Receiver Selection and CalibrationUnit for EHT-SPT (RESCUES)

Item Type text; Electronic Thesis

Authors Nguyen, Chi Hanh

Publisher The University of Arizona.

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ABSTRACT

The Event Horizon Telescope (EHT) uses a technique known as Very Long Baseline Interferometry (VLBI)

to combine radio telescopes all over the world into the equivalent of an Earth-size mirror. It will directly

image the immediate surroundings of Sagittarius A* (Sgr A*), the supermassive black hole (SMBH) at the

center of our galaxy. The 10-m South Pole Telescope (SPT) will provide EHT with its longest baselines,

which will significantly improve EHT resolution. To prepare SPT for EHT, a VLBI receiver was built with

its own optics system. The third mirror in the system requires a removable and repeatable mount that

can rotate, while being light and compact with excellent thermal insulation. In addition, the VLBI receiver

needs to have a motorized and remotely-controlled thermal calibration system to aid with data analysis.

This project, titled Receiver Selection and Calibration Unit for EHT-SPT (RESCUES), presents a mount

design that successfully satisfies the listed requirements. Finite Element Analysis (FEA) predicted that

RESCUES had high performance with deformation on the scale of µm, which is within the tolerance of the

optics system. Physical tests agreed with FEA simulations, confirming RESCUES reliability. RESCUES

was installed at the SPT and first light was detected in 2015.

ACKNOWLEDGEMENT

I wish to express my sincere gratitude to Dr. Daniel Marrone, my Thesis Advisor, for granting me an

incredible opportunity to work on this project. I am greatly indebted to him for his continuous guidance

and encouragement for my work as an undergraduate Astronomy major.

I am grateful to my family and relatives for their ceaseless support and faith in me, especially to my

parents and my elder sister who have never stopped believing in my ability.

I wish to extend my thanks to the faculty, staffs, and students at The University of Arizona for their

valuable inputs and technical assistance throughout the course of this work. My special appreciation goes to

Dr. Christopher Greer, Junhan Kim, Vanessa Bailey, the staff at the Arizona Radio Observatory, and the

staff at Steward Observatory machine shop.

I also want to send my thanks to Event Horizon Telescope and South Pole Telescope collaborators who

have aided in the progress of this work.

This material is based upon work supported by the National Science Foundation under Grant No. AST-

1207752.

DISCLAIMER

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the

author(s) and do not necessarily reflect the views of the National Science Foundation.

Contents

1 Introduction 1

1.1 Motivations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Design challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Calibration requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Project outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Removable and repeatable mounting 5

2.1 Design solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Kinematic mounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.2 Two-plate top structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.3 Hexapod design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.4 Material selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Design analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.1 Simulation setups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2.2 FEA predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Physical testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.1 Test setups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Thermal calibration system 16

3.1 System design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1.1 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1.2 Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2 System control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4 Test observations 18

5 Conclusion 19

References 20

List of figures i

List of tables iii

Appendix - Program to control motor iv

1 Introduction

1.1 Motivations

Supermassive black holes (SMBH) play crucial roles in modern astronomy and physics. They provide as-

tronomers with an opportunity to study General Relativity (GR) in its extreme regime, investigate flow and

accretion processes, and test models of black hole physics [1, 9] By studying SMBH and their immediate

environments, we can test the predictions of GR and the behavior of radiations in strong gravitational field.

It is now believed that most galaxies have a SMBH at their center. Our own galaxy, the Milky Way,

harbors a SMBH at its center, known as Sagittarius A* (Sgr A*) [8]. At 8 kpc away, Sgr A* is the closest

SMBH and the largest in angular size [5]. Its proximity to Earth allows astronomers to study SMBH with

details that cannot be obtained with other extragalactic targets. A compact source of 4x106 M� [2, 5], Sgr

A* has strong emissions in the submillimeter (submm), near infrared (NIR), and X-rays frequency ranges

[2]. The submm window has favorable conditions for detailed observation of Sgr A*. Sgr A* has a “submm

bump” due to a transition from optically thick to optically thin synchrotron emission that comes from the

immediate region surrounding the black hole [3, 4, 6, 7]. Thanks to this transition, we can, in theory, image

the surrounding environment of the black hole against the background emission [3]. The emission would

appear to be like a “shadow,” where the light on one side is red-shifted, while light on the other side is

enhanced by lensing [8, 2].

In addition to the physical motivation, observation made in the submm frequency window is also favored

in term of instrumentation. In the submm regime, data from many radio telescopes can be recorded and

combined in order to provide high-resolution images. This technique is known as Very Long Baseline In-

terferometry (VLBI) [10]. For a single-dish telescope, the angular resolution, meaning the smallest angle at

which two objects can be distinguished, is limited by diffraction. Diffraction limit is set by the diameter of

the primary mirror and the wavelength it observes [11]:

θ =λ

D(1)

where θ: angular resolution [rad], λ: wavelength of interest [length], and D : primary mirror diameter [length].

The angular size of Sgr A* shadow is about 45µarcsec [5]. At 1.3mm wavelength, the mirror needs to be the

size of the Earth, with surface precision on the scale of µm, in order to obtain angular resolution comparable

to that of Sgr A* shadow. However, it is impractical to build such an instrument. This is where VLBI

provides a solution. Unlike single dish, the resolution of VLBI is determined by the baseline, which the

distance between two telescopes [10]:

θ =λ

B(2)

where B : baseline [length]. By joining radio telescopes all over the world into an Earth-size array, we can use

VLBI to construct an equivalence of a giant radio telescope. This is the goal of the Event Horizon Telescope

(EHT) international collaboration [1]. Unlike conventional radio interferometer in which all radio dishes

record data simultaneously, EHT stations around the world will observe Sgr A* and record the data on

magnetic tapes, which will then be sent to the MIT Haystack Observatory for analysis and image synthesis.

This delayed process requires the data to have very high precision, especially for the phase information. As

a result, VLBI stations need specialized instruments for data collection.

