LBT Rigid Secondary Alignment Procedure
Ross Zhelem and Andrew Rakich
July 30th, 2007
Version 3
Scope: This document provides a complete plan for the mechanical alignment of the
rigid secondary mirror and primary mirror on LBT. The optical axes will be aligned
to lie along the rotation axis of the “un-folded” Gregorian rotator. Expected
accuracies of the proposed method are also discussed.
1. Setting the optical axis of an alignment telescope to the rotation axis of the
unfolded-path Gregorian rotator
During all of the following operations the telescope will be locked at Zenith pointing.
(Check, would it be more convenient to do this initially at horizon pointing, then tip to
zenith and tweak, also interesting to see how much things have moved).
The rotational axis of Gregorian rotator (GR) is to be defined by a Taylor Hobson Micro
Alignment telescope (part # 112/2582). The method of setting the “A-Scope” to the
Gregorian rotator rotation axis is described here.
1) The A-Scope is mounted, aimed towards the secondary mirror, on a stage
allowing both fine rotational and translational adjustment. This stage is in turn
mounted on a rigid C-Beam that crosses the centre of the GR and will rotate when
the rotator is rotated. Initially the A-Scope is placed roughly aligned to the rotator
axis, and this can be tuned by rotating the GR and setting a dial indicator to the
side of the A-Scope at different heights of the tube, and adjusting the A-Scope in
tip/tilt and de-centre until the run-out of the dial indicator gauge is minimized.
Using this method it should be possible to set the A-Scope parallel and centered
on the GR axis to within ~1-2 arc minutes and ~50 microns respectively.
2) A target flat mirror of ~ 2” diameter is also mounted onto the primary mirror cell
structure, and does not rotate when the rotator is turned. The mirror is mounted
roughly centered on and normal to the GR axis. This mirror is mounted on a stage
enabling fine rotational and translational adjustment. This mirror has a small,
roughly central target feature scratched into the aluminium coating with a razor
blade or similar. This target mirror will be rigidly attached to the Primary Mirror
cell and should not vibrate or otherwise displace as the rotator is turned.
3) The target cross is then viewed through the A-Scope, and adjusted on its XY stage
until it is centered in the field of view of the A-Scope.
Following this rough alignment, fine alignment is achieved by utilizing the autocollimation and direct imaging modes of the A-Scope respectively to (mostly) de-couple
the rotational and translational aspects of the alignment of the A-Scope to the GR axis.
First the auto-collimation mode is used to set the axis of the A-Scope parallel to the GR
axis as follows.
4) The A-Scope illuminator is switched on and the A-Scope is focused to infinity.
The returned image of the reticule is sought, and at this stage it is possible that the
tip-tilt stage of the target flat mirror will have to be adjusted to bring it into range.
As a precursor to this step, the objective target of the A-Scope could be used here
as it has a greater angular range/less angular sensitivity, once this is centered by
the appropriate tip/tilt adjustment of the target mirror the auto-collimation
returned image will be within capture range.
5) Once the image is located it should be driven to the centre of the A-Scopes FOV
(aligned with the target reticule as shown in figure 1, a) and b). All tip/tilt
adjustments are made with the target mirror tip/tilt stage at this point.
Figure 1. Shown here is the view through the alignment telescope when used in autocollimation mode, in the case a), where the return mirror is aligned so that the A-Scope
axis is normal to the target mirror surface and in the case b) where there is an angular
deviation from normal.
Now the Target mirror will be normal to the axis of the A-Scope to within an accuracy of
approximately 3 arc-seconds. The A-Scope axis will at this stage have some arbitrary
angular relationship to the GR axis, within the error envelope of the coarse setup done
previously with the dial-indicator gauge.
6) Once the image is centered on the object reticule, the GR is turned through 180
degrees. In general the image of the A-Scope focal plane reticule will displace
away from the centre, indicating an angular (angular only) misalignment of the AScope and GR axes.
7) Now, the image should be returned to centre. Importantly here, this time half of
the adjustment should be made by rotation of the A-Scope, and half by rotation of
the target mirror, as shown in figure 2.
The procedure described in 6) and 7) should be repeated until the A-Scope autocollimation reticule and its image show no perceptible motion. At this point the A-Scope
optical axis has been set parallel to the GR axis to within approximately 4 arc seconds,
but still has an arbitrary centration to the GR axis, within the error envelope of the coarse
setup done previously with the dial-indicator gauge.
8) To centre the A-Scope axis on the GR axis the illuminator is switched off and the
A-Scope is focused on the central target of the target mirror. This target is first
driven to the centre of the FOV of the A-Scope using the target mirror XY stage
only.
9) Next the GR is turned through 180 degrees. In general the target cross-hair will
have displaced from the centre of the FOV of the A-Scope. The target must now
be adjusted back on to the centre of the A-Scope target crosshair, half of the
adjustment being done with the target mirror XY stage and half of the adjustment
being done with the A-Scope XY stage.
The procedure described in 8) and 9) should be repeated until there is no discernable
movement in the target cross. At this point, assuming a distance of 1.0 m between target
mirror and A-Scope objective, the centration should be good to the resolution limit of the
A-Scope, approximately 15 microns.
