Sky & Telescope - April 2019 - 71

most round. Leave it there and adjust the primary mirror to
bring it back to the center. (It's often best to have a helper
with this process, since it's difﬁcult to watch the image and
move the mirror at the same time.) You may have to repeat
this moving-away-and-recentering several times, but you
should eventually be able to get a symmetrical out-of-focus
image in the center of the ﬁeld of view.
To conﬁrm your collimation, bring the image to focus. It
should shrink to a sharp spot, with faint hints of diffraction
rings around it if the seeing is exceptionally good. (Note that
you'll never get a pinpoint star image. The nature of light limits the smallest image your telescope can create, so you'll get
what's called an Airy disk - a tiny disk surrounded with faint
diffraction rings - and you'll only see that under excellent
atmospheric conditions.)
When you get a symmetrical out-of-focus donut and sharp
focus, you're done! Enjoy the view through your perfectly
aligned telescope.
Oh, okay. You've got a laser and you want to use it
(because lasers are cool!). Here's how:
You'll ﬁrst need a center marker on your primary, and the
very center of that marker needs to be open. (That's why a
self-adhesive hole reinforcing ring is useful: It has a hole in
the middle.)
After you've determined that the secondary is centered in
the view of the focuser (because the laser can't tell you that),
stick the laser in the focuser and aim its side window toward
the primary mirror so you can see the window while turning
the collimation screws. Then snug up the clamp just as you
would on an eyepiece.
Turn on the laser and look down the front of the telescope
tube. Does the laser beam hit the center of the primary's center marker? If not, adjust the secondary until it does. Remember to use the tilt screws only for motion toward and away
from the focuser, and rotate the mirror for sideways motion.
Once the laser beam is centered on the primary, look at
the side window of the laser collimator. The return beam
should go right back into the hole it came out of. If you don't
see the bright spot, the beam is either already dead-on or it's
so far off that it's not hitting the window. You can wiggle the
collimator to see which it is.
If the beam is misaligned, turn the primary mirror's
collimation screws to bring it to center, then lock down the
primary with the lock screws (if you have them). Then star
test as above.
So what about the Cheshires mentioned earlier? They
merely help you do the same as the peephole method, by giving you better reference points in the form of crosshairs and
an illuminated circle. I highly recommend using a Cheshire (I
prefer them to lasers), but it's an enhancement on the peephole method, not a replacement for it.
A couple of notes: The faster your scope, the more critical
the collimation. The "sweet spot" in the ﬁeld of view where
you get a perfect out-of-focus donut gets smaller and smaller
with faster primaries. If you want good views through the

p FOLLOW THE DOT A laser collimator is quick and convenient, but it's
not a complete replacement for the eyeball method.

eyepiece, keep fast scopes well collimated. Slower than f/5 or
so, you can get away with a lot of slop (comparatively speaking), because the sweet spot will probably not be far from the
center of the ﬁeld even if your collimation is off a little.
Several months ago a reader asked if the secondary offset
I mention above would affect the star test. Wouldn't the
secondary shadow at the middle of the light donut be offset
a little? And would that offset be enough to throw off the
alignment if you centered the secondary shadow? Yes and no.
Technically the offset should be visible. Just hold your hand in
front of the scope while performing a star test, and you can
see that any obstruction will appear offset if that obstruction
itself is offset. But a typical secondary offset is on the order of
an eighth to a quarter of an inch. On an 8-inch primary, that
means it's about 1.5% to 3% of the light donut's diameter.
You're unlikely to see that small a deviation by eye.
Like collimation itself, this effect will become more
pronounced with faster scopes because the secondary offset
is often greater. It's probably never going to be enough to
matter, but if so, the solution is simple: Cut out a circle of
paper a little larger than your secondary and set that on top
of the spider, making sure it's perfectly centered. As long as
the paper's diameter is large enough to completely mask the
secondary, the light donut will be symmetrical when the
scope is collimated.
If you followed the procedure in the previous article in this
series, you now have a clean and collimated telescope. There
are as many nuances to the procedures I set forth as there are
amateur astronomers. There are other methods I haven't even
mentioned. (Google "collodion mirror cleaning" for a thrill.)
But the methods I've described will serve you well and will
keep your Newtonian telescope functioning like new.

¢ Contributing Editor JERRY OLTION star tests every time he
goes out, but that's because he's, shall we say, fussy.
sk yandtele scope.com * A PR I L 2 019

http://www.skyandtelescope.com

Sky & Telescope - April 2019

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