The University of Arizona is contributing to this ongoing effort by incorporating the South Pole Telescope

(SPT) in Antarctica into EHT. Antarctica offers great weather condition for radio observation, including

1

high atmospheric submm transparency and low humidity [12]. More importantly, thanks to its strategic

location, SPT will provide EHT with the longest baseline and continuous observations of Sgr A* (figure 1).

Originally constructed as a 10-m single-dish telescope for high-resolution imaging of the Cosmic Microwave

Background (CMB) [12], SPT is not equipped for VLBI. As a result, a new VLBI receiver is being built at the

Department of Astronomy. The new receiver has its own optics and an atomic clock that can record phases

to very high precision, as required by VLBI. This new SPT VLBI receiver will have state-of-the-art detectors

and electronics, allowing SPT to obtain data with signal-to-noise comparable to that of other stations like

the Combined Array for Research in Millimeter-wave Astronomy (CARMA).

Figure 1: Incorporating SPT into the EHT array will significantly extend the baselines (blue lines), turningthe array into the equivalent of an Earth-size telescope. Consequently the angular resolution will be tripled,pushing EHT closer to achieving the sensitivity needed to image the immediate environment of Sgr A*. Theyellow line shows the test VLBI observation between SPT and the Atacama Pathfinder Experiment (APEX)in January 2015, with baseline spanning over 7000 [km]. Image credit: D. P. Marrone, The Department ofAstronomy, The University of Arizona.

1.2 Design challenges

One big issue the SPT VLBI receiver faces is that its new optics system requires the third mirror, also know

as the tertiary mirror, to be mounted right on top of the receiver. The tertiary mirror will be needed when

SPT collects VLBI data; but it should be removed when SPT is observing as a single-dish telescope. This

operation calls for several requirements that the tertiary mirror mount needs to address. In order to be

properly implemented in the SPT VLBI receiver, the mount has to satisfy the following requirements:

1) Rotatability : One of the most important features of SPT VLBI receiver is that it will observe at two

frequency windows, namely at 230 GHz and 345 GHz. Each frequency window has its own feedhorn. Due to

the size limit imposed by the available space at the SPT, the two feedhorns are placed at an angle of 5.73°from the vertical axis of the receiver (figure 2a). The tertiary mirror needs to be rotated by an angle of 5.73°to guide the light from its surface into one of the two feedhorns, depending on which frequency that we are

observing at. The rotation axis is defined by the chief ray, the central path that radiation follows through

SPT VLBI optics system (figure 2b). The mirror mount has to provide the necessary rotation to couple the

light with the desired frequency window for each VLBI run.

2) Compact size and weight : the receiver will be placed inside a cabin that currently houses many of SPT

2

(a) Two-feedhorn design (b) Rotation axis

Figure 2: (a) The inside of the receiver with one of the two feedhorns (230GHz window, in blue) assembled.The tertiary mirro must rotate around the chief ray from the secondary to direct light to the two feedhorns,which are tilted by 5.73° from the vertical. (b) Side view of the path that the chief ray travels: (A) from theSPT 10-m primary mirror to the secondary mirror (left), (B) light reflected off the secondary to the tertiarymirror (right), and (C) light from the tertiary mirror to one of the feedhorns. Line (B) defines the rotationaxis for the tertiary mount. The angle between (B) and (C) is 82.55°.

electronics. The small size of the receiver cabin and existing instruments limit how large the mirror mount

can be. Moreover, SPT VLBI optics defines a different light path than the one currently used by SPT as a

single-dish telescope. As a result, some optics elements of SPT VLBI receiver, including the tertiary mirror,

stay in the light path of SPT single-dish optics. When SPT makes observations as a single-dish telescope, the

tertiary mirror will need to be taken off the receiver. The limited space in the cabin poses another problem:

not only does the mount have to be removable, it also has to be light in weight and compact enough that a

person can carry it 20 feet up a ladder in the antarctic winter safely.

3) Strength, precision, and accuracy : While SPT VLBI receiver will be kept inside a cabin, the tertiary

mirror has to stay outside due to its location in the VLBI optics. Consequently, the mirror mount will be

partially exposed to the harsh antarctic environment, including snow, winds, and low temperature (-50°C).

The mount needs to be strong and stiff to support the weight of the mirror when exposed. At the same time,

its components must be built to high precision and accuracy (down to tens of µm) so that the mirror can

be maintained at the right position in the optics system. Additionally, the partial exposure means that the

mount needs excellent thermal insulation between the exterior and interior components, to avoid introducing

temperature fluctuations to the data.

4) Repeatability : Due to the necessary frequent removal and re-installation, it is crucial that the mount has

excellent repeatability to guarantee exact optics setup and reproducible data between different observations.

This is the most important challenge facing the mirror mount of SPT VLBI receiver.

1.3 Calibration requirement

In addition to the need for a removable and repeatable mounting, the SPT VLBI receiver also need a

conversion factor to convert the electrical response (in voltage) into a temperature measurement. We can

then use this temperature measurement to interpret meaningful physical data of the observed astronomical

3

target.

One way to calibrate the data is by observing the blank sky with no strong radio source and recording

its temperature as Tsky, then observing a hot load with known temperature Thot. Since we know the

temperatures of both the sky and the hot load, we can match them with the voltage measurements of the

receiver and establish a thermal scale for our data. Due to difficulty in accessing the receiver once the SPT

is observing, a motorized and remotely controlled thermal calibration system is needed. This system will

move a hot load to cover the windows of the receiver for Thot measurement, and then move the load off the

windows to record Tsky.

1.4 Project outline

This paper will report a design that successfully addresses the above challenges in mounting and provides

a system to calibrate the temperatures of the data. The finished mount is titled Receiver Selection and

Calibration Unit for EHT-SPT (RESCUES). The computer-aided design (CAD) was devised and studied

using Autodesk Inventor® [18], and constructed at The Department of Astronomy, The University of Ari-

zona.

This project was proposed and approved in May, 2014. Literature review and initial design were conducted

during the following summer. Material selection and structure analysis were carried out in August. The

design was finalized and parts fabrication took place over the next three months. Initial physical tests were

done in the lab before RESCUES was deployed to Antarctica, together with the SPT VLBI receiver, at the

end of 2014. RESCUES saw its first test operation at the SPT in January, 2015.