10) It is possible that crosstalk from the XY stages and rotation mounts will degrade
the results of steps 6) through 9). To check that accurate alignment has been
achieved check that the auto-collimation target is still properly centered and
maintains centering as the GR is turned. Also focus back on the target mirror
cross and check to see that the target is still properly centered and maintains
centering as the GR is turned. If either shows signs of imperfect alignment, reiterate steps 6)-10).
Once no perceptible change in the images are observable, the A-Scope and GR rotation
axes have been brought into coincidence to within ~ 4 arc seconds of angular deviation
and ~ 15 microns of lateral displacement. Note that this 15 microns resolution limit is
about the same as the bearing run-out for the GR bearing, as documented in LBT project
document 670S006.doc “Instrument Rotator and Cable Chain System Analysis”.
Figure 2. After initially setting the return image to the axis of the A-Scope, the GR is
turned 180 degrees, giving some displacement as in a). Half of this displacement is
corrected using the tip/tilt adjusters on the A-Scope mount giving the situation in b). The
other half of the adjustment is made using the tip/tilt adjusters on the target mirror
mount, driving the image back onto the A-Scope axis c). Note that in this figure the object
target is used, the same principle applies to the auto-collimation reticule as shown in
figure 1.
2. Secondary mirror alignment
1) The secondary mirror is initially mounted with some arbitrary centering and
tip/tilt with respect to the A-Scope optical axis. The target mirror used in Part 1
above is removed and the A-Scope is used to sight on the secondary mirror.
Note that once this mirror has been removed extreme care must be taken not to knock or
roughly move the A-Scope. If it is knocked the target mirror must be replaced and the
steps outlined in part 1 must be repeated.
2) The secondary mirror has circular target pattern engraved around the optical
center by the mirror manufacturer. The tolerance on location of this target feature
to the conicoid vertex of the mirror is 500 microns.
3) First, the secondary mirror target pattern is centered on eyepiece crosshair by
adjusting the mirror in translation. Assuming a range of 11.5 m, and a 3 arc
second resolution limit in the A-Scope, this centering should be possible to an
accuracy of approximately 175 microns.
4) Next the A-Scope is re-focused so that the image of the objective lens target
reticule is visible in the A-Scope. Now the secondary mirror is tip/tilted about the
pole of the mirror until the image of the target reticule is centered on the AScope’s focal plane reticule. Using the resolution and distance assumptions stated
in (3), and discounting centration errors, this step should be capable of setting the
axis of the secondary mirror parallel to the A-Scope axis to within ~12 arc
seconds.
5) Finally, for improved sensitivity, the A-Scope illuminator is turned on and the AScope is focused on the centre of curvature of the secondary mirror,
approximately 9.5 m in front of the A-Scope. By adjusting the rotation of the
secondary mirror about its pole until the returned image is centered on the AScope in auto-collimation mode, it should be possible to reduce the axis
parallelism error between the A-Scope and secondary mirror axes to
approximately 6 arc seconds.
Once axis parallelism is achieved the A-Scope should be re-focused on the secondary
mirror central target and if necessary steps 2) – 5) should be repeated until the image of
the secondary mirrors central target remains centered and the auto-collimation target
remains centered when the A-Scope is switched from focusing on the secondary mirror
surface to auto-collimation mode
In the worst case, where the 500 micron error mentioned in 2) and the 175 micron error
mentioned in 3) add linearly, the total centration error possible would be 675 microns.
When added in quadrature, the resulting “expected error” is ~530 microns.
3. Primary mirror alignment
The secondary mirror is manufactured so that the outer edge is concentric with the
circular target pattern and the optical center in the middle to within 1000 microns.
With the dome opened providing daylight illumination around the mirror perimeter or
with an extra flood light behind the mirror, the edge of secondary is reimaged by both
primary and secondary mirrors. This image of the secondary will be observed in the AScope. Any observed de-centration of the image of the secondary mirror will be primarily
due to tilt error on the primary mirror. The primary mirror is tip-tilted to ensure
concentricity of secondary mirror image.
Images visible through the alignment telescope include:
1) The image of the central hole in the primary mirror. This image does not change
position as the primary mirror is adjusted in tip/tilt, and should be well centered in
the A-Scope.
2) The image of the secondary mirror, imaged through the primary mirror then itself.
This image will subtend an angle of approximately 30 arc minutes, and will move
as the tip/tilt of the primary mirror is adjusted.
3) The image of the primary mirror, as imaged first by the secondary, then the
primary then the secondary again. This image is slightly smaller than the image
(2) above, subtending approximately 27 arc minutes in the A-Scope, this image
will tip in the same direction and at about the same rate as the image in 2 as the
primary mirror is rotated.
1 arc-minute of tip/tilt rotation of the primary mirror will correspond to about a 1.4% decentration of the images 2) and 3), so this method should be sensitive to approximately
the 30-60 arc-second level.
Note that this final method assumes that the primary mirror is perfectly centered, in
general the primary mirror will be de-centered with a compensating tip/tilt when aligned
using this technique. Therefore one would expect to see coma in the images on first
inspection at the start of optical alignment, as the tip/tilt compensation for de-center
achieved using this alignment technique is actually half of that required for coma
compensation.
The achievable accuracies discussed above represent sufficient accuracy for the
mechanical alignment of the primary and secondary mirrors to the Gregorian Rotator
axis.