Facts Underlying the Ultra-high
Precision of the VERA Antennas
VERA is an acronym for “VLBI Exploration of Radio Astrometry.” VERA uses
the relative VLBI method to triangulate maser source positions within the Milky
Way Galaxy with a precision 100 times better than ever before. This project
aims to draw a three-dimensional map of the Milky Way. Researchers invarious fields from many universities and research institutions are participatingin this project, with the National Astronomical Observatory of Japan playing acentral role. VERA uses a type of radio interferometry called VLBI (Very LongBaseline Interferometry) to combine observations from 20 meter diameter radio
telescopes stationed at foursites throughout Japan (Mizusawa, Iriki, Ogasawara, andIshigaki-jima) to provide a
resolution equivalent to that of a
2,300 km diameter radio
telescope. Such precision
provides a position determination accuracy equivalent to thesize of a one-yen coin on thesurface of the Moon whenviewed from the Earth.
A major feature of VERA is its
specialization in the relative
VLBI observation technique.
With standard observation methods, atmospheric fluctuations make it impossible to accurately measure the positions of stars. In the VERA system, twoclose together objects are observed simultaneously to compensate for atmospheric fluctuations, enabling precision position measurements. VERA
conducts relative VLBI efficiently through the use of “dual-beam antennas.”
One of these antennas can observe 2 objects up to 2 degrees apart simultaneously with the 2 pairs of receivers installed at the focal point. VERA was thefirst in the world to attempt observations with this kind of dual-beam antenna.
Think about the secrets behind this high-precision as you make your own radio
telescope paper model.

20-m Radio Telescope at the Ishigaki-jima Station

The mirror surface accuracy of the 20-m
antenna is maintained to 250 μm or less,allowing the antenna to efficiently captureradio waves of up to 43 GHz. Weighing
380 tons, with a survival wind speed of 90
m s-1,, it’s well equipped for the weather ofIshigaki-jima, known to be a busy typhoon
pathway.

Dual-beam receivers →To simultaneously observe two objects,
one pair of receivers is equipped for each of
the 22 GHz and 43 GHz frequencies. Thepositions of these receivers can be adjusted
three-dimensionally in order to observevarious pairs of objects. Also, in ordercorrect for field rotation caused by thediurnal motion of celestial objects, the entire
platform rotates.
Designed by Seiichi Sakamoto (NAOJ)(c) NAOJ

VERA Antenna Paper ModelConstruction Project for a Child with
Adult Assistance, Expert Course(Crafting Instructions 1)
Tools and materials: box cutter, glue (wood glue works well)

1. Preparation: Copy the three paper model cutout sheets onto Kent paperor similar thick paper, scaled so that the dimensions of the outer edge ofeach copy is 17 cm × 26.5 cm, and cut out all 33 model parts by cutting
along the bold lines. The nine parts with an asterisk (*) by their name labels
can be omitted if work time is limited. Next, make incisions and cut out theholes indicated by an “X” by cutting along the bold lines. Mountain-foldalong the dot-dash lines, and valley-fold along the dashed lines. Note thatthin dotted lines indicate an overlap width for gluing parts together andshould not be creased.

↑Photo showing the parts after being cut out and folded
2. Assemble the feedome: Curl the feedome side into a ring, and then attach
the feedome top from inside to form a shape like Mt. Fuji.
3. Assemble the subreflector: Crimp thesubreflector into a convex shape with yourfingernails. Attach the subreflector to the tip of
the triangular pillar shaped subreflector drive,and then glue the subreflector stays to thesubreflector drive so that they wrap around thethree sides of the drive to form a tripod structure.
When assembling the tripod structure, make surethe subreflector faces downward.
4. Assemble the main reflector：Form the mainreflector into a horn shape with the printed
surface facing inward, and insert the legs of thesecondary mirror stays into the three slits; thenglue them in place from the back side.

5. Assemble the main reflector back-up structure: Form the main reflector backup structure into a horn shape with the printed surface facing outward. Next
glue the part labeled “main reflector back-up structure outer edge” to the mainreflector back-up structure so that the side labeled “main reflector side” faces
outwards and the seams of the overlap tabs for gluing on both parts arealigned.
6. Assemble the center hub and receiver cabin: Curl the receiver cabin side intoa cylindrical shape. Guide the elevation axes through the holes from inside, andthen glue them together. Referring to the photo, glue the receiver cabin bottom
to the bottom side (the side closer to the elevation axis); then glue the center
hub bottom to the outside at a position a little higher than the elevation axis.
After that, glue the field rotator base to the top of the assembly. Fold the mobile
receiver platforms to form an “M” shape and attach them to the field rotator
platform. Fold the four tabs of the field rotator platform downward, and insert
them in the hole of the field rotator base (do not glue them). Curl the center hub
side into a cylindrical shape and glue the top on it. Put it over the receivercabin’s top part, and glue it to the center hub bottom.
7. Attach the sector gear: Glue together the two sector gear parts, and thenglue them to the receiver cabin side and center hub bottom.
8. Assemble the mount: Assemble the lower instrument cabin to form abottomless box. Attach the elevation drive to the second-floor part. Assemble
the mount except for the front side (bottom-side portion of the model cutout),
and glue the lower instrument cabin to the first floor of the mount at the marked
areas. To attach the second-floor part, first glue its front (concave) edge to theoverlap width on the front side of the mount; then assemble the front side of themount. Form the azimuthal drive wheels into wedge shapes and then attach
them, facing downward, to the backs of the marked areas in the four corners ofthe mount first floor.
9. Assemble the base: Glue the azimuth axis at the center of the base.

