each time an adjustment is made on the screws, the star, which has moved in the field because of the adjustment, must be precisely recentered. the rotations made on the screws must be small: a very misaligned mirror can need 1/2 turn, but the final alignment is made by fractions of a turn, nearly by a flexure of the tool. when a screw is unscrewed, the two others must be screwed. the three screws must be screwed or unscrewed in moderation, no screw being overtightened or totally unscrewed, the central screw, which holds the secondary mirror, must never be touched, The process of collimation is iterative (check-align-check-align etc.) and is neither difficult nor risky, if some simple precautions are followed: Three screws (push or pull), or three couples of screws (push-pull) on certain telescopes, allow to modify the orientation of this mirror. On SCTs, the only accessible alignment is the one of the secondary mirror. The idea is that the alignment becomes as familiar as checking the oil level or the pressure of the tires in a car before a trip ! Not checking the collimation before a planetary session. If a small misalignment can be tolerated in deep-sky observation, it is taking a big risk This is the reason why it is advised to choose a star in the same area as the object of interest (planet or Moon). German mount, an interesting experiment is to point at the same star at the meridian successively from the left and the right of the mount, to observe the modification of collimation due to the reversal of the tube). Collimation can even vary according to the orientation of the optical tube (with a The constraints are so high that a simple car trip always changes the alignment in smallĪmounts, and sometimes in large amounts. People who think that a SCT does not need to be collimated very often probably do not realize the level of precision required for this type of instrument. The observation of a star at high magnification is also the best way of knowing if all the conditions are favourableįor high resolution observation or imaging, because it allows to check if the instrument isįully operational (thermal equilibrium, no vibration etc.) and to estimate the level of atmospheric turbulence in a more precise way than the observation of a planet. This technique needs no specialized device, just a good eyepiece and eventually a good Barlow lens. Of observing, at high magnification, a defocused and focused star to detect any asymmetry in the diffraction patterns,Ī sign of a more or less pronounced misalignment. Precise it allows reducing the final misalignment of the telescope to a practically negligible amount. After this step, planetary images can still lose more than 50 % of their contrast. This step is indeed necessary, on a very misaligned reflector, but it is not sufficient because Aditionally, the ones who venture to do it generally stop at theĬentering of the shadow of the secondary mirror on a much defocused star. What is the precision of the proposed method ?īecause of a lack of information, few reflectors users dare The benefits of good optics vanish at the smallest misalignment. The diffraction laws, give better planetary images than larger reflectors with good optics (see What are the effects of obstruction ?). It is one of the reasons why small refractors, seeming to defy all Misalignment their collimation can be destroyed by a fraction of turn of a collimation screw. These telescopes are extremely sensitive to Reflectors it is certainly the main cause of the mediocre reputation of SCTs. In the best interest of an amateur who would not want to collimate his reflector to turn to a refractor of moderate size. It is as important as the tuning of a musical instrument: the images given by a misaligned telescope can be as awful as the sound given by a out-of-tune piano. Collimation is not a superfluous technique destined for opticians and purists. Image processing is incapable of compensating for the damages caused by a misalignment. No acceptable result can be achieved at high resolution without an irreproachable collimation The collimation is a way of improving the In addition, no telescope, even if it has been adjusted in the factory, keeps its alignement a very long time. Performance of a reflector collapses if its optics are misaligned. Moreover, even if these books do not talk much about the damages caused by an misalignment, one must know that the Optical books present us, for every type of telescope,ĭesign principles and theoretical performances, which implicitly suppose that this alignment is perfectly realized. Computer generated images, drawings and texts are property of the author and may not be reproduced or used without consentĬollimation is the adustment of the alignement of each optical element of a telescope with regard to the others.
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