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Coma in a parabolic mirror. Or, a bit of mathematics

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By way of an introduction, it took me over a year of sporadic work to get this paper together. I know I’m essentially reinventing the wheel, but I found it intriguing to study the geometry, trigonometry, and mathematical modelling of reflections of light in a parabolic mirror. It’s especially relevant as I changed jobs, moving from a pure optical shop to Australia’s only telescope factory. I describe the change as like moving from a Toyota dealership to the Ferrari factory. The Astroworx brand telescopes made by Sidereal Trading are, for the moment, Newtonian astrographs, and they use parabolic mirrors, which are precisely what this blog is all about. This paper is what happened when I wondered why we need coma correctors for Newtonian telescopes. Being a refractor specialist, I knew only a small amount about coma, which is the aberration that you find away from the centre of the field, making stars look - not just out of focus, but also smeared in a triangular pattern away from th
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Processing an LHaRGB image using monochrome and colour cameras, different pixel scales and Astro Pixel Processor

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Nearly nobody reads these blogs, and so I’m going to write one which is pretty much for my own benefit. I normally take notes about my workflow. I rarely actually follow them, normally getting distracted and going off on tangents, but in this case I was so out of my depth (I even needed help from a Dutch expert) I decided to figure it out beforehand, actually follow it, and record it here. Perhaps someone will benefit from the work I've done. Perhaps that person will be me. If you’ve found this blog using Google, and you’re after the actual workflow, scroll down to the section called “processing the data”. Finally, a chance for a photo A couple of weeks ago I visited the ASV’s dark sky site. It was the first time I'd been there for a while (thanks to successive COVID lockdowns) and I was looking forward to taking a decent image. In the time, I’d taken other astrophotographs, but the aim of those was more as a test for equipment, rather than the image itself. No, this was a t
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Frankenscope and NGC1365 - the Great Barred Galaxy

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Finally, an image! I've finally got around to processing an image of NGC1365 - the Great Barred Galaxy - that I got from the ASV's dark sky site in Central Victoria a couple of weekends ago. It isn't the best image I've taken, but it's interesting, and it's my first serious image since before the pandemic! But it was a challenge - I made things hard for myself by using two scopes, two cameras and one very rusty astrophotographer.  Frankenscope! The colour information in the image was gathered in 51 five-minute exposures using a colour camera on a Sidereal Trading modified telescope. This is one crazy machine.  The front half - the objective - of the scope is a saxon 102mm FCD100 triplet, which is a beautiful piece of glass. We removed the somewhat unworthy focuser and replaced it with an Astroworx Crayford focuser made in our own factory. This focuser will soon be launched onto the market, and we hope it will do well. The guide scope was made by Sky-Watcher, t
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The tale of a cataract as told by an astrophotographer

Sight, in humans, is quite a popular thing. People regard it highly, and do seem to think it's an overall good idea. My experience of sight has been, like most other people, a given. It started great, and as a kid my family would occasionally use me to find things, like shop signs, in the distance. Of course, it does deteriorate over time, and I've worn glasses since early university. It started just for reading, but soon I was wearing them full-time.  It turned out that apart from being long-sighted, I had astigmatisms which were reasonably severe, and getting worse.  You get old Eventually I wasn't able to look through telescopes any longer: unless I had an eyepiece that I couldn't afford, the eye relief - the required distance between the eyepiece and your eyeball - was so short that I had to remove my glasses. Without glasses, of course, all I could see were comets. No, they weren't meant to be there.  Resignedly, I gave up visual astronomy and went deeper into
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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
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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-Chrétien, 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-Chrétiens 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
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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
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