The Modulation Transfer Function (MTF) of an imaging device is a strong indicator of the resolution limited performance. The MTF at the system level is commonly treated as separable, with the optical MTF multiplying the postoptic (detector) MTF to give the system MTF. As new detector materials and methods have become available, and as the manufacturing of detectors has been separated from the optical system, independently measuring the MTF of the detector is of great interest. In this correspondence, a procedure for measuring the post-optic MTF of a mid-wave (3-5 micron) sampled imager is described. This is accomplished through a careful measurement of a reference optic that is later installed to allow for a final system MTF measurement. The key finding is that matching the chromatic shape of the illumination between the optic and system MTFs is critical, as in both measurements the effective MTF is scaled by the source and detector spectral shapes. This is most easily accomplished through the use of narrow bandpass filters. Our results are consistent across bandpass filter cut-on and F/number.
The increased desire for multispectral infrared optical systems in compact packages significantly
complicates the optical design of such systems. Add in the fact that multiple spectral bands are now
imaging on the same detector, and the optical designs become quite challenging. The availability of
materials over the desired multiple spectral bands that transmit well with the desired dispersion properties
further complicates the design. Designing optics for good performance in the SWIR (1.0 - 2.0μm) and the
MWIR (3.5 - 5.0μm) bands is an example where this challenge can be significant. Many of the preferred
optical materials in the MWIR start to cut off prior to reaching 1μm, or have dispersions that are very
difficult to control over this broad of a spectral band. Reflective designs are often limited because of
packaging limitations. In this paper, multiple approaches are designed and examined to find the best
balance between risk, performance, and size. The analyzed design studies include the use of traditional
MWIR materials, harmonic diffractive lenses, and alternative materials that will require further
development to be used in a tactical environment.
Laser beams for guide star generation are a potential hazard for aircraft. At the MMT telescope located on Mt. Hopkins
in Southern Arizona, a constellation of five Rayleigh guide stars is created with a total of 25 W of projected power at
532 nm wavelength. We report operational results from an automatic system deployed at the MMT that is designed to
detect aircraft and shut down the lasers if a collision with the beams appears likely. The system, building on a previous
prototype, uses a wide-angle CCD camera mounted with a minimally unobstructed view to the optical support structure
at the top of the telescope. A computer program reads the camera once every two seconds and calculates the difference
between adjacent image pairs. The anti-collision beacons required on all aircraft by the Federal Aviation Administration
appear as streaks in the field. If an airplane is detected, it is located in the field relative to the laser beam and its path is
projected. If aircraft are detected near or appear that they will approach the beam, the laser's safety shutter is closed and
warning messages are sent to the laser operator. Failsafe operation is assured by a "heart beat" signal continuously sent
from the detection system to the laser controller, and by the fact that the safety shutter must be energized to open. In the
event of a power failure, the system must be manually reset by the Laser Safety Officer before the laser beam can again
be propagated.
Over the past several years, experiments in adaptive optics involving multiple natural and Rayleigh laser guide stars
have been carried out by our group at the 1.5 m Kuiper telescope and the 6.5 m MMT telescope. From open-loop data
we have calculated the performance gains anticipated from ground-layer adaptive optics (GLAO) and laser tomography
adaptive optics corrections. In July 2007, the GLAO control loop was closed around the focus signal from all five laser
guide stars at the MMT, leading to a reduction in the measured focus mode on the laser wavefront sensor by 60%. For
the first time, we expect to close the full high order GLAO control loop around the five laser beacons and a tilt star at the
MMT in October 2007, where we predict image quality of < 0.2 arc seconds FWHM in K band (λ = 2.2 μm) over a 2 arc
minute field. We intend to explore the image quality, stability and sensitivity of GLAO correction as a function of
waveband with the science instrument PISCES. PISCES is a 1-2.5 µm imager with a field of view of 110 arc seconds, at
a scale of 0.11 arc seconds per pixel. This is well matched to the expected FWHM performance of the GLAO corrected
field and will be able to examine PSF non-uniformity and temporal stability across a wide field. FGD.
