William Optics GTF 81 Five Element APO User manual

2013 Edition by James J. Yang
GTF 81 Five Element APO
User Guide

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Contents
Getting to know your telescope
GTF 81 APO Specifications
GTF 81 APO Accessories Chart
Connection Instructions
Usage
Storage and Cleaning
Caution and Safet
Caution and Safety, Optional Equipment
Recommended Products
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7
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DO NOT USE THIS TELESCOPE UNDER ANY
CIRCUMSTANCES TO DIRECTLY VIEW THE SUN.
It could easily cause instant blindness or serious damage to your
eyes. To view the sun, use only appropriately designed solar filters
that will reject 99.96% of the sun light and heat. Educate your
family on how to use this telescope properly during day and night
time observations. For further information please contact your local
time observations. For further information please contact your local
dealer.
CAUTION!!
WARNING!

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GTF 81 APO Overview
ON/OFF Button
Battery Socket
(1.55v/FR44 Battery)
Set to Zero
by pushing
the button.
Dew Shield
Lock Screw
Finderscope
Rings
1/4-20 for a
standard photo
tripod.
Digital Readout
Gauge
Digital Readout Gauge

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GTF 81 APO Specifications
Aperture
Focal Ratio
Focal Length
Objective Type
Resolving Power
Limiting Magnitude
Lens Shade
Lens Shade
DDG Focuser
1.25" Adapter
L-type Mount
Field Stops
Tube Diameter
Tube Length
T
Tube Weight
81mm
F/5.9
478 mm
Five element with FPL-53 Air Spaced
(2 groups), STM coatings.
1.43 seconds of arc
12.2
12.2
Retractable
2" R&P 360° Rotating Camera-angle
Adjuster, 1:10 fine focus Focuser
Travel Length 3.15" (80mm)
1.25" Adapter
2" Brass Compression Rings
Integrated Integrated L Bracket (fits most EQ mount)
knife edge Baffles
90 mm
440mm Fully Retracted
484mm Fully Extended
3.7 kg

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iOptron IEQ30
T-mount
GTF 81 APO
50mm Finder Scope
2” Photo Adapter
CCD Camera
GTF 81 APO Accessories Chart

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Connection Instructions
3.) Tighten both lock screws
after diagonal was attached.
4.) Insert eyepiece for visual
observation and tighten diagonal
lock screw.
2.) Remove 1.25” adapter then
insert 2” diagonal.
1.) Loosen both lock screws
for release 1.25” adapter.

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Usage
The WO (William Optics) GTF 81 is a 5 element APO telescope designed for
both visual and photographic use, and is an outstanding travel scope. We recom-
mend purchasing a WO 2” star diagonal and a 2“ photo adapter for best use of
this telescope. The GTF 81 is well suited to viewing nebulae, clusters, large gal-
axies and comets, so the use of high quality wide-angle eyepieces is highly rec-
ommended. The WO SWAN, UWAN and SPL series eyepieces are available in a
variety of focal lengths. Viewing the moon and planets at magnifications of 180X
is spectacular, when seeing conditions permit.
To calculate the magnification of your telescope and eyepiece combination,
divide the telescope focal length in mm by the eyepiece focal length in mm.
Keep in mind that the atmosphere plays an important role in seeing conditions,
and only the best seeing conditions will support high power viewing. Additional
power under less than ideal seeing conditions will not increase viewable details,
and lower powered eyepieces should be used under those conditions. It’s easi-
est to locate objects using low power eyepieces (20X magnification and lower)
and then zoom in by switching to higher powered eyepieces. The largest field of
and then zoom in by switching to higher powered eyepieces. The largest field of
view can be seen by using a 12x low power-wide field of view eyepiece.
A stable tripod or mount is highly recommended for optimal viewing. This includes
high quality photography tripods, stable Alt-Atz mounts, and German Equatorial
mounts, which are designed for astronomical use with the ability for precise track-
ing of celestial objects. Optional accessories from WO include mounting rings and
plates, and super high quality 2” Quartz star diagonals which provide for superior
viewing and high quality images.
The GTF 81 telescope is perfect for large-field astrophotography. The GTF 81 has
superb color correction at a fast speed of f5.9. Please see the WO website for ex-
amples of photos taken with this telescope. The Accessories Charts on page 5 and
page 10 provides additional details on photographic accessories for various CCD,
digital and film cameras.
Never aim your telescope or finder scope at the Sun without proper Solar filters in-
stalled on the front of the telescope. Doing so for even a moment may permanent-
ly damage your vision. Proper Solar filters consist of filters made by reputable
manufacturers, designed to fit tightly over the front of the dew shield. Solar eye-
piece filters are not considered safe, and should not be used. With proper front
mounted Solar filters, the telescope will not be harmed by viewing the Sun. Con-
tact your Authorized WO Dealer for further information on the brands, sizes, and
prices of proper solar filters.

