AO document

Document prepared by M.V.Amaresh Kumar, email: mamar1982@gmail.com 1
AO Test Bench Simulation Documentation
Prepared by
M.V.Amaresh Kumar
IUCAA, Instrumentation Lab
Friday, September 30, 2011

This documentation is prepared by M.V.Amaresh Kumar (email:
mamar1982@gmail.com). The AO (adaptive optics) test bench is simulated
using ZEMAX software, which allows us to analyze the ray path geometry
and to enhance the desired output by using various advanced functions that
are available in the software. Softwares like Zemax that are available in the
market are OSLO, CODEV etc. At IUCAA I am having an opportunity to
use ZEMAX as the professional software to accomplish the task of
simulating an AO test bench.

Primary Data:
The aim of the AO test bench (experiment) is to emulate atmospheric
distortions and their corrections for an astronomical target at the center, with
an NGS (natural guide star) at the isoplanatic angle, which is considered to
be 30 arc second as a scenario. And a laser guide star in the system located
between the target and the NGS as shown in the Figure1. The telescope that
is presently in mind is 2m class present at IGO in Girawali, INDIA which
has the secondary mirror with f/10.
Figure1: Schematic diagram:‘A’ is an astronomical target, ‘B’ is laser guide
star (LGS) and ‘C’ is natural guide star (NGS). The dotted circle represents
the isoplanatic area.
In the AO test bench the target and the NGS are apertures with a source
in a black box and the LGS is an external laser impinging between the target
and the NGS as shown in Figure2, which is an illustration of how target,
NGS, LGS can be emulated in the laboratory environment. The wavefronts
(U) emanating from the target, NGS and LGS will be the starting point of
ZEMAX simulation. The flowchart of AO test bench is shown in Figure 3
A
B
C

which outlines the basics needed for the ZEMAX simulation and Figure 4
shows the schematic diagram of AO test bench.
Figure2: The schematic diagram of simulating a source (Q) in a black box
(P) with apertures (S, T) which are acting as a target and NGS, where (R) is
the isoplanatic angle and (V) is the LGS.
Figure3: Basic AO test bench flowchart.
P
Q R
S
T
U
V
Mirro
r

Figure 4: Schematic diagram of the AO test bench.
ZEMAX Simulations of AO test bench
ZEMAX is the software that optical engineers and designers around the
world choose for optical design, illumination, laser beam propagation, stray
light, freeform optical design and many other applications. In our case we
are using ZEMAX to simulate an AO test bench at IUCAA, Instrumentation
Lab. The aim of the simulation is to identify the optical components and
their parameters which best fit the test bench, Figure 5.shows the 3D layout
obtained from ZEMAX simulation.

Figure5: 3D layout of AO test bench with dimensions less than 45 cm length
and breadth.
Figure 5: Optical Elements and numbers
(1) Source position.
(2) Off axis parabolic mirror (for making the f/10 beam parallel with 2cm
beam diameter).
(3) Off axis parabolic mirror (using 3,4,5 convert the size of beam
diameter from 2 cm to 3 mm).
(4) Plane fold mirror.
(5) Off axis parabolic mirror (converting the beam to 3 mm in diameter).
(6) Dichoric plate with 2mm thickness used to move the LGS from the
main beam.

(7) Tip-tilt mirror (for 1st
order corrections).
(8) Plane fold mirror.
(9) Off axis parabolic mirror (using 9, 10 to separate the NGS from the
target).
(10) Mirror with radius of curvature 22 mm.
(11) Circular obscuration to stop the NGS and project it on the wavefront
sensor. Assuming the test bench will have a non changing NGS position.
(12) Off axis parabolic mirror.
(13) Plane fold mirror to project the target onto the Deformable mirror.
(14) Deformable mirror with both the LGS wavefront and the target
wavefront on the same plane, see figure 5.
(15) Fold mirror to direct the target light into science instrument.
(16) Wavefront sensor related to Deformable mirror.
Technical Details:
In the technical details I will call all the Zemax elements with numbers as
indicated in the above figure.
(a) Between 2 and 3: The beam diameter is 2 cm and the distance
between the two mirrors is flexible without any change of other
parameters. This is the place where the atmosphere can be simulated
as shown in the Figure 6. Also, I chose the beam diameter to be 2 cm
so to have enough spatial flexibility to conduct atmospheric
disturbance experiments with ease.
(b) 3,4,5 converts the beam diameter from 2 cm to 3 mm, keeping in
mind the size of the DM available commercially of the shelf which is
generally around few mm. Until the DM is particularly chosen and
procured we cannot specify the size of the beam diameter after
element 5 accurately. So, this tells that element 5 (only) might change
according to the choice of DM and can be corrected using Merit
function in ZEMAX.
(c) The spaces between 5, 7 and 8, 9 are depending on each other. This
distance will allow flexibility in placing the optomechanics around
the wavefront sensor which is placed at 11.
(d) 9, 10 and 11, if you want to work on these optical elements, work
with prior some knowledge on separating the NGS from target. One
of my suggestions to work with NGS is to have a beam sampler
which will take some amount of light onto the wavefront sensor.

