Q1: Why deploy N wavefront sensors on a three mirror anastigmat (TMA) and not N + 1?
Q2: Why measure M Zernike coefficients and not M + 1?
Q3: Why control L rigid body degrees of freedom (total) on the secondary and tertiary and not L + 1?
The usual answer: “We did a lot of ray tracing and N, M, and L seemed OK.”
We show how straightforward results from aberration theory may be used to address these questions. We consider, in particular, the case of a three mirror anastigmat.
The Magellan Baade and Clay telescopes regularly produce images of ~0.5" in natural seeing. We review efforts to
improve collimation, active optics response, and telescope guiding and pointing to optimize the performance of the
telescopes. Procedures have been developed to monitor and analyze image quality delivered by the imaging science
instruments. Improved models have been developed to correct for flexure of the telescope and primary mirror under
gravity loading. Collimation has been improved using a "two-probe" Shack-Hartman technique to measure field
aberrations. Field acquisition performance has been improved by implementing an open loop model for the primary
mirror control. Telescope pointing has been improved by regular monitoring and adjustments to improve acquisition
We describe the construction and commissioning of FIRE, a new 0.8-2.5μm echelle spectrometer for the Magellan/
Baade 6.5 meter telescope. FIRE delivers continuous spectra over its full bandpass with nominal spectral
resolution R = 6000. Additionally it offers a longslit mode dispersed by the prisms alone, covering the full z to
K bands at R ~ 350. FIRE was installed at Magellan in March 2010 and is now performing shared-risk science
observations. It is delivering sharp image quality and its throughput is sufficient to allow early observations of
high redshift quasars and faint brown dwarfs. This paper outlines several of the new or unique design choices
we employed in FIRE's construction, as well as early returns from its on-sky performance.
FIRE (the Folded-port InfraRed Echellette) is a prism cross-dispersed infrared spectrometer, designed to deliver singleobject
R=6000 spectra over the 0.8-2.5 micron range, simultaneously. It will be installed at one of the auxiliary
Nasmyth foci of the Magellan 6.5-meter telescopes. FIRE employs a network of ZnSe and Infrasil prisms, coupled with
an R1 reflection grating, to image 21 diffraction orders onto a 2048 × 2048, HAWAII-2RG focal plane array.
Optionally, a user-controlled turret may be rotated to replace the reflection grating with a mirror, resulting in a singleorder,
longslit spectrum with R ~ 1000. A separate, cold infrared sensor will be used for object acquisition and guiding.
Both detectors will be controlled by cryogenically mounted SIDECAR ASICs. The availability of low-noise detectors
motivates our choice of spectral resolution, which was expressly optimized for Magellan by balancing the scientific
demand for increased R with practical limits on exposure times (taking into account statistics on seeing conditions).
This contribution describes that analysis, as well as FIRE's optical and opto-mechanical design, and the design and
implementation of cryogenic mechanisms. Finally, we will discuss our data-flow model, and outline strategies we are
putting in place to facilitate data reduction and analysis.
It has recently been suggested that up to half of the wavefront variance can be removed from the total atmospheric distortion by correcting only the lowest seeing layer (Rigaut 2000, 2001). This Ground-Layer AO (GLAO) correction could provide improved image quality over a very wide field of view; however, no development work has been done on existing telescopes. The implications are profound for optical designs of future AO optimized telescopes (e.g. the ELTs) as accurately compensating for this ground-layer strongly favors an adaptive element conjugated to the median height of the ground-layer. The gains of GLAO are tantalizing but substantially unproven, and thus, the Giant Magellan Telescope (GMT) project has developed a multi-phased study with the goal of providing an on-sky demonstration of GLAO technology at the Magellan Telescopes.
The first phase of this experiment is to measure the the height and
boundary of the ground-layer through multiple, fixed wavefront sensors
on very bright cluster fields over the full 24 arcminute Magellan
field of view. With a typical wind speed of 9 m/s and a presumed secondary ground-layer conjugation error of 100 m, the equivalent decoherence time is approximately 0.04 seconds. Therefore, we have designed and constructed high resolution Shack-Hartmann sensors running at 100 frames per second with coarse, 0.6m sub-apertures.
We present a technical description of the wavefront sensors and image
analyzer, as well as current results from the first deployment of
this instrument at Magellan. In addition, we discuss the implications
for ground-layer modeling and describe the next phases of the GMT's
The Magellan active optics system has been operating continuously on the Baade 6.5-m since the start of science operations in February 2001. The active optical elements include the primary mirror, with 104 actuators, and the secondary mirror, with 5 positional degrees of freedom. Shack-Hartmann (SH) wavefront sensors are an integral part of the dual probe guiders. The probes function interchangeably, with either probe capable of guiding or wavefront sensing. In the course of most routine observing stars brighter than 17th magnitude are used to apply corrections once or twice per minute. The rms radius determined from roughly 250 SH spots typically ranges between 0.05" and 0.10". The spot pattern is analyzed in terms of a mixture of 3 Zernike polynomials (used to correct the secondary focus and decollimation) and 12 bending modes of the primary mirror (used to compensate for residual thermal and gravitational distortions). Zernike focus and the lowest order circularly symmetric bending mode, known affectionately as the "conemode," are sufficiently non-degenerate that they can be solved for and corrected separately.