![]() If it isn’t, adjust the collimation bolts until it is centered – this may result in your target star moving outside the field of view, but it’ll always move towards the direction the mirror has been tipped – if you’ve tightened a bolt, it’ll move toward that direction, and if you’ve loosened it’ll move away from the direction of that bolt. If the “donut” (the shadow of the secondary mirror) is centered on the diffraction rings of the star, your collimation is good. You should see one of the images on the left. ![]() Point your scope at Polaris if you’re in the northern hemisphere, or a bright star near the pole in the southern hemisphere, switch to a high-power eyepiece and slightly defocus it. However, I still recommend at least checking collimation on a star to be sure you’ve done a good job. You can rotate the secondary mirror by loosening a center screw or nut and retightening it – be careful not to loosen it all the way or your secondary mirror will fall onto the primary and damage both.Ĭollimating on a star is a relatively simple, tool-free method, but with a long scope, it’s annoying to fiddle with the screws and then comes back to the eyepiece. You can usually adjust your secondary’s tip/tilt with 3 or 4 small Allen or Philips head screws recessed in the spider/secondary mirror holder. The primary rests on a series of supports on a ring or cell, which sit on springs that loosen/tighten when the collimation bolts are adjusted.īarring a huge bump or similar, the secondary mirror in a Newtonian rarely goes very far out of alignment. Just remove the locking screws.Īdjusting the collimation bolts adjusts the tip and tilt of the primary mirror for alignment. ![]() In practice, they do nothing and serve as a mere hindrance to collimation, and in some cases could hit the back of your primary mirror if the scope is dropped and damage it as a result. Most mass-manufactured telescopes also have 3 locking bolts or screws, which are in theory meant to keep the collimation more stable. Your primary mirror will always have 3 collimation bolts/knobs at the back of its supporting cell. If you have a compression ring focuser or adapter, you won’t have to worry much about this issue. I do this by pressing the collimator firmly into the focuser then quickly tightening the thumbscrew. ![]() I recommend using #1 or #2 and at least quickly checking with method #3 once you’re observing.īefore doing anything with your collimation tool, make sure it is square with the focuser and not tilted by the thumbscrew. Using a Cheshire collimator or collimation caps.In order of increasing accuracy and sophistication they are: Newtonians are unique in that you can collimate them with several different methods. I recommend checking collimation every time you take your Newtonian out for observing, as there is usually a 50% chance it needs to be tweaked at the very least. Newtonians are the most likely to need collimation. If your refractor doesn’t have any screws in its lens cell, you’re out of luck. Just adjust until the star looks perfectly symmetrical both in and out of focus. The in-focus images are at the top.Īdjusting the collimation of the objective lens is usually done with either 1 or 2 sets of 3 screws at the front of the lens cell, which can be accessed by pulling off or unscrewing the dew shield. The bottom images show what a star with a mis-collimated refractor looks like out of focus. Funny enough, I’ve had it happen more often with expensive refractors than cheap ones.Īn out-of-collimation refractor will usually show elongated stars like coma in a Newtonian, like in the image below. * Notwithstanding that the two mirrors could be misaligned so badly that this could occur at some particular point.Believe it or not, refractors can be out of collimation occasionally. It does not ensure that the secondary mirror is aligned correctly with respect to line-of-sight through the eyepiece, and neither with respect to the primary mirror. Any adjustment of the secondary to reflect the laser back to the origin only ensures that the incident surface of the secondary mirror is perpendicular to the line-of-sight through the eyepiece holder. While it is possible to shine the laser through the eyepiece holder to the secondary mirror, and the mirror can be adjusted to reflect the laser back to the point of origin, it cannot be reflected onto the primary and then back to the secondary*. The primary, usually a parabolic mirror but always concave, is adjusted so that the beam is reflected back to the secondary mirror such that the reflected beam returns to its point of origin.Ī RCT, or other Cassegrain-like telescopes have a convex, possibly hyperbolic, mirror as the secondary. The Newtonian laser collimator works by shining a laser through the eyepiece holder to the secondary mirror which reflects the beam onto the primary. No, the fundamental designs of the two types of telescope are in this respect quite different.
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