Optical Specifications
This. section. is. primarily. intended.
for. the. more. curious. or. technically.
minded. who. may. want. to. know. just.
what. makes. the. Astroscan. “tick”. on.
the.inside..However,.it.also.covers.the.
related. matters. of. how. magnification.
and.field.of.view.are.determined,.which.
should.be.of.interest.to.most.users..As.
already. mentioned,. the. Astroscan. is.
a. classical. Newtonian. reflector.. The.
other. two. basic. types. of. telescope.
in. addition. to. reflectors. (or. mirror.
type).are.refractors.(or.lens.type).and.
catadioptrics.(or.compound.lens-mirror.
type).. Edmund. Scientific. markets. all.
three.forms.but.is.perhaps.best.known.
for.its.innovative.Astroscan.
The. accompanying. diagram. shows. the. position. and.
relationship. of. the. Astroscan’s. four. primary. optical.
components.. The. heart. of. the. system. is. the. 4
1
∕
8
-inch.
(105.mm).clear-aperture.primary.mirror.at.the.bottom.
of.the.surrounding.spherical.housing..(The.mirror.itself.
actually.has.a.4
1
∕
4
-inch.diameter.but.a.small.portion.of.its.
outer.edge.is.contained.within.the.mirror.cell.mounting..
Thus.the.term.“clear.aperture.”).Its.focal.ratio.is.f/4.2,.
resulting.in.a.focal.length.of.17.5.inches.(445.mm),.and.
its.aluminized.and.overcoated.reflective.front.surface.is.
parabolic.to.an.accuracy.of.
1
∕
8
.wave.–.or.to.within.just.
a. few. millionths. of. an. inch. of. being. optically. perfect!.
(Those. wishing. to. delve. into. the. matter. of. optical.
accuracy. should. consult. the. diagram. above,. entitled.
“
1
∕
8th
.Wave.Optics.”)
The usual tolerance for high-precision optics is one-quarter of the
wavelength of light – no part of the glass surface must depart more than
1
∕
4
wave or 5
1
∕
2
millionths of an inch from the specified shape. Compare
this with a sheet of paper, which has a thickness of about 200 waves! The
Edmund Scientific Astroscan betters this tolerance with optics of
1
∕
8
wave.
In the case of a telescope mirror where light transverses the distance twice,
a
1
∕
8
wave defect on the mirror will result in a
1
∕
4
wave error at the image
plane as shown. This gives nearly perfect imagery. Further narrowing of the
tolerance to
1
∕
10
wave or less is more in the nature of advertising claims than
any appreciable gain in definition.
ONE
WAVE
LIGHT
WAVE
( )
GREEN
LIGHT
ONE WAVE = .00022" (22 millionths)
1/8 WAVE = .000027" ( 2 3/4 millionths)
{
( )
(1/8 )
NORMAL
IMAGE
PLANE
TELESCOPE MIRROR
1/8 WAVE DEFECT
1/4 - WAVE ERROR
1
/
8
th
.Wave.Optics
Parabolic
Primary Mirror
Secondary Mirror
(Diagonal)
Anti-Reflection
Coated Window
Focusing
Knob
Image Plane
(Focal Plane)
Eyepiece
Eye Lens
Compound
Field Lens
Field Stop
Eye Relief
Distance
Observer's
Eye
Near.the.top.of.the.telescope.tube.is.the.optically-flat,.
plane-parallel. glass. window.. It’s. coated. with. a. bluish-
looking. anti-reflection. coating. (like. those. seen. on.
binocular. and. camera. lenses). to. prevent. unwanted.
reflection. of. incoming. light. from. its. surfaces. (just. the.
opposite. of. the. primary. mirror,. which. is. coated. to. be.
as.reflecting.as.possible!)..Attached.to.the.inside.of.the.
window.is.the.small.optically-flat.diagonal.mirror.whose.
front. surface. is. coated. the. same. as. the. main. mirror..
Light.entering.the.telescope.passes.through.the.optical.
window.down.onto.the.parabolic.primary.which,.being.
concave,. reflects. it. back. up. the. tube. in. a. converging.
cone..The.diagonal.intercepts.and.redirects.the.light.90.
degrees,.where.it.comes.to.focus.at.the.eyepiece.on.the.
side.of.the.tube.for.viewing..Modern.eyepieces.consist.
Astroscan's.Primary.Optical.Components