§2.1 describes the design of the removable and repeatable mount, while §2.2 presents how RESCUES

was tested with stress analysis and shows the predicted effects of gravity on the performance of the mount.

§2.3 analyzes physical testing data. §3 shows the thermal calibration unit, including the components and

control system. §4 presents preliminary observations from SPT. A summary of the project and conclusion

are presented in §5.

4

2 Removable and repeatable mounting

2.1 Design solutions

In order to overcome the challenges described in §1.2, many design options and materials were evaluated.

The final design of RESCUES (figure 3) encompasses the following features, which effectively solve the

issues at hand: kinematic mounts to ensure repeatability, two-plate top structure to allow the mirror to turn

relative to the mount legs, hexapod design to provide rigid support and versatility, and fiberglass material

for strength and thermal insulation.

(a) Front view (b) Side view

Figure 3: (a) Front and (b) side view of RESCUES when the mirror is mounted and rotated to 5.73°fromvertical axis. Main components are color coded for easy identification: (A - Magenta) Ring, (B - Green)Main plate, (C - Black) Legs, (D - Yellow) Fixed plate, (E - Dark blue) Rotating plate, (F - Grey) SPTVLBI tertiary. mirror.

2.1.1 Kinematic mounts

Kinematic design is a well-known solution for constrained motion. Kinematic design works by removing

some or all of the six degrees of freedom available to a rigid body (three translations and three rotations)

[13]. When all six degrees of freedom are constrained, the position of the structure is fully defined. One

way to remove all six degrees of freedom is by mating three spherical balls, placed at 120° apart from each

other and at the same radius from the center of a surface, with three v-shaped grooves that also form 120°angular separations. Each ball makes two points of contact to the groove that it sits in, and therefore three

pairs of balls and grooves limit all translational motions. When a flat surface is mounted on top of the balls,

stopping them from rolling, all rotational motions are prevented. Kinematic balls and grooves with high

precision are readily available from the industry, with spherical surface uncertainty of up to ±2 [µm] [20].

Kinematic design is applied in RESCUES by inserting three sets of Bal-tecTM kinematic mounts in its

base. Each mount has two components: a vee block that serves as the groove, and a ball that sits on top of

5

the vee block. The vee blocks are fixed into a ring that will stay on top of the SPT VLBI receiver permanently

(figure 4a). The balls are attached to the underside of the RESCUES main plate (figure 4b) that will then

sit on top of the ring. This plate is where the legs and other supporting instruments of RESCUES are kept.

A gap of 6.35mm is left between the main plate and the ring. This gap enables the kinematic mounts to

dictate the position of the main plate on top of the ring.

(a) Ring fixed on top of VLBI receiver (b) Bottom view of underside plate

Figure 4: Kinematic mounts incorporated into RESCUES design. (a) Three vee blocks are attached onto aring that will sit permanently on top of the SPT VLBI receiver. The vee blocks form an equilateral triangle.(b) Three precision balls are used to mate with the vee blocks. Each ball sits inside a counterbore hole thatis fractionally larger than the vee block. The counterbore holes reduce the distance between the main plateand the ring, while still maintaining a gap between them so that the kinematic mounts have full control onwhere the main plate sits relative to the ring so that the mounts determine the location of the main platerelative to the ring.

Once the main plate is assembled on top of the ring, the legs can be adjusted to bring the tertiary mirror

into its correct position. The legs are then locked down to fix the mirror’s position. When the mirror is not

needed, only the main plate needs to be dismounted from the ring. The mirror mount can be safely and

quickly assembled again by attaching the plate on top of the ring. The kinematic mounts ensure that the

plate and the ring are accurately and quickly relocated in the same position.

2.1.2 Two-plate top structure

To provide the tertiary mirror with a rotation on the scale of a few degrees, the top part of RESCUES is

split into two pieces.

The first piece is a fixed plate that the legs are mounted to (figure 5a). This plate features a small

enclosure at the back that acts as a mount for the second piece. This part of the plate was originally

designed to resemble a closed box with a motorized system to rotate the mirror. However, due to the weight

limit the motorized system was not implemented, and the sides of the closed box were hollowed to reduce

weight and allow easy access and assembly. Adjusting the height and inclination of the fixed plate will bring

6

the tertiary to its position in the optics system. As its name suggests, this plate will stay in one fixed position

during observations.

The second piece that makes up the top of RESCUES is a rotating plate, which has a shaft that is inserted

into the enclosure at the back of the fixed plate. The rotating plate is designed to be parallel to the back of

the tertiary mirror. This configuration allows the rotating plate to stay close to the mirror, hence reducing

the torque produced by the mirror on the plate, as well as making the mount more compact. However, this

design requires the shaft to be raised to an angle of 54.96° from the plane of the tertiary mirror’s back so that

the shaft can lie along the chief ray (figure 5b). The fact that the shaft is not perpendicular to the rotating

plate introduces some limitation into the rotation of this plate relative to the fixed plate. This limitation

was solved by smart choices in the shapes of both plates, allowing one plate to rotate up to 15° without

running into the other.

(a) Fixed plate. (b) Rotating plate.

Figure 5: (a) Fixed plate with an enclosure (A) in the back to support the second piece. The plate is actuallymade up of several pieces that are designed to be light-weighted and use as little space as possible. (B) Sidepieces are where the legs will be mounted to. (C) A rib running across the widest part of the plate preventsbending. (D) A small tab at the top of the plate is used to keep the rotating component of the top structurein place. (b) Notice the unusual angle cuts in the shaft placement and the shape of the rotating plate. Theangles are defined by the rotation axis, as well as the limits posed by the size of the mirror, the plates, andtheir relative positions.

The tertiary mirror is mounted to the rotating plate at three points, using steel leveling sets. The leveling

sets allow the distance between the back of the mirror and the rotating plate to be finely adjusted to high

accuracy [21]. The rotating plate is attached to the fixed plate with a series of sleeve bearings and shaft

collars. The fixed plate also has a movable metal tab that will lock the rotating plate into position once it is

rotated to the right angle that couples light from the sky into the observing horn (figure 5a, also see parts

D, E, and F in figure 3b).

