How To Align Your Newtonian Reflector Telescope (How to Collimate Your Telescope)
If you have a Musical instrument, that sound and play beautifully or a car that runs efficiently when You first get them. But after a while, you notice that it has gone slightly out of tune or Your car starts run erratically. What do you do? Learn how to tune Your instrument so it plays beautifully again. Take the car to a mechanic to get it running at it’s optimum performance again.
The same applies to Your Reflector Telescope. If the images are are misshapen or have double images then it is likely that Your Telescope needs to be fine tuned. This is done by realigning the primary and secondary mirror so that they reflect a perfect image in the Eyepiece. This is called COLLIMATION.
Your Newtonian reflector will give great images of stars and planets — but only as long as you keep it fine tuned. The “Alignment” of a telescope is known as collimation. You may think that it is difficult, fiddly and laborious and best left to the experts. It can be if You don’t know what your doing. But that is why your here. To learn how to Realign Your Reflector Telescope.
You will be able to Collimate your Scope in a matter of minutes if You follow these instructions carefully.
You should already be acquainted with the optical parts of your telescope,
You will be lining up:
The Primary Mirror.
This is the paraboloidal mirror at the bottom of the tube. It has an aluminized surface that reflects light to form an image at the focal plane. It has an axis of symmetry — the optical axis. On this axis, at the focal point, is a “sweet spot” which is dead centre of the Mirror. This is where images of stars and planets are as sharp and crisp as they can be. Outside the sweet spot, an aberration known as coma visibly degrades the image. Coma makes stars appear asymmetric even if the telescope is perfectly focused — the farther the star is from the center of the focal plane, the worse it gets. In particular, this aberration can dramatically reduce the clarity of planetary detail. Surprisingly, the size of the “sweet spot” depends only on the main mirror’s focal ratio (the mirror’s focal length divided by its diameter) and not its size. For instance, even a perfect f/4.5 mirror, small or large, can provide “diffraction limited” performance only within a 2-millimeter (0.08-inch) circle at the focal plane. An f/10 paraboloid’s sweet spot, by contrast, spans 22 mm (0.87 inch). (For the mathematically inclinded, the sweet spot’s diameter is proportional to the cube of the f/ratio.) The primary mirror is held in an adjustable cell designed to support the mirror without deforming it. By adjusting the collimation screws, at the back of the Telescope, we can fine-tune the mirror’s tilt and accurately position the sweet spot where we want it. Because the sweet spot can be very small, this is by far the most critical part of collimation.
Take a look at your telescope and make sure you know where these adjustment screws are and how they work. To make collimation easy, the center of the mirror should be marked in some way. I use an adhesive binder reinforcement ring, the kind used in offices and schools to keep files in 3-ring binders.
The Secondary Mirror.
This is a small, flat mirror at the front of the Telescope that serves to reflect the image by the primary to the hole in the side of the tube, or viewer where it is viewed with an eyepiece. To minimize harmful diffraction effects, the secondary, or diagonal, mirror is generally only large enough to let the central portion of the focal plane receive light from the whole primary mirror. You should center this fully illuminated area in the eyepiece by positioning the secondary in the correct location. The secondary is attached to an adjustable holder suspended on a spider — often a cross made from thin sheet metal. Identify the adjustment screws for the secondary holder and the spider.
The Eyepiece .
The third optical component in the telescope system is the Eyepiece. It is a complex magnifying lens used to view the image formed at the focal plane. Like the primary mirror, the eyepiece has an optical axis, and this axis should be aimed at the center of the main mirror for best performance — though in practice it is the center axis of the focuser drawtube that you aim at the primary mirror. A good eyepiece will show a sharp image in the central parts of the field of view (its sweet spot), but toward the edge not even the best and most expensive eyepieces can produce a perfect image. For this reason it is important to make sure that the sweet spots of the primary mirror and the eyepiece match up — the ultimate goal of Collimation
This is best done during daylight, with the telescope aimed at the ceiling or the sky (be careful to avoid the Sun). Look into the empty focuser and try to identify the optical parts just described.
The illustration to the right shows what you should see: the secondary mirror in its holder, its elliptical face tilted 45° and appearing circular. With your eye close to the focuser, you can see the primary mirror reflected in the secondary, and the secondary and its spider in turn reflected in the primary. Finally, inside this reflection of the secondary, you can see the focuser drawtube and your eye.
How to Align Your Newtonian Reflector Telescope – 3 Easy Steps Once you are acquainted with the telescope’s optical parts and what they look like in the focuser, you’re ready to proceed. To get your telescope collimated, here is what you need to do:
Step 1: Center the secondary mirror on the axis of the focuser drawtube.
Step 2: Aim the eyepiece at the center of the primary mirror.
