4-inch Telescopes (100-120mm)

4-inch telescopes are basically for amateur astronomers and kids. On average, they provide  450-1350 mm focal lengths, and a range of focal ratios from f/3.9-f/9. They can magnify objects 204-236x times the human eye. They also can resolve double stars at 1.15 arcsec. You can get 4-inch telescopes from different brands in the price range of 262-5,200$.

What is a 4-inch telescope?

A 4-inch telescope has an aperture (the diameter of the objective lens) of 4-4.75 inches (100-120 mm). 4-inch telescopes are in the category of small-size telescopes.

What can you see with a 4-inch telescope?

  • The Moon and its craters.
  • The phases of Venus and Mercury.
  • Mars’ polar ice caps.
  • Jupiter’s cloud belts, the Great Red Spot, etc.
  • Saturn’s rings, the division in them, and several moons.
  • Uranus and Neptune as dots.
  • Stars in the galaxies like in M13, M15, M92, etc.
  • Nebulae, such as M27, M57, the Cat’s Eye, etc.
  • NGC objects like M42, the Orion Nebula, and M8, the Lagoon Nebula.

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 telescope5-inch telescope
Aperture4-4.75 inch (100-120 mm)5-inch (127-130 mm)
Focal Ratiof/3.9-f/9f/5-f/15
Focal Length450-1350 mm650-1900 mm
Highest Magnification204-236x225-260x
Price262-5,200$279-8,400$

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 telescope6-inch telescope
Aperture4-4.75 inch (100-120 mm)6-inch (150 mm)
Focal Ratiof/3.9-f/9f/3.9-f/10
Focal Length450-1350 mm162-1800 mm
Highest Magnification204-236x295-320x
Price262-5,200$310-15,070$

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 telescope7-inch telescopes
Aperture4-4.75 inch (100-120 mm)7-inch (180 mm)
Focal Ratiof/3.9-f/9f/2.8-f/15
Focal Length450-1350 mm500-2700 mm
Highest Magnification204-236x360x
Price262-5,200$1,350-5,500$

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. 

4-inch telescope8-inch telescope
Aperture4-4.75 inch (100-120 mm)8-inch (203 mm)
Focal Ratiof/3.9-f/9f/4-f/11.5
Focal Length450-1350 mm800-2415 mm
Highest Magnification204-236x300-480x
Price262-5,200$570-5,000$