P1 and H1 will be delivered to the Eastman Kodak Corporation (EKC) for alignment and assembly as part of VETA-2. EKC will use these elements to check the alignment equipment and for mechanical tests. Following these tests, all mirror elements will be shipped to Optical Coating Laboratories, Inc. (OCLI) where the final cleaning and coating (with sputtered Iridium) will take place. OCLI first will verify the sputtering geometry on test samples which duplicate the required geometry for the optical elements. The final coating of the optical element (and witness samples) will be performed after satisfactory preliminary qualification runs are obtained. The mirror elements then will be shipped back to EKC for final alignment and assembly.
The final alignment and assembly at EKC will be performed in a vertical
tower which is inside a class 100 clean area. The mirror elements,
composite support sleeves, and aluminum center aperture plate all
must be supported in a strain free manner. The mirror elements will be
positioned above an optical flat located at the bottom of the assembly
tower. The optical flat will be leveled to gravity, and the
optical reference assembly mounted on the center aperture plate
will be made parallel to the optical flat. The inner paraboloid (P6)
then will be mounted so that its axis of symmetry is normal to the
optical flat. The optical alignment sensor used for this purpose
illuminates the paraboloid from near its focus; light passes through
the paraboloid, reflects from the flat, and returns to a quad cell
detector near the paraboloid focus. The software does a fourier
decomposition of the centroid coordinates as a function of the
azimuthal angle illuminated. The paraboloid is aligned normal
to the flat when the centroid of the returned light does not show a
dependence upon the azimuth angle (
). The
paraboloid focus is determined by finding the point where the centroid
does not show a
dependence upon azimuth angle; proper
axial and lateral alignment is achieved when the paraboloid focus is
coincident with the center of a sphere which is part of the
optical reference assembly mounted on the center aperture plate.
This technique was developed on a technology development program, and
shown to be sensitive to alignment errors of less than 0.02 arc seconds.
The next smaller paraboloid (P4) then is added and aligned so that
it is co-axial with P6 and the two paraboloid foci are coincident.
The paraboloid focal lengths are about twice the system focal lengths,
and this extra length is accommodated by fold flats. These fold flats
then are removed and the first hyperboloid (H6) is added. The
alignment is similar to that of the paraboloids; a
azimuthal
dependence of the image centroid indicates that the hyperboloid
focus is displaced laterally from that of the associated paraboloid. The
position of the system focus can be adjusted laterally without any
loss of resolution by rotating the hyperboloid about the common focus
it shares with the associated paraboloid; the position of the system
focus can be adjusted axially with small loss of resolution by
displacing the hyperboloid axially. The position of H6 is adjusted
to yield a coma-free (no
centroid dependence) image
which is coincident with the center of a second sphere on the
optical reference assembly. The next paraboloid (P3) then will be
aligned so that its focus is coincident with that of P4; then H4
will be added so that its focus is coincident with that of H6, and so
forth through P1, H3, and H1.
The mirror then will be shipped to the NASA Marshall Space Flight Center for final X-ray calibration, where the X-ray performance will be determined and compared with the expected results based upon the metrology data and the calculated degradation from gravity, finite source distance, detector resolution, and so forth. We had excellent agreement for the only previous mirror for which metrology adequate for this task existed, and we hope no surprises will be found.
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