Sky & Telescope - April 2019 - 69

Spider
tu ALIGNMENT ANXIETY One of the
most frequently noted disadvantages
attributed to the Newtonian reflector is its
need for regular collimation of its two mirrors. But this supposed disadvantage can
be reduced to a minor task if approached
methodically.

tic
al
ax
is

Scratch that. Unless somebody
drop-kicked your scope across the
Eyepiece
observing ﬁeld, your focuser is ﬁne.
Focal plane
So start by making a peephole about
Secondary mirror
1/8″ in diameter that goes in the
focuser like an eyepiece. A 35-mm
Secondary
ﬁlm container with the bottom
mirror holder
cut off and a hole drilled in the
as choosing whether to cut the red
cap works great for this if you can
wire or the green wire when defusﬁnd one. In a pinch, you can tape
ing a bomb, but it's really pretty
some paper over the focuser and
simple. The hardest part is probably
poke a hole in it. The purpose of the
learning to say it properly. (It's collPrimary mirror cell
peephole is to force your eye to stay
i-ma-tion, not cul-min-a-tion or
Primary mirror
centered over the drawtube.
coll-im-na-tion.)
Check ﬁrst to see if the secondCollimation means lining the
ary is too high or too low in the
mirrors up in a straight line. The
optical tube. It probably won't be, but it might be if someone
primary mirror needs to be pointing more or less straight out
ﬁddled with it. You can use the far end of the focuser's drawof the tube, the secondary mirror needs to be more or less
tube as a reference. The shiny part of the secondary should
centered over the primary, and it needs to bounce incoming
look centered within that circle. (You might have to put a
light down the center of the focuser's drawtube. I say "more
piece of white paper behind the secondary and wave a ﬂashor less" because the primary's aim down the tube isn't all
light around to see where the edges of the secondary are.)
that critical, and the secondary is seldom truly centered.
A Cheshire eyepiece (a collimation tool consisting of a sight
In fact, it's usually deliberately off-center. We'll get to the
tube containing a beveled reﬂective surface and crosshairs)
"why" of that in a minute.
makes this a little easier by constricting the diameter of the
reference circle, but you can do ﬁne with just your centered
Begin With the Secondary
eyeball. This adjustment doesn't need to be exact; just get it
You start your adjustments at the top of the scope. If you've
close, both vertically and horizontally.
already looked up collimation instructions online, you may
The next step is to align the secondary mirror so you
have found some whose ﬁrst step is "Square the focuser."

Figure 3
Spider vanes and reﬂection of focuser drawtube
should be centered. If you
look in the open focuser
without the peephole,
you'll see your eyeball
centered too!

Spider
vane

Center
marker
Cheshire's
cross-hairs

Ignore the extra
bulge of the
secondary offset

Cheshire's
illuminated window
reﬂected in primary
Cheshire's crosshairs reﬂected in
primary
Cheshire's
peep sight

p IN PERFECT TUNE Left: When both mirrors are aligned properly, everything will be centered except possibly the outline of the secondary mirror.
Right: A Cheshire eyepiece provides crosshairs and a bright inner reference circle. This view also shows the optics in collimation.

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Sky & Telescope - April 2019

Table of Contents for the Digital Edition of Sky & Telescope - April 2019