Low power Huygenian Designing making a low power

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.

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

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

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece § • 4) Field lens lies @ F 1 in front of PP 1; PP 1 & F 1’ coincide

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 6) Construct line from intersection of PP 1 & axis to edge of field lens

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 9) Construct perpendicular to axis @ F 2 behind eye lens § 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.

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 10) Construct line from intersection of (8) & (9) @ F 2 behind eye lens through intersection of (6) & eye lens to cross (5)

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 11) Where (5) & (10) cross construct perpendicular to axis _ PP 2

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 12) Field stop lies @ F 2 in front of eye lens

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 14) Construct line from intersection of PP 1 @ axis to edge of field lens. Where it crosses PP 2 gives the radius of the field stop. Angle subtended from Fc @ axis gives geometric afov semi-angle

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

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 17) Construct line from intersection of (15) & (16) through intersection of PP 2 & axis

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 _ PP 2’

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • 19) Construct line through intersection of image plane Fp & axis & intersection (17) & field lens, to intersect (18) @ PP 1’ & F 1’

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • (20) • (21) • (22) Construct rays from field lens intersecting lower edge of field stop to eye lens Construct chief ray from F 2 to Fc @ axis Construct oblique rays parallel to it. Ray height @ Fc gives exit pupil

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece • (23) Fc - F 2 = Eye Relief

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

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.

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.

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

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

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

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece The improvised optical bench §Even an ATM optical bench is expensive. But you can always improvise with plasticine and a carpenter’s 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 1 m 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 Gullstrand’s equation to find the true focal length and the position of the principal planes.

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 machinist’s job simpler. §Finished parts can be fastened together using hidden compression rings & circlips (not shown). §CZ 121 tube & barstock costed @ £ 43. 48 inc VAT. Compared to £ 83. 19 inc VAT for 3” round x 6” length. §Alternative is scrap brass pipe & fittings.

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece prepping 2". 75 OD x 4" length brass barstock for use as mandrel, held in 3 jaw scroll chuck & live centre

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

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece enlarging pilot hole

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

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

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece the finished solderment normalising

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece parting off trapping circlip

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

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

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

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

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

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

LOW POWER HUYGENIAN Constructing a Huygenian eyepiece assembled eyepiece

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

The Achromatic Refractor