4-inch Telescopes (100-120mm)

What is a 4-inch telescope?

A 4-inch telescope is an amateur telescope with an aperture size of 4 inches or approximately 100mm. The diameter of the objective lens or mirror in a 4-inch telescope is typically around 4 inches, which is smaller than larger telescopes, resulting in a narrower field of view. The focal length of a 4-inch telescope can range from around 400mm to 1000mm, with the focal ratio influencing the telescope’s speed, field of view, and potential magnification. The magnification power of a 4-inch telescope is around 200x, with a maximum usable magnification of 50x per inch of aperture. The gathering power of a 4-inch telescope is significantly higher than that of the naked eye, allowing for detailed observations of celestial objects. The resolution of a 4-inch telescope is determined by its aperture, enabling the telescope to distinguish small details in observed objects. Equatorial mounts are commonly used with 4-inch telescopes, providing stability and ease of tracking celestial objects. Alt-Azimuth mounts are also used with 4-inch telescopes, offering simple movement in both vertical and horizontal directions.

What is a 4-inch refractor telescope?

A 4-inch refractor telescope is an optical instrument with an aperture of 4 inches (102mm) that determines its light-gathering capacity and image resolution. These telescopes usually have a focal length ranging between 450mm to 1350mm, with an average focal ratio of f/3.9 to f/9. They provide a maximum magnification of around 204x to 236x, allowing observers to view celestial objects such as star clusters, nebular clouds, and planets with sharp clarity.

A 4-inch refractor telescope can weigh between 4.4 lbs to 32 lbs, with an average weight of 17 lbs, making it relatively light and easy to transport. Some 4-inch refractor telescopes incorporate ED or apochromatic glass to reduce chromatic aberration and enhance image quality, especially for astrophotography. These telescopes are typically mounted on an equatorial mount for stability and accurate tracking of sky objects.

Constructed with quality glass, including options like FPL-53 and Lanthanum, a 4-inch refractor telescope may also incorporate ED or apochromatic glass to reduce chromatic aberration and enhance image quality, especially for astrophotography. The telescope’s dew shield prevents moisture buildup on the lens during night observations, ensuring clear views of the night sky.

Overall, the attributes of a 4-inch refractor telescope, such as its compact size, quality glass optics, and essential accessories, make it an ideal choice for both terrestrial and astronomical observations, providing a balance of performance and portability for stargazing enthusiasts.

What is a 4-inch reflector telescope?

A 4-inch reflector telescope is an optical instrument that uses a 4-inch mirror as its primary light-gathering component, making it suitable for observing celestial objects such as planets, the Moon, and double stars. The telescope’s aperture of around 114mm determines its light-gathering capacity and resolution, enabling the observation of fainter objects with greater clarity. With a magnification power of approximately 200x, a 4-inch reflector telescope offers detailed views of astronomical phenomena. The ideal focal ratio for a 4-inch reflector telescope ranges from f/4 to f/5, ensuring a balance between light-gathering ability and depth of field. Mounted on an Equatorial EQ-1 mount, a 4-inch reflector telescope offers stability and precision during observations. The portability, lightweight design, and smartphone compatibility of a 4-inch reflector telescope make it a versatile and user-friendly instrument for both novice and experienced astronomers.

What space objects can you see with a 4-inch telescope?

A 4-inch telescope with a gathering power of 102x is capable of observing all the official planets, numerous small solar system bodies, and thousands of deep sky objects such as planetary nebulae and asteroids. Some specific examples of deep sky objects that can be observed include galaxies such as the Andromeda Galaxy, star clusters such as the Pleiades, and nebulae such as the Orion Nebula. The moon and sun are great to observe in a 4-inch telescope, with the maximum power on a super good 4-inch scope being 100x per inch on the moon and 300x on Jupiter before getting dim. Under reasonably dark skies and depending on the observer’s skill and local light pollution, every object listed in “entry level” star atlases can be within reach of a 4-inch telescope. The 4-inch refractor telescope is also capable of observing the phases of Mercury and the details of Mars at opposition. The impact of light pollution on the observable objects and details through a 4-inch telescope is that it can make it more difficult to see fainter objects and details, and may require the use of filters to enhance contrast.

Types and Technical Specifications of 4-inch telescope

Types (Optical Design)

Doublet Refractor: Example: Takahashi FC-100DF f/7.4 Fluorite APO Refractor, Sky-Watcher Evostar 100 ED APO Refractor Telescope, Celestron Omni XLT 120 Telescope, Celestron Omni XLT 120 Telescope, Celestron Omni XLT 102 Telescope, Celestron AstroMaster 102AZ f/6.5 Doublet Refracting Telescope, Sky-Watcher StarTravel 102AZ3 f/4.9 Doublet Refracting Telescope, Takahashi FC-100DC F/7.4 Fluorite Refractor, Sky-Watcher Evostar 120ED APO – 120mm Refractor Telescope, Celestron StarSense Explorer DX 102AZ Smartphone Telescope, Celestron Inspire 100AZ Refractor

Triplet Refractor: Example: Sky-Watcher Esprit 100mm ED Triplet APO Refractor, Orion EON 115mm Triplet Apochromatic Refractor, Sky-Watcher Esprit 120mm ED Triplet APO Refractor, Takahashi TS-120 120 APO Refractor with 3-inch FeatherTouch Focuser from SI

