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Collimating a Telescope - AstroShop

Our guide to collimating a refractor and Newtonian telescope
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. For this general visual process we highly recommend the Farpoint visual collimation eyepiece.

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.


Collimation Part 1
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.
Collimating a mirror Part 2

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.

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.

Front view of refractor objective

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.

Poor Collimation Good Collimation

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