Evolution of the Astronomical Eyepiece - Brayebrook Observatory

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EVOLUTION of the ASTRONOMICAL EYEPIECE NAGLER I - A 2-1-2-2 design by Al N a g l e r, having a Sm yth achro m a t i c field flattener to widen the geometric apparent field from 52˚ to 82˚. However rectilinear distortion is 28%. Eye clearance is 1.2Fe. The Smyth lens enlarges the beam leading to a very bulky eyepiece in focal lengths greater than 13mm, and raised production costs. It also suffers markedly from spherical aberration of the exit pupil, more so th an most other ult ra-w ide a ngle designs. NAGLER II - A 2-1-2-1-2 modification by Al Nagler, sharing the same fundamental design and field, but having reduced spherical aberration of the exit pupil. It is still more noticeable than the military Bertele, and the Galoc. MEADE UWA - A 2-1-2-1-2 design by Meade, having strong similarities to the Nagler II. The apparent field is 84˚ and eye clearance 1.5Fe. The number of air-glass surfaces and lens thicknesses l ea d to signi ficant transmission losses. Axial point images also tend to be larger th an those formed by either the Orthoscopic or Monocentric. 40
WHAT ARE THE BEST DESIGNS EVOLUTION of the ASTRONOMICAL EYEPIECE Without doubt the best design types are the Monocentric, in its various forms, the Orthoscopic in its plethora of variants, the König in its various forms, the Bertele, the Galoc and Galoc II, and the modern Galoc-Bertele hybrid. Choosing an eyepiece depends very much on the nature of the observing you intend using it for. Monocentrics are ideal for double star and planetary work where the restricted field is an advantage, as is the high image contrast. The Orthoscopic is a good all round design, useful for most types of observing, as is the König II. The Bertele and the Galoc lend themselves to low power, wide field work, and are to be recommended above the more recent ultra-wide angle negative-positive designs. Where image contrast is an important consideration, as in the detection of low surface brightness, diffuse objects, or faint planetary detail, eyepieces with more than 6 air-glass surfaces are to be discouraged, particularly when used in conjunction with a coma corrector or focal reducer. Also, where sketches of star fields are to be made, the use of ultra-wide angle designs which suffer fr om rectilinear distortion, should be avoided. For example, the geometric field of the Nagler II is only 59˚; rectilinear distortion introduced by the Smyth lens, enlarges the apparent field a further 30˚. The Galoc possesses an apparent field between 70˚ and 80˚, yet achieves it with only 4 air-glass surfaces and alm ost zero recti linea r distortion. Images of the Moon at medium to high power, seen in the presence of more than 20% rectilinear distortion cause the crater detail at the edge of the field to elongate, and as the image is moved, gives t he a ppearance of peeri ng through a gold fish bowl. I t sh oul d be born in mind that beca use eye relief is reduc ed at sl ow fo ca l ra tios, th e ap par ent fi eld a ppl ies ONL Y at the des ign (usually cri tic al ) foc al rat io, which in ul traw id e a ngle designs i s typical ly f /4. At slower foca l ra tios, as the eye reli ef decrea ses, the apparent field e xpands. What ever th e design , off a xi s perf o rm anc e i s al w ays i m p roved with slower fo cal ra tio objectiv es, although some Barlo w- e yepi ece combinations do not w ork well, and some RFT a pochro m a t s com pensa te eyepiece Petzval curvat u re. In general the mo st eff e c t i v e c ombin ati ons a re objectives wi th fo ca l ra tios sl ower than f/10, an d e ith er Mono cent ri c or O rthoscopic eyepieces. Em ployin g a Barl ow in c onj uncti on wi th a f ast primary is no su bst itute which is why long fo cu s refr acto rs and Cassegr ain ref lectors te nd to have superior visua l i magery. Fi nal ly, im a ge b rightness in extended objects is a function of the e xit p upi l, not th e objective f ocal rati o. Maximum image brightn es s occurs where th e exit pupil m atc hes the pupil lia ry aperture , an d can never exceed th e object’s vi sual bright ness w ith the unai ded eye. W hen all owa nces are made for refl ecti on and absorpt ion losses in th e obj ective an d eyepi ece, the i ma ge brightn ess o f ex ten ded obje cts is alw ays s lightly less than its naked eye bright ness. It i s not t h e r e f o r e n ec essa ry, as is often m istake nly stated to be the case, to us e a f ast Newton ian in o rd er to detect f aint com et s an d galaxies. It is al so n ot n ec essa ry to use h ighly c o r r ect ed eye pieces with a slow fo ca l ra tio ob je ctive. Indee d, the o ff -ax is im ag es presented for exampl e by th e Bertel e at f/16 a re superior i n ev ery respe ct t o t hose obta ined a t f /4 with a Nagler II . 41
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Page 5 and 6: THE EVOLUTION of the ASTRONOMICAL E
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Page 37 and 38: ZEISS ASTROPLANOKULAR - A 2-1-2 eye
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Page 43 and 44: THE EVOLUTION OF EYEPIECE DESIGN (C
Page 45 and 46: TESTING EYEPIECES EVOLUTION of the
Page 47 and 48: TESTING EYEPIECES (cont): EVOLUTION
Page 51 and 52: appendix APPARENT LUMINANCE of TELE
Page 53 and 54: appendix APPARENT LUMINANCE of TELE
Page 55: BIBLIOGRAPHY LABORATOR Y INSTRUMENT
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Evolution of the Astronomical Eyepiece - Brayebrook Observatory

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