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With its completion in the mid-1840's the 72" Reflective telescope at Birr Castle was the largest telescope in the world and held this title for about 75 years.

The telescope we see today at Birr Castle is a reconstruction. Click on the photo above to see a larger version of the photo.

Optical Telescopes work best when the air is clear, dry and there is very little air movement. As you might have guess, Ireland's weather does not often contain these characteristics. Never the less, using this telescope, the 3rd. and 4th Earls of Rosse were able to contribute to the advancement of Astronomy.

Return to Birr Castle top page

More pages on Birr Castle and its gardens:

The telescope
The gardens
Ireland's Historic Science Center
Outside link on Reflecting telescopes


Prior to the construction of the 72 inch reflecting telescope the third Earl of Rosse constructed a 36 inch reflective telescope.

One of the keys to making a fine telescope is the precise grinding of its mirror and the quality of the material (blank) the mirror is composed of. The goal is to produce a mirror free of pitting, highly reflective, and of a material that will not distort due to changes in temperature or other environmental factors. To archive the quality required, the Earl not only designed and constructed his own steam powered mirror grinder but using an alloy developed from his own experimental research, casting the mirror blake right on the castle grounds,

Reflecting Telescopes or Refractive Telescope, what is the difference?

In order to see faint objects you need a way to gather more light from the object than what your eye is capable of capturing all by itself. With optical telescopes you gather the light in one of two ways, you use a big lens to focus (refract) the light or you use a parabolic mirror to (reflect and) focus the light. Refracting telescopes use a lens to focus light, Reflecting telescopes use a mirror to focus light.

After succeeding with his 36 inch telescope and tantalized by what it reveled the the Earl set his sights on the construction of a 72 inch reflective telescope. Once cast, the mirror's weight came in at over three tons. The completed mirror was then mounted in a large tube. Besides protected the mirror from the weather, birds etc, the tubes inner walls absorb scattered light which can dull the telescope's imaging of stellar object.

Photo Right - The 72 inch telescope in its mounting. While the telescope could be position to look at any spot from the horizon to directly up it had limited movement from side to side.


Two tower walls were build so the 3 ton mirror and its tube could be moved to view different parts of the sky. Due to the positioning of the towers, and the earths rotation, viewing of stellar objects were limited to a short time frame of 50 minutes to 2 hours.

For much of its existence, the 72 inch telescope was focused on nebula and star clusters. Sometimes, it was able to see spiral arms on what was previously viewed as gas clouds. These spiral nebulas would eventually be called spiral galaxies. The Earl's research help to establish that these spiral nebulas were not made of gas but individual stars, residing out side our own milky way galaxy.

Given the Irish weather and its limited tracking capacity the ordinal telescope had an amazing list of achievements. The mirror, being made from metal, tarnished quickly from Irelands humid climate. Never the less, with regular maintenance the telescope was keep in use through generations. It was only after the death of the fourth Earl of Rosse that the telescope was dismantled its usefulness having been replaced by larger telescopes else where.

Go to the Birr Castle top page

Check out the Gardens at Birr Castle

Ireland's Historic Science Center


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Much of the information on this website was gather during my families July 2004 visit to Ireland