A method of coarse alignment has been proposed for a primary telescope mirror that comprises multiple segments mounted on actuators that can be used to tilt and translate the segments to effect wavefront control in increments as fine as a fraction of a wavelength of light. The method was originally intended for application to the Next Generation Space Telescope (NGST), which will have an aperture about 8 m wide and will include nine primary-mirror segments (a central segment and eight outer segments). The method could also be applied to other lightweight telescopes that are similarly designed to rely on active wavefront-control systems instead of traditional massive structures to ensure high optical quality.
A method of coarse alignment is needed because thermal deformations, mechanical loads, manufacturing errors, and other phenomena can give rise to large initial misalignments of the segments; for example, immediately after deployment, the segments of the NGST could be misaligned by as much as millimeters in piston (displacement along the nominal optical axis) and milliradians in tilt. The present method, to be implemented by the computer that would control the telescope-aiming mechanisms and mirror-segment actuators, provides for reduction of errors in the positions and orientations through the following steps:
- Aim the telescope at a bright distant point source of light (e.g., a star).
- Systematically scan the tilts of each mirror segment while repeatedly acquiring star images on an imaging photodetector array at the telescope focal plane. By an algorithm that involves comparison of images with previous images, those images that initially do not fall on the detector can be brought onto the detector and images formed by all the segments can be centered on the detector (see figure).
- Temporarily tilt the mirror segments to form separated images on the detector. Operating on each image in turn, vary the piston adjustment of the corresponding mirror segment to concentrate as much light as possible into a smaller focal spot of a given size. Repeat the process with a smaller spot size. After a large focus adjustment, recenter the spots according to step 2. Repeat all of the foregoing until the maximum amount of light is concentrated in the smallest possible focal spot.
- Form a dispersed-fringe sensor as follows: Insert a grism (a right-angle prism with a transmission grating on the hypotenuse face) into the optical path. Tilt all of the outer segments except one to move their images off the detector and to form a dispersed-fringe image by use of the central mirror segment and the remaining outer mirror segment. Analyze the modulation period and orientation of the fringes to determine the magnitude and sign of the piston error between the two segments. Use this error to perform a coarse-phase piston adjustment of the affected outer mirror segment. Repeat this procedure until all of the outer segments have been adjusted.
- Perform a somewhat finer coarse phase (image-sharpening) adjustment by use of white-light interferometry: Remove the grism from the optical path, then in a manner similar to that of step 4, use the central mirror segment plus one outer mirror segment at a time to form a white-light image of the star. Adjust the outer segment in piston to obtain the highest peak intensity in the image. Repeat the procedure for the remaining outer segments.
This work was done by Scott Basinger, Andrew Lowman, David Redding, and Fang Shi of Caltech and Chuck Bowers of Goddard Space Flight Center for NASA's Jet Propulsion Laboratory.
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Coarse Alignment of a Segmented Telescope Mirror
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