COLLIMATING YOUR REFRACTOR:

One of the biggest myths about refractor collimation is that "all refractors are collimated at the factory and never need adjustment." Unfortunately perpetuated in many handbooks for beginners, all quality refractors have been fitted with adjustable objective cells for years, and while they seldom lose alignment once adjusted, it is helpful to know how and to be able to check with either a new instrument, one that has travelled a lot or has been unused for a long period of time or subject to large changes in temperature. This short article explains how and the tools to make it easy. The one exception that I am aware of is TeleVue as these scopes are factory collimated and must be returned to Tele Vue if they require re-collimation.

Collimation is the process of aligning the lenses of your telescope so that the light they collect will focus at the right spot at the back of your telescope for your eyepieces to work. Collimation is a simple process and works like this: Pull off 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. There are a number of devices available for collimation. The best one is your eyepiece and the North Star. The easiest way to find the North Star is to look for the Big Dipper. Draw an imaginary line along the two end stars in the bowl of the Big Dipper. The first star you come to along this line is Polaris. It is best for this purpose that your telescope not be polar aligned, in fact point the mount head due east or west. This is because German Equatorial Mounts have a blind spot around the pole. Also turn off your motor drive if you have one attached to the mount. Use your lowest power (largest number eyepiece) to acquire Polaris, 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 can't reach focus then start with: 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. The actual process is to slightly loosen the pair on the side the deflection is, slacken the Allen head screws then 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 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. It helps to have a friend to help for collimation. Have your partner adjust the screws according to your directions while you look in the eyepiece.

CHESHIRE EYPIECE COLLIMATION

Antares Cheshire eyepiece for refractors.

The Cheshire eyepiece easily collimates refractors. Remove the star diagonal and cap the front of the tube and put the Cheshire into the focuser. Using daylight or an artificial lightsource shining into the opening with the angled reflective surface, look into the small hole at the top. Looking into the Cheshire, you will see two or more ghostly images that look like doughnuts. If there are more than one, usually overlapping, adjust the objective cell push-pullbolts until all images merge into one. The field should appear as a pale blue image with one dark circle concentric with the outer edge, with a black dot in the center which is the return reflection orifice. All of this assumes that your focuser is aligned properly with the tube. Instructions for laser checking of focuser alignment are below. The star diagonal may now be replaced, any deviation from the previous image will indicate mis-alignment of the diagonal, check to see if it is seated correctly in the drawtube and recheck the image. Some diagonals may have to have the mirror or prism shimmed to give an aligned image.

What you should see...

What you might see...

Testimonial as to the effectiveness of this method:

I want to thank you for posting such an informative and easy to understand tutorial on collimating a refractor. I was having difficulty in collimating my refractor because according to the instructions I was to take it outside or in a well lit room and point the scope at a uniform dark object and the distance of that object was not an issue. After several hours standing outside in the hot sun pointing at a dark wall across a corn field and not able to determine what the rings I was trying to center even looked like I had to give up. I could not see any rings.

I had no idea the collimator tool was even called a Cheshire until I read your site. Following your instructions I was able to collimate my refractor in about a half hour, not bad since I never collimated a scope before. Again I want to thank you for your web site.

Stephen M. Roberts

Network Systems Administrator

Interlogic Outsourcing Inc.

REFRACTOR LASER COLLIMATION

If you have upgraded to an adjustable focuser the following will be the primary steps of laser collimation:

1. Take the dewshield off the telescope. Measure the inner diameter of the cell.
2. Draw on a paper sheet (thin paper preferrably) a circumference the same diameter as the front aperture size of your objective cell. At the geometrical centre your should draw a reticule (cross). This reticule indicates the optical axis of the system.
3. Place the sheet on the front of the objective (outside), and make the circumference coincident with the inner circumference of the cell. You can use tape to secure the paper-sheet.
4. Place a laser colimator in the focuser. Screw tight the compression ring (or holding screws if using the original Synta's stock focuser), the same way as if your using one of your eyepieces.
5. Turn the laser on. You will see a red dot coming from the laser and hitting the paper. If the dot is far away form the center then your focuser is uncolimated. You will need to either shim the stock focuser or adjust your replacement focuser so that it leans toward the center of the tube. Do this until the laser beam is in the physical center of the objective lens, then rotate the laser to see if it stays in the center, if not, then further adjustment is required.

The next step is to adjust the angular alignment of the objective so that the beam, which is reflected from the exact center of the rear surface of the objective, is returned to the collimator face and centered on the laser aperture. The holographic collimator should be put in single beam mode for this, so that the diffracted pattern does not annoyingly interfere with viewing the reflected beam impact on the collimator face, viewed through the objective. This will also make the central beam brighter, which will be helpful. Enough light is reflected even from an anti-reflection coated lens surface to perform this adjustment. Some light will be reflected from each lens surface within the objective, although the reflection from the rear surface will probably be brightest. If the elements of the objective are accurately centered and collimated with respect to each other and have no wedge, all the reflections will coincide, forming one reflected spot centered on the laser port. Adjust the cell push-pull screws until the laser beam is centered on the output hole of the laser. If there are multiple reflected spots, it is an indication of alignment problems within the objective. The collimator can then be used to diagnose and correct the inter-element misalignment.

See also how to Star Test Your Refractor

Helpful websites:

Sky Watcher Lens Collimation

Laser Collimation

Tectron Telescopes

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