What is Newtonian Telescope? | Working | Important Advantages and Disadvantages


The Newtonian reflecting telescope (also known as the Newtonian reflector) is the oldest reflecting telescope variant developed by Sir Isaac Newton in the year 1668. This design uses a primary concave mirror and a secondary flat diagonal mirror. The Newtonian telescope is popular because of its simple yet effective design lauded by amateur telescope makers. Nowadays, these telescopes are found in several design variations and sometimes incorporate additional optical elements to enhance image quality or mechanically improve the image position.

Newtonian telescope
Newtonian telescope
Image source: User:Solipsist (Andrew Dunn), NewtonsTelescopeReplicaCC BY-SA 2.0

How does a Newtonian telescope work?

Initially, Sir Isaac Newton created mirrors on his own by mixing copper and tin on the back. This mirror reflected a huge amount of light and was comparably less costly than the silver used during that time. The telescope was designed to capture light from the top of the tube that pointed towards the sky.

The lower end of the telescopic design has a parabolic or spherical primary mirror used to capture light emitted by the celestial bodies. The light (image) captured by the primary mirror is then redirected/reflected towards the flat secondary mirror. This secondary mirror then further reflects the image towards the eyepiece. The eyepiece is generally located at around 90 degrees from the secondary mirror in the tube. The magnification and focus take place within the eyepiece.

Design of a Newtonian Telescope
Image source: KrishnavedalaNewtonian telescope2CC BY-SA 4.0

What are the advantages of the Newtonian telescope?

1. This particular telescope design ensures chromatic aberration-free image formation, unlike other refracting telescopes.

2. Newtonian telescopes are generally cheaper than other telescopes with a similar configuration for any given objective diameter (or aperture).

3. The production of these telescopes is simple. They involve only one surface that requires to be ground and polished into a complex shape. Early designs of reflecting telescopes such as Gregorians and Cassegrain used two surfaces that had to be figured. Objectives of achromatic reflecting telescopes had four surfaces that needed figuring.

4. This telescopic design presented a wider field of view as it is easy to obtain a short focal ratio in this.

5. The design has placed the eyepiece at the top end of the telescope. This eyepiece placement, along with short focal ratios, produces a compact mounting system, adds portability, and reduces cost.

Commercial Newtonian telescope design.
Image Source: Szőcs TamásTamasflexNewton01CC BY-SA 3.0

What are the disadvantages of the Newtonian telescope?

1. The Newtonian telescope, like any other reflector, is subjected to coma. Coma is referred to as an off-axis aberration due to which images tend to flare inwards in the optical axis direction. (Images of stars on the edge of the field of view appears to have a “comet-like” shape). There is so much observable flare on-axis. This flare is linearly increasing with field angle and is inversely proportional to the square of the mirror focal ratio (the ratio between the mirror focal length and the mirror diameter).

The third order tangential coma is given by the formula 3θ / 16F², θ stands for the angle off-axis to the image formed (in radians), and F stands for the focal ratio. A Newtonian telescope with a focal ratio, lower than or equal to f/6, is said to have a highly significant coma for visual or photographic use. Combining primary mirrors having a low focal ratio with lenses that correct for coma can enhance the sharpness of the image over the field.

2. Newtonian telescopes have a secondary mirror that causes a central obstruction in the path of light. Along with this obstruction, the support structure (called the spider) of the secondary mirror produces diffraction spikes. This phenomenon reduces image contrast. A two or three-legged curved spider is used to reduce these effects visually. This curved spider helps in reducing the diffraction sidelobe intensities to around four times less and improving image contrast. However, these circular/curved spiders are more susceptible to wind-induced vibrations.

http://By SvonHalenbach – Own work, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=1356068Newtonian optical assembly depicting the 1. tube, 2. the primary mirror, and 3. the spider support or secondary diagonal mirror support

3. Collimation tends to be an issue for portable Newtonian reflectors. Transportation and handling of telescopes may result in disturbing the alignment of the primary and secondary mirrors of the telescope. This creates an additional work of re-aligning or collimating the entire telescope set up every time it is moved. Certain telescopic designs such as refractors and catadioptrics (specifically Maksutov cassegrains) have solved collimation.

4. This telescopic design places the focal plane at an asymmetrical point located at the uppermost part of the optical tube assembly. Due to this, the eyepiece lands in an inferior viewing position, particularly on equatorial telescope mounts. In fact, the larger telescopes need ladders or other higher support structures to access the eyepiece. Specific designs have incorporated the mechanism for rotating the eyepiece mount or the entire tube assembly in order to obtain a better viewing position. The counterbalancing of these heavy instruments mounted at the focal plane is highly crucial for research purposes.

Variation of the Newtonian telescope:

Jones-Bird reflector

The Jones-Bird reflecting telescope (occasionally referred to as Bird-Jones) is a catadioptric (mirror-lens) variant of the customary Newtonian design. In place of the parabolic mirror in the Newtonian design, the Jones-Bird design has a spherical primary mirror. The spherical aberrations are fixed using a sub-aperture corrector lens that is mounted either in front of the secondary mirror or inside the tube. The main advantage of this design is that it reduces the size and cost of the telescope. The telescope tube length is considerably shorter.

Schmidt-Newtonian telescopes

The Schmidt-Newtonian telescopes mounts a corrector plate in front of the telescope. The plate ensures a closed tube system i.e. the air inside the tube remains unchanged and helps in keeping the temperature inside unaltered. This plate also fixes the errors or aberrations caused by the primary mirror of the telescope. In this design the secondary mirror is placed behind the plate in such a way that the spider support does not interfere with the image formation.

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About Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers.

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