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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