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APERTURE AND FOCAL LENGTH OF TELESCOPES

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APERTURE AND FOCAL LENGTH OF TELESCOPES

The width of the optical tube is known as its aperture. Technically, it is the area of the optics that determines how much light is gathered, but convention uses the width to describe aperture. This is what is referred to when you hear the term “8-inch scope” or “20-inch scope”. Every telescope’s primary optics, whether lens or mirror, has an eventual focal point. The distance from the Objective (the light gathering primary optical component) to the final focal plane of the telescope is called its Focal Length. In general, for a given aperture, longer focal lengths produce narrower fields of view, but higher magnifications, while shorter focal lengths produce wider fields of view but lower magnifications. The ratio of the focal length to the aperture is known as f/Ratio, and you will often see this quoted for a particular scope. F/2 to f/5 are considered short f/Ratios; f/6 to f/10 are considered medium f/Ratios, while f/12 to f/20 are considered long f/Ratios. It is the medium f/Ratios that are considered best for general use, where Moon, planets, stars, star clusters, nebulae, and galaxies are all going to be observed with the same instrument.

The longer the focal length of a scope, the larger and heavier the telescope will be. The exception is the catadioptric telescopes, where a longer focal length is compressed in a shorter tube. As an extra, catadioptric telescopes often come with computerized mounts, making finding objects very easy and simple.

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Bushnell 1250 x 90mm Motorized GoTo Maksutov-Cassegrain Telescope
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Bushnell 1250 x 90mm Motorized GoTo Maksutov-Cassegrain Telescope
Item No. 103-10005
Manuf. No. Bushnell 788890

Bushnell 788890 Telescope 1250 x 90mm Motorized GoTo Maksutov-Cassegrain Telescopes
MSRP: $651.00
Our Price: $449.99
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Bushnell NorthStar 4.0 inch/100 Catadioptric Telescope, 1300mm Maksutov-Cassegrain, GO-TOMotorized Altazimuth Mount
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Bushnell NorthStar 4.0"/100 Catadioptric Telescope, 1300mm Maksutov-Cassegrain, GO-TOMotorized Altazimuth Mount
Item No. 103-10000
Manuf. No. Bushnell 788840

Bushnell's NorthStar #788840 telescope offers amateur astronomers state-of-the-art computer-driven location and tracking capability with simple, push-button controls.
MSRP: $780.00
Our Price: $529.99
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Takahashi Mewlon-300 Complete System w/EM-400 GOTO Mount
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Takahashi Mewlon-300 Complete System w/EM-400 GOTO Mount
Item No. 111-10300
Manuf. No. Takahashi TLS3320

Mewlon-300 Complete System includes: Mewlon-300 300mm f/11.9, FL 3572mm tube assembly; EM-400 GOTO mount, 11x70 finder, tube holder, extra heavy duty metal tripod, LE-30mm ocular and diagonal.
 
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Celestron NexStar 127 SLT Maksutov-Cassegrain Telescope on GoTo Mount
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Celestron NexStar 127 SLT Maksutov-Cassegrain Telescope on GoTo Mount
Item No. 102-18150
Manuf. No. Celestron 22097

It includes a motorized altazimuth mount with computerized hand control and 4,000+ object database
MSRP: $850.00
Our Price: $549.95
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Celestron NexStar 90 SLT Maksutov-Cassegrain Telescope on GoTo Mount
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Celestron NexStar 90 SLT Maksutov-Cassegrain Telescope on GoTo Mount
Item No. 102-18140
Manuf. No. Celestron 22087

Fully coated optics, 1250mm focal length and f/13.9 focal ratio are capable of a stellar magnitude of 12.3 and 1.29 arc seconds of resolving power.
MSRP: $620.00
Our Price: $419.95
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Celestron CGEM 1100 Schmidt-Cassegrain Computerized GoTo Telescope
Celestron CGEM 1100 Schmidt-Cassegrain Computerized GoTo Telescope
Item No. 102-11520
Manuf. No. Celestron 11082

CGEM 1100 - The EdgeHD is Celestron's latest innovation that will surely be the system of choice for the serious astro-imagers. Available January 2010
MSRP: $7,240.00
Our Price: $3,999.00
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Celestron CGEM 925 Edge HD Computerized GoTo Telescope
Celestron CGEM 925 Edge HD Computerized GoTo Telescope
Item No. 102-11510
Manuf. No. Celestron 11081

