Date: January 22, 2011
Title: Telescope Eyepieces
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: Edgardo discusses the theories and facts behind telescope eyepieces.
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.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
Theory and facts behind telescope eyepieces
Hi again! This is your host Edgardo Molina. From Pleiades. Research and Astronomical Studies in Mexico City, Mexico. Today is my first podcast of the year and I am grateful for having the opportunity by the members behind curtains that move the threads to make this wonderful project possible.
Today I am embracing one of the most forgotten aspects of visual astronomy using a telescope. The eyepiece. This is the first of a two podcast series on the subject. This piece of equipment should be given the same respect as the rest of the optical tube assembly. In fact I have heard that the eyepiece should be considered to be an essential part of the optical train to allow the telescope to deliver the images to our eye in full capacity.
Just as what I am used to in the amateur radio world where there is a wise saying stating: “If you have 10 coins for a complete amateur radio station, invest 9 of them in the antennae system and 1 coin in the radio”. Here in the amateur astronomy world this¬†saying is also correct. Better invest in a 9 coin eyepiece and leave 1 coin for the telescope (of course please keep the quality of the scope to a decent minimum. No science fair plastic Wal-Mart with 800X magnification type of telescopes apply!)
You could be wasting precious telescope optics capacity by using a mediocre or poor eyepiece. If you are serious about your observations, better take care of the eyepiece investment as well as the telescope.
Nowadays the most respected names in the amateur and professional telescope arenas, are aware of using the best eyepieces they can afford to allow their customers to enjoy in full all what the optical characteristics of their telescopes have to offer when visual observing.
It is true that to really appreciate the quality of an eyepiece, you should look through several types and qualities on board the same telescope. If it is a good, really good telescope, the better the test, and if it is a refractor with no central obstruction on the business end, the evaluation of the eyepiece will be better defined. And at this point I must confess I am not printing here my signature for the love of fine refractor optics! ¬†Oh! Well…
Now lets go to the theory behind the eyepieces:
Definition:
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed at the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.
An eyepiece consists of several “lens¬†elements” in a housing, with a “barrel” on one end. The barrel is shaped to fit in a special opening of the instrument to which it is attached. The image can be¬†focused¬†by moving the eyepiece nearer and further from the objective. Most instruments have a focusing mechanism to allow movement of the shaft in which the eyepiece is mounted, without needing to manipulate the eyepiece directly.
The eyepieces of binoculars are usually permanently mounted in the binoculars, causing them to have a pre-determined magnification and field of view. With telescopes and microscopes, however, eyepieces are usually interchangeable. By switching the eyepiece, the user can adjust what is viewed. For instance, eyepieces will often be interchanged to increase or decrease the magnification of a telescope. Eyepieces also offer varying fields of view, and differing degrees of eye relief for the person who looks through them.
Modern research-grade telescopes do not use eyepieces. Instead, they have high-quality CCD sensors mounted at the focal point, and the images are viewed on a computer screen. Some amateur astronomers use their telescopes the same way, but direct optical viewing with eyepieces is still very common
Eyepiece properties
Several properties of an eyepiece are likely to be of interest to a user of an optical instrument, when comparing eyepieces and deciding which eyepiece suits their needs.
Design distance to entrance pupil
Eyepieces are optical systems where the entrance pupil is invariably located outside of the system. They must be designed for optimal performance for a specific distance to this entrance pupil (i.e. with minimum aberrations for this distance). In a refracting astronomical telescope the entrance pupil is identical with the objective. This may be several feet distant from the eyepiece; whereas with a microscope eyepiece the entrance pupil is close to the back focal plane of the objective, mere inches from the eyepiece. Microscope eyepieces may be corrected differently from telescope eyepieces; however, most are also suitable for telescope use.
Elements and groups
Elements¬†are the individual lenses, which may come as¬†simple lenses¬†or “singlets” and cemented¬†doublets¬†or (rarely)¬†triplets. When lenses are cemented together in pairs or triples, the combined elements are called¬†groups¬†(of lenses).
The first eyepieces had only a single lens element, which delivered highly distorted images. Two and three-element designs were invented soon after, and quickly became standard due to the improved image quality. Today, engineers assisted by computer-aided drafting software have designed eyepieces with seven or eight elements that deliver exceptionally large, sharp views.
Internal reflection and scatter
Internal reflections, sometimes called¬†scatter, cause the light passing through an eyepiece to disperse and reduce the¬†contrast¬†of the image projected by the eyepiece. When the effect is particularly bad, “ghost images” are seen, called¬†ghosting. For many years, simple eyepiece designs with a minimum number of internal air-to-glass surfaces were preferred to avoid this problem.
One solution to scatter is to use thin film coatings over the surface of the element. These thin coatings are only one or two wavelengths deep, and work to reduce reflections and scattering by changing the refraction of the light passing through the element. Some coatings may also absorb light that is not being passed through the lens in a process called total internal reflection where the light incident on the film is at a shallow angle.
Chromatic aberration
Lateral chromatic aberration is caused because the refraction at glass surfaces differs for light of different wavelengths. Blue light, seen through an eyepiece element, will not focus to the same plane as red light. The effect can create a ring of false colour around point sources of light and results in a general blurriness to the image.
