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.
DPhot is a "deep photometry" computer program for measuring faint point sources from an overlapping set of astronomical
images without making a mosaic. A grid of points is written on the sky. The point response function is used to calculate
influence coefficients between the grid points and the pixels. A least-squares fit to the pixel data gives flux density
measurements at small groups of grid points overlying images of sources. Summing and centroiding the groups of
measurements gives a source list of flux densities and coordinates.
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.
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.
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.
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.