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Low power Huygenian Designing & making a low power Huygenian eyepiece for a classical achromatic refractor constructing a simple low power Huygenian eyepiece using principal planes geometric ray tracing techniques.

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece RAY TRACING by PRINCIPAL PLANES _ ACHROMATIC HUYGENIAN EYEPIECE 1)PP1 EFL behind OG focal plane

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 2)Draw chief ray through OG centre to edge of image field (taken to be 2-inches) 3)Draw edge rays through OG top & bottom to edge of image field

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 4)Field lens F1 in front of PP1; PP1 & F1 coincide

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 5)Construct line parallel to axis touching edge of field lens

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 6)Construct line from intersection of PP1 & axis to edge of field lens

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 7)Eye lens 1/2 (F1+F2) behind field lens _ F2 8)Construct line parallel to (6) intersecting centre of eye axis

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece During acclimation, an f/14 achromat, compared to an f/7 apochromat would require ~43% less frequent focus correction over an 80% longer cool down time, over a focal shift 3 times greater. 9)Construct perpendicular to F2 behind eye lens

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 10)Construct line from intersection of (8) & F2 behind eye lens through intersection of (6) & eye lens to cross (5)

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 11)Where (5) & (10) cross construct perpendicular to axis _ PP2

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 12)Field stop F2 in front of eye lens

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 13)Where (10) crosses axis, lies the Ramsden Disc _ Fc

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 14)Construct line from intersection of axis to edge of field lens. Where it crosses PP2 gives the radius of the field stop. Angle subtended from axis gives geometric afov semi-angle

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 15)Construct line parallel to axis touching lower edge of field lens

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 16)Drop perpendicular from eye lens to (15)

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 17)Construct line from intersection of (15) & (16) through intersection of PP2 & axis

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 18)Construct line parallel to (17) through field lens & back to (16); where it intersects (15) construct perpendicular to axis _ PP2

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 19)Construct line through intersection of image plane Fp & axis & intersection (17) & field lens, to intersect PP1 & F1

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece (20)Construct rays from field lens intersecting lower edge of field stop to eye lens (21)Construct chief ray from F2 to axis (22)Construct oblique rays parallel to it. Ray Fc gives exit pupil

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece (23)Fc - F2 = Eye Relief

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece The apparent field of view:

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece The actual lenses are not thin, so the raytrace has to be revised to take into account the principal planes within the thick lenses. This is done using Gullstrand s equations. The lens radii are measured using a spherometer, and their focii and principal planes calculated. For the eye lens crown element: The achromatic is a cemented doublet so the internal radii cannot be directly measured. For calculations to determine the internal surface radii refer to end of article.

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece And for the flint lens element: The achromatic is a cemented doublet so the internal radii cannot be directly measured. For calculations to determine the internal surface radii refer to end of article.

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece For the field lens:

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece The lenses need to be drawn in CAD: The eye lens

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece The lenses need to be drawn in CAD: The field lens

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece And the revised raytrace:

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

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The improvised optical bench Even an ATM optical bench is expensive. But you can always improvise with plasticine and a carpenters rule. More precise separation mesures can be made with a vernier caliper. Lens radii can be measured using a spherometer. This is a Fob Watch style spherometer, graduated in Dioptres, and Pouces (a French inch - there are 36 Pouces in a Dioptre). A lens with a focal length of 1m has a power of 1 Dioptre. Surface powers can be added or subtracted to find the overall power of the lens. These values are then inputted into Gullstrands equation to find the true focal length and the position of the principal planes.

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece Machining the eyepiece barrel You could machine the barrel from brass barstock, but brass bar is very expensive, and since most of the barrel is hollow it would take an awfully long time at the lathe producing bin loads of swarf. The sensible thing to do, from both a machining time and cost viewpoint is to select suitable tubestock, and design the barrel from available stock tube outside diameters and wall thicknesses. Brass tubes are not always compatible so some barstock maybe required, but it can be kept to a minimum. Turning screw threads is awkward so a design that relies on good fits and soldered parts makes the machinists job simpler. Finished parts can be fastened together using hidden compression rings & circlips (not shown). CZ121 tube & barstock £43.48 inc VAT. Compared to £83.19 inc VAT for 3 round x 6 length. Alternative is scrap brass pipe & fittings.

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

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prepping 2".75 OD x 4" length brass barstock for use as mandrel, held in 3 jaw scroll chuck & live centre

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece turning down 2".75 OD x 4" length barstock to ID of 2".5 OD x 16swg tube for use as mandrel

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 2".5 OD x 16swg tube mounted on mandrel & turned to exact length 2".768

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece enlarging pilot hole

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece boring out barstock to 2".010 ID using single point tool

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece hard soldering 2".365 OD sleeve to 1"5 x 1/4" wall tubestock

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece Paul Schofield - the happy solderer

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece the finished solderment normalising

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece solderment mounted in 3 jaw scroll chuck

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece solderment trued up and being faced off with parting tool

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 2".0 OD x 16swg tube mounted on mandrel & held against a drill chuck in the tailstock

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 2".0 OD x 16swg tube held on mandrel being parted off at one end

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece 2".0 OD x 16swg tube sleeve being dressed up to shoulder

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece turning makeup piece from Dural bar, the 2".0 ODx 16 swg sleeve has been driven into it prior to parting off so as to ensure the brass tube and the makeup piece were dead concentric

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece a very delicate operation - counterboring the body tube to 2".382 ID using a single point boring bar. Overhang causes the tool to chatter if the cuts are more than a few thou deep. The tolerance is only a thou. The ID was checked at the end of each run, the runs were driven by the leadscrew to obtain the best surface finish.

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece body tube trapped on mandrel by masking tape and held on a dead centre, the OD dressed using a chisel point tool

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece dressing the body tube OD - tool lubricated with mineral oil

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece dressing the body tube OD applying mineral oil to obtain a smooth surface finish

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece body tube after polishing with emery paper down to 2500 grade - note the mirror finish

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece parting 0".864 length of 2".5 OD x 16swg tube trapped on mandrel using masking tape

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece boring out the trapping circlip

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece parting off trapping circlip held on live centre to prevent chattering

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece parting off trapping circlip

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece parting off field stop spacer to 0".1825 length

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece dressing field stop spacer to 2".382 OD exactly, mounted on mandrel to ensure concentricity

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece parting off eye lens retaining cap

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece Modified Huygenian mechanical components prior to finishing

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece Modified Huygenian mechanical components after finishing

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece lens edges painted matt black

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece assembled eyepiece

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LOW POWER HUYGENIAN Constructing a Huygenian eyepiece field lens & field stop

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The Achromatic Refractor

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