OMC
LABORATORY ASSEMBLY AND NAOMI INSTALLATION AT GHRIL
wht-naomi-92
Document number
AOW/GEN/RAH/8.0/09/97/ NAOMI Installation
Draft version date: 24
September 1997
This document describes the
initial laboratory assembly of the OMC and the installation of NAOMI at the
GHRIL. The former operation will be performed at ROE using a metrological
approach with a 3D measuring machine.The OMC will be disassembled when moved
between various locations, e.g. from the test focal station to the GHRIL,
whereas the wavefront sensor (WFS) remains as a complete assembly. The assembly
of the WFS will be covered by a separate procedure to be developed by the RGO;
this procedure will be primarily intended for internal use by RGO.
The installation of NAOMI at
the GHRIL relies heavily on the positional repeatability of the OMC components
on the OMC baseplate. Many steps in the installation procedure are designated
simply as checks to insure that the OMC components are really in their correct
location. Diagnostic procedures will be developed to isolate the source of the
problem if the check is unsatisfactory. In later versions of this document the
diagnostic procedures may be incorporated as hidden text if this approach is
found to be appropriate.
Alignment and performance
checks are marked “CHECK:”.
Steps are marked “STEP:”.
Notes are marked “NOTE:”.
[Control system implications
are indicated in square brackets as shown here.]
{A pre-configuration
requirement , when not indicated as a full step, is indicated in curved
brackets as shown here.}
This section outlines the
approach to be followed by ROE during the initial assembly of the OMC at ROE.
In addition to the use of a 3D measuring machine as mentioned above, the
approach depends on the use of the manufacturer’s data for the powered
surfaces. In particular the measured radii of curvature will be used to
re-optimise the positions of the optical components. Note that a more detailed
procedure will be prepared by ROE for presentation at the OMC CDR.
Components such as the
off-axis paraboloids (OAPs) will be mounted in invar mounting rings with lugs attached. An adhesive will hold
each component in its mounting ring. Eccentrics on each mount are adjusted so that the lugs are
moved to calculated positions. The assembly of a component in its mount will
usually involve the following operations that employ ROE’s 3D measuring
machine:
a) Measure the mirror surface
relative to its mounting ring.
b) Measure the mounting ring
to the lugs.
c) Measure the relative
positions of the calibrated eccentric adjustments.
When the optical components
have been installed in their mounts they are assembled to the alignment jig, i.e. the baseplate, and the positions
of all components are verified with the measuring machine. The location and
image quality of the final image will be checked using the NCU
diffraction-limited point source. Interferometric tests may also be performed
to provide additional verification of satisfactory image quality.
The features of the Nasmyth
Calibration Unit (NCU) are summarised below for information purposes only. Not
all features are used in the installation of NAOMI.
1. an on-axis
diffraction-limited (in visible region over full aperture) point source
2 a fast low-amplitude tip/tilt motion of the above source
3. an on-axis
non-diffraction-limited source (approximately 1 arcsecond)
4. a diffraction-limited (at
K band) point source close (2 -3 arcsec) to the axis for science instrument use
5. a 40-arcsecond diameter
flat-field source for IR and optical science instrument calibration
6. an on-axis f/11 laser beam
for initial alignment
7. a laser pencil beam
(temporarily deleted as a firm requirement but requested by RGO)
8 a WHT pupil simulator using
a mask
9. a feed for a
pre-correction camera
10. an array of off-axis
sources for mapping the AO optical system distortion and wavefront aberrations
over the field of view
11. a means of generating
known static aberrations
12. a turbulence generator
for use during laboratory tests
13. neutral density and
spectral filters for controlling the intensity and colour of all broad band
sources listed above.
The ING must provide an
alignment telescope which views the mechanical axis of the WHT image derotator.
The latter shall have been tested on sky to verify that the mechanical axis is
coincident with the optical axis of derotation. The ING should also verify the
stability of the WHT exit pupil with respect to the optical axis as the image
rerotator is moved through its full range. The pupil stability should be better
than 0.036 of the pupil diameter (preliminary specification). More detailed
requirements defining the interface between the WHT and the OMC will be the
subject of an interface document yet to be prepared.
For the initial installation
the baseplate is moved around without its components but with its alignment
targets installed. The GHRIL alignment telescope and the location of the
Nasmyth focus are used to position the baseplate. Vertical alignment is achieved
by changing the height of the GHRIL table, if required. The OMC has been
designed so that all components can be removed from the baseplate and replaced
with accuracies of better than ± 50 µm. Thus many steps in the installation are
simply checks to insure that the alignment has not been inadvertently
disturbed.