7

2.1.3 Hexapod design

RESCUES uses a hexapod design to support the tertiary mirror. Hexapod structure is versatile and allows

the position of the mirror to be adjusted in all directions (up-down, left-right, and front-back), as well as

providing tilt and tip for the mirror [14]. These motions are achieved by changing the lengths of the six legs

that make up the hexapod structure. In addition to versatility, hexapod design constrains the mirror in all

six degrees of freedom while being very compact and lean, allowing the legs to clear the light path.

Figure 6: Hexapod design.

Tapped hollow tubes and high precision rod ends [22] were used to construct the hexapod of RESCUES.

Each tube has a right-handed rod end screwed into one end, and a left-handed one into the other end. The

rod ends are then attached to the main plate at the bottom and the fixed plate at the top (figure 6). The

length of each leg can be changed by turning the tubes.

2.1.4 Material selection

The top structure of RESCUES and the mirror are exposed to antarctic environment. The bottom part

of RESCUES is kept inside a cabin with higher temperature. This results in a temperature gradient in

the mount. The legs of RESCUES can act like bridges, transfering heat from the bottom part into the

mirror. In order to avoid that scenerio, the legs are made up of garolite (also known as G-10), which is

fiberglass embedded in epoxy. G-10 has strength comparable to that of metals, while its density and thermal

conductivities are significantly lower (table 1). These qualities satisfy RESCUES’ requirements of strong,

well-insulated, and light-weighted structure.

The dimension of the legs are chosen by maximizing the stiffness of G-10 while minimizing the weight

and size. The lengths of the legs are constrained by the positions of the cryostat’s top and the tertiary

mirror. By studying a variety of G-10 tubes with different outer diameters (OD) and inner diameters (ID)

vs. displacements, we can identify the most suitable tube size. The smallest ID that was investigated is

0.9525 [cm], and the largest OD is 2.54 [cm]. A simple relationship between the load, displacement, and

8

stiffness of a cylindrical structure can be written by applying Hooke’s law:

P = kδ (3)

where P [N]: effective load, k [N/m]: material stiffness, δ [m]: displacement. The load in question is the

gravitational pull exerts on the legs due to the top structure and the tertiary mirror. Material stiffness can

be found by looking at the Young modulus and the geometry of the tube:

k =EA0

L0(4)

with E [pa]: the Young modulus, A0 [m2]: cross-sectional area parallel to the direction of the applied load,

L0 [m]: the relax, uncompressed length of the legs. Substitute equation (4) into (3) and solve for δ, we can

estimate the displacement as:

δ =PL0

EA0(5)

The longest leg of RESCUES needs to be L0 ' 0.29464 [m]. The total gravitational pull resulted from

the mirror and the top structure of RESCUES is overestimated to be P ' 200 [N]. Figure 7 shows the

estimated deformations of different tube sizes. The tubes are divided into five groups based on their ID and

wall thickness, which is one half of the difference between the OD and ID. The tubes with wall thicknesses

above 0.3 [cm] show a smaller range of displacements. For a cutoff displacement of 20 [µm] (≤ 0.01% of the

leg lengths), the smallest tube has OD = 1.9050 [cm] and ID = 0.9525 [cm]. This tube size was selected for

RESCUES legs.

Figure 7: Estimated displacement vs. dimensions for various G-10 tube sizes.

Computer simulations, as discussed in the next session, provides more precise predictions for deformations,

as well as predicted the changes in the effects of gravity when the mount is placed at different inclinations.

9

2.2 Design analysis

RESCUES design solution was tested by simulation before fabrication to ensure the safety and strength of

the design. In-lab physical tests were carried out after the parts were machined and assembled to compare

with simulated values.

2.2.1 Simulation setups

Finite Element Analysis (FEA) was used to test the tentative design of RESCUES. The FEA used in this

project was included in Autodesk Inventor®. FEA splits the total assembly into simplified models that can

be approximated and characterized with finite numerical operations [16]. The FEA performed on RESCUES

analyzed stress, in particular finding the bending of each component in all three dimensions due to a specified

load.

In order to obtain reliable predictions of the performance of RESCUES, it is essential that FEA inputs

are as accurate as possible. Important details for FEA include the physical and mechanical properties of the

materials that the design uses, the relationship to other instruments like the SPT VLBI receiver, and the

direction of the applied load. The mechanical properties were provided by the part supplier when applicable,

and from Autodesk material library. For mechanical properties with wide range of values, the lowest numbers

were used to avoid underestimating the strengths of the structure.

(a) (b) (c)

Figure 8: FEA simulations.(a) The design is split into smaller sections for FEA. (b) Gravity acting alongthe vertical orientation of the mount (arrow). (c) Gravity acting normal to the vertical orientation of themount (arrow).

Two simulations were conducted on RESCUES to understand the behavior of the design and evaluate

its ability to fulfill its scientific purpose. The first simulation defines the structure to be sitting in a straight

up position, with gravity acting directly downward (figure 8b). This is the rest position of RESCUES where

the mount and the receiver can be accessed for assembling and maintenance. In the second simulation, the

mount is rotated 90° so gravity is now pulling “horizontally” with respect to the mount orientation (figure

8c). This inclination is higher than the observing position ('30°), so this second configuration gives the

upper limits of displacements and bending in the mount due to gravity. In both cases, the ring and the main

plate were kept fixed. This is due to the fact that RESCUES will be sitting on top of the receiver, which can

firmly support these parts, and therefore gravity does not deform them significantly. The properties of the

materials used in the design are also kept constant for all simulations. In addition, the mirror was turned to

10

Mechanical and thermal properties of important materials in RESCUES design.

Properties G-10 Stainless steel Aluminum

Density [kg/m3] 1.91 8.08 2.71

Tensile strength [pa] x 108 2.41 5.40 3.10

Young modulus [pa] x109 19.9 193.0 68.9

Thermal conductivity† [W/m•K] 0.45 13.0 210

Linear contraction†† 0.20% 0.10% 0.15%

†: values at 225K.

