Low power Huygenian Designing & making a low power Huygenian eyepiece for a classical achromatic...

LowLowpowerpower

HuygenianHuygenian

LowLowpowerpower

HuygenianHuygenian

Designing & making a low power Huygenian eyepiece for a classical achromatic refractor

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.

constructing a simple low power Huygenian eyepiece using principal planes geometric ray tracing techniques.

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

RAY TRACING by PRINCIPAL PLANES _ ACHROMATIC HUYGENIAN EYEPIECE

•1) PP1 lies @ EFL behind OG focal plane

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•4) Field lens lies @ F1 in front of PP1; PP1 & F1’ coincide

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•5) Construct line parallel to axis touching edge of field lens

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•6) Construct line from intersection of PP1 & axis to edge of field lens

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•7) Eye lens lies @ 1/2(F1+F2) behind field lens _ F2•8) Construct line parallel to (6) intersecting centre of eye lens @ axis

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 axis @ F2 behind eye lens

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•10) Construct line from intersection of (8) & (9) @ F2 behind eye lens through intersection of (6) & eye lens to cross (5)

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•11) Where (5) & (10) cross construct perpendicular to axis _ PP2

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•12) Field stop lies @ F2 in front of eye lens

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•13) Where (10) crosses axis, lies the Ramsden Disc _ Fc

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•14) Construct line from intersection of PP1 @ axis to edge of field lens. Where it crosses PP2 gives the radius of the field stop. Angle subtended from Fc @ axis gives geometric afov semi-angle

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•15) Construct line parallel to axis touching lower edge of field lens

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•16) Drop perpendicular from eye lens to (15)

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•17) Construct line from intersection of (15) & (16) through intersection of PP2 & axis

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•18) Construct line parallel to (17) through field lens & back to (16); where it intersects (15) construct perpendicular to axis _ PP2’

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•19) Construct line through intersection of image plane Fp & axis & intersection (17) & field lens, to intersect (18) @ PP1’ & F1’

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 Fc @ axis•(22) Construct oblique rays parallel to it. Ray height @ Fc gives exit pupil

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

•(23) Fc - F2 = Eye Relief

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

The apparent field of view:

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

For the field lens:

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

The lenses need to be drawn in CAD:

The eye lens

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

The lenses need to be drawn in CAD:

The field lens

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

And the revised raytrace:

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 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 Gullstrand’s equation to find the true focal length and the position of the principal planes.

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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).

CZ121 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 HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

2".5 OD x 16swg tube mounted on mandrel & turned to exact length 2".768

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

enlarging pilot hole

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

boring out barstock to 2".010 ID using single point tool

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

hard soldering 2".365 OD sleeve to 1"5 x 1/4" wall tubestock

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

Paul Schofield - the happy solderer

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

the finished solderment normalising

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

solderment mounted in 3 jaw scroll chuck

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

solderment trued up and being faced off with parting tool

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

2".0 OD x 16swg tube mounted on mandrel & held against a drill chuck in the tailstock

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

2".0 OD x 16swg tube held on mandrel being parted off at one end

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

2".0 OD x 16swg tube sleeve being dressed up to shoulder

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing 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 HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

dressing the body tube OD - tool lubricated with mineral oil

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

dressing the body tube OD applying mineral oil to obtain a smooth surface finish

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

body tube after polishing with emery paper down to 2500 grade - note the mirror finish

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

parting 0".864 length of 2".5 OD x 16swg tube trapped on mandrel using masking tape

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

boring out the trapping circlip

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

parting off trapping circlip held on live centre to prevent chattering

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

parting off trapping circlip

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

parting off field stop spacer to 0".1825 length

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

dressing field stop spacer to 2".382 OD exactly, mounted on mandrel to ensure concentricity

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

parting off eye lens retaining cap

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

Modified Huygenian mechanical components prior to finishing

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

Modified Huygenian mechanical components after finishing

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

lens edges painted matt black

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

assembled eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

LOW POWER HUYGENIANConstructing a Huygenian eyepiece

field lens & field stop

The Achromatic Refractor