(e) 13,14,6. Allows the target and the LGS to fall at the same location on
the deformable mirror.
(f) Beams from 14 passes onto 15 and 16 are parallel beams. After
element 15.
Figure 6: The atm 1 is the simulated fine atmosphere aberrations and atm 2
is the wedge prism which simulates the tip-tilt. Combined together is the
effect of atmospheric aberrations. Done using Zemax.
Analysis:
The whole analysis will concentrate on the output which will be feed to the
science instrument.
Parameters: 30 arc second isoplanatic angle so 15 arc sec is from center to
the edge of the isoplanatic disc which is 0.002487 degrees. Considering
various wavelengths I have chosen 350 nm, 551 nm and 850 nm for the
documentation purposes and the simulation can be done for any wavelength
in the margin specified or can surpass it. I have chosen the central axis for
one field which is basically the target and an off axis field at the 15 arc
3
4
atm
1
atm
2

second range which is considered as the NGS. The location of the LGS is
what I consider as in between the target and NGS, for the present simulation
I am using the target location itself as LGS location.
Spot Diagram:
Figure7: Spot Diagram for central field with wavelength 350 nm.

Figure10: Spot Diagram for off axis field with wavelength 350 nm.

In micro
meters
Central
field
(1)350nm
Central
field
(1) 551
nm
Central
field (1)
850 nm
Off axis
field (2)
350 nm
Off axis
field (2)
551 nm
Off axis
field (2)
850 nm
Airy Radius 14.12 22.23 34.29 14.12 22.23 34.29
RMS Radius 0.171 0.171 0.171 0.892 0.886 0.884
Geometric
Radius
0.289 0.289 0.289 1.796 1.720 1.688
Table1: Spot Diagram data at two fields for three different wavelengths

Wavefront:
Figure13: Wavefront for central field with wavelength 350 nm.

Figure16: Wavefront for off axis field with wavelength 350 nm.

In waves Central
field
(1)350nm
Central
field
(1) 551
nm
Central
field (1)
850 nm
Off axis
field (2)
350 nm
Off axis
field (2)
551 nm
Off axis
field (2)
850 nm
Peak to
Valley
0.0063 0.004 0.0026 0.0437 0.0278 0.0180
RMS 0.0015 0.0009 0.0006 0.0088 0.0056 0.0036
Table2: Wavefront data at two fields for three different wavelengths
Encircled Energy:
Figure19: Geometric Encircled Energy for both the fields at 350 nm.

Figure 20: Geometric Encircled Energy for both the fields at 551 nm.
Figure 21: Geometric Encircled Energy for both the fields at 850 nm.

FFT of Point Spread Function:
Figure20: Log fft of Point Spread Function for central field.
Figure21: Log fft of Point Spread Function for central field.

Concluding Remarks
The AO test bench simulation that is presented here is good to work till the
deformable mirror (14), as the deformable mirror is not yet decided and will
be over the time, the successor user of the simulation should be able to work
confidently with the above documentation and the Zemax code and incase
any questions regarding the documentation please email at
mamar1982@gmail.com. 1
I would like to thank Dr. Hillol K Das, Mr Chaitanya, Mr Krishna, Mr
Swapnil and Miss Garima for all the support and well wishes. It is an
opportunity to work with Prof. Ramaprakash who allowed me to understand
the adaptive optics system and its use in optical Astronomy.

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