††: fractional contraction corresponding to a temperature change from 293K to 225K.

References: [15, 17, 19, 18].

Table 1: G10, stainless steel, and aluminum are the main materials in the majority of RESCUES parts. Itis noticeable that G-10 is very close to steel and aluminum in strength, but its thermal conductivity is lowerby an order of magnitude, making it a better insulating material.

the right by 5.73° to duplicate the rotation that the mount has to achieve when coupling the light into one

of SPT VLBI feedhorns.

2.2.2 FEA predictions

Table 2 shows the maximal displacements of RESCUES when gravity acts in two different directions.

Predicted displacements of RESCUES.

Force direction δ3D [µm]

Vertical ' 11

Horizontal ' 60

Table 2: FEA predictions for the bending of RESCUES design due to gravity. δ3D is the maximal displace-ment considering all three directions.

A visual display of the three-dimensional (3D) displacements is presented in figure 9. The displacements

in individual axis are shown in figure 10. Figures 9a and 9b show that the deformations in the legs of

RESCUES in the rest position agree with estimations based on Hooke’s law. The displacement values range

between 0 to 11 [µm], which is '84% the value calculated in §2.1.4. The horizontal case has higher levels of

deformations. However, even in this extreme case, the changes in the legs are roughly 30[µm], which is still

about 0.01% of the leg lengths.

In addition, it is worth noticing that there are some offsets in the displacements of the mirror between

x-, y-, and z-directions, most clearly in figures 10a, 10c, and 10f. The offsets come from the non-symmetric

placement of the mirror, corresponding to the 5.73° rotation. The mirror is moved more significantly in the y-

direction compared to the other two axes, which is to be expected since this axis is parallel to the applied load.

The horizontal case, however, shows no such offsets (figure 11). In the case of the horizontal configuration,

the x-axis, which lies parallel to the direction of gravitational pull, still shows the most displacement as

expected. It can be concluded that while all three axes are affected by the non-symmetrical set up of the

11

mount, y-axis is more affected by gravity when the mount is in the rest position. This knowledge is crucial

in aligning the mirror. Special attention needs to be paid to stiffening the mount in the y-axis before the

mount and the mirror are inclined into the observing position.

FEA provides an overall view of how the entire structure, including the mount and the tertiary mirror,

would be affected by an applied force equal that of their combined gravitational pull. The simulations confirm

that RESCUES is indeed strong enough to hold the mirror. The predicted bending and displacements in

the plates are to be compared with physical test.

(a) Front view, vertical case. (b) Back view, vertical case.

(c) Front view, horizontal case. (d) Back view, horizontal case.

Figure 9: Predicted 3D displacements for the case where applied gravity acts (a,b) vertically downward, asin figure 8b and (c,d) horizontally, as in figure 8c. In the vertical case, there is an offset in the direction ofthe displacement of the mirror, while in the horizontal case the displacement is almost symmetrical.

12

(a) X (b) X

(c) Y (d) Y

(e) Z (f) Z

Figure 10: Predicted displacements in individual x-, y-, and z-directions for the case where gravity actsvertically downward, as in figure 8b. Notice the scales are different between x-, y-, and z-directions, withy-axis having the most changes and z-axis having the least effect. This is due to the direction of the appliedload.

13

(a) X (b) X

(c) Y (d) Y

(e) Z (f) Z

Figure 11: Predicted displacements in individual x-, y-, and z-directions for the case where gravity actsvertically downward, as in figure 8c. Unlike in the previous figure, x-axis has the most displacements,consistent with the direction of the applied load.

14

2.3 Physical testing

2.3.1 Test setups

After being fabricated and assembled, RESCUES was evaluated in the lab. The physical tests consisted

of applying force on the mount and measuring the displacements of the plates on the mount with a dial

indicator. The dial indicator used in this project has a magnetic base that keeps it stand firmly on a metal

foundation. Its arm is sensitive to minuscule changes on the scale of µm, which is one order of magnitude

smaller than the tolerance that the mount needs to achieve. This level of precision is necessary in order to

properly documented the accuracy of this instrument.

Similar to the computer simulations, the weights are applied in two directions (figure 12). The instrument

and the dial indicator were mounted on an optics table to isolate the structure from environmental vibrations.

(a) (b)

Figure 12: Test setups. The arrows show the directions of applied loads (a) vertically downward and (b)horizontal.

2.3.2 Results

Test results are summarized in table 3. The average measurements of displacements are smaller than the

simulations predicted by an order of magnitude. An explanation for the overestimation is that the Young’s

modulus of G10 is in fact greater than assumed in FEA, hence the mount is stiffer in reality. Thanks to

the conservative choice of strength, FEA did not underestimate the deformation. Additionally, the tests

show that the displacement in the horizontal configuration is higher than that of the vertical configuration.

This trend is consistent with what the simulations suggest. The physical tests confirm that RESCUES can

provide reliable mounting for the tertiary mirror.

Test results.

Force directions δ3D [µm]

Vertical ' 1

Horizontal ' 4

Table 3: Average displacements as measured by the dial indicator. Notice that the displacements are anorder of magnitude lower than predicted.

15

3 Thermal calibration system

3.1 System design

3.1.1 Components

The thermal calibration system consists of five main parts: a programmable motor [23], a lead screw [24], a

shaft support [19], a hot load glued to a thin aluminum tray, and two limit switches to control the positions

of the load. The full thermal calibration system is mounted on top of the RESCUES main plate (figure 13).

Each limit switch is mounted at one end of the plate, on the side of the shaft support. The switches are

places as close to the edge of the plate as possible so that the load can have the largest range of motion.

Figure 13: The thermal calibration system on the main plate. The parts are color coded for easy identifica-tion. (A - black) The motor, (B) the lead screw, (C) the hot load (dark grey) glued to an aluminum tray(magenta), and (D - olive) the shaft support. The black arrows show the directions of motions correspondingto “off window” and “on windows” configurations. In this figure, the load is in “on windows” configuration.Not pictured: two limit switches, one for “on” and one for “off” position.