Step 3: Center your primary mirror’s sweet spot in the eyepiece’s field of view.
In most cases, only the last of these three steps will need to be repeated regularly; the first two are more or less set-and-forget operations.
Collimating Your Reflector.
Step 1: Begin by making sure that the focuser and the secondary mirror are lined up. The simplest and best tool is a Site Tube.
You put the Site tube into the focuser, like you would an eyepiece, and look through the tube’s peephole at the secondary mirror.
If the secondary is well out of adjustment, you should tilt and rotate it to get the reflection of the spot on the primary mirror centred in the sight tube before you proceed. It can be hard to distinguish the edge of the secondary mirror from the reflected edge of the main mirror, so place a piece of white cardboard between the secondary mirror and the primary mirror.
The elliptical secondary mirror should appear round and well centered in the circular opening of the sight tube. If it is, Carry on to step 2.
Step 2: Here you adjust the tilt of the secondary mirror to aim the focuser’s axis at the center of the primary. First, remove the cardboard from the spider. Now, while viewing through the sight tube, carefully adjust the screws that tilt and rotate the secondary until the primary mirror’s reflection appears centered in your field of view. If your sight tube has cross hairs, align the primary’s center spot with them; otherwise, center the outer edge of the primary within the sight tube. (Make sure that the sight tube is racked in far enough to let you see the whole primary mirror.)
A laser collimator is even better for this step.
— just center the laser beam on the primary’s center spot. A small error in secondary alignment is usually not a problem. As long as the pointing error is no more than 1 or 2 percent of the main mirror’s diameter, it makes no visible difference. However, if you plan to use a laser collimator in Step 3, you should be aware that even a tiny misadjustment here will throw off the final collimation.
With a solid-tube reflector, you need only check this once in a while.
Step 3: In this, the final and most important step, you need to tilt the main mirror to center its sweet spot (and its optical axis) in the focuser. This procedure should be done at the beginning of each observing session and checked occasionally during the night, since temperature changes or routine handling may cause your telescope’s components to shift enough to change collimation.
The best tool for this procedure is a Cheshire Eyepiece.
Put it in the focuser and observe the reflection of its shiny 45°-angle face in the primary. By turning the primary’s adjustment screws you can move this reflection until it appears centered on the primary mirror’s center spot. If you can make these adjustments while looking in the Cheshire, so much the better; otherwise an assistant can be very helpful. Most mirror cells have three adjustment screws or three pairs of push-pull adjustments. For simplicity’s sake,
I recommend using only two of the adjustments — the third one can be left alone unless you run out of adjustment on one of the others.
When Step 3 is done, the optical axis is accurately centered in the focuser, and collimation is complete. However, if you look carefully you will notice that the Cheshire eyepiece does not appear exactly centered inside the shadow of the secondary. Don’t worry; this is in fact how things should look because the secondary mirror is slightly offset.
A laser collimator is often used for Step 3, by centering the returning beam on the laser’s faceplate which looks like a target . Warning: The laser Collimator might not be a snug fit in the view Finder barrel so it may tilt to one side. Throwing the laser beam off centre and thus give incorrect collimation.Put some tape around it ( I use plumbers tape as it’s thinner and gives a better fit)
Make sure the laser beam is IN the CENTRE SPOT of the primary Mirror then adjust the mirror so that the beam is reflected back to the centre of the Collimators target faceplate.
Re-check that the Laser Beam is still in the centre of the primary Mirror.
A Cheshire eyepiece is better for the final adjustment. Star-Testing Your Collimation Once your telescope has cooled down and is well-collimated, it should be ready to perform at its best. At high magnification (25× to 50× per inch of aperture, or 1× to 2× per mm of aperture) and in good seeing conditions, stars at focus should appear in the eyepiece as tight, symmetric disks. However, if stars at the center of the field show the telltale asymmetry of coma, double-check your collimation with the Cheshire eyepiece. If the center spot still looks centered, then it isn’t located at the primary mirror’s true optical center. If this is the case with your mirror’s center spot, ignore it for now and try tweaking the primary’s collimation, in small steps, until you have centered the best image in the field of view.
The Cheshire will indicate the position of the primary mirror’s true optical center. If necessary, move the spot to the correct position or put another, larger piece of tape on top of it. If you know that your primary mirror spot is okay (and in most cases it will be, if carefully centered), there is no need to routinely fine-tune your collimation with a star test — the Cheshire eyepiece is not only easier to use, but it is more accurate if the seeing is less than ideal, which it is most nights. Now your telescope is in perfect tune, and the improvement in performance should be obvious. If not, try to deliberately miscollimate the primary, and see what it does to a high-magnification view of a planet.
Now You know how to collimate your Telescope.
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