| Collimation is the process of aligning the lenses or mirrors of your
telescope so that the light it collects is focused correctly for optimal viewing and
sharpest photographic results. There are a number of devices available (Cheshire
Cross Hair and Laser) for general collimation and the process is relatively simple.
The better collimated your telescope is, the sharper the images it produces. In general,
you can simply collimate every now and then but if the instrument is moved a lot, then it
may require more regular checking for accuracy. I personally collimate my Newtonian every
observing session, first with a SkyWatcher
collimation tool and then with a laser device for fine tuning and then sometime during
observing session - but that's just me..most people don't need to go to these extremes. Collimating a Newtonian
Remove the dust cover at the front of the
telescope and look down the optical tube. At the bottom you will see the primary mirror
held in place by three clips 120º apart, and at the top the small oval secondary mirror
held in a support and tilted 45º toward the focuser outside the tube wall. The secondary
mirror is aligned by adjusting the central bolt behind it, (which moves the mirror up and
down the tube), and the three smaller screws surrounding the bolt, (which adjust the
mirrors angle). The primary mirror is adjusted by the three adjusting screws at the back
of your scope. The three locking screws beside them serve to hold the mirror in
place after collimation.
Aligning the Secondary Mirror
Point the telescope at a lit wall and
insert the collimating cap or Cheshire collimating device into the focuser in place of a
regular eyepiece. Look into the focuser through your pinhole collimating device. You may
have to twist the focus knob a few turns until the reflected image of the focuser is out
of your view. Note: If collimating without a collimating device, keep your eye against the
back of the focus tube . Ignore the reflected image of the collimating cap or your eye for
now and instead, look for the three clips holding the primary mirror in place. If you
can't see them, this means that you'll need to adjust the three bolts on the top of the
secondary mirror holder, (possibly using an Allen key wrench or Phillip's screwdriver).
You will have to alternately loosen one and then compensate for the slack introduced by
tightening the other two. Stop when you see all three mirror clips. Make sure that all
three small alignment screws are tightened to secure the secondary mirror in place.
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Aligning the Primary
Mirror
If you don't have a laser collimator, it helps to have a friend to help with the primary
mirror collimation process. There are usually 3 locking bolts and 3 Phillip's head screws
or knurled thumb screws at the back of your telescope. The larger Phillip'shead or
chrome knurled thumb screws are the mirror tilt adjusters. Use an Allen key wrench to
loosen the hex bolts by a few turns. Now run your hand around the front of your telescope
keeping your eye to the focuser, you will see the reflected image of your hand. The idea
here being to see which way the primary mirror is defected. You do this by stopping at the
point where the reflected image of the secondary mirror is closest to the primary mirrors'
edge (Step 3). When you get to that point, stop and keep your hand there while looking at
the back end of your telescope, is there a adjusting screw there? If there is you will
want to loosen it (turn the screw anti-clockwise) to bring the mirror away from that
point. If there isn't a adjusting screw there, then go across to the other side and
tighten the adjusting screw on the other side. This will gradually bring the mirror into
line until it looks like Get your friend to adjust the adjusting screws according to your
directions while you look in the focuser. |
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Collimating a Refractor with Adjustable Lens Cell using a Star.
You can also use a star to check correct
collimation. This applies to Newtonians ,Cassegrains or refractors. By observing
outoffocus star images, you can test whether your telescope's optics are aligned.
Place a star in the centre of the field of view and adjust the focuser so that the image
is slightly out of focus. If the seeing conditions are good, you will see a central circle
of light (the Airy disc) surrounded by a number of diffraction rings. If the rings are
symmetrical about the Airy disc, the telescope's optics are correctly collimated.
Adjusting your Refractor
| Remove the dew cap at the
front of your telescope and look into the scope. The pair of lenses are held in a cell by
a threaded ring. This cell is held in place by three pairs of screws spaced 120 degrees
apart. The larger Phillip's head
screws actually hold the cell on, while the smaller, buried Allen screws push against a
ledge at the front of the tube and allow the cell to tilt slightly, by tension against the
Phillips screws.The idea being to alternately loosen and tighten each against the other
until you have a round star image. |
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Use your lowest power (largest number
eyepiece) to acquire your star, centre it using your slow motion controls. Now switch to
your next higher power eyepiece, while keeping the image
centred. The in-focus star image will have a bright innermost point, a slightly fainter
inner ring and a fainter still outer ring that is hard to see. If it doesn't look like
this, or you are unable to reach focus, then take out your star diagonal and look at the
image slightly out of focus, this will allow you to gauge the deflection. A typical
off-collimation image will have a bright spot off to one side when you bring the focus out
(see image at left below).
The actual process is to slightly loosen
the pair on the side the deflection is, then slacken the Allen head screws then next
tighten the Phillip's head screws against them again.
Check the star image again after moving it
into the centre of the eyepiece. If you find your image is getting worse, then go the
other way, or slacken the other two Allen screws a little. Once you have a round star
image you are set. |