A DC power supply is needed for the motor. The motor uses RS-485 serial communication, with four built-

in I/O channels. A serial-to-USB encoder [25] is used to connect the motor to a computer. A user interface,

known as Motion Control Programmer, is provided by the manufacturer. Motion Control Programmer can

set up a terminal to send direct commands or download programs to the internal memory of the motor.

The motor has its own programming language, which allows us to define simple tasks and use conditional

statements. A program was written to run the motor to and from the windows, using two I/O channels,

where each channel corresponds to one limit switch. The program can be found in the Appendix.

3.1.2 Wiring

A shielded cable is used to connect the motor with a DC power and the serial port. Figure 14 shows the

wiring diagram of the thermal calibration system.

16

Figure 14: The wiring diagram of the thermal calibration system, connecting the motor to the serial-to-USBencoder, a DC power source, and the switches.

3.2 System control

The program controls the motor using two branches: one branch, labelled Aa, moves the load to the “on

windows” configuration, while the other branch, labelled Ab, moves the load to the “off windows” one.

Each branch works by checking the status of the I/O channel at its corresponding position. For example,

when the load is completely off the windows, the “on windows” switch is open and the “off windows” switch

is closed. The I/O channels were set up such that an open switch equals an active channel and vice versa, a

closed switch means an inactive channel. When the motor receives an “active” status from the “on windows”

switch, it knows that it can move toward the windows. It can then execute branch Aa to move the load

toward the windows. At the same time, the motor recognizes that it is at the “off windows” position because

the channel there is inactive. When it is at this position, the motor cannot go any further off the windows

or else it would run into the base of RESCUES legs. Thanks to the inactive status, the motor knows where

the load it, and that it cannot proceed any further in the “off windows” direction. Therefore, branch Ab

cannot be executed and the motor will return a number, letting us know the current position of the thermal

load.

When the load reaches the “on windows” position, the switch there will be closed and its I/O channel is

updated to inactive status. The motor responds by coming to a stop and returning a number corresponding

to this position, updating the user with where the load is. “Off windows” position is then available, and

branch Ab can be executed to bring the load away from the windows.

17

4 Test observations

RESCUES, together with the SPT VLBI receiver and supporting electronics, were deployed to Antarctica

and installed on the SPT in late 2014 (figure 15). First light was obtained on January, 2015.

The first detection of the new SPT VLBI receiver was the Moon (figure 16a). Additionally, a giant

molecular cloud near Sgr A*, namely Sagittarius B2 (Sgr B2), was mapped to confirm that the new receiver

could point at the galactic center. The VLBI capability was evaluated in partnership with the Atacama

Pathfinder Experiment (APEX) station in Chile. Observational results verified that SPT can now perform

high-quality VLBI observations with other telescopes around the world, providing excellent data and tripling

the angular resolution of EHT.

(a) (b)

Figure 15: RESCUES at the SPT. (a) The mount (foreground) in its rest position in the optics system. (b)Side view of RESCUES, with the tertiary mirror, sitting on top of the SPT VLBI receiver. The receiver(purple) and the two circular windows can be seen through the hollow space in the center of the main plate.(Photos credit: D. P. Marrone, The Department of Astronomy, The University of Arizona.).

(a) Submm wavelength. (b) Visible wavelength.

Figure 16: Images of the Moon at two different parts of the electromagnetic spectrum. Notice that the (a)submm map measures the surface temperature, rather than the reflected light of the Sun that is seen in the(b) optical image. The red regions have higher temperature. The crescent is waning so it was previouslymore illuminated, resulting in a wider crescent (dark red) in surface temperature.

18

5 Conclusion

The study of SMBHs holds enormous potential for our study of physics in extreme gravitational field. Sgr A*,

the SMBH at the center of the Milky Way, is the best candidate for direct imaging of a black hole’s immediate

environment thanks to its proximity and its characteristic emission. With the help of VLBI, EHT array aims

to achieve resolution several times smaller than the angular size of the event horizon. Incorporating the SPT

station into EHT will significantly improve the resolution of the array, pushing the project closer to their goal.

In order to prepare the SPT for VLBI observations, a new receiver and optics system have to be implemented.

One element of the optics system, the tertiary mirror, requires special mounting and positioning. Its mount

needs to be removed and re-installed between different kinds of observations, so repeatability is the foremost

quality. Moreover, it has to provide some rotations to couple signals to one of the off-center feedhorns.

Additionally, it also has to be strong to support the mirror, while being light-weighted and compact to be

removed and re-installed manually. Another design requirement is excellent thermal insulation to minimize

instrumental uncertainties in the data.

The mount design that successfully addressed the above challenges, named RESCUES, was devised and

investigated. RESCUES applies kinematic mounting elements to achieve highly precise repeatability. It uses

a two-part top structure to rotate the mirror to couple signals to the receiver, while its hexapod design

reduces weight and size while maintaining rigid support for the mirror. In addition, garolite legs provide the

mount with excellence strength and thermal insulation.

Computer simulations evaluated the stiffness of the mount. The simulations used two different config-

urations, where gravity acted along the vertical axis of the mount in one case, and perpendicular to the

vertical axis in the second case. Simulation results confirmed that RESCUES had small displacements of

0.01% of the lengths of the legs. The simulations also documented the level of deformations in different areas

of the mount, which helped improve the accuracy of the optics alignment. Physical testing was conducted

in a controlled environment, where force was applied in two directions corresponding to the two simulations,

and the deformations of the mount were recorded. The average displacements were found to be an order of

magnitude smaller than predicted by the simulations. These results confirmed the stiffness and reliability of

the mount.

In addition to repeatable and removable mounting, RESCUES provides the SPT VLBI receiver with a

motorized and remotely controlled thermal calibration system. The thermal calibration system is mounted

on top of the main plate. It can communicate with a computer using serial port. A program was written to

control the load, utilizing the motor I/O channels and two limit switches.

RESCUES was installed at SPT in late 2014 and test observations confirmed its excellent performance.

With the implementation of the new SPT VLBI receiver and RESCUES, the SPT now has the capability to

operate as an EHT station.

