Patrick L. Thompson

Diffracted radiance: a fundamental quantity in nonparaxial scalar diffraction theory.

Most authors include a paraxial (small-angle) limitation in their discussion of diffracted wave fields. This paraxial limitation severely limits the conditions under which diffraction behavior is adequately described. A linear systems approach to modeling nonparaxial scalar diffraction theory is developed by normalization of the spatial variables by the wavelength of light and by recognition that the reciprocal variables in Fourier transform space are the direction cosines of the propagation vectors of the resulting angular spectrum of plane waves. It is then shown that wide-angle scalar diffraction phenomena are shift invariant with respect to changes in the incident angle only in direction cosine space. Furthermore, it is the diffracted radiance (not the intensity or the irradiance) that is shift invariant in direction cosine space. This realization greatly extends the range of parameters over which simple Fourier techniques can be used to make accurate calculations concerning wide-angle diffraction phenomena. Diffraction-grating behavior and surface-scattering effects are two diffraction phenomena that are not limited to the paraxial region and benefit greatly from this new development.

Grazing-incidence hyperboloid–hyperboloid designs for wide-field x-ray imaging applications

The classical Wolter type I grazing-incidence x-ray telescope consists of a paraboloidal primary mirror and a confocal hyperboloidal secondary mirror. This design exhibits stigmatic imaging on-axis but suffers from coma, astigmatism, field curvature, and higher-order aberrations such as oblique spherical aberration. Wolter-Schwarzschild designs have been developed that strictly satisfy the Abbe sine condition and thus exhibit no spherical aberration or coma. However, for wide-field applications such as the solar x-ray imager (SXI), there is little merit in a design with stigmatic imaging on-axis. Instead, one needs to optimize some area-weighted-average measure of resolution over the desired operational field of view. This has traditionally been accomplished by mere despacing of the focal plane of the classical Wolter type I telescope. Here we present and evaluate in detail a family of hyperboloid-hyperboloid grazing-incidence x-ray telescope designs whose wide-field performance is much improved over that of an optimally despaced Wolter type I and even somewhat improved over that of an optimally despaced Wolter-Schwarzschild design.

Systems engineering analysis of aplanatic Wolter type I x-ray telescopes

It is well known that normal-incidence aplanatic telescope de- signs perform better at small field angles than ones corrected only for spherical aberration. This is why most large astronomical telescopes fab- ricated in the past fifty years have been of the Ritchey-Chretien (aplanatic) design rather than of the classical Cassegrain design. For the relatively new field of x-ray astronomy, the Wolter type I grazing inci- dence design has been extensively utilized. It consists of a paraboloidal primary mirror coaxial with a confocal hyperboloidal secondary mirror. Aplanatic versions of the Wolter type I grazing incidence x-ray telescope have been discussed in detail in the literature, and are widely touted as being superior designs. However, scattering effects from residual optical fabrication errors and other practical engineering error sources prevent these grazing-incidence telescopes from being near diffraction-limited (even on axis) at the very short operational x-ray wavelengths. A sys- tems engineering analysis of these error sources indicates that they will dominate coma at the small field angles, and of course astigmatism, field curvature, and higher-order aberrations dominate coma at the large field angles. Hence, there is little improvement in performance when going to an aplanatic design. Comparison of performance predictions for the clas- sical versus aplanatic Wolter type I x-ray telescope are presented for the special case of the Solar X-Ray Imager (SXI) baseline design. SXI is expected to become a standard subsystem aboard the next generation of NOAA/GOES weather satellites.

ATLAST-9.2m: a large-aperture deployable space telescope

We present results of a study of a deployable version of the Advanced Technology Large-Aperture Space Telescope (ATLAST), designed to operate in a Sun-Earth L2 orbit. The primary mirror of the segmented 9.2-meter aperture has 36 hexagonal 1.315 m (flat-to-flat) glass mirrors. The architecture and folding of the telescope is similar to JWST, allowing it to fit into the 6.5 m fairing of a modest upgrade to the Delta-IV Heavy version of the Evolved Expendable Launch Vehicle (EELV). We discuss the overall observatory design, optical design, instruments, stray light, wavefront sensing and control, pointing and thermal control, and in-space servicing options.

Hyperboloid-hyperboloid grazing incidence x-ray telescope designs for wide-field imaging applications

The classical Wolter Type 1 X-ray telescope consists of two grazing incidence mirrors, a confocal paraboloid and hyperboloid. This design exhibits perfect geometric imaging on-axis (i.e., no spherical aberration) but suffers from severe field curvature, coma, astigmatism, and higher-order aberrations such as oblique spherical aberration. The Wolter-Schwarzschild design, consisting of two general aspheric grazing incidence surfaces, is corrected for both spherical aberration and coma, thus yielding very good geometrical performance at small field angles that becomes severely degraded at large field angles. The image quality criterion for stellar (small-field) X-ray telescopes is frequently expressed in terms of an on-axis fractional encircled energy, with the off-axis performance being dictated by the field-dependent aberrations characteristic of the design. A more appropriate image quality criterion for wide-angle applications is some area-weighted-average measure of resolution that maximizes the number of spatial resolution elements over a given operational field-of-view (OFOV). In practice, scattering effects from residual optical fabrication errors and detector effects (finite pixel size and charge spreading) dominate geometrical aberrations for small field angles whereas the geometrical aberrations dominate the image degradation at large field angles. Under these conditions, there is little merit in a telescope design corrected for coma (or even spherical aberration). Our new image quality criterion has led us to a whole new class of generalized Wolter Type I (hyperboloid- hyperboloid) designs that can be optimized for a given OFOV. A specific design and its predicted systems performance for the Solar X-ray Imager mission are described in detail.

