In this paper we present optical designs for an entirely new approach to extremely large
telescopes and telescopes with spherical primary mirrors. The key feature is a novel
optical system referred to as the improved spherical aberration corrector (ISAC). This
corrector works exceptionally well for post prime focus applications, as well as for
Cassegrain and Couder/Schwarzschild-like optical systems with both spherical primary
and spherical secondary mirrors. The Couder/Schwarzschild configuration also adapts to
a pancake configuration where the telescope is physically shorter than its aperture.
We describe a 1p8m f/6 Cassegrain optical system that creates a 1.42° FOV with near diffraction
limited images from 400nm to 1100nm with full-field distortion less than 0.01%. The
astronomical application for this optical system is the CCD/Transit Instrument with Innovative
Instrumentation (CTI-II), designed to produce a highly precise photometric and astrometric survey
of a complete strip of sky in the northern hemisphere. We describe the scientific observation
program and supporting optical design for the telescope. The all-spherical, five lens field
corrector represents a very capable optical system that works well with many other astronomical
telescopes such as SDSS, Pan-STARRS, SkyMapper, ESO's VST, the WIYN ODI, and the MMT
WFC. In many cases, using a five lens corrector exceeded the optical performance of the original
published system designs. Conversely, these and other optical concepts compromised the
performance of the CTI-II design. The CTI-II design is similar to many other wide-field telescope
and imaging camera designs, thus the design is of potential general use in astronomy.
Earth's atmosphere represents a turbulent, turbid refractive element for every ground-based telescope. We describe the
significantly enhanced and optimized operation of observatories supported by the combination of a lidar and
spectrophotometer that allows accurate, provable measurement of and correction for direction-, wavelength- and timedependent
astronomical extinction. The data provided by this instrument suite enables atmospheric extinction correction
leading to "sub-1%" imaging photometric precision, and attaining the fundamental photon noise limit. In addition, this
facility-class instrument suite provides quantitative atmospheric data over the dome of the sky that allows robust realtime
decision-making about the photometric quality of a night, enabling more efficient queue-based, service, and
observer-determined telescope utilization. With operational certainty, marginal photometric time can be redirected to
other programs, allowing useful data to be acquired. Significantly enhanced utility and efficiency in the operation of
telescopes result in improved benefit-to-cost for ground-based observatories.
We propose that this level of decision-making will make large-area imaging photometric surveys, such as Pan-STARRS
and the future LSST both more effective in terms of photometry and in the use of telescopes generally. The atmospheric
data will indicate when angular or temporal changes in atmospheric transmission could have significant effect across the
rather wide fields-of-view of these telescopes.
We further propose that implementation of this type of instrument suite for direct measurement of Earth's atmosphere
will enable observing programs complementary to those currently requiring space-based observations to achieve the
required measurement precision, such as ground-based versions of the Kepler Survey or the Joint Dark Energy Mission.
Ground-based telescopes supported by lidar and spectrophotometric auxiliary instrumentation can attain space-based
precision for all-sky photometry, with uncertainties dominated by fundamental photon counting statistics. Earth's
atmosphere is a wavelength-, directionally- and time-dependent turbid refractive element for every ground-based
telescope, and is the primary factor limiting photometric measurement precision. To correct accurately for the
transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the direction
and at the time that the supported photometric telescope is acquiring its data. While considerable resources have been
devoted to correcting the effects of the atmosphere on angular resolution, the effects on precision photometry have
largely been ignored.
We describe the facility-class lidar that observes the stable stratosphere, and a spectrophotometer that observes NIST
absolutely calibrated standard stars, the combination of which enables fundamentally statistically limited photometric
precision. This inexpensive and replicable instrument suite provides the lidar-determined monochromatic absolute
transmission of Earth's atmosphere at visible and near-infrared wavelengths to 0.25% per airmass and the wavelengthdependent
transparency to less than 1% uncertainty per minute. The atmospheric data are merged to create a metadata
stream that allows throughput corrections from data acquired at the time of the scientific observations to be applied to
broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean
atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere. We demonstrate
application of this instrument suite to stellar photometry, and discuss the enhanced value of routinely provably precise
photometry obtained with existing and future ground-based telescopes.
