of.combinations.of.several.lenses.or.sets.of.lenses.and.
have. a. “field. stop”. which. gives. a. nice. sharp. edge. to.
the.circular.field.of.view..In.essence,.the.primary.mirror.
collects.light.and.forms.an.image.at.its.focus,.while.the.
eyepiece.magnifies.that.image.
The.magnifying.power.of.a.telescope.is.found.simply.by.
dividing.the.focal.length.of.the.primary.mirror.(or.objective.
lens.for.a.refractor).by.the.focal.length.of.the.eyepiece.
being.used..This.is.often.expressed.as:.X.(or.power).=.
Focal.Length/focal.length,.where.the.capitalized.words.
represent.the.focal.length.of.the.primary.mirror.and.lower.
case.words.represent.the.focal.length.of.the.eyepiece..
In.the.case.of.the.Astroscan.used.with.its.standard.28.
mm.eyepiece,.this.becomes.445/28.=.16X..Notice.that.
this.is.in.millimeters..It.can.also.be.done.in.inches,.as:.
17.5/1.1.=.16X..Also.note.that.to.increase.magnification,.
you. decrease. the. focal. length. of. the. eyepiece. –. not.
increase.it,.as.many.believe!.So.to.change.the.Astroscan.
to. a. higher. power,. replace. the. 28. mm. Plossl. with. the.
supplied. 15. mm. one. and. you. have:. 445/15. =. 30X..
Other.magnifications.can.be.achieved.using.additional.
eyepieces,.and.the.power.of.any.given.eyepiece.can.be.
more.than.doubled.by.using.the.Edmund.Scientific.2.5X.
Barlow. lens.. (See. the. “Accessories”. section. for. these.
and.other.useful.items.).
It. needs. to. be. stressed. here. that. your. Astroscan,. as.
a. low-power. wide-field. telescope,. was. designed. for.
optimum.performance.in.the.magnification.range.of.15X.
to.35X..While.it.can.certainly.be.used.at.higher.powers.
(the.useful.upper.limit.to.magnification.for.any.telescope.
is.actually.around.50X.per.inch.of.aperture),.there.will.be.
a.corresponding.reduction.in.the.size.of.the.field.of.view.
(further. described. in. the. last. paragraph. on. this. page).
and.with.it.increasing.difficulty.in.finding.objects..There’s.
also. increased. sensitivity. to. atmospheric. turbulence.
as. power. is. increased. and. precise. focusing. becomes.
ever.more.critical..Experienced.observers.typically.use.
their. lower. powers. most. of. the. time,. reserving. higher.
ones.for.steady.nights.and.close-up.views.of.the.Moon,.
planets. and. tight. double. stars.. It’s. been. said. that. a.
magnification. of. 30X. will. reveal. just. about. everything.
the.casual.stargazer.would.want.to.see.in.the.universe,.
given.enough.aperture.and.good.optics.–.both.of.which.
the.Astroscan.has!
While.on.the.subject.of.power,.it.should.be.realized.that.
there.are.actually.three.types.of.telescope.“power.”.Most.
obvious.(especially.to.beginners).is.the.one.we’ve.been.
talking.about.–.magnifying.power,.or.how.much.larger.
(and.closer).a.telescope.makes.things.look..But.of.more.
importance.(especially.to.professional.astronomers,.or.
amateur. stargazers. who. enjoy. viewing. “faint. fuzzies”.
like. galaxies). is. light-gathering. power. –. or. how. much.
brighter.a.telescope.makes.objects.appear..And.the.third.
type.is.resolving.power.–.or.how.sharp.and.detailed.an.
image.the.telescope.shows..As.aperture.increases,.so.
do.both.of.these..For.its.size.and.intended.use,.your.new.
Astroscan.balances.all.three.types.of.power.nicely,.as.
explained.in.the.following.sections.of.this.guide..
One.other.important.matter.related.to.magnifying.power.
itself.is.the.angular.field.of.view..As.mentioned.above,.
as.power.increases,.the.amount.of.sky.seen.decreases..
There. are. really. two. different. types. of. “field. of. view.”.
One.is.the.“actual.field,”.or.amount.of.sky.shown.by.the.
telescope.itself..The.Astroscan.covers.a.full.3.degrees.or.
six.full-Moon.diameters.with.its.28.mm.(16X).low-power.
eyepiece..The.other.type.of.field.is.the.“apparent.field”.of.
view.of.the.eyepiece.being.used..The.Plossl.eyepieces.
supplied.with.your.telescope.have.fields.of.50.degrees..
This.is.the.angle.you.would.see.holding.the.eyepiece.up.
close.to.your.eye.and.pointing.it.at.the.daytime.sky.or.
other.bright.surface..The.actual.field.is.found.simply.by.
dividing.the.apparent.field.of.the.eyepiece.by.the.power.
being.used,.or.Actual.Field.=.Apparent.Field/X..For.the.
Astroscan.used.with.its.16X.28.mm.eyepiece.this.would.
be.50/16.=.3.1.degrees..If.you.switch.to.the.30X.15.mm.
eyepiece,.it.would.be.50/15.=.1.7.degrees,.or.only.about.
half.as.much.actual.sky.coverage.