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We suggest placing all accessories inside an airtight container with desiccative
control. Remember to store the telescope in a non-humid environment, and
never leave moisture build up.
Be particularly careful after a night observation, that the lens has no dew on it before
storing the telescope away.
In case the lens surface becomes dusty, smeared, or gets fingerprints or fungus
build-up on it, first remove any surface particle by using an air blowe
build-up on it, first remove any surface particle by using an air blower, then
carefully proceed by wiping the lens gently with a lint-free proper soft cloth. Use an
optical quality liquid cleaner to get the best cleaning result. Change your cloth
to a new one from time to time.
The above-mentioned indications are not necessary if you carefully protect your
telescope.
The beautiful finish of your GTF 81 is very durable. For proper cleaning, take care
of the exterior body tube by wiping it down with a slightly damp soft cloth from time
of the exterior body tube by wiping it down with a slightly damp soft cloth from time
to time. Water should be enough to do the job. Please do not use any organic
solvent on your telescope like alcohol, benzene and other hazardous
chemical, as this might ruin it.
Storage and Cleaning
A
B
B
A
To mount on tripod
or EQ Mount
1:10 Ratio Knob
Heavy Duty R&P Focuser
All WO telescopes, are now fitted with a smooth and precise dual speed
1:10 focusing system, which means that for every ten full rotations of the
small fine-focus knob the large knob rotates once.
The focuser drawtube tension can be adjusted using a 2.5 mm Allen key
and tension should be adjusted only when necessary by progressively
tightening the screw small amounts until you reach the desired tension.
1.5mm Screw
Adjustable tension screw
for loading capacity
(2.5mm Screw)
Focus Lock Screw
Please use
1.5 / 2.0 / 2.5mm Allen Keys
2mm Screw
2mm Screw

Caution and Safety, Optional Equipment
Optional Equipment
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In order to operate your GTF 81 you will need the following minimum
equipment:
A sturdy tripod for astronomical or birding usage or a mount (equatorial or ALT-
AZ).
A mirror diagonal or erecting prism (The WO 2” Dielectric Diagonal is
strongly recommended for better results in star observation and digital
photograph
photography. A top-quality Dielectric Quartz model is also available now).
At least one or two good eyepieces (6 to 40 mm wide-field recommended)
depending on the application.
Other accessories depending on application.
Caution! Never directly view the sun light with your telescope.
Do not aim your GTF 81 at the Sun because this might damage your vision. Loss
of eyesight can occur when your telescope is not properly protected by solar
viewing filter. (Solar filters can be easily purchased through your WO
Authorized Dealers).
Always place the optical telescope assembly (OTA) on a completely flat
surface, unstable placement of telescope may cause it to fall, and unless it is
surface, unstable placement of telescope may cause it to fall, and unless it is
handled with caution, it may easily injure yourself and others.
Never use your telescope under rainy conditions: this telescope is not designed
to be water-proof. If your telescope accidentally gets caught in rain, please
wipe down the water with a dry clean cloth, but if the lens gets totally soaked by
water, please contact your WO Authorized Dealer right away for details on a
service solution.
Do not disassemble or attempt repairing your telescope without a written
Do not disassemble or attempt repairing your telescope without a written
authorization from William Optics Corp. This violates the warranty terms under
the limited product warranty section and invalidates any guarantee. Always consult
with your local WC Authorized Dealer or with William Optics Corp. for details
on how to service your telescope.

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Recommended Products
50mm Finder / Guiding Scope. 2" Dielectric Carbon Digonal 2” Photo Adapter
iOptron IEQ30 Mount