Esi Alignment Plan Draft III A. Sheinis
3/19/99
The ESI instrument will have the ability to be aligned modularly:
The optical subsystem (OSS) is an optical and mechanical subassembly consisting of the camera , dewar, grating, both prisms and both fold mirrors, along with the associated mounting hardware. All these components are mounted into an optical subframe which can be assembled and aligned off-line on an optical bench. The components of the OSS are aligned to an internal axis, defined by the moving prism mounting plate. The goal will be to be able to remove the entire OSS and replace it without impacting its internal alignment, .
The camera is assembled and aligned as a subassembly prior to its installation into the OSS.
A spaceframe will serve as the structure which defines the location of the OSS, collimator, tv guider system, slit and filter wheel assemblies.
Upon installation in the ESI, the OSS should be aligned as a unit to the instrument axis. An alignment telescope placed in the locating fixture of the collimator will be used to set the position of the OSS. The angular orientation of the OSS can be determined using an inclinometer.
The collimator and the TV guilder system will be able to pick up the instrument axis independent of the OSS via the above fixture.
The ESI spaceframe will not be aligned to the rotator bearing! Only the optic components within the space frame will be aligned to this axis. Keck personnel will be responsible for the alignment of the rotator bearing to the telescope axis. We will need to know the accuracy of this alignment from them.
Definitions:
Axes refer to local coordinates at each surface. Z axis is always the optical axis, positive in the direction of the light transmission. X axis is chosen along the ruling direction of the grating. Y axis follows from the right hand rule.
The camera will be assembled off-line as a subassembly prior to itĘs installation in the OSS. See assembly notes compiled by Dave Hilyard at the end of this document.
Regardless of which degrees of freedom are adjusted during the alignment the tilt and spacing of each optical element should be monitored as it is installed .
Optical elements are placed in one at a time , with tilts and decenters being measured for each successive element. Lateral registration is done to machine shop tolerances (MST), with possible shimming to minimize error stack up.
Once all elements are in place, final wavefront errors should be measured interferometrically. The interferogram can be used to approve the final alignment wavefront error.
Lastly image quality and chromatic errors should be evaluated with a star test in the jig, both on and off axis i.e. a single pass star test using a white light collimator.
The OSS is assembled and aligned as a unit on a bench off-line. The optical axis for the OSS will be established on fixed prism for position and relative to the moving prism mounting plate for angular orientation. These are the Zero references. In other words, everything is aligned angularly relative to the moving prism mounting plate and laterally relative to the fixed prism axisą.
Translational location in the X,Y and Z axes for the grating and prisms will be established initially on the bench mechanically.
A)Grating:
Alignment of the X and Y rotation of the grating can be achieved on a bench or in the telescope by first measuring the orientation of the grating mounting plate relative to the OSS fiducial (moving prism mounting plate). Tilt about x and y are adjusted relative to the grating mounting plate such that the angles are correct relative to the fiducial plane. This is done by setting the heights of the three grating kinematics on a mill table.
Fine adjustment of the grating is achieved by comparing Echellette spectra to theoretical spectra. The size of the spectra (dispersion amount) pertains to the tilt of the grating, the location of the spectra pertain to the tilt of the camera.
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B)LD Mirror:
The alignment of the X and Y rotation for the Ldmirror can be achieved initially on a bench via the following: (figure 6)
1) Set up an AT aligned axially with the LD mirror mounting plate in the OSS. Install the LD Mirror, in place in the OSS.
2) Measure the location of the LD mirror angularly with respect to the mounting plate. A reference mirror placed against the mounting plate will provide the return from the plate. The orientation of the mounting plate relative to the Moving prism mounting plate (our zero reference) is measured via the inclinometer. Thus, manufacturing errors in the oss plates are compensated. Final alignment can be done by measuring the center of the field relative to the Echellette spectrum and compensating of necessary.
3) Small piston errors of the mirror will contribute to beam decenter at the camera entrance aperture. These errors are not likely to impact the image quality.
Fine adjustment of the LD mirror is achieved by comparing low dispersion spectra to theoretical spectra. The location of the spectra pertain to the tilt of the ld mirror.
6.Collimator to ESI: (Figure 9) Focus for the collimator will be measured by the following: Set collimator focus by autocollimating between a flat and a pinhole placed at the slit plane. Then measure the collimator distance using metering rod and an inside micrometer from center of the collimator active area to the center of the slit. Saggital difference will need to be calculated since the collimator is an off-axis segment.
Tilts and centration are measured with AT #1 in the collimator hole projecting to a target close o the collimator and collimatng off a small mirror. By rotating the cass bearing the relative location of the AT and the bearing axis can be determined. Adjustments are made by lengthening or contracting each strut. The amount is determined by moving the mirror the appropriate amount in AutoCad and measuring the strut length change. Be sure to rotate the collimator in AutoCAD about the center target! (note this will produce a small focus change since you are not rotating about the collimator vertex.)
7.OSS to Collimator:
First the angle and position of the OSS need to be measured relative to the bearing axis. Angular orientation is measured using an inclinometer. Measure the average orientation of the bearing. Then measure the orientation of the moving prism mounting surface. The difference of these two measurements should be 40.4 +/- 0.1 degrees about the x axis and 0 about the y axis. Z axis rotations are not considered here as a misalignment in Z rotation will be easily compensated by a position angle offset in the cass rotator bearing.
Position of the OSS is determined by measuring the location of a target placed on the fixed prism with the AT placed in the alignment hole of the collimator (collimator should be aligned to the bearing axis previously). The target shows the location of the intersection of the fixed-prism-cover plane and the telescope axis. Rotate and translate the OSS about the center of the target, by the appropriate amount. Measure corresponding strut length changes in AutoCAD. Re-shim struts on the instrument.
8.Camera to OSS: Camera angle relative to the bearing axis is measured using the inclinometer. Strut lengths are adjusted to change the camera angle. Camera position is determined by measuring the location of the spot produced on the OSS by a laser that has been bore-sighted to the camera. This has been adjusted by varying the strut lengths
Fine adjustment of the camera is achieved by comparing Echellette spectra to theoretical spectra. The size of the spectra (dispersion amount) pertains to the tilt of the grating, the location of the spectra pertain to the tilt of the camera.
4.Slit Wheel: Focus for the slit wheel will be measured via metering rods from the collimator. Position is measured via an alignment telescope referencing the bearing axis from the front of the instrument.
8.TV Guider :
The TV guider can be aligned in place by placing a laser at the pupil location and projecting to the center of the field mirror. Adjust each optics such that the spot is centered in the next optic on down the lineą. Pupil location is determined using a tape measure, and an alignment telescope referenced to the bearing axis
Tooling List
This fixture will attach collimator. It will have locating holes precision bored to accept an alignment telescope (AT). The axis of the hole will determine instrument axis for the on axis field (of the telescope).
Alignment Telescope/autocollimator
Alignment Telescope/autocollimator
This camera will be required to view the alignment telescope reticle with both higher precision than the naked eye and in locations that cannot accept the size and weight of a human being.
Image relay lens and tube for above camera:
Required to relay alignment telescope reticle to camera
Frame grabber for above camera.
B) Camera Alignment:
Required to test the moncromatic wavefront quality of the assembled camera.
This will provide the Dspace reference for the camera elements.
For the camera star test.
For camera star test.