19

References

[1] Doeleman, S., et al., Imaging an event horizon: submm-VLBI of a super massive black hole (2009)

Astro2010: The Astro. and Astrophys. Dec. Survey, Science White Papers, no. 68.

[2] Doeleman, S., Weintroub, J., Rogers, A. E. E., Plambeck, R., Freund, R., Tilanus, R. P. J., et al,

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre (2008)

Nature, 455, 78-80.

[3] Falcke, H., & Markoff, S. B., Toward the event horizon — the supermassive black hole in the Galactic

Center (2013) Class. Quantum Grav., 30.

[4] Falcke, H., Goss, W. M., Matsuo H., Teuben, P., Zhao, J. H., & Zylka, R., The simultaneous spectrum

of Sagittarius A* from 20 centimeters to 1 millimeter and the nature of the millimeter excess (1998)

ApJ, 499, 731-34.

[5] Ghez, A. M., Duchene G., Matthews K., Hornstein, S. D., Tanner A., Larkin, J., et al, The first

measurement of spectral lines in a short-period star bound to the galaxy’s central black hole: a paradox

of youth (2003) ApJ Letters, 586, L127-131.

[6] Serabyn, E., Carlstrom, J., Lay, O., Lis, D. C., Hunter, T. R., Lacy, J. H., & Hills, R. E., High-frequency

measurements of the spectrum of Sagittarius A* (1997) ApJ Letters, 490, L77-81.

[7] Mezger, P. G., Zylka, R., Salter, C. J., Wink, J. E., Chini R., Kreysa, E., & Tuffs R., Continuum

observations of SGA A at mm/submm wavelengths (1989) Astro. Astrophys., 209, 337-48.

[8] Melia, F., & Falcke, H., The supermassive black hole at the galactic center (2001) Ann. Rev. Astro. and

Astrophys., 39, 309-52.

[9] Broderick, A. E., Johannsen, T., Loeb, A., Dimitrios, P., Testing the no-hair theorem with Event

Horizon Telescope observations of Sagittarius A* (2014) ApJ, 784, 14pp.

[10] Thompson, A., Moran, J., Swenson, G. W., Interferometry and Synthesis in Radio Astronomy, Second

Edition (2001) Wiley-VHC.

[11] Rieke, G. H., Measuring the Universe: A Multiwavelength Perspective (2012) Cambridge University

Press.

[12] Carlstrom, J. E., Ade, P. A. R., Aird, K. A., Benson, B. A., Bleem, L. E., Busetti, S., et al., The 10

Meter South Pole Telescope (2011) Pub. of the Astro. Soc. of the Pacific, 123, 568–81.

[13] Moore, J. H., Davis, C. C., Coplan, M. A., Building Scientific Apparatus, Second Edition (1991) Perseus

Books Publishing.

[14] Chini, R., The Hexapod Telescope - A never-ending story (2000) Rev. Mod. Astro. 13, 257-68.

[15] Goodzeit, C. L., An introduction to mechanical design and construction methods (2001) US Particle

Accel. School (2001).

[16] Szabo, B. A., & Babuska, I., Finite Element Analysis (1991) John Wiley & Son, Inc.

[17] NIST Material Measurement Laboratory, Cryogenic Technologies Group.

20

[18] Autodesk Inventor®. Autodesk Inc. (2015) www.autodesk.com

[19] McMaster-Carr Supply Company (2013) www.mcmaster.com

Shaft support part number: 1049K130.

[20] Bal-tecTM, Inc. (2015) www.precisionballs.com

Vee-block: VB-75-SM.

Precision ball: 750-TBR-T.

[21] J. W. Winco, Inc. (2015) www.jwwinco.com

Leveling set: 660MCF0-AK.

Spherical washer: 25WLA8.

[22] RBC Bearing, Inc. (2015) www.rbcbearings.com

Rod end: HM4.

[23] Schneider Electric Motion USA (2015) www.motion.schneider-electric.com

Motor series: MDrive®14 Plus Motion Control.

Part number: MDI1CRZ14A4.

[24] MISUMI USA: Industrial Configurable Components Supply (2015) www.us.misumi-ec.com

Lead screw: MTSWK12-250-S10.

Support units: MTWZ-Set.

[25] CommFront Communications (2015) www.commfront.com

Encoder: USB-Serial-3 Opto-isolated Converter.

21

List of Figures

1 Incorporating SPT into the EHT array will significantly extend the baselines (blue lines),

turning the array into the equivalent of an Earth-size telescope. Consequently the angular

resolution will be tripled, pushing EHT closer to achieving the sensitivity needed to image

the immediate environment of Sgr A*. The yellow line shows the test VLBI observation

between SPT and the Atacama Pathfinder Experiment (APEX) in January 2015, with baseline

spanning over 7000 [km]. Image credit: D. P. Marrone, The Department of Astronomy, The

University of Arizona. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 (a) The inside of the receiver with one of the two feedhorns (230GHz window, in blue) assem-

bled. The tertiary mirro must rotate around the chief ray from the secondary to direct light

to the two feedhorns, which are tilted by 5.73° from the vertical. (b) Side view of the path

that the chief ray travels: (A) from the SPT 10-m primary mirror to the secondary mirror

(left), (B) light reflected off the secondary to the tertiary mirror (right), and (C) light from

the tertiary mirror to one of the feedhorns. Line (B) defines the rotation axis for the tertiary

mount. The angle between (B) and (C) is 82.55°. . . . . . . . . . . . . . . . . . . . . . . . . 3

3 (a) Front and (b) side view of RESCUES when the mirror is mounted and rotated to 5.73°from

vertical axis. Main components are color coded for easy identification: (A - Magenta) Ring,