Generalized Wolter type I design for the solar x-ray imager (SXI)

The off-axis aberrations of the classical Wolter Type 1 X- ray telescope consisting of a confocal paraboloid and hyperboloid severely limit the useful angular field that can be imaged onto a flat detector. A family of alternative designs based on two hyperboloids has been developed that improves the wide-field imaging performance by aberration balancing. A particular member of this family, designated as H-T number 17, has been selected for use in the SXI instrument. The development of the SXI optical prescription and its defining parameters are first presented, then detailed otpical performance predictions are compared to those of the SXI baseline design.

Visible Nulling Coronagraph Testbed Results

We report on our recent laboratory results with the NASA/Goddard Space Flight Center (GSFC) Visible Nulling Coronagraph (VNC) testbed. We have experimentally achieved focal plane contrasts of 1 x 108 and approaching 109 at inner working angles of 2 * wavelength/D and 4 * wavelength/D respectively where D is the aperture diameter. The result was obtained using a broadband source with a narrowband spectral filter of width 10 nm centered on 630 nm. To date this is the deepest nulling result with a visible nulling coronagraph yet obtained. Developed also is a Null Control Breadboard (NCB) to assess and quantify MEMS based segmented deformable mirror technology and develop and assess closed-loop null sensing and control algorithm performance from both the pupil and focal planes. We have demonstrated closed-loop control at 27 Hz in the laboratory environment. Efforts are underway to first bring the contrast to > 109 necessary for the direct detection and characterization of jovian (Jupiter-like) and then to > 1010 necessary for terrestrial (Earth-like) exosolar planets. Short term advancements are expected to both broaden the spectral passband from 10 nm to 100 nm and to increase both the long-term stability to > 2 hours and the extent of the null out to a ~ 10 * wavelength / D via the use of MEMS based segmented deformable mirror technology, a coherent fiber bundle, achromatic phase shifters, all in a vacuum chamber at the GSFC VNC facility. Additionally an extreme stability textbook sized compact VNC is under development.

Evaluation of the polarization properties of a Philips-type prism for the construction of imaging polarimeters

The design and construction of wide FOV imaging polarimeters for use in atmospheric remote sensing requires significant attention to the prevention of artificial polarization induced by the optical elements. Surface, coatings, and angles of incidence throughout the system must be carefully designed in order to minimize these artifacts because the remaining instrumental bias polarization is the main factor which drives the final polarimetric accuracy of the system. In this work, we present a detailed evaluation and analysis to explore the possibility of retrieving the initial polarization state of the light traveling through a generic system that has inherent instrumental polarization. Our case is a wide FOV lens and a splitter device. In particular, we chose as splitter device a Philips-type prism, because it is able to divide the signal in 3 independent channels that could be simultaneously analyze to retrieve the three first elements of the Stoke vector (in atmospheric applications the elliptical polarization can be neglected [1]). The Philips-type configuration is a versatile, compact and robust prism device that is typically used in three color camera systems. It has been used in some commercial polarimetric cameras which do not claim high accuracy polarization measurements [2]. With this work, we address the accuracy of our polarization inversion and measurements made with the Philips-type beam divider.

Performance of the engineering model x-ray mirror of the Solar X-ray Imager (SXI) for future GOES missions

We have measured the x-ray imaging performance of a grazing incidence telescope mirror, the HT #17, employing a hyperboloid-hyperboloid design. This design provides improved wide-field imaging compared to an optimally defocused Wolter Type I mirror. This improvement will be advantageous for future Geostationary Operational Environmental Satellite (GOES) missions that will provide full disk images of the sun with the Solar X-ray Imager (SXI). The x-ray measurements were made in the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center and the results are presented here.

Generalized Wolter Type I design for wide-field x-ray imaging applications

Surface scatter effects are a dominant image degradation mechanism for very short X-ray wavelengths, even for state- of-the-art optical surfaces. And the severe off-axis aberrations of the classical Wolter Type I grazing incidence X-ray telescope consisting of a confocal paraboloid and hyperboloid severely limit the useful angular field that can be imaged onto a flat detector. A new family of alternative designs based on two hyperboloids has been developed that improves the wide-field imaging performance by aberration balancing and a compete optical systems engineering analysis. A particular member of this family, designed as H- T number 17, has been selected for use in the Solar X-ray Imager (SXI) instrument to be integrated into the NOA GOES satellite. The development of the SXI optical prescription and its defining parameters are first presented, then detailed image quality predictions, as degraded by diffraction effects, geometrical aberrations, surface scatter effects, and all other residual errors in the mirror manufacturers error budget tree are presented. This new optimized design yields an 80 percent increase in the number of spatial resolution elements over the full solar disc when compared to the classical Wolter Type I design that was the SXI baseline design.