8 Gpixels

    • Integrated front-end electronics

    • 16 bits/pixel, 2 sec readout time  2.8 GB/sec

       Parallel readout

  • Housings (environmental control)

  • Filters

  • Optics

  • Mechanisms

    • L2 position varies with wavelength (filter)

    • Filter insertion

    • Mechanical shutter

Camera Challenges

  • Detector requirements:

    • 10 mm pixel size

    • Pixel full-well > 90,000 e–

    • Low noise (< 5 e– rms), fast (< 2 sec) readout ( < –30 C)

    • High QE 400 – 1000 nm

    • All of above exist, but not simultaneously in one detector

  • Focal plane position precision of order 3 mm

  • Package large number of detectors, with integrated readout electronics, with high fill factor and serviceable design

  • Large diameter filter coatings

  • Constrained volume (camera in beam)

    • Makes shutter, filter exchange mechanisms challenging

  • Constrained power dissipation to ambient

    • To limit thermal gradients in optical beam

    • Requires conductive cooling with low vibration

LSST optical array

2.8 Gigapixels

8.6 square degrees

LSST optical array

2.8 Gigapixels

8.6 square degrees

To same angular scale

LSST optical array

2.8 Gigapixels

8.6 square degrees

SNAP optical array

0.44 Gigapixels

0.34 sq.deg total

To same angular scale

LSST Data Rates

  • 2.8 billion pixels read out in less than 2 sec, every 12 sec

  • 1 pixel = 2 Bytes (raw)

  • Over 3 GBytes/sec peak raw data from camera

  • Real-time processing and transient detection: < 10 sec

  • Dynamic range: 4 Bytes / pixel

  • > 0.6 GB/sec average in pipeline

  • 5000 floating point operations per pixel

  • 2 TFlop/s average, 9 TFlop/s peak

  • ~ 20-30 Tbytes/night

Figures of Merit for Survey Science

Relative Survey Power


  • Wide-field imaging from the ground will soon be approaching a golden age, with a number of very powerful facilities operating around the globe.

  • The science that will be addressed is very broad - ranging from potentially hazardous near-Earth asteroids to the fundamental physics of dark energy.

  • The ground-based facilities offer complementary capabilities to wide-field experiments which, hopefully, will also be fielded in space. We need both to keep this field vibrant.

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