HEIDI grids on an optical bench, illuminated with a Helium Neon laser. The reflected and transmitted beams are complementary to each other. The black bars, seen orthogonal to the slits, are interference fringes resulting from the small, deliberate misalignments between the grids. The anticipated diffraction effects were, thus, visually confirmed.

This test was the precursor to the complete laser calibration of the HEIDI telescope described in the abstract below.

Quantitative Characterization of the X-Ray Imaging Capability of Rotating Modulation Collimators Using Laser Light

C. J. Gaither, III*
NAS/NRC Fellow
Goddard Space Flight Center, Code 682
E. J. Schmahl
University of Maryland
C. J. Crannell, B. R. Dennis, F. L. Lang**, L. E. Orwig
C. N. Hartman
Goddard Space Flight Center, Code 735
G. J. Hurford
California Institute of Technology

Accepted by Applied Optics, August, 1996.

Abstract

We have developed a method for making quantitative characterizations of bi-grid Rotating Modulation Collimators (RMCs) that are used in a Fourier Transform X-ray imager. With appropriate choices of the collimator spacings, this technique can be implemented with a beam-expanded HeNe laser to simulate the plane wave produced by a point source at infinity even though the RMCs are diffraction-limited at the HeNe wavelength of 632.8 nm. The expanded beam passes through the grid pairs at a small angle with respect to their axis of rotation, and the modulated transmission through the grids as the RMCs rotate is detected using a photomultiplier tube. In addition to providing a quantitative characterization of the RMCs, the method also produces a point response function and provides an end-to-end check of the imaging system. We applied our method to the RMCs on the High Energy Imaging Device (HEIDI) balloon payload in its preflight configuration. We have computed the harmonic ratios of the modulation time profile from the laser measurements and compared them with theoretical calculations including the diffraction effects on irregular grids. Our results indicate the 25'' X-ray imaging optics on HEIDI are capable of achieving images near the theoretical limit and are not seriously compromised by imperfections in the grids.
Key Words:
Fourier transform imaging, imaging systems, laser applications, optical testing, rotating modulation collimators, X-ray imagers.

* Currently at Analytic Services, Inc., Arlington, VA.

**Also Department of Physics, The Catholic University of America, Washington, DC.

Postscript version of published paper.

Figs 3a and 3b

Figs 7c and 7d

Fig 11

Last revised November 26, 1996.