↑Photo showing all of the units which are now almost completed.
(c) NAOJ

VERA Antenna Paper ModelConstruction Project for a Child with
Adult Assistance, Expert Course(Crafting Instructions 2)
10. Assemble entire model: Guide
the receiver cabin’s elevation axis
through the elevation axis bearingson the mount (do not glue), andthen insert the sector gear into theslit in the elevation drive on themount. Glue together the mainreflector and main reflector back-up
structure so that the positions of thesubreflector stay’s legs and thepositions of the main reflector backup structure’s subreflector support pillars are aligned, and the mainreflector structure forms a single-piece unit. Attach this main reflector
structure to the top portion of the center hub so that the subreflector stays
form an inverted-Y shape when viewed from the front. Be careful not toapply glue to the four slits. Insert the feedome’s four tabs into these fourslits (do not glue). Finally, place the radio telescope on the top of its base tofinish assembly.

The subreflector support pillars
also form an inverted-Y shapewhen viewed from the back.
The main reflector back-up
structure and the back-up
structure outer edge should bealigned to form intersections
where 8 pillars converge.

The dual-beam receivers can beseen on the mobile receiverplatforms when the feedome isremoved; thus, the space
between the receivers can bechanged, and the receivers canbe rotated as well.

The subreflector stays form aninverted-Y shape when viewedfrom the front.

Do not apply glue to thefeedome.
The side with the concave
second-floor part is the front of
the mount.

(c) NAOJ

VERA Antenna Paper ModelConstruction Project for a Child with
Adult Assistance Expert course (1/3)

Feedome sideMain reflector

Receiver cabin bottom

Field rotator platform*

Subreflector
stay
Feedome top
Mobile receiverplatform*

Subreflector drive
Center hub side *Subreflector

Center hub top

BottomField rotator
platform base *Elevation axis
Receiver cabin side(c) NAOJ

VERA Antenna Paper ModelConstruction Project for a Child with
Adult Assistance Expert course (2/3)
Azimuth drive wheels *Elevation drive

Second ﬂoor part

MountLower instrument cabin

Sector gear

Sector gear(c) NAOJ

VERA Antenna Paper Model
Construction Project for a Child with
Adult Assistance Expert course (3/3)

Main reflector
back-up structure
Base

NAOJ Mizusawa VLBI Observatory
20-m Radio Telescope
1/200 Scale Model

http://www.miz.nao.ac.jp/en/

Azimuth axis *(Main reflector side)

Main reflector back-up structure(Main reflector side)(c) NAOJ

Telescope Fun Facts

1.3 Fast-switching method“De-twinkling” the Stars

Unlike the Hubble Space Telescope and other telescopes in space, large
terrestrial telescopes such as VERA, the Subaru Telescope, and ALMA are
influenced by absorption (light reduction) and atmospheric fluctuations dueto the Earth’s atmosphere. Atmospheric fluctuations cause the twinkling of
the stars and heat haze shimmering, which may blur obtained images, give
inaccurate star locations, etc. Because of this, various measures (referredto as phase correction methods) are taken in order to eliminate theseeffects. The typical methods are introduced below, using the VERA
antennas as examples.

1. Method to directly detect star twinkling and remove it
1.1 Self-calibration method
By monitoring a small, bright object in the telescope’s field of view andobserving the target object for shorter intervals than the speed of thereference object’s twinkling, the twinkling of the stars can be eliminated.
The wavefront compensation method used by optical/infrared telescopes
like the Subaru Telescope is also based on this principle of utilizing a guidestar within the field of view.

s＊／sQSO五星＊[I/

1.2 Paired antenna method
Generally, there are not many cases where a small and sufficiently brightobject can be found within thefield of view. Using two antennas
Reference object
as a pair, one of them observing
Target object (QSO, etc.)
the target object while the otherone observes a nearby reference
object, provides a similar effect.
In VERA, each antenna isequipped with two pairs ofreceivers, which areindependently controlled tosimultaneously observe twoobjects, thus achieving the sameeffect.

The fast-switching method performs* §'observations of two objects with one antenna,switching the field of view between them at ahigh speed to produce the same effect. TheALMA (Atacama Large Millimeter/Submillimeter Array) antennas use thismethod combined with the radiometer method(described below). Since the ALMA antennas
perform observation at extremely high radio
frequencies, the number of objects that aresufficiently strong to serve as reference
objects at those frequencies is extremely limited. Because of this, ALMA
uses the following method as well: observing a reference object at a lowfrequency of about 100 GHz; converting the measured twinkling speed tothe corresponding speed at the observation frequency for the main target;
and using the converted twinkling frequency for correction.

2. Methods to estimate and correct phasefluctuation＊

2.1 Radiometer method／

In radio wave observations, the main reason
stars twinkle is water vapor. If the amount ofwater vapor can be measured by some means,then the effects from the stars twinkling can beremoved. The radiant intensity of water vapor
emission lines at 183 GHz and 22 GHz
frequencies, and the radiant intensity of infrared
rays near 10 µm are used to estimate theamount of water vapor.

3. Technique to facilitate phase fluctuation correction／｀気．
fFor multi-element radio interferometers, it is also possible to use anantenna array that makes the correction of atmospheric fluctuation effects
easier (e.g., an array where the antenna baseline redundancy has been
intentionally increased).

(c) NAOJ