![]() ![]() That is field curvature in the optical system and is present in many telescopes. If you get elongated stars that get worse in the corners and that point to the center of the field in long exposures, that is not a tracking or guiding problem. Here the upper right corner of the frame shot with a fast 24mm lens shows these aberrations. Stars that look like seagulls or angels in the corners of the field are caused by lens aberrations in the lens or telescope. This is not a problem with guiding or tracking in the mount. Stars that are elongated in the corners where the elongation points to the center of the field, and where the elongation is worse farther away from the center of the field, are caused by curvature of field in the optical system and can be corrected with a field flattener. This is the upper left corner of an image showing field curvature. Stars that are round in short exposures but elongated or trailed in longer exposures are caused by problems in the mechanical setup of the scope, tracking, guiding, or autoguider settings. Halos, reflections, distorted stars, stars with irregular diffraction spikes are all symptoms of problems in the optics. A scope with optical problems, like bad collimation in a Newtonian or Schmidt-Cassegrain, can produce funky-looking stars that to the inexperienced eye may look like tracking or guiding problems. We first need to tell if the cause of our problem is optical or mechanical. It's even possible to have what seems like excellent guiding and still get elongated or trailed stars. The problem is that many gremlins can creep into the works and cause trailed stars. Usually the software presents its results with a graph or readout that shows the guiding accuracy and corrections being issued. As the guide star moves slightly due to causes such as flexure, periodic error, or slight polar misalignment, the software interfaces with the mount and issues brief commands to move the mount a small amount to bring the guide star back to its original position, preventing trailing. The autoguider is a separate camera that only examines the position of one star every few seconds with software that calculates its position to sub-arcsecond accuracy. That means using an autoguider either piggyback with a separate guide scope, or with an off-axis guider. This is because no mount is mechanically perfect and atmospheric refraction causes the positions of celestial objects to vary slightly as they change elevation in the sky.įor critical work, we need to guide. Merely tracking the sky at the sidereal rate is often not good enough and can result in stars that are either elongated or trailed. For visual use and for short exposures at short focal lengths, this is often all we need for a mount that is reasonably well polar aligned.įor longer exposures at longer focal lengths, we need to follow the stars with precision - sometimes to an accuracy on the order of just a few arcseconds, which can be very critical indeed. This image is enlarged 200 percent to make the trailing more easily seen.Įquatorial telescope mounts are made to track celestial objects as they move across the sky due to the Earth's rotation. Trailed stars are seen in this image of the Orion Nebula shot with a 50mm lens on a tracking mount. We'll try to cover some of the most common ones here and how to fix them. There can be many reasons you may get trailed, or elongated, stars in your images - seemingly as many causes as there are stars in the sky. In this case, they aren't "happy" trails at all - in fact, they are evil, and can drive you crazy. ![]() But if you can't get it out of your head when it comes to stars in long-exposure tracked and guided astrophotos, you've got a problem. Singing or humming the song " Happy Trails" is a fine choice for when you are taking circumpolar star trail images on a fixed tripod.
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