Newtonian: Example: Celestron AstroMaster 114EQ Telescope, Celestron Explorascope 114AZ f/10 Modified SCT Reflecting Telescope, Celestron Cometron 114 AZ Telescope, Takahashi TSA-120 Triplet APO Refractor, Celestron PowerSeeker 114 EQ Telescope, Unistellar eVscope 2 Smart Telescope & eVscope Backpack, Celestron 114LCM Computerized Telescope, Celestron 114LCM Computerized Telescope, Unistellar eVscope eQuinox Smart Telescope, Celestron StarSense Explorer LT 114AZ Smartphone Telescope

Sextuplet: Example: Sharpstar Z4 F/5.5 Fully Air-Spaced Sextuplet, 

Petzval: Example: Takahashi FSQ-106EDX4 f/5 Petzval Refracting OTA Telescope,

Maksutov Cassegrain: Example: Celestron Nexstar 4SE GoTo Telescope, Explore Scientific Firstlight MAK100mm White Tube Maksutov Cassegrain with Twi-Nano

Technical Specifications

Aperture: 4-4.75 inch (100-120 mm)

Focal Length: 450-1350 mm 

Focal ratio: f/3.9-f/9

Highest Magnification: 204-236x

Light Gathering Power: 212-270x

Price: 262-5,200$

Images taken with 4-inch telescopes

Examples of 4-inch telescopes

4-inch

• Takahashi FC-100DF f/7.4 Fluorite APO Refractor
• Sky-Watcher Esprit 100mm ED Triplet APO Refractor
• Sky-Watcher Evostar 100 ED APO Refractor Telescope
• Celestron Omni XLT 102 Telescope
• Sharpstar Z4 F/5.5 Fully Air-Spaced Sextuplet
• Celestron AstroMaster 102AZ f/6.5 Doublet Refracting Telescope
• Sky-Watcher StarTravel 102AZ3 f/4.9 Doublet Refracting Telescope
• Takahashi FC-100DC F/7.4 Fluorite Refractor
• Celestron Nexstar 4SE GoTo Telescope
• Celestron Inspire 100AZ Refractor
• Celestron StarSense Explorer DX 102AZ Smartphone Telescope
• Celestron 114LCM Computerized Telescope
• Explore Scientific Firstlight MAK100mm White Tube Maksutov Cassegrain with Twi-Nano

4.25-inch 

• Takahashi FSQ-106EDX4 f/5 Petzval Refracting OTA Telescope 106 mm (4.25″)

4.5-inch

• Celestron StarSense Explorer LT 114AZ Smartphone Telescope (114 mm (4.5″))
• Celestron AstroMaster 114EQ Telescope (114 mm (4.5″))
• Orion EON 115mm Triplet Apochromatic Refractor 115mm (4.5″)
• Celestron Explorascope 114AZ f/10 Modified SCT Reflecting Telescope 114mm (4.5″)
• Celestron Cometron 114 AZ Telescope 114 mm (4.5″)
• Celestron PowerSeeker 114 EQ Telescope 114 mm (4.5″)
• Unistellar eVscope 2 Smart Telescope & eVscope Backpack 114 mm (4.5″)
• Unistellar eVscope eQuinox Smart Telescope 114mm (4.5″)

4.75 inch

• Celestron Omni XLT 120 Telescope (120 mm (4.75″))
• Sky-Watcher Esprit 120mm ED Triplet APO Refractor 119 mm (4.75″)
• Takahashi TSA-120 Triplet APO Refractor 119 mm (4.75″)
• Sky-Watcher Evostar 120ED APO – 120mm Refractor Telescope 120 mm (4.75″)
• Takahashi TS-120 120 APO Refractor with 3-inch FeatherTouch Focuser from SI (119 mm (4.75″)

How do 4-inch telescopes compare to other size-telescopes

4-inch vs 5-inch telescope

5-inch telescopes provide a focal length of 650-1900 mm with a focal ratio of f/5-f/15. In contrast, 4-inch telescopes vary from 450-1350 mm in focal length with a focal ratio of f/3.9-f/9. Moreover, 5-inch telescopes provide 225-260x the highest magnification, versus the 204-236x of 4-inch telescopes.

4-inch vs 6-inch telescope

4-inch telescopes have less aperture than the 6-inch telescopes. They offer a focal ratio of f/3.9-f/10 whereas the 4-inch telescopes vary from f/3.9-f/9. However, 6-inch telescopes offer a focal length of 162-1800 mm whereas the 4-inch telescopes vary from 450-1350 mm in focal lengths. 6-inch telescopes provide the highest magnification of 295-320x, whereas 4-inch telescopes can magnify 225-260x. 

4-inch vs 7-inch telescopes

4-inch telescope has less aperture than the 7-inch telescopes. They offer a focal ratio of f/2.8-f/15 whereas the 4-inch telescopes vary from f/3.9-f/9. Besides, 4-inch telescopes offer a focal length of 450-1350 mm whereas the 7-inch telescopes start from 500 mm to 2700 mm mm in focal lengths. Moreover, 7-inch telescopes provide 360x the highest magnification, whereas 4-inch telescopes can magnify up to 225-260x.

4-inch vs 8-inch telescope

8-inch telescopes provide focal lengths of 800-2415 mm (f/4-f/11.5), while 4-inch telescopes vary in focal lengths from 450-1350 mm (f/3.9-f/9). On average 8-inch telescopes provide the highest magnification of 300-480x, whereas 4-inch telescopes can magnify 225-260x.