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ON CHOOSING GOOD ASTRONOMICAL EYEPIECES K Siva Senthil Amateur Astronomer Association Delhi Abstract: An astronomical observing system’s performance is dependant both on the optical performance of the primary mirror as well as the eyepieces. In fact choice of a good eyepiece becomes a critical factor in image quality especially with amateur equipment. In this paper we review basics of an eyepiece system along with an a survey of what kinds of eyepiece systems are available to the average amateur astronomer I-INTRODUCTION Eyepiece is assembly of lenses placed at focal point of objective of a telescope. They are also known as ocular lens. Eyepiece is used to magnify the image formed by objective lens or mirror of the telescope. Eyepieces are specified by their focal length, apparent field of view (FOV). These parameters can be used to derive any other information required. Magnification that an eyepiece can produce depends on which telescopes specifications. For telescopes with longer focal length an eyepiece with small focal length will produce more magnification compared to that of when used with telescope of smaller focal length. While buying an eyepiece following properties of eyepiece should be checked for 1) Optical quality - There are many factors that influence optical quality. Anti-reflection coating used to avoid ghost images and total internal reflection; blackened lens edges to minimize scattered light, various kinds of aberrations caused by elements of eyepiece are few of the factors that should be considered while purchasing an eyepiece. 2) Field of view - There are two taxonomies involved with FOV, they are Apparent FOV and Actual FOV. Actual FOV is expressed in degrees, minutes, or arc seconds which is the actual portion of sky that is covered by looking through the eyepiece. Apparent FOV is an abstract number that is calculated from constants which specify the eyepiece. Formula used to calculate apparent FOV is as follows S/L = 2 * tan (α/2) S = Aperture of entrance field stop L = Focal length of eyepiece α = Apparent FOV Apparent FOV and Actual FOV are related as follows – FOV actual = FOV apparent / Magnification Deep sky observations require eyepieces with low magnification and large FOV. Since, deep sky objects are very faint and to get good contrast and light collection eyepieces with low magnification are preferred. Planetary observations are usually done in high magnification and small FOV eyepieces. Since, in planetary observations we often like to look in closer details the planetary object. 3) Eye relief - It is the distance between the eyepiece and observers eye where the entire field of view is visible. Observation is comfortable with eyepieces of fairly large eye relief. If the eye relief is too large then observation becomes uncomfortable as it will be difficult to hold the eye at one position for a long time. For people wearing glasses eye relief is of primary concern. As wearing glasses one will find it difficult to observe through an eyepiece of low eye relief. As a rule of thumb eye relief decreases as focal length of eyepiece decreases i.e. magnification increases. 4) Focal length - This is the distance from the principal plane (for an ideal lens it will be the plane passing through the center of lens) of an eyepiece to the point where the rays converge to a point. Combined with the focal length of telescope it yields the magnification that a particular eyepiece can produce. Plane passing through the focal point is known as focal plane. In some eyepiece location of focal plane is also considered as it helps to place a graticule without affecting the image quality. There are different kinds of eyepieces. They differ in number of elements and their assembly. Initial designs of eyepieces had single elements and are known as positive eyepiece and negative eyepiece. These types of eyepiece suffered from all kind of defects, distortions, and narrow fields. First eyepiece was designed by Hans Lipershy in 1608-1609 using standard spectacle. It had a positive lens. This was followed by Galileo Galilei in 1610, who produced an eyepiece with negative lens. This lens suffered from all known defects; surprisingly it is still in use in toy binoculars. between the two lenses. Huygenians has a large field lens and small eye lens. Location of focal plane in this assembly prevents placement of graticule. Advantages 1) Absence of cementing in combining the lenses makes it a perfect choice for solar projection. 