Story Behind the Birth of
91-cm Reflector Telescope
The 91-cm Reflector Telescope, the first large-aperture telescope
manufactured in Japan, was produced by Nippon Kogaku K.K. (nowNikon Corporation). Since the telescope has an equatorial fork
mount and is designed to use the Cassegrain focus exclusively, theentire structure is compact. While the 188-cm Reflector Telescope isused primarily for researching stars through spectroscopic
observations,the main purpose of the91-cm reflector telescopeis photometric researchthrough photoelectric
photometry observations;thus, these two telescopes
complement one another.
Photoelectric photometry,
which became popular in
the 1950s, was frequently
used for variable starobservations and stellar
color measurements due
to its high photometricaccuracy. Because of this,photoelectric photometry
instruments graduallycame to dominant theobserving time on existingtelescopes in many
countries.This createdmomentum to establish
dedicated telescopes forphotoelectric photometry
instruments in order tosecure observing time.

As a result, 91-cm telescopes were built in Europe and the UnitedStates. Following suit, Japan also considered introducing a 91-cm
equatorial reflector telescope for photoelectric photometry. But atthat time, Japan had already ordered a 188-cm telescope from aforeign manufacturer, and purchasing another from overseas was
not considered viable. Through the efforts of the Director Generalof Tokyo Astronomical Observatory and others who felt that itshould be possible to produce this level of telescope domestically,
it was established that Nippon Kougaku could make such atelescope. What’s more Nippon Kougaku agreed to produce thetelescope at cost.
Today, the 91-cm Reflector Telescope has been reconfigured intoa wide-field near-infrared camera and continues to be used forobservations.
With this story in mind, try your hand at crafting a paper model ofthe first Japan-made large-aperture telescope. It’s a great way tolearn about telescope structure.

The 91-cm Reflector Telescope’s
main specifications

Primary mirror diameter:
91 cm (focal length: 12.0 m)
Primary mirror weight: 262 kg (Pyrex)
Focus type: Cassegrain
Mount: Equatorial fork mount
Weight: 6 tons

Dome

Secondary mirror
Guide telescope

Guide Telescope

Declination axis

Aperture: 20 cm(focal length: 250 cm)
Field of vision: 1° 7’Fork

極軸

Cassegrain focus

昇降床ピアーDesigned by Seiichi Sakamoto (NAOJ)(c) NAOJ

91-cm Reflector Telescope Paper Model
Construction Project for a Child with Adult
Assistance, Expert Course (craftinginstructions)Tools and materials: box cutter, ruler, aluminum tape, glue
(wood glue works well)
1. Preparation: Copy the two paper model cutout sheets onto Kent paper orsimilar thick paper. Next, paste aluminum tape on the reverse side of theprimary and secondary mirrors. After that, cut out all 23 model parts by
cutting along the bold lines on the model sheets. Next, make incisions andcut out the holes indicated by an “x” by cutting along the bold lines.
Mountain-fold along the dot-dash lines, and valley-fold along the dashedlines. Note that thin dotted lines indicate an overlap width for gluing parts
together and should not becreased.
2. Assemble the primary
mirror and telescope bodytube: First assemble thetelescope body tube, andattach the baffle to the holein the center of the primary
mirror. After that, attach theprimary mirror to the primary
mirror cell, and fix them tothe bottom end of the bodytube. Finally, attach thedeclination axis gears to theoutside of the mirror tube’s
declination axis.

3. Assemble the secondary
mirror: Assemble the top unit,
and fix the secondary mirror
to the tip of the unit. After that,assemble the spiders andattach the top unit to theintersection of the spiders.
4. Assemble the fork: First
attach the polar axis (forkside) to the hole at the centerof the fork, and then startworking on the fork’s prongs.
Complete the assembly while
taking care to make sure thatthe declination axis tabs arefolded out to pass through theholes in the declination axis
gears and the body tube. After
that, put the declination axis tabs through the declination axis holes on thebody tube from outside and then spread them outward to temporarily fix
them. Fold the declination axis tabs back down inside the body tube asneeded to secure them to the declination axis. Note that the declination axis
tabs should not be bonded to the body tube.
5. Assemble the mount: Adjust the diameter of the polar axis (mount side) sothat the fork-side polar axis can slide snugly into it during assembly. Then fix
the mount-side polar axis to the inside of the mount. Finally, fix those parts tothe base.
6. Attach the secondary mirror: Attach the spiders to the inside of the bodytube top ring at the support strut joints so that the spiders from a cross when
seen from above.
7. Assemble the guide telescope: Assemble the guide telescope, glue
together the eyepiece, body tube (1), body tube (2), and body tube (3) in thatorder, and then use the attachment to fix them to the main telescope’s bodytube. Coloring: You can color you telescope any color you’d like. Refer to theNAOJ webpage for pictures of the actual telescope coloration.

(c) NAOJ

91-cm Reflector Telescope Paper Model
Polar axis (fork side)
Construction Project for a Child with Adult
Assistance, Expert Course (1/2)
Primary mirror cell

Polar axis (mount side)×

BaffleGuide telescope eyepiece×Declination axis gear×

Secondary mirror
attachment×

SpiderGuide telescope attachment

××××

×

Primary mirror

Secondary mirror

×

Stick aluminum tape on the back here

Guide telescope body tube (1) (2)(3)(c) NAOJ

91-cm Reflector Telescope Paper Model
Construction Project for a Child with
Adult Assistance, Expert Course (2/2)

2cm = 7mfork

×
Base

Declination axis tabs
×
×

Mount

NAOJ Okayama AstrophysicalObservatory
91-cm Reflector Telescope
1/35 Scale Model

http://www.oao.nao.ac.jp/(c) NAOJ