What is Dobsonian Telescope ? | Working | Important Advantages


What is a Dobsonian telescope?

Dobsonian telescope refers to a type of Newtonian telescope that comes with an altazimuth-mount, design by John Dobson in the year 1965. Dobsonian telescopes have contributed to the increasing size of the telescopic designs preferred by amateur astronomers. Dobsonian telescopes contained a basic mechanical design that could be easily manufactured from available types of machinery to build a massive, mobile, low-cost telescope. This telescopic configuration is augmented for detecting faint, deep-sky objects such as stars, galaxies, and nebulae. Observations of this type need the usage of a large objective diameter for an increased light-gathering power, of a comparatively shorter focal length, and of a portable set-up for traveling to less light-polluted sites.

dobsonian telescope
Dobsonian telescope

How does a Dobsonian telescope work?

The Dobsonian telescope follows the Newtonian design that uses a primary concave mirror and a secondary flat diagonal mirror. The lower part of the telescope is fixed with a parabolic or spherical primary mirror that is used for capturing the light rays from different celestial bodies. The light incident on the primary mirror is then directed towards the flat secondary mirror by reflection. The secondary mirror receives the light rays and then reflects them further towards the eyepiece. The telescope eyepiece is placed orthogonally to the secondary mirror in the tube. The image focusing and magnification occur within the eyepiece.

What are the advantages of the Dobsonian Telescope over the Newtonian Telescope?

Diagram of the Altazimuth Mount of a Dobsonian telescope.
image source: KoukarineDobCC BY-SA 4.0

Advantage of Dobsonian Telescope:

The Dobsonian design has the following advantages:

Altazimuth mount:

  • * The Altazimuth mount is the central distinguishing part of the Dobsonian telescope from the Newtonian telescope. The latter consisted of an equatorial telescope mount with a clock drive that had a massive size, was less portable, expensive, and made it very hard to access the eyepiece positions. The Altazimuth mounts reduce the weight, size, cost, and increase the entire telescopic set-up’s portability.

* It also helps to keep the eyepiece in a position that is comfortable to access the telescope’s size. The Dobsonian telescope is set-up by simply placing the altazimuth mount on the ground, and then fitting the tube on over it. The altazimuth mount also allows us to rotate the telescope in both vertical and horizontal axes simultaneously. 

However, the altazimuth mount has a set of drawbacks. An Un-driven altazimuth mount requires being manually aligned in every few minutes along both axes to compensate for the Earth’s rotation for keeping an object in view. Equatorial mounts have a single axis and are un-driven. Aligning the axes turn out to be very challenging with higher magnifications.

The altazimuth mount does not permit the usage of conventional setting circles that assist in targeting the telescope at the known coordinates of several objects. These mounts are considered to be difficult for pointing at objects near the apex, essentially because a vast azimuth axis movement is required to maneuver the telescope directing by even an insignificant amount. Altazimuth mounts are also not considered to be suitable for astrophotography.

Large objective diameter compared to mass/cost

o  Low cost to objective size ratio: The Dobsonian design generally provides a more objective diameter per unit of the cost compared to other telescopic designs.

o  Low mass to objective size ratio: The structure of the Dobsonian design, when measured in terms of weight and volume, turned out to be comparatively minimal for any specified objective diameter when matched to other telescopic designs.

Good “Deep Sky” telescope

The Dobsonian telescopic design with the maximized objective diameter and improved portability makes the set-up perfect for detecting nebulae, dim star clusters galaxies (deep sky objects). These types of observations need broad objectives and unpolluted dark sky scenes. Generally, telescopes are difficult to ship from one place to another, but the Dobsonian design allows portability without compromising the configuration. These objects are relatively large, so it is possible to observe them at low magnifications that do not necessitate a clock-driven mount.

Balance Issues:

Generally, telescopic designs that operate with a telescope tube fixed in association to its altitude bearings are more vulnerable to lose balance by the addition or removal of other equipment such as thin films, cameras, finder scopes, or even unusually heavy eyepieces. The Dobsonian telescopes are designed to withstand such minor imbalances. These telescopes have sufficient friction in their bearings for resisting a reasonable amount of imbalance. However, this friction is also responsible for making these telescopes challenging to position accurately. At times, counterweights are bolted or hooked onto the mirror’s back for rectifying the imbalance caused.

Portable mount
Image source: Rob Teeter, Teeter 16 inch telescopeCC BY-SA 4.0

What is a Digital Dobsonian Telescope?

Digital Dobsonian relector mount.
Image source:anonymous, Dobson-mount, marked as public domain

Advancement and development in the field of consumer electronics have benefited amateur astronomy. Modern designs of Dobsonian mounts are capable of finding popular deep-sky objects automatically. New automated variants also solve the problem of manual alignment by compensating for the rotational drift accurately. This design combines advanced functions with reasonable prices, making it highly suitable for amateur astronomers.

To know more about telescopes visit https://lambdageeks.com/reflecting-telescope/ & https://lambdageeks.com/steps-to-use-a-telescope-parts-of-a-telescope/

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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