2) Cheap design Disadvantages 1) Short eye relief, narrow field of view, 2) High image distortion, 3) Chromatic aberration. Mittenzway Eyepiece This eyepiece is modification of Huygens eyepiece. This eyepiece replaces the field plano convex lens with a meniscus lens. II - POPULAR TYPES Some of the popular types of eyepieces are listed below with their functionalities, discussing their advantages and disadvantages. Positive Eyepiece - This is more refined version of Lipershy's eyepiece. This has a positive lens. They are now used in optical labs for testing telescopes. Huygens Eyepiece - This eyepiece has two elements known as eye lens and field lens. Both the lenses are plano convex lenses separated by air gap with plane side facing outwards. Focal plane is located Ramsden Eyepiece - This eyepiece has two elements of same focal length. Two lenses are separated by air gap. This distance is typically less than one focal length. Lenses are plano convex lenses with curved surfaces facing each other. Separation between lenses is adjustable to remove the transverse chromatic aberration. Focal plane of this eyepiece is outside the lens arrangement and is thus accessible for a graticule to be placed. Advantages 1) It is free from coma, 2) Cheap design 3) Absence of cementing in combining the lenses makes it a perfect choice for solar projection. Disadvantages 1) They suffer from chromatic and spherical aberration to an extent, 2) Narrow field of view. Kellner Eyepiece - This is a three element design. An achromatic doublet is used as eye lens. This doublet is cemented together. This doublet helps in removal of residual transverse chromatic aberration. Advantages 1) Absence of transverse chromatic aberration, 2) Inexpensive and good image quality, 3) Good field of view (approx - 40 - 50 degrees), 4) Good eye relief, 5) Works well with faster telescopes (f/8, f/6 etc.). Disadvantages 1) It produces ghost images, 2) Has considerable internal reflection. RKE Eyepiece - This is a variant of Kellner eyepiece. This has slightly larger FOV compared to Kellner eyepiece. It was developed by Dr. David Rank for Edmund Scientific. Abbreviation RKE stands for Rank Kellner Eyepiece. Orthoscopic Eyepiece This is a four element design. It has a triplet field lens with double convex eye lens. This was designed by Ernst Abbe. They provide more magnification compared to eyepieces mentioned till now. They are used for planetary observations. Advantages 1) Good eye relief, 2) Nearly zero chromatic and spherical aberration, 3) Fairly flat field of view, 4) Wide field of view, 5) Low internal reflection, 6) Low distortion of image. Erfle Eyepiece - This is typically five element design. However, six element design also exist. This design has typically two achromats with double convex lens or three achromats with double convex lens. This was designed by Heinrich Erfle during world war for war purposes. Erfle eyepiece with six elements was designed by Kasperiet, associate of Erfle. They are perfect for deep sky observations. Advantages 1) Large field of view (up to 80 degrees), 2) Good eye relief. Disadvantages 1) Astigmatism at the edges, 2) Low image quality. Köing Eyepiece - This is a short focal length version of Erfle eyepiece. This contains up to seven elements with cemented singlets and doublets. This gives high magnification and FOV up to 70 degrees. Plössl Eyepiece - This is a four element design. It has two sets of doublets with convex and concave lenses attached together. It is one of the most popular eyepieces today. This eyepiece is used for both deep sky and planetary observations. Advantages 1) Large FOV, 2) Better quality of image. Disadvantages 1) Short eye relief. Nagler Eyepiece - This is a patented design which is also a trademark from TeleVue. This design has 7 to 8 lenses with varied density of glasses. This eyepiece, with its wide (82 degree) field of view and high optical quality, works well even on telescopes with fast focal ratios. This is attributed to the design of eyepiece as the first two element act like Barlow lens. The disadvantage of this design is loss of brightness and contrast due to interaction with many surfaces. Barlow Lens is not an eyepiece. They are attachments to any of the above mentioned eyepieces. These lenses are achromatic diverging (convex) elements. Their functionality is to increase the focal length of the telescope. They are specified magnification of typically 1.5x, 2x, 3x etc. Please note that Lumicon, Orion, Panoptic, TeleVue, University or Brandon, Meade are manufacturers. They are not classified as types of eyepieces. In amateur astronomy we hardly talk of types of eyepieces but they are very essential to know as they are used as standards when amateur astronomer is submitting reports / observations to some organisations. There are few types of eyepieces which are also trademarks like Nagler and RKE. In the new age of space observatories, even ground based observatories are switching from eyepieces to CCD chips. These are connected to Computers which generate images. However, eyepieces maintain their importance in the field of Amateur Astronomy where these new techniques of observations are not used. REFERENCES Content - World Wide Web 1) 2) 3) 4) Eyepieces on Wikipedia GATFAQ website Frosty Drew Observatory Tom Lougheed ’s formula in astronomy 5) John Savard ’s pages 6) Astro-tom’s pages 7) The Belmont Society Image Credit 1) Frosty Drew Observatory