Abstract
The Navy Precision Optical Interferometer, located near Flagstaff, Arizona, is a ground-based interferometer that collects, transports, and modulates stellar radiation from up to six primary flat collectors, known as siderostats, through a common vacuum relay system to a combiner. In the combiner, the modulated beams are superimposed, fringes obtained, and data recorded for further analysis to produce precise star positions or stellar details. The current number of observable stellar objects for the astrometric interferometer can increase from 6000 to at least 47,000 with the addition of full-aperture 20-deg down-tilting beam compressors in each optical train. Such an aperture increase, from the current 12.5 to 35 cm, opens the sky to many additional and fainter stars. Engineering analysis of our beam compressor primary mirror shows that the maximum allowable sag, 21 nm, occurs prematurely at 2.8-deg down-tilt angle. Furthermore, at the operational down-tilt angle of 20 deg, the wavefront deformation increases to 155 nm. We present a finite element analysis technique and design modification concept to reduce tilt-induced deformation on the mirror surface. This work is a first pass to determine the feasibility for a mechanical solution path forward. From this analysis, we found that four outwardly applied 17.8-N forces on the rear surface of the mirror could reduce sag from 155 to 32 nm at 20-deg down-tilt angle.
Highlights
The Navy Precision Optical Interferometer (NPOI), near Flagstaff, Arizona, makes use of smaller separate optical stations spaced along a Y-array and used simultaneously to simulate an equivalent single larger telescope
We determined the maximum allowable tilt-induced peak-to-valley deformation of the primary mirror surface to be 21 nm and found this to occur at 2.8-deg down-tilt angle
At the NPOI operational down-tilt angle of 20 deg, the peak-to-valley displacements increase to 155 nm, 134 nm beyond our allowable
Summary
The Navy Precision Optical Interferometer (NPOI), near Flagstaff, Arizona, makes use of smaller separate optical stations spaced along a Y-array and used simultaneously to simulate an equivalent single larger telescope (see Fig. 1). There are currently two separate instruments combined at the NPOI: a 4-element stationary astrometric array and a 6-element reconfigurable imaging array Both use flat-mirror tracking siderostats as primary light collectors that redirect stellar radiation through an evacuated beam relay system to a beam combiner station, where the beams are superposed, fringes obtained and modulated, and data recorded for further analysis. The transport optics,[3] beginning inside the ported cylinder just beneath the tip/tilt mirror, is housed in an evacuated piping system, along a Y-array configuration (see Fig. 1), the arms of which range out to 240 m in length This evacuated transport system provides transfer of the stellar wave fronts (optical beams), up to nearly 800 m in horizontal length that are 1 to 2 m above ground level, to the beam combiner without generating additional thermally induced wavefront distortions such as those that naturally occur from the earth’s thick atmosphere. The present work was initiated to characterize deviations of the reflective surface on the primary mirror with respect to tilt angle and, if necessary, to perform a first-order analysis to determine whether a simple conceptual mechanical counterweight-type fix is feasible
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