LINEAR STAGES TEST
Matteo Lombini
and
Carmelo Arcidiacono
7 ottobre 2004
1
Introduction
In the LOWFS for MAD each star enlarger is moved in the 2 arcmin field
of view by a couple of motorized linear stages mounted orthogonally. A not
perfect planarity in the stage movement over which the star enlargers are
mounted produce a tip-tilt misalignment of the star placed at infinity with a
consequent pupils decentering on the CCD (fig 2). The moving stages have
a clearance that provokes rotations around the three axis (fig. 1). We are
interested only to pitch and roll movements.
yaw
Z
roll
X
dX
pitch
Y
dY
dZ
Figura 1: The pitch is the rotation around the y-axis, the roll is the rotation
around the x axis the yaw is the rotation around the z-axis.
1
2
TEST
Reference star
Star
enlarger
Tip-tilt
mount
Linear
stages
Figura 2: This drawing shows the effects of pitch and roll of the linear stages
on the star enlarger (SE). The result is a tip-tilt misalignment of the SE that
causes a degrade of the pupil image.
2
Test
The goal of this test is to measure the pitch and the roll values of the linear
stages during their movements to be below our specifications (10 arcsec).
The verification was necessary because the constructor guarantees only the
pitch and the yaw and not the roll. The linear stages are commercial M405CG micro positioners produced by Physik Instrumente.
Above the stage we placed a mirror orthogonal to the beam. To measure
the pitch and the roll we needed the mirror surface to be parallel respect to
the bench while to measure the pitch and the roll the mirror surface had to
2
2
TEST
Figura 3: Picture of set up per pitch and roll tests.
be perpendicular to the bench. In the figure 2 and 3 is shown the set up for
the pitch and roll measurements.
3
2
1. Laser
2. Filter
3. Spatial filter
4. Lens L1
5. Diaphragm 1
6. Mirror S1
7. Beam splitter
8. Mirror S2
9. Mirror S3
10. Stage
11. Power source
12. Lens L2
Figura 4: Drawing of the set
up for pitch and roll tests.
13. Diaphragm 2
14. CCD
4
TEST
3
3
SET UP VERIFICATION
Set up verification
The first results of the measurements were often out of specification. That’s
why we decided to verify how much our errors in positioning and aligning of
the set up components influenced the test results. In the list below there is
a list of the preliminary tests for the confidence of the pitch hand roll tests:
• Measure uncertainty: the movement of the laser spot centroid in
the CCD can be caused by air turbolence in the lab. In this test the
stage was in quiet. As it can be seen in the figure 6 the measure
uncertainty is of the order of 2 arcsec in pitch and roll. This value is
our measurements error.
• Diaphragm : the last optical component of the set up is a lens that
focuses the parallel beam onto the CCD (see fig. 10). To avoid off-axis
aberrations and deviations the laser beam should pass across the center
of the lens. The alignment of this lens could be not enough precise so
we placed a diaphragm to block off axis rays. The comparison with
the tests without this diaphragm gave not appreciable differences.
• Stage inclination: our set up should be insensible to a wrong positioning of the stage or of the reflecting mirror. In this tests we placed
one or two spacers below the mirror (0.5 mm of width) in order to
cause a systematic tilt. As it can be seen it the figures 11 and 12 the
tests confirmed the insensibility of a not perfect alignment of the stage
or of the mirror.
• Mirror planarity: The 100 mm mirror (n. 9 in figure 4) indicate a
λ/10 planarity in the reflecting surface. We made a first test rotating
the mirror of 90 degrees to control the surface isoplanarity and a second
test in which we placed three little spacers between the mirror and the
stage to check eventual deformations of the surface. The results (fig.
11) do not differ from the previous ones more than our uncertainty.
• test repeatability : this is maybe the strongest verification of our
measurements. We made tests over the same stage after several days,
re-aligning the various components in the set up. In the picture 13 are
confronted the pitch and roll measurements of the stage #6 taken in
different days.
• Overload of the stages: the tested stage was loaded by another
one orthogonally, the same configuration for the LOWFS reference
positioning (fig. 2). An eventual clearance between the rail and the
stage was expected to decrease with a weighted on load. We did not
find any difference in this test. In fact being the weight of one stage
(2 kg) too small respect to the maximum loading (20 kg).
