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Showing posts from April, 2021

Asteroid Apophis - existential threat or spectacular sky show?

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This blog was posted (slightly edited) as an article in Crux, the Magazine of the Astronomical Society of Victoria, vol.39 no.2 April 2021. Our neighbours the asteroids Asteroids are pretty much everywhere in our solar system. Typically, they have highly elongated orbits, meaning they can cross the orbits of the planets, as the planets have much less eccentric (that is, nearly circular) orbits. There are several classes of asteroid, determined by the amount if time they spend inside or outside Earth's orbit. Atira - spend their entire time inside the Earth's orbit Aten - cross Earth's orbit but spend the majority of time nearer to the Sun Apollo - cross Earth's orbit but spend the majority of time further out than the Sun Amor - spend their entire time outside Earth's orbit Most of the Atira and Amor classes are quite safe, although some of them can approach Earth's orbit closely. However, the Aten and Apollo types don't only approach,

Peter Kinchington's Carina

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Peter Kinchington sent me this dramatic image of the Carina Nebula in narrowband a while back. He took it using an ED80 refractor from his place in Mooroolbark (not far from the shop). For those of you who are used to seeing Carina as pink, this is a false colour image, using the "Hubble Palette". Peter has used a monochrome camera (in this case, a QHY163M) which captures the light produced by ionised Hydrogen, Oxygen and Sulphur in the nebula. Visible light is made up of a whole spectrum of wavelengths from different sources. A good number of these sources are ones we don't want. For example, normal DSLR photos pick up extraneous light from terrestrial light pollution from street lights and your own house. It'd be nice to be able to get rid of this light. On the other hand, you really do want the light from (say) ionised Hydrogen, because this makes up the bulk of gas in most nebulas. Using the correct filter, you can. Ionised Hydrogen shines in a highly-specif

Celestron StarSense Explorer LT 70AZ - who would have thought something so small would be so much fun?

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The other day I was alone, imaging up at the dark sky site. My rig is autonomous, which means that once it's going, I can pretty much leave it be. I normally hang about watching it, listening for alarms, and I like to watch it during a meridian flip, but sometimes I go to bed for a few hours. But that night I had a gadget with me to play with, a Celestron StarSense Explorer, the 70mm refractor. It's the baby of the bunch. I hadn't mucked about with it at night before, and this was a good opportunity. I'd already installed the software on my phone, so I set up the scope and fired up the app. I had to align it, and this involved two steps. First, I had to position my phone in the cradle so that the camera is above the middle of the StarSense's mirror. There are a couple of knobs that move the phone around. Second, the camera angle is wide. Somewhere in there is the spot where the scope is pointing, and the app needs to know exactly where this is. I

When things go too well ... at first. Vela supernova remnant.

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Do you remember those old TV westerns where the hero whispers "it's quiet... too quiet"? During the Easter break, I went to the dark sky site to photograph the Vela SNR (that's supernova remnant, not signal-to-noise ratio). This fibrous structure shows well in Oxygen and Hydrogen, but I don't think there's much Sulphur, so I decided to do a bicolour image. Things went swimmingly. I'd polar aligned, centred on the target, and taken a couple of test images through the two filters before it got properly dark. When the time came, I chilled the camera and began taking images. Guiding was going well, and a meridian flip went flawlessly. After this, it was about five hours of just steady as she goes. During that time, the rig would refocus every hour and change to a Hydrogen filter. I didn't have to do anything until about 3:30am when I would cover the scope for calibration photos. It was going so well I decided to retire to my tent. I woke

Whiffling goose

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This mind-bending image was taken by @Vincent TC (https://www.facebook.com/VincentTC1971) in the Netherlands a couple of weeks back. I was in two minds about how to present the photo - it was tempting to put it upside down! (Look at the bottom,,,) Vincent was out birding with his camera in bad weather (actually, because of the bad weather!). He got some shots of a Sea-Eagle and a Spoonbill (both different to the species we get here in Australia, of course), but this goose (I think it's a Greylag) landing in very strong wind caused a bit of a stir on Facebook. Consider the following. A bird is flying and suddenly decides to land, effectively dropping vertically to the ground. This might be due to very strong wind, or if there are raptors (or shooters) around. The bird can’t simply dive to the ground, as this would mean it would gain a large amount of speed. Pulling out of a dive like this before hitting the ground would place huge strain on the bird’s wings. The bi

The open cluster, M47

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You've got a new telescope, and you've seen the Moon and something of the planets (yes, they're small), and you're wondering about what to look at next. If you live in the city or the suburbs, you will probably already have found that nebulas and galaxies need telescopes with large apertures, such as a Dobsonian. They will also have to wait until you can get into the country (this was written while we were still locked down in Victoria). Instead, have a look for star clusters. They're just stars, so you can see them through the city's haze. What's more, they're pretty, and all different. Generally, they come in two classes, open (meaning scattered) and globular (meaning a swarm). Closed clusters are normally pretty small, but open ones can be large or small. This is my photo of an open cluster, M47. The poor thing doesn't seem to have a common name like a lot of them do. Here's how to find it: At this time of year (Apr

How to clean a reflector's mirror

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A little while back we received a 10" Dobsonian reflector that had been stored for several years. The mirror was very dirty and the owner was a bit intimidated by having to remove, clean, replace and recollimate it, so he got us to do it. Reflector telescopes make up probably half of the scopes we sell. Refractors and Schmidt Cassegrains make up the rest, of course. Reflectors, especially Dobsonians, with their large apertures, give their owners a good bright view of the night sky. But yes, sometimes you have to clean the mirror. When do you need to do it? New owners of reflector telescopes always ask how often they need to clean the mirror. It's a reasonable question. I recommend cleaning mirrors as rarely as possible, and not until the mirror seriously needs cleaning. Mirrors, especially large ones, are surprisingly forgiving. As long as they're collimated (that is, adjusted to they're perpendicular to the telescope's axis), they will continue to give a

Photographing the asteroid Apophis

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Never heard of it 99942 Apophis is an asteroid whose orbit - somewhat worryingly - intersects the Earth's. It's defined as a hazardous object. Apophis orbits nine times for every eight Earth orbits, and because of where the intersection is located, the danger period is every eighth April. Wait, what? The intersection is on the ascending node, meaning Apophis approaches the Earth from "below" (assuming Earth's northern hemisphere is "up"). This means Apophis is visible from the Southern Hemisphere before the encounter, and it falls below the horizon, becoming visible from the Northern Hemisphere after the encounter. There's a super close (but safe) pass coming up in April 2029, meaning about now is the last encounter before then. So of course I want a photo In preparation for writing an article about the asteroid, I wanted to actually get a photo of it. Because it's not a very close pass, it wanders by rather than zooms, and so I had a good