Last updated: 3 July 2003

ETX-125 collimation - it ain't that hard

Sent:	Wednesday, July 2, 2003 13:19:32
From:	pavilion@pinefields.com (Richard B. Emerson)
Collimating ETX's seems to be shrouded in mystery and warnings of dire
consequences.  First off, because the procedure does involve
disassembling part of the telescope and opening the optical elements to
outside air, with its dirt, dust, and contaminants.  This is a procedure
to be done only after a "is this really necessary" check.  Furthermore,
it is possible to damage the mount and/or OTA while doing the procedure
described below.  Do a "is this really, really necessary" check.  If you
think you're Freddy Fumblefingers or given to dropping things, consider
alternate plans.  But with a little care, attention to detail, and a
good set of hex wrenches, it is possible to collimate an ETX.  "It ain't
that hard."

The basic approach is simple cut and try (twist and try?).  Look at the
collimation target, take the OTA off the mount, twist an adjusting
screw, put the OTA back on the mount, and look at the target again. 
Taking the OTA off the mount is surprisingly easy.  The surprise is the
OTA does come out by removing the three hex screws on the back of the
OTA.  There is *no* need to loosen, release, or even mess with the four
screws on the side of the OTA, at the back of the mounting arms.

The procedure is as follows:
1) Remove the focus knob.

2) Put the cover on the front of the OTA (we don't want fingerprints or
scratches on the optics).  Remove the screw below the focus knob and
then screw to the left of the optical port (this assumes you're standing
with the eyepiece and flip mirror knobs close to you and the OTA lens
pointing away from you).  Do NOT remove the screw above the focus knob

3) Support the OTA from underneath (that means at least one steady hand
under the OTA and *not* the mount or tripod head!) as you loosen and
remove the last screw.  Failure to properly support the OTA once the
last screw is removed puts the OTA at risk!  Got that?  Good!  If you
fail to support the tube and the mount is tilted down, getting good
collimation will be the least of your worries.  You'll be too busy
looking around for a dust pan to sweep up the glass from what used to be
your telescope.

4) Slide the OTA forward (towards the correcting lens and away from the
eyepiece and flip mirror) and the OTA will come free of the mount.  It's
that simple!

Remember the back of the OTA is open to dust, etc.  Dropping anything
down that hole in the middle of the back could ruin your day.  Yes, you
can unscrew the corrector lens to get out that dropped screw but maybe
the screw damaged the secondary mirror when it hit? Don't drop stuff
down there!  Even if you didn't damage the mirror or lens, twisting the
lens off the OTA tube could alter the optical train's alignment, meaning
you'll have to start all over.  Don't drop stuff down there!

To re-mount the OTA, get the focus shaft more or less lined up with its
opening in the mount and gently slide the OTA back.  You'll have to
jiggle and twist the OTA a bit to get things lined up.  (A little
practice re-inserting the OTA in the mount after removing the OTA the
first time is a good idea.)  From here, repeat steps 4, 3, 2 (remove the
lens cover after step 2, of course) and leave the focus knob off until
you're done.

On the back of the OTA, once it's removed from the mount, you will see
six hex screws.  There are pairs of screws at 12, 3:30, and 7:30
(viewing the OTA back as a clock).  Ignore the ones with the wide heads.
 It's the smaller ones that count here.  They are sealed with red thread
sealant.  A firm but moderate twist from a hex key should break the

I find that putting a broad pointer just in front of the scope while
looking at a *centered* set of out of focus diffraction rings indicates
the screw (or screws) needing adjustment.  Of course it's not a good
idea to touch the lens with the pointer...  I put the pointer where the
shadow of the secondary mirror is closest to the outside of the ring and
observe the pointer's shadow on the diffraction ring; the screw closest
to the shadow gets a visit from Mr. Wrench.

