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Showing posts from July, 2021

Video - unboxing the saxon Astroseeker 15075

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We've uploaded another video onto the Optics Central YouTube channel. This one is me unboxing the saxon Astroseeker 15075. This is a 6-inch Newtonian on a light computerised alt-azimuth mount. The mount is a nifty little unit, well capable of carrying the weight of the tube. It's a heavier duty mount than the SkyWatcher mini AZGTi, and is capable of taking a heavier tube than the Newtonian here. In fact, this mount is often found with a 127mm Maksutov. The mount can be controlled by the hand controller that's included in the box. Alternatively, because the newest versions of the mount have a built-in WiFi connection, you can also control it using the SynScan app for your phone. The tube is a standard Newtonian, with a parabolic mirror and a focal length of 750mm. This makes it an f/5, meaning it's good for deep-space objects, such as galaxies and nebulas. Star clusters and the Moon are also well within your reach, especially in light polluted areas, but planets wi

Abbe numbers and refractive indices

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I'm a refractor guy. I've seen these things called Schmidt-Cassegrains, and someone told me they can do astronomy with mirrors. One day I might be beguiled by the hyperbolic surfaces of a Ritchey-Chr├ętien, or the pure beauty of a well-machined truss tube. For now, though, I reckon that if it was good enough for Galieo, Kepler, Brahe and Copernicus, then it's good enough for me. But refractors have their problems. Chromatic aberration, where the different colours components in the light from stars don't focus at the same point, is the bugbear of the design. Of course, other designs do have their problems. Newtonians have coma, Schmidt-Cassegrains have astigmatism, and Ritchey-Chr├ętiens have an air of intolerable smugness (with apologies to the late Douglas Adams) So how do we manage chromatic aberration? We have two main ways of controlling the way light changes as it passes into and out of a glass lens. First, the amount the light bends is determined by the

Prasun's Uluru - a double-stacked astroterrestrial photo

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I've shown astroterrestrial photos before, photos with a subject on the ground with a night sky background. Here's a stunning example. It's Uluru (obviously) from the viewing area with the South Eastern sky behind it. The photo was taken by Prasun Agrawal a little while back, and I was knocked out by its quality. I love the hook shape of the Rho Ophiuchi cloud complex near Antares. But there's more to the image than than this. You can easily see details in the rock, such as Kantju Gorge on the mid-left, that interesting line of circular features in the centre of the rock, and bushes in the foreground. It's an unusually high level of quality. How did Prasun do it? It's a composite: a common way of producing this type of image. Single images are more challenging as you need to focus separately on foreground and background. With a composite, you take a single foreground image and combine this with a "stacked" image of the sky. The stac