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SKY & TELESCOPE
; BOTTOM: BOB’S KNOBS
is in precise focus, it is not a minute
pinpoint. It is actually a small disk,
known as the Airy disk, surrounded by a
series of diffraction rings, with the first
ring being prominent and the rest dim.
Once again, centre the star in
the field and switch to a high-power
eyepiece, magnifying the view enough
sothatwhenthestarisinfocusyou
canseetheAirydiskandthefirst
diffractionring.FortheAirydisk
andringtobevisible,theatmosphere
must be steady and the telescope well
acclimated to outdoor temperatures.
Oftenthebesttimetotryin-focus
collimationisassoonaftersunsetas
possible.
Examinethefirstdiffractionring.Is
itunbrokenaroundtheAirydisk?Or
isitincomplete?Ifitisbroken,move
the collimation screws by tiny amounts
untilitisaperfectring.Isin-focus
collimationnecessary?Ifyouintend
toobserveorimagetheMoonand
planetsathighpower,yes,itcanmakea
significant difference in images.I mentioned earlier that it’s possible
to collimate with an artificial star. You
can buy commercial artificial stars,
which are battery powered and use
high-intensity LEDs, or you can make
one by punching a small hole in a piece
of aluminium foil with a needle and
placing that in front of a bright light
source. The hole needs to be small to
produce a suitably pinpoint ‘star’. In a
pinch, however, I’ve used the filament
of a miniature Christmas tree bulb as
an artificial star. That’s not perfect, butit can get a telescope well into the fine-
collimation stage. An illuminated ball
bearing also works well, particularly in
the daytime when no stars are available.
Place the artificial star far enough
from the telescope so it can be
brought close enough to focus to show
diffraction rings. The star must be far
enough away that it will almost come
to perfect focus. When the rings are
visible, perform fine collimation as
normal. In-focus collimation should
be done on the truly pinpoint source
of a real star, but an artificial star
collimation can be good enough for
most purposes.
How long does it take to collimate
a Schmidt-Cassegrain? When you’re
new to the process, and especially if you
have to begin with ballpark or rough
collimation, expect to spend as much as
an hour adjusting the telescope. Once
you’ve gained some experience and have
got your SCT’s collimation dialed in well
enough that it only needs minor tweaks,
you’ll likely be finished and ready to
observe in around 5 to 10 minutes.
Collimating a Schmidt-Cassegrain
sounds tedious, but it gets easier with
practice and the payoff is huge. I’ve seen
good collimation turn an optically so-so
SCT into a planetary powerhouse. In
some cases, it can be like having a whole
new telescope. Enjoying sharp images
under a beautiful, starry sky is more
than enough reward for sweating in the
dark over those three pesky little screws. ROD MOLLISE is a well-known expert
on all aspects of Schmidt-Cassegrain
telescopes.S TOOL-FREE Replacement thumbscrews that eliminate the need for additional tools are
available from several manufacturers. Be careful to replace only one screw at a time so that
you keep all the screw holes aligned and avoid dropping the secondary mirror onto your
telescope’s primary.S IN FOCUS Final adjustments to establish perfect collimation should be done under good-
to-excellent seeing conditions. Focus the star while it’s centred in the field and look for the first
diffraction ring around the Airy disk. If it appears like the arc at left, adjust the corresponding
screw using very small turns until it appears to surround the star as seen at right.