Date: September 26, 2010
Title: A Primer on High End Refractor Telescope Optics. Part 1
Podcaster: Edgardo Molina
Organization: Pleiades. Research and Astronomical Studies A.C. www.pleiades.org.mx (web site soon to be presented also in English)
Description: In this first of a three-part series of podcasts Edgardo Molina provides a history of refractor telescopes.
Bio: Edgardo Molina. B.S. in Mechanical Engineering from the Anahuac University in Mexico City. Post graduate studies in IT Engineering and a Masters Degree in IT Engineering. Working for IPTEL, an IT firm delivering solutions to enterprises since 1998. Space exploration enthusiast who participated in several Mexican space related activities. Licensed amateur radio operator with call sign XE1XUS. Amateur astronomer since childhood and actual founder and president of the Pleiades. Research and Astronomical Studies A.C. in Mexico City, Mexico. Avid visual observer and astrophotography fan. Public reach through education in exact sciences, engineering and astronomy. Lectures and teaching in several universities since 1993.
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A Premier on High End Refractor Telescope Optics Part I
Hello again. This is Edgardo Molina, from Pleiades. Research and Astronomical Studies in Mexico City, Mexico. I am your host today for this episode of the 365 Days of Astronomy Podcast.
Today I am starting a series of 3 podcasts related to telescope technology.
Refractor telescopes. High end refractor optics. These telescopes are the oldest in design and nowadays they offer the discerning amateur with exotic choices to fulﬁll the most demanding customers. The series will be divided as follows:
Part I: Refractor Telescope History. A little bit of background on the subject.
Part II: Telescope Optics. The physics involved in the subject.
Part III: High End Refractor Optics. Who is who in the market nowadays for high end refractor optics. The uses of this kind of telescope and how they perform side by side with other types of telescope optics.
Now, let’s go to Part 1. Refractor Telescope History.
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magniﬁed image for making a photograph, or collecting data through electronic image sensors.
There are three primary types of optical telescopes: Refractors (Dioptrics) which use lenses, Reﬂectors (Catoptrics) which use mirrors, and Combined Lens-Mirror Systems (Catadioptrics) which use lenses and mirrors in combination; for example, the Maksutov telescope and the Schmidt camera.
A telescope’s light gathering power is directly related to the diameter (or aperture) of the objective lens or mirror. The larger the lens is, the more light the telescope can collect. What is commonly described as a telescope’s power, its magniﬁcation, is a function of both the objective’s focal length and that of the eyepiece.
The telescope is more a discovery of optical craftsmen than an invention of scientist. The lens and the properties of refracting and reﬂecting light had been known since antiquity and theory on how they worked were developed by ancient Greek philosophers, preserved and expanded on in the medieval Islamic world, and had reached a significantly advanced state in by the time of the telescopes invention in early modern Europe. But the most significant step cited in the invention of the telescope was the development of lens manufacture for spectacles, ﬁrst in Venice and Florence in the thirteenth century, and later in the spectacle making centers in both the Netherlands and Germany. It is in the Netherlands in 1608 where the ﬁrst recorded optical telescopes (refracting telescopes) appeared. The invention is credited to the spectacle makers Hans Lippershey and Zacharias Janssen in Middelburg, and the instrument-maker and optician Jacob Metius of Alkmaar. Galileo greatly improved upon these designs the following year and is generally credited with being the ﬁrst to use a telescope for astronomical purposes. Galileo’s telescope used Hans Lippershey’s design of a convex objective lens and a concave eye lens and this design has come to be called a Galilean telescope.
The original design Galileo came up with in 1609 is commonly called a Galilean telescope. It uses a convergent (plano-convex or bi-convex) objective lens and a divergent (plano-concave or bi-concave) eyepiece lens. Galilean telescopes produce upright images.
Galileoʼs best telescope magniﬁed objects about 30 times. Because of ﬂaws in its design, such as the shape of the lens and the narrow ﬁeld of view, the images were blurry and distorted. Despite these ﬂaws, the telescope was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was the ﬁrst to see craters on the Moon and four moons orbiting Jupiter.
Parallel rays of light from a distant object would be brought to a focus in the focal plane of the objective lens. However, the (diverging) eyepiece lens intercepts these rays and renders them parallel once more, but traveling at a larger angle to the optical axis. This leads to an increase in the apparent angular size.
The ﬁnal image is a virtual image, located at inﬁnity and is the same way up as the object.
Johannes Kepler proposed an improvement on the design that used a convex eyepiece, often called the Keplerian Telescope.
The Keplerian Telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo’s design. It uses a convex lens as the eyepiece instead of Galileo’s concave one. The advantage of this arrangement is the rays of light emerging from the eyepiece are converging. This allows for a much wider ﬁeld of view and greater eye relief but the image for the viewer is inverted. Considerably higher magniﬁcations can be reached with this design but to overcome aberrations the simple objective lens needs to have a very high f-ratio, which is the focal lenght of the telescope divided by it’s aperture, (Johannes Hevelius built one with a 45 m (150 ft) focal length and even longer tubeless telescopes were constructed). The design also allows for use of a micrometer at the focal plane (used to determining the angular size and/or distance between objects observed.
The next big step in the refractors development was the advent of the Achromatic lens in the early 18th century that corrected chromatic aberration seen in Keplerian telescopes up to that time, allowing for much shorter instruments with much larger objectives.
The Achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, “crown” and “ﬂint” glass, to limit the effects of chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. The era of the Great refractors in the 19th century saw large achromatic lenses culminating with largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900.
Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of ﬂuorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Such telescopes are sold in the high-end amateur telescope market. Apochromatic refractors are available with objectives of up to 553 mm in diameter, but most are between 80 and 152 mm. Prices for these kind of optics are very high. Refractors are nowadays the most expensive telescopes per unit of objective area.
In our next podcast we will embrace the physics of optical trains used in refractor telescopes. Stay tuned.
Please join me again for the next episode of these series related to high end refracting telescopes. Until then I wish you all clear skies!
For the 365 Days of Astronomy Podcast, this is Edgardo Molina, from Pleiades. Research and Astronomical Studies in Mexico City, Mexico. Thank you for listening!
End of podcast:
365 Days of Astronomy
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