CGEM 925 Edge HD - The EdgeHD is Celestron's latest innovation that will surely be the system of choice for the serious astro-imagers.
MSRP: $6,398.00
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Celestron CGEM 800 Schmidt-Cassegrain Computerized GoTo Telescope
Celestron CGEM 800 Schmidt-Cassegrain Computerized GoTo Telescope
Item No. 102-11500
Manuf. No. Celestron 11080

CGEM 800 - The EdgeHD is Celestron's latest innovation that will surely be the system of choice for the serious astro-imagers. Available January 2010
MSRP: $4,558.00
Our Price: $2,499.00
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Celestron CGE Pro 1400 Schmidt-Cassegrain Computerized GoTo Telescope
Celestron CGE Pro 1400 Schmidt-Cassegrain Computerized GoTo Telescope
Item No. 102-11620
Manuf. No. Celestron 11094

CGE PRO 1400 - Celestron's newest line of observatory class Schmidt-Cassegrain telescope, comes mounted on the state of the art CGE GoTo German Equatorial mount.
MSRP: $17,998.00
Our Price: $9,999.00
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Celestron CGE Pro 1100 Schmidt-Cassegrain Computerized GoTo Telescope
Celestron CGE Pro 1100 Schmidt-Cassegrain Computerized GoTo Telescope
Item No. 102-11610
Manuf. No. Celestron 11093

CGE PRO 1100 - Celestron's newest line of observatory class Schmidt-Cassegrain telescope, comes mounted on the state of the art CGE GoTo German Equatorial mount.
MSRP: $13,998.00
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Celestron CGE Pro 925 Schmidt-Cassegrain Computerized GoTo Telescope
Celestron CGE Pro 925 Schmidt-Cassegrain Computerized GoTo Telescope
Item No. 102-11600
Manuf. No. Celestron 11092

CGE PRO 925 - Celestron's newest line of observatory class Schmidt-Cassegrain telescope, comes mounted on the state of the art CGE GoTo German Equatorial mount. Available January 2010
MSRP: $12,598.00
Our Price: $6,899.00
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Takahashi TOA-130FEMT Apochromatic Refractor Telescope Complete System w/ 4.0 inch Focuser w/ EM-200 Temma-2 GOTO Mount
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Takahashi TOA-130FEMT Apochromatic Refractor Telescope Complete System w/ 4.0" Focuser w/ EM-200 Temma-2 GOTO Mount
Item No. 111-10190
Manuf. No. Takahashi TSS1036

Takahashi - TOA-130F 5.1" f/7.7 Ortho-APO Refractor with 4" Focuser on EM-200 Temma II Mount
 
Our Price: $20,694.95
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Takahashi TOA-130EMT Apochromatic Refractor Telescope Complete System w/ 2.7 inch Focuser w/ EM-200 Temma-2 GOTO Mount
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Takahashi TOA-130EMT Apochromatic Refractor Telescope Complete System w/ 2.7" Focuser w/ EM-200 Temma-2 GOTO Mount
Item No. 111-10180
Manuf. No. Takahashi TSS1045

The newest Takahashi refractor, the triplet TOA-130, is the first of the new Ortho-Apochromat Series.
 
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Takahashi FSQ-106ED New Q Apochromatic Refractor Telescope Complete System w/ EM-200 Temma-2 GOTO Mount
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Takahashi FSQ-106ED New Q Apochromatic Refractor Telescope Complete System w/ EM-200 Temma-2 GOTO Mount
Item No. 111-10130
Manuf. No. Takahashi TSS1064

Complete system includes the tube assembly, EM-200 Temma-2 700x GOTO mount with all standard accessories
 
Our Price: $17,283.95
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Takahashi TSA-102S / EM-200 Temma-2 Apochromatic Refractor Telescope Complete GOTO System
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Takahashi TSA-102S / EM-200 Temma-2 Apochromatic Refractor Telescope Complete GOTO System
Item No. 111-10080
Manuf. No. Takahashi TSS1023

The TSA-10S2 is the first in the new TSA line of Takahashi triplet apochromats. The color free images produced will delight the lunar, planetary and deep sky observer with very high contract images with pin point stars
 
Our Price: $14,277.95
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