STEP: Use dowels if available
otherwise follow next step.
STEP: Install the two OMC
alignment targets on the baseplate. The target that defines the input focus is
placed in nominal position of the on-axis Nasmyth focus. Its centration is
checked with the GHRIL micro-alignment telescope. The baseplate is pivoted on
the bench surface about the Nasmyth
focus until the second target is coincident with the line of sight of the
alignment telescope.
CHECK: Check NCU f/11 beam
and its direction using the pre-aligned mask placed on jig then removed.
NOTE: The DM will ordinarily
be replaced by a dummy flat until the operations in Section 3.4.6; this dummy
flat is a replacement for the whole DM assembly with its 2-axis stage (rather
than just the DM or top part of the module). This alleviates the need to
maintain a flat DM during much of the installation and it allows a more
reliable assessment of the image quality of the other optics.
CHECK: Check image quality
after field lens using a video-camera alignment aid. The camera will have a
graticule set at the desired focal position, together with suitable optics and
ND filters (if needed). The camera may also be set at a pre-determined position
to check the image quality with the field lens and dichroic removed.
CHECK: Put on pupil mask between fold and 2nd OAP slide up and down to
check beam position/direction.
CHECK: Install a second video camera at the science focus and check the beam
position/quality. (The possibility of using the same camera for both focus
checks should be investigated.)
NOTE: The lenslet arrays are
potted into holders which are keyed into the wheel; inadvertent lenslet
rotation should not be an issue.
NOTE: The atmospheric dispersion
corrector (AtDC) configuration should not matter for alignment with a HeNe
laser but it should be set to zero dispersion for broadband operations.
NOTE: 2-CCD issues
á
The straight-through CCD
is the datum -assume using this one if not otherwise specified.
á
The top CCD is moved in
x,y to take out any beamsplitter error.
STEP: Place the WFS on table
using the dowels.
NOTE: Details of initial
installation without dowels are required.
CHECK: Install the pupil
fiducial and examine alignment with a laser.
STEP: Turn on the WFS electronics.
STEP: Set AtDC to zero
deviation
CHECK: With the acquisition
lens in place check the acquisition of
the WFS calibration source [could use EPICS CCD test GUI to visualise].Verify
that the co-ordinates of the acquisition source are correct.
NOTE: One may wish to perform
other checks with the WFS calibration source at this point.
STEP: Acquire the on-axis,
visible point source from the NCU.
CHECK: {acq lens} Check that
the AtDC rotation gives spot elengation in the right direction
CHECK: {lenslet} Check that
spot array is regular, on-axis and in focus on both CCDs
CHECK: {no lenslet} [
flat-field CCDs make available to RTCS?]
CHECK: {lenslet} Check that the spot illumination
balanced -this verifies the pupil
illumination
¬
IF NOT horizontally
balanced: then move WFS "rotation" control
CHECK: {lenslet} Check the output focus signal.
CHECK: Install the NCU array of point sources Check
the x-y pick-off measurements of
relative-motion deltas, focus and repeatability at four field points
distributed over the field.
NOTE: One cannot use DM dummy
flat beyond this stage
NOTE: May need more frequent
repetition (eg. per night)
[all control systems fully
functional]
STEP: Employ 2-axis DM
alignment procedure using illumination in adjacent offset segments. An
automated operational procedure will be developed to cover this step.
CHECK: DM flat?
NOTE: May need more frequent
repetition
STEP: Flat-field CCD
STEP: Calculate WFS offsets
for off-axis angle [science camera] [for lab. test use 50/50 beamsplitter and
visible camera at the IR science port]
CHECK: Perform closed-loop
tests [optionally without science] (remove spot array).
CHECK: Test the WFS transfer
function.
CHECK: Test closed loop performance on static spot (to evaluate noise)
CHECK: Perform image-motion
closed -oop test using the NCU tip/tilt mirror. Check the FSM first then all DMsegments: obtain image
motion power spectra before and after - check input power spectrum, 0dB point,
noise above+below 0dB, spikes?
CHECK: Verify that the image
derotator centre is coincident with the NAOMI axis.
NOTE: WFS {requires DM
flattened and FSM mid-range} and pre-corr display (poss offset) available - as
ELECTRA
STEP: Use the {zero pick-off
offset} WFS display as a calibration reference. This must be equal to the
derotator centre. The pre-correction
camera may be used for acquisition purposes.