The MMT's five Rayleigh laser guide star system has successfully demonstrated open loop wavefront sensing for both
ground-layer and laser tomography adaptive optics (AO). Closed loop correction is expected for the first time in the
autumn of 2006. The program is moving into its second phase: construction of a permanent facility to feed AO
instruments now used with the telescope's existing natural star AO system. The new facility will preserve the thermal
cleanliness afforded by the system's adaptive secondary mirror. With the present laser power of 4 W in each of the
Rayleigh beacons, we will first offer ground-layer correction over a 2 arcmin field in J, H, and K bands, with expected
image quality routinely 0.2 arcsec or better. Later, we will also offer imaging and spectroscopy from 1.5 to 4.8 μm with
a tomographically corrected diffraction limited beam. The development of these techniques will lead to a facility all-sky
capability at the MMT for both ground-layer and diffraction-limited imaging, and will be a critical advance in the tools
necessary for extremely large telescopes of the future, particularly the Giant Magellan Telescope. We describe the
present state of system development, planned progress to completion, and highlight the early scientific applications.
Experiments have been carried out at the MMT telescope in June 2005 and again in April 2006 to validate open loop tomographic wavefront reconstruction using five dynamically refocused Rayleigh laser beacons (RLGS) and multiple tilt natural guide stars (NGS). Wavefront sensing in this manner is recognized as a critical precursor to the development of adaptive optics for Extremely Large Telescopes. At the MMT, wavefronts from the laser beacons are recorded by five 60-element Shack-Hartmann sensors implemented on a single CCD. A wide-field camera measures image motion from multiple field stars to calculate global tilt and distinguish effects of contributions to second order aberrations from low and high altitude turbulence. Together, the signals from these sensors are used to estimate the first 45 Zernike modes in the wavefront of a star within the LGS constellation. The reconstruction is compared off line to simultaneous wavefront measurements made of the star with a separate Shack-Hartmann sensor. We will present the results in this paper and quantify the wavefront improvement expected from tomographic adaptive optics correction.
Simultaneous wavefront measurements are planned at the 6.5 m MMT telescope of five dynamically refocused Rayleigh laser beacons (RLGS) and a bright natural star to demonstrate tomographic wavefront reconstruction. In this paper, we summarize preliminary data recorded from the five laser beacons during the first telescope run at the MMT in June 2004. Beam projection is from behind the secondary of the MMT to form a regular pentagon of beacons on the sky with a radius of 60 arcseconds around the natural star. Beacon images are recorded over a range gate from 20 to 30 km, with dynamic refocus optics in the focal plane to remove perspective elongation (Stalcup, et. al., these proceedings). Separate externally synchronized Shack-Hartmann sensors record wavefront measurements of the beacons and the star, which will yield the first 33 Zernike modes from each wavefront measurement. A linear tomographic reconstructor, implemented as a matrix multiplication of the combined Zernike modal amplitudes from all five RLGS, has been computed to estimate contributions to the atmospheric aberration in two layers at 0 and 6 km. To validate the tomographic approach, the wavefront of the natural star will be predicted by computing the sum of the aberration in the direction of the star, and the prediction compared to simultaneous measurements recorded from the star directly.
A new requirement for astronomical adaptive optics is the simultaneous measurement of wavefronts of multiple natural or laser guide stars. We have devised a new implementation of the Shack-Hartmann method to image multiple spot patterns on a single imaging array. An image of the telescope pupil is formed on a multifaceted prism with rings of subapertures. All beacons in the field are then imaged by a camera lens to form the same spot pattern repeated over the detector format. The facets are fly-cut in polycarbonate, tangent to a convex surface. In order to minimize scattering and aid manufacturing, the prism angles are exaggerated, and an index-matching fluid is used to reduce the refracted angles by a factor of 15. Results from lab and telescope tests are presented.
A demonstration of tomographic wavefront sensing has been designed, fabricated, and tested. The last of the initial testing of the dynamic refocus system at the 61" telescope on Mt. Bigelow, Arizona is presented, along with the first results from the system after its transfer to the 6.5 m MMT on Mt. Hopkins, Arizona. This system consists of a laser beam projector, and a wavefront sensor at the telescope's Cassegrain focus. The projector transmits 5 pulsed 532 nm beams in a regular pentagon of 2 arcminutes diameter from behind the telescope's secondary mirror that in good seeing can yield sub-arcsecond beacons over a 20-30 km altitude range. The wavefront sensor incorporates a dynamic refocus unit to track each returning laser pulse, and a multiple laser beacon Shack-Hartmann wavefront sensor using a novel substitute for the traditional lenslet array. A natural guide star wavefront sensor was also fielded to collect ground-truth data to compare with wavefronts reconstructed from the laser wavefront sensor measurements. All of the subsystems were shown to work, but bad weather ended the testing before the final data could be collected.
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