Comparison of optical observational capabilities for the coming decades: ground versus space Matt Mountain1, Roeland van der Marel1, Remi Soummer1, Anton Koekemoer1, Harry Ferguson1, Marc Postman1, Donald T. Gavel5, Olivier Guyon3, Douglas Simons2, Wesley A. Traub4 ABSTRACT Ground-based adaptive optics (AO) in the infrared has made exceptional ad- vances in approaching space-like image quality at higher collecting area. Optical- wavelength applications are now also growing in scope. We therefore provide here a comparison of the pros and cons of observational capabilities from the ground and from space at optical wavelengths. With an eye towards the future, we focus on the comparison of a ∼ 30m ground-based telescope with an 8–16m space-based telescope. The parameters relevant for such a comparison include collecting area, diffraction limit, accessible wavelength range, background emis- sion, atmospheric absorption and extinction, Strehl ratio, field of view, temporal and spatial PSF stability, and target accessibility in time and on the sky. We review the current state-of-the-art in AO, and summarize the expected future improvements in image quality, field of view, contrast, and low-wavelength cut- off. We compare the depth that can be reached for imaging and spectroscopy from the ground and from space in the V and J bands. We discuss the exciting advances in extreme AO for exoplanet studies and explore what the theoretical limitations in achievable contrast might be. Our analysis shows that extreme AO techniques face both fundamental and technological hurdles to reach the contrast of 10−10 necessary to study an Earth-twin at 10 pc. Based on our assessment of the current state-of-the-art, the future technology developments, and the in- herent difficulty of observing through a turbulent atmosphere, we conclude that there will continue to be a strong complementarity between observations from the ground and from space at optical wavelengths in the coming decades. There will continue to be subjects that can only be studied from space, including imag- ing and (medium-resolution) spectroscopy at the deepest magnitudes, and the exceptional-contrast observations needed to characterize terrestrial exoplanets and search for biomarkers. additional contact information primary author: Matt Mountain, STScI, [email protected], 410-338-4710 1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 2Gemini Observatory, 670 N. A’ohoku Pl., Hilo, HI 96720 3Subaru Telescope, 650 N. A’ohoku Pl., Hilo, HI 96720 4JPL, M/S 301-451, 4800 Oak Grove Drive, Pasadena, CA 91109 5UCO/Lick Observatory, UCSC, 1156 High Street, CfAO Building Santa Cruz, CA 95064 –1– 1. Introduction The future of optical observational astronomy looks bright, with excellent prospects for the advent of larger aperture telescopes both on the ground and in space. As the telescope size increases, so does the cost and complexity of a project. The 2010 Decadal Survey will therefore face difficult choices for ground- and space-based astronomy investments in the coming decade. To facilitate these choices, it is important to understand the relative advan- tages of ground- and space-based facilities for observational astronomy. In particular, the ever increasing capabilities of adaptive optics (AO) from the ground motivate a critical side- by-side comparison of the observational parameter space accessible to either type of facility. We focus here mostly on comparison of the capabilities of a ∼ 30m ground-based tele- scope with an 8–16m space-based telescope. Examples of the former are the Thirty Meter Telescope (TMT; Stone et al. 2009) and the Giant Magellan Telescope (GMT; McCarthy et al. 2009). Examples of the latter are the Advanced-Technology Large-Aperture Space Telescope (ATLAST; Postman et al. 2009) and the Terrestrial Planet Finder Coronagraph (TPF-C; Levine et al. 2009). We compare only briefly the capabilities of current ground- based 8–10m class telescopes to smaller space-based telescopes such as the 2.4m Hubble Space Telescope (HST), the 6.5m James Webb Space Telescope (JDEM), or the proposed Joint Dark Energy Mission (JDEM/IDECS; Gehrels et al. 2009). We restrict attention to the regime below ∼ 1µm, for two reasons. First, this is where future large space-based proposals are likely to focus their attention; JWST will already provide an important new large aperture facility for science longward of ∼ 1µm. Second, below ∼ 1µm is where the future capabilities of AO from the ground are most uncertain and least documented. AO corrections in the infrared (IR) are typically more capable and less technically challenging than in the optical (see Section 3). However, in comparing performance of space versus ground in the IR, careful consideration must be given to the role of background emission in achieving a given signal-to-noise ratio S/N (see Section 4). We will not discuss here the relative cost for new optical observing facilities on the ground or in space. Observational facilities in space are generally more expensive to build and operate than those on the ground. So they are usually pursued only if they open up parts of parameter space that are not accessible from the ground. We highlight which parts of observational parameter space are uniquely accessible only from space. Scientific motivations for access to this parameter space are briefly mentioned where relevant. Costs and science drivers are discussed in detail in other submissions to the Decadal Survey. 