Anything more than a 1/4 turn of a screw is a *lot* of motion.  This is
something to be done in 1/6th or less turns with lots of checking along
the way.  Don't forget to re-center the target after remounting the OTA
and looking through the eyepiece.  Because the mount gets moved around a
lot when removing and inserting the OTA, it's best to use the Sun's
reflection off a curved or spherical reflector (a shiny spoon back or
Christmas tree ball work well here) as the target "star".  I found that
with a little care I found that I could preserve the mount's azimuth but
the elevation changed a lot with each cycle of replacing the OTA. 
Polaris is also a useful target if your finder scope is aligned properly
so you can come back to Polaris using the finder scope.  Forget trying
to use any other stars as you'll get frustrated with having to realign
the scope every time you remount the OTA.

General collimation comments:  If you don't understand what collimation
is about or how to diagnose collimation problems, leave your telescope
alone!  At the least, you could mess up a good telescope and at the
worst you could break something permanently.  If, however, you're bound
and determined to go ahead, here are some tips on what to look for.

If you're an SCT owner and feel comfortable with collimating, the above
will seem very familiar (except for pulling the OTA off the mount,of

If you try to focus on a bright star and get little "rays" radiating
away from mostly one point and spread over a relatively small arc of the
star's circumference, you may have a problem.  Don't confuse a fuzzy
image from too much magnification or poor seeing for poor collimation. 
For people in the northern hemisphere, Polaris is a good test target
(sorry, folks south of the Equator, you'll have to pick your own
target).  Keep in mind, though, the screws you're removing from the OTA
back are black and tiny.  Drop one at night and you could still be
looking for it the following morning...

Center the test star in the field of view with a 26 mm eyepiece.  Focus
the image as carefully as you can.  If the image is uniformly noisy or
surrounded almost evenly by diffraction rings (even though air
turbulence makes them dance around), you're done and things are fine. 
If, however, the image doesn't seem to click into focus (the focus point
is fairly small so it's easy to "skid" past it but when you're there,
things seem to almost pop into clarity), defocus the image until you see
a donut perhaps 1/4 or 1/8 the diameter of the field of view.  The
lighter ring is the diffracted, out of focus image of the star forming a
disk, interrupted in the middle by the shadow from the secondary mirror.
 In a well collimated telescope, the central shadow is extremely well
centered within the diffraction disk.  If the central shadow is
noticeably off-center, it's time to re-collimate.  In general, ETX's
don't need re-collimation once put into proper collimation.  Some scopes
have been received out of collimation and a severe blow to the OTA
(signs of this should be obvious) cause problems; in general, though,
ETX's shouldn't need re-collimation.  By comparison, SCT's frequently
need a collimation touch-up and Newtonians usually need re-collimation
before every use.

For a number of reasons, using a daytime target is preferable to using a
star for a test target.  The Sun's reflection off a curved shiny surface
makes an excellent "star" (hmmm... well, the Sun is a star, after
all...).  The back of a spoon, if it's polished, or a Christmas tree
ball (about 2" in diameter) gives a good tight point-like target.  The
further away the target is, the more effective it will be as a test
star.  Keep in mind, however, that convection in the intervening air
mass will blur the image.  For this reason testing early in the day,
before solar heating becomes a problem, is a good idea.

The "star" must be centered in the eyepiece's field of view!  Trying to
diagnose collimation with an off-center target is a waste of time! 
De-focusing in either direction will be equally effective for
collimation.  That is, whether you turn the focus knob clockwise or
counterclockwise to get past sharpest focus doesn't really matter.  Once
you've got good collimation with the 26 mm eyepiece, make the
diffraction circle smaller  (by turning the focusing shaft slightly) and
check the results.  (Keep in mind that as you get closer to being in
focus, "reading" the diffraction pattern becomes harder)  Once you're
satisfied with the results with the 26 mm eyepiece, try something
shorter: a 12 or 10 mm eyepiece.  If you can get a centered diffraction
ring with this eyepiece, you can check you work with something still
shorter (5-6 mm) but remember you're working close to the optical
train's limits, making it harder to interpret the results.  Do *not* use
a Barlow to achieve a shorter focal length unless it's in place all the
time.  The collimation settings with the added lenses will be slightly
different from collimation without them.

Oh, and when you're done with re-collimation... put the focus knob back

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