THE GODDARD GRID CHARACTERIZATION FACILITY

Summary

The GCF is a system that is used to analyze geometrical aspects of grids.

Primarily (but not necessarily) for grids used in Fourier transform telescopes

Determines their geometrical qualities and suitability for imaging.

Originally, the system was designed to characterize the HEIDI grids.

It is currently being updated for finer grids.

The Existing Hardware

The GCF hardware includes a vision system, a motion system, and a computer.

The vision system consists of a CCD camera, a grid display monitor, and an IPS vision board.

The motion system consists of an X-Y (Daedal) translation table, two micro-stepping motors, and computer interface.

The computer used to characterize the HEIDI grids was a 286 PC. The present computer is a 486/DX66.

Mode of Operation

A light box mounted on the X-Y table, with the grid supported on a rigid frame attached to the table.

The light transmitted by the grid passes into a zoom CCD camera which feeds into the PC.

The X-Y table can be driven manually or under computer controlusing the stepping motors in the horizontal plane.

The range of the X- and Y-coordinates is large enough to accommodate scans of grids or assemblies up to about 300 mm in any linear dimension.

Steps can be made in sub-micron increments with position reproducibility of about 2 to 3 microns.

The frame holding the grid permits adjustable rotations about the optical axis, and incremental changes in the angle of incidence.

The monitor has 512 x 484 pixels (all but the outer 30-pixel border is visible),

The zoom lens can display fields from a few tens of microns to a few thousand microns.

In a typical mode of operation, the field of view is about 100 microns, giving a nominal resolution of better than 0.2 micron/pixel.

Existing Software

The GCF software system consists of scanning and analysis programs.

The main scanning program drives the grid to find and record edge locations of each slat in the grid.

Other similar programs locate fiducials, shift the grid under real-time control, etc.

Another series of programs analyze the data, producing mean and RMS values of the pitch, slat sizes, cumulative deviations of slat positions.

Goals of the GCF

to characterize grids.

to compute the predicted point response function (PRF).

Grid Characterization Parameters

Mean and RMS Pitch

Maximum Deviation of Edge Position

Edge Straightness, Splay

Slit/Slat Ratio

Camber--as in airplane wings-- )))) or ((((

Venetian Blinding--like //// or \\\\

Transmittance

Grid Point Response (PRF)

We will calculate the PRF for each grid pair. (Assume they are ideally aligned.)

Predict the response of the laser calibration for each collimator.

Deviations from the predicted response would be an indication of misalignment.

Predict the response of the ensemble of collimators. (Alignment must be assumed.)

Proposed GCF Upgrades

Contamination Containment

Reduction of Table Error

Reduction of Thermal Effects

Minimization of Diffraction

Methods for Independent Corroboration

Disk Space Expansion