(B - Green) Main plate, (C - Black) Legs, (D - Yellow) Fixed plate, (E - Dark blue) Rotating

plate, (F - Grey) SPT VLBI tertiary. mirror. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Kinematic mounts incorporated into RESCUES design. (a) Three vee blocks are attached onto

a ring that will sit permanently on top of the SPT VLBI receiver. The vee blocks form an

equilateral triangle. (b) Three precision balls are used to mate with the vee blocks. Each ball

sits inside a counterbore hole that is fractionally larger than the vee block. The counterbore

holes reduce the distance between the main plate and the ring, while still maintaining a gap

between them so that the kinematic mounts have full control on where the main plate sits

relative to the ring so that the mounts determine the location of the main plate relative to

the ring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 (a) Fixed plate with an enclosure (A) in the back to support the second piece. The plate

is actually made up of several pieces that are designed to be light-weighted and use as little

space as possible. (B) Side pieces are where the legs will be mounted to. (C) A rib running

across the widest part of the plate prevents bending. (D) A small tab at the top of the plate

is used to keep the rotating component of the top structure in place. (b) Notice the unusual

angle cuts in the shaft placement and the shape of the rotating plate. The angles are defined

by the rotation axis, as well as the limits posed by the size of the mirror, the plates, and their

relative positions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

6 Hexapod design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

7 Estimated displacement vs. dimensions for various G-10 tube sizes. . . . . . . . . . . . . . . . 9

8 FEA simulations.(a) The design is split into smaller sections for FEA. (b) Gravity acting

along the vertical orientation of the mount (arrow). (c) Gravity acting normal to the vertical

orientation of the mount (arrow). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

i

9 Predicted 3D displacements for the case where applied gravity acts (a,b) vertically downward,

as in figure 8b and (c,d) horizontally, as in figure 8c. In the vertical case, there is an offset in

the direction of the displacement of the mirror, while in the horizontal case the displacement

is almost symmetrical. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

10 Predicted displacements in individual x-, y-, and z-directions for the case where gravity acts

vertically downward, as in figure 8b. Notice the scales are different between x-, y-, and z-

directions, with y-axis having the most changes and z-axis having the least effect. This is due

to the direction of the applied load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

11 Predicted displacements in individual x-, y-, and z-directions for the case where gravity acts

vertically downward, as in figure 8c. Unlike in the previous figure, x-axis has the most dis-

placements, consistent with the direction of the applied load. . . . . . . . . . . . . . . . . . . 14

12 Test setups. The arrows show the directions of applied loads (a) vertically downward and (b)

horizontal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

13 The thermal calibration system on the main plate. The parts are color coded for easy iden-

tification. (A - black) The motor, (B) the lead screw, (C) the hot load (dark grey) glued to

an aluminum tray (magenta), and (D - olive) the shaft support. The black arrows show the

directions of motions corresponding to “off window” and “on windows” configurations. In this

figure, the load is in “on windows” configuration. Not pictured: two limit switches, one for

“on” and one for “off” position. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

14 The wiring diagram of the thermal calibration system, connecting the motor to the serial-to-

USB encoder, a DC power source, and the switches. . . . . . . . . . . . . . . . . . . . . . . . 17

15 RESCUES at the SPT. (a) The mount (foreground) in its rest position in the optics system.

(b) Side view of RESCUES, with the tertiary mirror, sitting on top of the SPT VLBI receiver.

The receiver (purple) and the two circular windows can be seen through the hollow space in

the center of the main plate. (Photos credit: D. P. Marrone, The Department of Astronomy,

The University of Arizona.). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

16 Images of the Moon at two different parts of the electromagnetic spectrum. Notice that the

(a) submm map measures the surface temperature, rather than the reflected light of the Sun

that is seen in the (b) optical image. The red regions have higher temperature. The crescent

is waning so it was previously more illuminated, resulting in a wider crescent (dark red) in

surface temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

ii

List of Tables

1 G10, stainless steel, and aluminum are the main materials in the majority of RESCUES

parts. It is noticeable that G-10 is very close to steel and aluminum in strength, but its

thermal conductivity is lower by an order of magnitude, making it a better insulating material. 11

2 FEA predictions for the bending of RESCUES design due to gravity. δ3D is the maximal

displacement considering all three directions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Average displacements as measured by the dial indicator. Notice that the displacements are

an order of magnitude lower than predicted. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

iii

Appendix – Program to control motor 'PROGRAM TO CONTROL MDRIVEPLUS MOTOR, 'USED TO MOVE THERMAL LOAD CALIBRATION FOR RESCUES 'CHI HANH NGUYEN 'DEPARTMENT OF ASTRONOMY AND STEWARD OBSERVATORY 'THE UNIVERSITY OF ARIZONA 'UPDATED 12/01/2014 'THIS PROGRAM CAN BE TYPED INTO A TERMINAL THAT IS CONNECTED TO MDRIVE MOTOR. ‘DO NOT TYPE IN THE COMMENTS. PG 2 'RUN PROGRAM AT ADDRESS 2, ENTERS PROGRAM MODE S4=0,0,0 'SET I/O 4 AS INPUT, ACTIVE LOW, SINK S2=0,0,0 'SET I/O 2 AS INPUT, ACTIVE LOW, SINK LB Aa 'MAIN ROUTINE TO MOVE LOAD TO WINDOWS, LABELED Aa CL Xx, I2=0 'IF I/O 2 IS ACTIVE, EXECUTE BRANCH Xx CL Xz, I2=1 'IF I/O 2 IS INACTIVE, EXCUTE BRANCH Xz BR Aa 'LOOP OVER THIS PROGRAM LB Xx 'ROUTINE TO MOVE LOAD H 100 'HOLD 100 MICROSECS BEFORE CONTINUING SL -300000 'SLEW MOTOR AT #STEPS A SEC RT 'RETURN TO Aa LB Xz 'ROUTINE TO STOP MOTOR SL 0 'SLEW AT 0 STEP, I.E STOP PR "1" 'RETURN "1" TO TERMINAL E 'END PROGRAM LB Ab 'MAIN ROUTINE TO MOVE LOAD OFF WINDOWS, LABELED Ab CL Yx, I4=0 CL Yy, I4=1 BR Ab LB Yx 'ROUTINE TO MOVE LOAD H 100 SL 300000 'SLEW IN OPPOSITE DIRECTION NOW RT LB Yy 'ROUTINE TO STOP MOTOR SL 0 PR "0" 'RETURN "0" TO TERMINAL E PG 'EXIT PROGRAM MODE