In the quest to design large and extremely large telescopes, one of the first questions encountered
is that of basic optical configuration and shape of the primary mirror. Spherical mirrors have a
number of advantages in production, testing and alignment but suffer from substantial spherical
aberration, thereby requiring some form of corrective optics. Many designs for spherical
aberration correctors are present in the literature, but each has its strengths and weaknesses. We
present the design for a new spherical aberration corrector which is believed to offer higher
performance with less complexity than previous approaches. The new design is substantially
more compact and uses slower optical surfaces than most axially symmetric designs. It can scale
to accommodate apertures as large as 100m, and adapts equally well to post prime focus and
Cassegrain-like focus applications.
Refractive elements are commonly used on Cassegrain-form telescopes to correct off-axis
aberrations and both widen and flatten the field. Early correctors used two lenses with spherical
surfaces, but their performance was somewhat limited. More recent correctors have three or four
lenses with some including at least one aspheric surface. These systems produce high resolution
images over relatively wide fields but often require the corrector and mirrors to be optimized
together. Here we present a new corrector design using five spherical lenses. This approach
produces high image quality with low distortion over wide fields and has sufficient degrees of
freedom to allow corrector to be optimized independent of the mirrors if necessary.
We are implementing the second-generation CCD/Transit Instrument (CTI-II), a unique 1.8-m imaging astrometric and photometric telescope. We discuss design aspects of CTI-II, including the optical system, structure, focal plane mosaic and the detector readout system that allows precise astrometric and photometric measurements. The scientific design drivers for the imaging telescope include discovery and measurement of motion and distance for late M, L and T stars, synoptic photometric monitoring of active galactic nuclei (AGN), and discovery and near real-time spectroscopic followup of distant supernovae and AGN outbursts. These projects drive the design of the wide field-of-view stationary telescope that employs the time-delay and integrate (TDI) readout mode for CCD detectors to produce a deep, multicolor image of the sky every clear night. Nightly observation of the same strip of the sky produces the time domain photometric and repeated astrometric measurements required by the science drivers. The telescope, its focal plane mosaic and the data system all incorporate unique and innovative elements that support an unbiased survey of the sky with intensive time-domain sampling. We review these aspects of the project, and describe steps taken to support the astrometric and photometric precision required by the scientific mission of the telescope.
The last decade has seen significant interest in wide field of view (FOV) telescopes for sky survey and space
surveillance applications. Prompted by this interest, a multitude of wide-field designs have emerged. While all designs
result from optimization of competing constraints, one of the more controversial design choices is whether such
telescopes require flat or curved focal planes. For imaging applications, curved focal planes are not an obvious choice.
Thirty years ago with mostly analytic design tools, the solution to wide-field image quality appeared to be curved focal
planes. Today however, with computer aided optimization, high image quality can be achieved over flat focal surfaces.
For most designs, the small gains in performance offered by curved focal planes are more than offset by the complexities
and cost of curved CCDs. Modern design techniques incorporating reflective and refractive correctors appear to make a
curved focal surface an unnecessary complication. Examination of seven current, wide FOV projects (SDSS, MMT,
DCT, LSST, PanStarrs, HyperSuprime and DARPA SST) suggests there is little to be gained from a curved focal plane.
The one exception might be the HyperSuprime instrument where performance goals are severely stressing refractive
prime-focus corrector capabilities.
We present a technique for the absolute measurement of very low-level scattering. The method is absolute in that it relies on fundamental physics and does not require calibration against standards maintained by the National Institute of Standards. In a bi-directional mode-locked ring laser, a difference in longitudinal mode frequency between the two senses of circulation of the intracavity pulses can be measured as a beat note between the corresponding outputs. A very small amount of light backscattered by a sample (located at the pulse crossing) from one sense of circulation into the other, causes the beat note to vanish. The threshold in mode frequency difference that causes locking is a measure of the scattering amplitude.