David G. Elliott

Traveling-wave induction launchers

An analysis of traveling-wave induction launchers shows that induction is a feasible method of producing armature current and that efficient accelerators can be built without sliding contacts or arcs. In a traveling-wave induction launcher the armature current is induced by a slip speed between the armature and a traveling magnetic field. At 9-m/s slip speed a 9-kg projectile with an aluminum armature weighing 25% of the total mass can be accelerated to 3000 m/s in a 5-m-long barrel with a total ohmic loss in the barrel coils and armature of 4% of the launch kinetic energy and with an average armature temperature rise of 220 degrees C, but a peak excitation frequency of 8600 Hz is required. With a 2-kg launch mass the ohmic loss is 7%. A launcher system optimized for rotating generators would have a peak frequency of 4850 Hz; with an aluminum armature weighing 33% of the launch mass and a slip speed of 30 m/s the total ohmic loss in the generators, cables, and accelerator would be 43% of the launch kinetic energy, and the average armature temperature rise would be 510 degrees C. >

Mesh-matrix analysis method for electromagnetic launchers

The mesh-matrix method is a procedure for calculating the current distribution in the conductors of electromagnetic launchers with coil or flat-plate geometry. Once the current distribution is known the launcher performance can be calculated. The method divides the conductors into parallel current paths, or meshes, and finds the current in each mesh by matrix inversion. The author presents procedures for writing equations for the current and voltage relations for a few meshes to serve as a pattern for writing the computer code. An available subroutine package provides routines for field and flux coefficients and equation solution. >

The state of the focus and image quality of the Spitzer Space Telescope as measured in orbit

We describe the process by which the NASA Spitzer Space Telescope (SST) Cryogenic Telescope Assembly (CTA) was brought into focus after arrival of the spacecraft in orbit. The ground rules of the mission did not allow us to make a conventional focus sweep. A strategy was developed to determine the focus position through a program of passive imaging during the observatory cool-down time period. A number of analytical diagnostic tools were developed to facilitate evaluation of the state of the CTA focus. Initially, these tools were used to establish the in-orbit focus position. These tools were then used to evaluate the effects of an initial small exploratory move that verified the health and calibration of the secondary mirror focus mechanism. A second large move of the secondary mirror was then commanded to bring the telescope into focus. We present images that show the CTA Point Spread Function (PSF) at different channel wavelengths and demonstrate that the telescope achieved diffraction limited performance at a wavelength of 5.5 μm, somewhat better than the level-one requirement.

Engineering design of the Wide-Field Infrared Explorer (WIRE)

The Wide-Field Infrared Explorer is a cryogenically-cooled infrared telescope designed to study the evolution of starburst galaxies. This survey mission, proposed as part of the NASA Small Explorer program, takes advantage of recent advances in infrared detector technology to detect distant galaxies in 12 and 25 micrometers wavelength bands. The WIRE instrument is designed to be integrated with a spacecraft bus provided by Goddard Space Flight Center and launched into a 500 km orbit on a Pegasus XL launch vehicle. Most of the mission will be split between a moderate depth survey requiring 14 minutes exposure time per field and a deep survey requiring 4-8 hours per field. The WIRE telescope has an aperture of 300 mm, focal length of 1105 mm and field of view of 31.6 arcmin. A dichroic beam splitter separates the beam into the two wavelength bands. The two sensors are 128 X 128 Si:As arrays with 75-micrometers pixels operating in the blocked impurity band (BIB) mode. The focal plane arrays are cooled by solid hydrogen to 7.5 K and the optics and baffles are cooled by solid hydrogen to below 19 K.

Traveling-wave synchronous coil gun

An outline is presented of the coilgun concept, excitation, switching, brush commutation, power supply, and performance. It is shown that a traveling-wave synchronous coilgun permits independent adjustment of the magnetic field and armature current for high velocity at low armature mass fraction. Magnetic field energy is transferred from the rear of the wave to the front without passing through the power supply. Elaborate switching is required. >

Thermo-magnetic Surgery for Experimental Renal Cancer

Thermo-magnetic surgery is an innovative modality in the treatment of malignancies. This unique combination can produce selectively controlled destruction of deep tumors. Heating of the magnetic metallic compounds of ferrosilicone by hysteresis produces temperatures which are incompatible with cancer cell survival. The intense focal heating causes a coagulation necrosis. Damage to surrounding normal tissue can be avoided by careful temperature monitoring and power control of the magnetic field. Cell destruction in the target tissue has been demonstrated by light and electron microscopy. Reproducible and predictable VX2 renal carcinomas in rabbits have been produced and used as the tumor model in the initial experiments of this research project. A clinical trial with selected cancer patients, performed with this technique of thermo-magnetic surgery, is now a reasonable and feasible procedure.

Photometry of infrared space telescope images using the grid PRF method Dphot

The grid PRF photometry method Dphot detects and measures point sources and extended sources using the Basic Calibrated Data images from the Spitzer Space Telescope without prior source position information. A mosaic is not used. Point sources with separations as small as 1.0 arcsec can be detected and measured. Examples of Dphot photometry are presented for 47 Tuc, the Galactic Center, Einstein Cross, and galaxies in GOODS North.

Deconvolving infrared telescope images as point sources on a grid

Deconvolution of infrared telescope images can partially recover the distribution of light in the sky. The light from point sources and extended sources is modeled as a grid of closely-spaced points. A matrix of influence coefficients contains the response of the telescope-instrument combination to light at the grid points. A non-negative least-squares routine finds the sky flux densities that reproduce the instrument data. A personal computer program and examples of infrared telescope data deconvolution are presented.

SIRTF stray light analysis

The Space Infrared Telescope Facility (SIRTF) is a 1-meter cryogenic infrared telescope. Stray light is kept below the natural background by restrictions on sun, Earth, and moon off-axis angles; by conservative baffle design; by the use of advanced diffuse black coatings; and by superfluid helium cooling. The aperture stop is located at the primary mirror rather than at the secondary mirror to increase the aperture and reduce the central obscuration. Stray light from off-axis sources is greater with the aperture stop at the primary than with the aperture stop at the secondary, but the modulation of the signal produced by tilting of the secondary mirror for chopping is less. Stray light from telescope thermal emission is lower with the aperture stop at the primary.