![]() ![]() Strain can cause parts of the mirror to expand and contract differently with temperature, so although it might have no astigmatism and good performance at one temperature, it might function differently on a warmer or cooler night. Either your primary mirror is inherently astigmatic, or it has a strain in the glass. If astigmatism does rotate with the primary mirror and is thus caused by an issue with it, there are two possibilities as to what’s wrong. ![]() Alternatively, it could be your mirror cell, particularly if your telescope is on the larger end. If astigmatism rotates with it, you have an astigmatic primary mirror, which we’ll talk about next. Simply rotate the primary mirror in its mirror cell. That being said, determining whether your primary mirror or either the secondary mirror or the primary mirror cell is at fault is fairly simple. The most common reason Newtonians have astigmatism actually tends to be related to the secondary mirror. If you’re using a Newtonian reflector, you might still be able to solve the problem yourself, so read on. You likely have a defect in your telescope’s optics that cannot be fixed and should sort out the issue with the telescope manufacturer or discuss it on a forum or a local astronomy club. You’re using a refractor or catadioptric scope and still see astigmatism, you can stop reading after this paragraph. If you’re already sure you’ve got perfect vision and have determined it’s definitely not your eyes. If the problem is indeed your eyes, the solution might be as simple as wearing glasses when using lower magnifications or just ignoring the issue. At higher magnifications, astigmatism in your eyeball becomes less of a problem – as opposed to astigmatism from the telescope, which becomes more of a problem at higher magnifications. ![]() If the orientation of astigmatism changes as you move, it’s not your telescope – you may just have very mild astigmatism in your eyes that hasn’t been noticed by a doctor. First, if you have any doubts, rotate your head/eyes and look through the eyepiece. It can be caused by any of the following in a Newtonian reflector:įinding the cause of regular astigmatism is pretty much a process of elimination. The center of the field of view shows equally astigmatic stars as the edge, and stars look like ovals when out of focus that change orientation if you are inside versus outside the focal plane. Stars will look like crosses (not to be confused with the diffraction spikes in a Newtonian reflector) and at high magnifications become fuzzy and hard to focus at all. The more complicated and harder of the two to solve is regular astigmatism at the focal plane, which is an issue that primarily affects Newtonian reflectors. It’s caused by light focusing differently on different axes, so one “side” focuses ahead or behind the other.Īstigmatism can manifest itself in two ways at the telescope. Visibility: In-focus stars look like crosses out-of-focus stars look oval and rotate 90 degrees depending on which side of the focal plane you’re onĪffects: All telescope designs, but usually reflectorsĪstigmatism is an optical aberration you might already have heard of, as it’s a common defect in our eyeballs – indeed, roughly a third of the population suffers from it. Regardless, aberrations are annoying, and any telescope user wants to figure out what they are and how to eliminate them. Others are visible as defects in focus either with an eyepiece or camera. Some optical aberrations can be seen during a star test, which is a process in which a bright star is examined out of focus at high magnification and the diffraction rings are examined. Each inherently has its own aberrations essentially by default, which we’ll talk about in a bit. There are essentially two methods of focusing light in any format: by refraction through lenses and by reflection from the surfaces of mirrors. The key to good telescope performance is compromising on what optical aberrations can be tolerated and making their effects negligible whenever possible. Particularly with eyepieces, it is basically inevitable that any telescope will have aberrations at the focal plane in some fashion, and they will always exist. All telescopes – and indeed, most optical devices – work by collecting light at the objective lens or primary mirror, and then converging these light rays into a spot called the focal plane. ![]()
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