5
3 SET UP VERIFICATION
• Tests repetitions: We a result was strongly out of the specifications
we controlled the set up and repeated the test to avoid rough errors in
the alignment. Unfortunately the repetition of some tests confirmed
our previous results (fig. 7).
• Yaw tests: The Physik Instrumente guarantees the pitch and yaw
movements less than our specification but did not for the roll movement. We decided to test the yaw movement for the stages out of
specification in roll in order to send back the ones out of spec also in
yaw for a reworking. This test made us a confirmation of the measurements because the pitch value for two different mirror placing was the
same.
6
4
INTERFEROMETRIC TESTS
4
Interferometric tests
Our tests on the stages gave often pitch and roll values out of spec, differently from the company test graphs for each stage. We ware enough confident
of our measurements (see section above), nevertheless we decided to make
an interferometric test of pitch and roll movements as the company did. We
reproduced the set up used by the company (fig. 5) mounting a mirror in
front of a WYKO interferometer. In figures 15 and 14 are showed respectively the differences of our interferometric tests with the company and the
laser ones. The results of our different measurements correspond below the
laser tests uncertainty (sect. 3) while the company plots show a similar
trend in the shape but a smaller Peak to Valley.
Mirror
Steel
plate
Interferometer
Dichroic
Stage
Interferometer
Iron Stage
plate
Granite
plate
Honeycomb plate
Honeycomb benchco
(b)
(a)
Figura 5: a) company set up; b) our set up with the interferometer.
7
5 RESULTS
5
Results
Takin under regard our measurements uncertainty we decided to accept also
the stages with pitch and/or roll slightly out of spec. The stages with pitch
and/or yaw out of spec was sent back to the company to be reworked. The
ones with just roll out of spec could not be sent back because the company
did not guaranteed for this movement. So the two worst stages was chosen to
be used for the CCDs focus positioning (n◦ 4 e n◦ 9) where a great precision
is not requested. the other 16 stages will move 8 star enlargers the field
of view of the LOWFS. In the tabular below are listed the results of our
measurements in pitch and roll or the 18 stages received.
Motore
N◦
1
2
3
4
0
0
7
8
9
Pitch
(arcsec)
7
13
7
13
9
8
26
14
21
Roll
(arcsec)
5
22
7
08
6
17
8
8
40
Motore
N◦
10
11
12
93
14
18
16
15
18
Pitch
(arcsec)
7
10
11
10
5
12
12
6
9
Roll
(arcsec)
11
6
15
8
8
13
5
27
6
Tabella 1: Pitch and roll measurements. The measurement uncertainty is 2
arcsec.
8
6
6
PLOTS
Plots
The plots below show the various tests made to verify our set up and to
compare our measurements with the company ones.
9
6 PLOTS
10
Figura 6: Istogram of unbertainty measurement error in our tests. In the
y-axis there are the number of displacement of the centroid in function of
the centroid distance from the initial center.
6
PLOTS
Figura 7: Test of the stage n◦ 5. The solid lines are the pitch and the dashed
lines are the roll values.
Figura 8: Test of the stage n◦ 9. The solid lines are the pitch and the dashed
lines are the roll values. This stage (out of specs)is used for CCD focus
positioning.
11
6 PLOTS
Figura 9: Overloading test of the stage n◦ 7. The solid lines are the pitch
and the dashed lines are the roll values.
Figura 10: Diaphragm test of the stage n◦ 2. The solid lines are the pitch
and the dashed lines are the roll values.
12
6
PLOTS
Figura 11: Mirror test of the stage n◦ 3. The solid lines are the pitch and
the dashed lines are the roll values.
Figura 12: Tilt test of the stage n◦ 2. The solid lines are the pitch and the
dashed lines are the roll values. The black lines indicate the test without
spacers, the dark grey lines indicate the test with a spacer and the grey lines
the test with two spacers. The differences are below our uncertainty.
13
6 PLOTS
Figura 13: Comparison between two different set ups (horizontal and vertical
mirror). Only the pitch value is measured in both tests. The solid line is
the pitch for the set up with the horizontal mirror and the dashed line is
the set up with the vertical mirror.
Figura 14: Comparison between the tests with (solid) and without (dashed
) interferometer.
14
6
PLOTS
Figura 15: Comparison between our (solid) and PI (dashed) interferometric
tests.
15