2. Characteristics of Observational Parameter Space 2.1. Telescope Size One of the main advantages for ground-based facilities is that they can generally be constructed with larger telescope diameters D than what is possible in space, which impacts two important characteristics: the collecting area and the diffraction limit. 2.1.1. Collecting Area The collecting area of a telescope scales as D2. This implies that large-aperture ground-based telescopes are generally able to collect more photons than space-based telescopes, which is one of the factors that determines the achievable depth and signal-to-noise ratio (S/N; see Section 4). 2.1.2. Diffraction Limit The diffraction limit of a telescope scales as λ/D. This implies that large-aperture ground-based telescopes generally have a smaller diffraction limit than –2– space-based telescopes. However, due to atmospheric turbulence it is more difficult for ground-based telescopes to achieve image quality near the diffraction limit than it is for space-based telescopes, especially at optical wavelengths (see Sections 2.3, 3 and 5). 2.2. Atmospheric Emission and Absorption One of the most fundamental advantages of space-based over ground-based observations is the fact that light can be observed unaffected by the atmosphere. This impacts several important characteristics of the observational parameter space. 2.2.1. Accessible Wavelength Range and Absorption Ultra-Violet observations are not possible at all from the ground, due to atmospheric absorption. This blocks an entire wavelength range in which many astrophysical problems can be uniquely studied. Even in the optical there are spectroscopic absorption features (possibly time-variable) associated with telluric bands. These need to be corrected using observations of standard stars, which themselves are only characterized to finite accuracy. 2.2.2. Background The atmosphere creates a background in ground-based observations that needs to be subtracted. Figure 1 compares the background at Mauna Kea and L2. The shot noise from the background affects the the achievable depth and S/N (see Section 4). In addition, background variations can lead to systematic errors due to imperfect subtraction. 2.2.3. Line Emission In addition to a continuum background, ground-based observations accumulate photons from atmospheric emission in well-defined bands with many narrow emission lines (see Figure 1). The presence of these emission lines increases towards the IR, and their strengths vary with time. This limits the ability to study spectral features in astronomical objects at particular wavelengths. 2.2.4. Atmospheric Extinction Atmospheric extinction must be corrected on the basis of airmass estimates and standard star observations, to obtain absolute photometry. The accuracy with which these corrections can be done are critical for certain areas of science. 2.3. Image Quality The image quality depends both on the diffraction limit of the telescope (Section 2.1.2) and the ability of the telescope to reach this diffraction limit. 2.3.1. Strehl Ratio (SR) The SR is the ratio of the peak flux in the normalized point- spread function (PSF) to that for a diffraction limited system. It (and other measures of image quality) describe the extent to which the light is concentrated in the PSF core as opposed to the wings. A space-based telescope is typically designed to be diffraction limited (commonly defined as SR > 80%), at some target wavelength driven by the science. By contrast, large ground-based telescopes need an AO system that aims to optimize a combination of the complimentary goals of high SR and large field of view (see Sections 3 and 5). Even when an AO system achieves a diffraction-limed core, a low SR can severely limit the science. For example, in a crowded field the PSF wings of bright stars create an elevated background that drowns out the light of fainter stars. 2.3.2. Field of View (FOV) The FOV sizes for space- and ground-based telescopes are both limited by technological, design, and cost constraints related to their optics and detectors. However, for ground-based observations the FOV is further limited by the area –3– over which AO correction is possible (see Section 3). This limits the ability to do certain kinds of science, e.g, wide area surveys at high-resolution. 2.3.3. Stability Another advantage of space observations is the long-term stability of the environment, with no gravity, no seismic disturbances, no weather, and generally smaller thermal fluctuations.