Reflecting Telescopes

A reflecting telescope uses a mirror instead of a glass lens to collect the light. Thus the objective lens is now a single concave mirror. Since these mirrors are more economical to make, it is possible to construct some pretty big mirrors and thus greatly increase the potential magnification power of a telescope. The optical design is found below in a simple concave mirror. Light comes in toward the mirror and is reflected to a focal point in front of the mirror instead of behind it.

A reflecting telescope design is found below, where light comes toward the primary mirror in the back of the telescope and is reflected back to a secondary mirror, which can then redirect the light to an eyepiece location which differs from telescope design to design. The model invented by Isaac Newton is shown at the upper right of the image. This model of telescope offers the advantage of a large light-collecting surface in the primary mirror, and the magnification can be changed with a simple change in the eyepiece. These mirrors do suffer a problem called spherical aberration which means that the concave mirror cannot bring all of the light rays to the same focus. By grinding such a mirror at the center to a few millionths of a meter deeper, a parabolic shape is achieved in cross section which corrects for this problem. Then all the rays can be brought to a single, sharp focus. Now it is only a matter of placing the secondary mirror and the eyepiece at different locations for ease of use and portability. The options are found in the image below.

This image is a Cassegrain Reflector of the type which I own as well as the school where I teach. The secondary mirror is actually in the light's path, but the mirror diameter is so large that enough light is collected to compensate for the lost space. The significant advantage to a Cassegrain telescope is the compact tube size. This makes a relatively large amateur reflecting telescope more portable, and portable is good when you live in the metro area and can barely see stars.

The value of a reflecting telescope is in the relatively economical price tag. For instance, the Keck Telescopes which sit atop Mauna Kea had a pricetag of roughly $1 billion. When I read of stuff like that, the word "economical" makes no sense, but I also know that the price for the Keck scopes would be much larger if anyone tried to grind out a pair of 10 meter diameter lenses. Even larger telescopes are being constructed, and very shortly, if not already by the time you read this page, the 4 giant 8 m mirrors in Chile will be completed and tied together to act as a super telescope.

A photograph of the Keck Telescopes at rest during the day atop Mauna Kea. At an altitude of 13,500 ft above sea level, these telescopes are well above the majority of the polluting effects of our atmosphere, in a colder and more stable air pattern, and in a very dark sky site. Students are always surprised to learn that snow covers this Hawaiian peak for 3 months each year and temperatures have dipped to 10oF with 110 mile an hour winds. Snow plows are ordered from Wisconsin to keep the dirt roads clear for astronomers. One of the problems with using these scopes at high altitudes is the reduction of oxygen in the thin air. This lack of oxygen is a deprivation to the brain, and causes people to think poorly until acclimated after several days. Scientists and engineers have recently made electrical connections to the sea level site and can operate the telescopes from down there. The second photo is a cutaway view of the interior of the Keck pair. Please notice the cars and trucks at the front entry way to the Keck scopes to give you a sense of scale. The other pair of photos below the cutaway shows me standing outside the Gemini facility with the Keck twin scopes behind, as well as the Japanese Suburu scope off my left shoulder, and then there I am at the visitor center.

 

These are the Kitt Peak, Arizona telescopes which are used by continental USA astronomers as well as foreign visitors. While not as large as the Keck telescopes, wonderful work is still carried out from this fine collection.

 

 

 

 

 

 

The final drawing is given here to let you see a comparison of the giant mirrors which are either already in service or soon to be.

This is a photograph I took in the summer of 2002. The background telescope is the Canada-France telescope operated by the University of Hawaii. It was the first large scope to be constructed atop Mauna Kea, and the place where my Uncle Bill first worked when he moved to Pahala in 1980. He designed a small fixture that would hold an 8" diameter mirror spotting telescope so astronomers could point the instrument at objects in the sky. A story is told of a horrific ice storm that froze the dome opening. A engineer had to climb up the round structure and chip away the ice, and in the process slipped and was left dangling from a rope in 110 mile per hour winds and 15 degree temperatures. He was rescued with no harm done, but it goes to show that the weather can be pretty tough atop a Hawaiian volcano. The foreground telescope is the recently completed Gemini facility. It is 15 stories high, and the dome top weighs 628 tons, yet can turn noiselessly around full circle in 2 minutes. The actual telescope with an 8.1 meter mirror is 7 stories tall and yet can be easily pushed into position with a touch of a hand, the unit is so finely tuned and designed. There is an identical twin to this telescope at an 8000' mountain in Chile. Together, the Gemini twin telescopes can simultaneously image a deep space object for better resolution. Many of the photographs in this course are from these telescopes, and whenever you see NOAO/AURA in the credits, the pictures are from this instrument pair. To learn a little more about the Gemini telescope, go to my Gemini page and see some really nice pictures of this cool facility.

This is a photograph I took in the summer of 2002 showing the Subaru telescope operated and owned by the Japanese astronomers. It is the only facility atop Mauna Kea that is completely run from the site. All the other scopes are operated at sea level to reduce the impairment in judgment and seeing from the high altitude. The scope is unusual in its dome shape. The "tear drop" shape of the entire building is to reduce wind effects. Like the Gemini telescope, the panels at the sides open to allow wind to pass completely through and keep the telescope from shaking.

 

 

 

 

 

 

 

 

The VLT mirror which is shown in the above diagram is one of four such 8 meter mirrors at the Chilean site seen in this image to your right. These are the four giant telescopes that will be circuited to act as one giant telescope, and promises to be the finest telescope in the world. The site is still under construction as of the summer of 2002, but will soon be fully operational

From here, you should proceed to the pages that teach about Radio Telescopes and these different instruments astronomers have developed to unlock the starlight secrets from radio signals, or you can return to the Introduction to Light and Telescope Page, the Syllabus, or the Home page.


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