One solution is to reduce the aberration by using multiple elements of different types of glass. Achromats are lens groups that bring two different wavelengths of light to the same focus and exhibit greatly reduced false colour. Low dispersion glass may also be used to reduce chromatic aberration.
Longitudinal chromatic aberration is a pronounced effect of optical telescope objectives, because the focal lengths are so long. Microscopes, whose focal lengths are generally shorter, do not tend to suffer from this effect.
Focal length
The focal length of an eyepiece is the distance from the principal plane of the eyepiece where parallel rays of light converges to a single point. When in use, the focal length of an eyepiece, combined with the focal length of the telescope or microscope objective, to which it is attached, determines the magnification. It is usually expressed in millimetres when referring to the eyepiece alone. When interchanging a set of eyepieces on a single instrument, however, some users prefer to refer to identify each eyepiece by the magnification produced.
Magnification increases when the focal length of the eyepiece is shorter or the focal length of the objective is longer. For example, a 25 mm eyepiece in a telescope with a 1200 mm focal length would magnify objects 48 times. A 4 mm eyepiece in the same telescope would magnify 300 times.
Amateur astronomers tend to refer to telescope eyepieces by their focal length in millimetres. These typically range from about 3 mm to 50 mm. Some astronomers, however, prefer to specify the resulting magnification power rather than the focal length. It is often more convenient to express magnification in observation reports, as it gives a more immediate impression of what view the observer actually saw. Due to its dependence on properties of the particular telescope in use, however, magnification power alone is meaningless for describing a telescope eyepiece.
Location of focal plane
In some eyepiece types, such as Ramsden eyepieces (described in more detail in our next podcast), the eyepiece behaves as a magnifier, and its focal plane is located outside of the eyepiece in front of the field lens. This plane is therefore accessible as a location for a graticule or micrometer crosswires. In the Huygenian eyepiece, the focal plane is located between the eye and field lenses, inside the eyepiece, and is hence not accessible.
Field of view
Actual field of view
Due to the effects of these variables, the term “field of view” nearly always refers to one of two meanings: the angular size of the amount of sky that can be seen through an eyepiece when used with a particular telescope, producing a specific magnification. It is typically between one tenth of a degree, and two degrees.
Apparent field of view
this is a measure of the angular size of the image viewed through the eyepiece, in other words, how large the image appears (as distinct from the magnification). This is constant for any given eyepiece of fixed focal length, and may be used to calculate what theactual field of view will be when the eyepiece is used with a given telescope. The measurement ranges from 35 to over 80 degrees.
Barrel diameter
Eyepieces for telescopes and microscopes are usually interchanged to increase or decrease the magnification and to allow the user to select a type with a certain performance characteristic. To allow this eyepieces come in standardized “Barrel diameters”.
Telescope eyepieces
0.965 inch (24.5 mm) РThis is the smallest standard barrel diameter and is usually found in toy store and shopping mall retail telescopes. Many of these eyepieces that come with such telescopes are plastic, and some even have plastic lenses. High-end telescope eyepieces with this barrel size are no longer manufactured, but you can still purchase Kellner types.There are three standard barrel diameters for telescopes. The barrel sizes (usually expressed in inches) are:
1¼ inch (31.75 mm) Р1¼ inch is the most popular telescope eyepiece barrel diameter. The practical upper limit on focal lengths for eyepieces with 1¼ inch barrels is about 32 mm. With longer focal lengths, the edges of the barrel itself intrude into the view limiting its size. With focal lengths longer than 32 mm, the available field of view falls below 50°, which most amateurs consider to be the minimum acceptable width. These barrel sizes are threaded to take 30 mm filters.
2¬†inch¬†(50.8¬†mm) – The larger barrel size in 2¬†inch eyepieces helps alleviate the limit on focal lengths. The upper limit of focal length with 2¬†inch eyepieces is about 55¬†mm. The trade-off is that these eyepieces are usually more expensive, won’t fit in some telescopes, and may be heavy enough to tip the telescope. These barrel sizes are threaded to take 48¬†mm¬†filters¬†(or rarely 49¬†mm).
Eye relief
The eye needs to be held at a certain distance behind the eye lens of an eyepiece to see images properly through it. This distance is called the eye relief. A larger eye relief means that the optimum position is further from the eyepiece, making it easier to view an image. However, if the eye relief is too large it can be uncomfortable to hold the eye in the correct position for an extended period of time, for which reason some eyepieces with long eye relief have cups behind the eye lens to aid the observer in maintaining the correct observing position. The eye pupil should coincide with the Ramsden disc, the image of the entrance pupil, which in the case of an astronomical telescope corresponds to the object glass.
Eye relief typically ranges from about 2 mm to 20 mm, depending on the construction of the eyepiece. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic. Until recently, and still quite commonly, eyepieces of a short-focal length have had a short eye relief. Good design guidelines suggest a minimum of 5–6 mm to accommodate the eyelashes of the observer to avoid discomfort. Modern designs with many lens elements, however, can correct for this, and viewing at high power becomes more comfortable. This is especially the case for spectacle wearers, who may need up to 20 mm of eye relief to accommodate their glasses.
In our next podcast, I will discuss the types of eyepieces from a historical point of view and also will land some facts on the state of the art eyepieces one can find nowadays in the market.
Stay tuned! For the 365 Days of Astronomy Podcast this is Edgardo Molina, signing off and wishing you clear skies.
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.