The Scruffy Astronomer

During the latest lockdown in Melbourne, I was at home, but had taken with me a small Newtonian telescope and a guide camera. I was wondering what sort of photo of Jupiter I could get with simple and inexpensive equipment. I put the scope up onto my NEQ6 mount, but it didn't have a finderscope. Complicating matters, the guide camera was set into the focuser of the telescope and it was nicely focused. I didn't want to move it. How was I going to see where the scope was pointed? How was I going to align the scope to the sky? Reusing old equipment I have an old guide scope at home - an Orion. It's not as good as the saxon one, but it does the job. I've also got an ancient illuminated reticule eyepiece which I rescued out a junk bin at an astro meet once. I planned to use this in the guide scope. The eyepiece was filthy. When I looked through it, all I could see was nicely-focused grit and dust across the whole field. I had to pull the eyepiece apart to c...

Classifying telescopes When clients come in and ask about a new telescope, the first thing we normally ask is what they want to see with it. People often look at us weirdly when I ask this. "The sky?" they tend to venture. We're after an idea whether they want to look at planets or deep sky targets like nebulas and galaxies. What we're getting towards is what type of telescope is best for them. Telescopes aren't the same. They're highly specific tools, each with their own area of specialisation. Get the wrong one and it's not going to give you what you want. Ferraris and Land Cruisers We explain by asking people what is the best type of car: a Land Cruiser or a Ferrari? They're both great for what they're intended for, but used for a different purpose... not so much. So how do we decide what type? Aperture and focal length Telescopes are fairly comprehensively described in two measurements, aperture and focal length. Aperture - how much ligh...

I got a Bird-Jones telescope in for some maintenance the other day. It needed its mirror realigned. It's a bit of a pain to work on because it's got an additional lens in the focuser that disrupts the laser we use to guide the adjustment. What's a Bird-Jones? The Bird-Jones reflector telescope (also known as a Jones-Bird) is a variant of the Newtonian design. These guys look pretty much like normal Newtonian telescopes, with an open aperture at the front, a mirror down the bottom, a flat secondary at the top and the focuser at the side front. So what's the difference? The mirror is a subtly different (and cheaper) shape. A true Newtonian design has a parabolic primary mirror at the bottom of the tube. A Bird-Jones uses a spherical primary mirror. Have a look at my two scribbled diagrams. The first one shows a parabolic mirror. Parallel light rays from a star are coming in from the right, and bounce off the mirror. No matter whether they hit the m...

Ever since the Copernican Revolution , we have known that the Sun is the centre of the Solar System. Orbits of the planets don't need to be circular (in fact, Mars' orbit is quite oval), but the one thing that's correct is that the sun is at the centre. Right? Well, not quite. Moving to a heliocentric view of the Solar System was a step forward, but it's more accurate to say that everything in our solar system, including the Sun, orbits the centre of gravity of the whole system. Time for a thought experiment. Imagine you've got a dinner plate, with some items on it. There's a large burger in the middle, a pile of chips on one side, and a restrained amount of salad on the other side (we wouldn't want to overeat, would we?). Now imagine you want to balance the plate on your finger, circus-style. If you don't want to drop your food on the floor, you're going to have to choose very carefully where your finger goes. If you balance the plate...

A random walk through the sky When I first started in astronomy ( back at school ), our astro club's observation nights were unplanned affairs. We'd turn up, maybe have a quick squiz at Norton's Star Atlas to see what's up, and then grab a scope. We'd wander randomly from one target to another, exploring the sky on our own. With a plan, you don't miss a target Inevitably though, the day afterwards, someone would start talking about the Lagoon Nebula or some other cool target. When we'd find out where it was so we could see it, we'd find it was just setting in the evening and we'd have to wait six months to see it. (Geek bullying - find some really cool object in the sky, then wait to tell your victim about it until it's too close to the Sun to observe. How very Niles Crane .) Observation nights always go better when you can plan your targets. And it's not just visual observers, either. Astrophotographers want to expose the target for as...

Chromatic aberration can have a significant effect on your binocular's performance, especially when looking at views with high contrast. it results in poor focus and colour fringing. This article explains what chromatic aberration is, and why high contrast makes things worse. When I'm talking about binoculars to customers, most often bird watchers, but also hunters, we often discuss how they might perform in high contrast environments. What, you may ask, are these? And why is it important? The answer is associated with chromatic aberration . A while back, I was out at Lake Acraman in the South Australian outback. We were out for geology, but of course, we were also looking for birds. Because we were in the desert, the sun was shining strongly, and the shadows cast by the trees are deep and sharp. My mate Dean had a pair of Vortex Razor binoculars - a magnificent tool. Being newer to birding, on the other hand, I had a far less salubrious pair - a stopgap while I saved ...

What's the deal with magnification? When using a telescope for visual observations, it's a simple matter to calculate how much bigger everything looks. But with a camera it's more complicated. Here, two concepts  replace magnification: your field of view and your camera's resolution. One of the most common questions people ask me is about the magnification on telescopes. For visual telescopes, it's actually pretty simple. The magnification is just the ratio of the focal length of the main mirror or lens to the focal length of the eyepiece you're using. So, for example, if you've got a telescope with a 900mm focal length, and you use a 20mm eyepiece, the magnification is 900/20, or 45 times. Swap to a 10mm eyepiece and the magnification jumps to 90 times. This is why shorter eyepieces are more powerful. But people also ask me about about magnification when you're using a camera. There's no eyepiece, so it's more complicated.  It's about fie...

An unusual Barlow that's shipped with (at least some) Celestron StarSense Explorer telescopes has to be assembled in an unusual way in order to be able to focus on stars. We’ve been selling the new Celestron StarSense Explorer telescopes for a few months now. The StarSense Explorer is a nifty new development in the world of beginner to intermediate telescope mounts. It uses the camera on your mobile phone to determine which way the scope is pointed by looking at the stars and consulting a database. It then guides you to whatever you want to look at. Very clever. Of course, our stock sold out before we were able to grab one to use as a display model, so I still have yet to play with one. But that’s not the point of this post. The point is the strange Barlow lens that comes with the StarSense Explorers. A Barlow is inserted into the converging rays of the telescope, making them converge less. Put simply, this pushes the focal point (where the light actually converges) further back fr...

Focusing with a small Newt and a DSLR A slightly longer version of this is on the Optics Central blog page . One of the most common (and irritating) problems that owners of Newtonian telescopes have is getting a DSLR onto them – and actually get the image in focus. Well of course you want to take a photo! One of the most common (and irritating) problems that owners of Newtonian telescopes have is getting a DSLR onto them – and actually get the image in focus. Of course, people want to take photos of the Moon or other things when they’ve got a telescope and a DSLR – who wouldn’t? But actually getting the two things together is a little complicated. To make the physical connection, you need at least two things. First the telescope’s focuser has to have a specific thread (called a t-thread) cut into it. If your focuser doesn’t have one of these you can get a little tube with an eyepiece-shaped tube and the required thread. Second, you need an adapter called a t-ring, whi...

4 December 2019 I've talked about different types of telescopes before, but I'm having another go - this time with some hand-drawn diagrams that I've scribbled. This blog was originally three Facebook posts, so they haven't really edited together perfectly... Chromatic aberration in refractors It wasn't Galileo who invented the telescope - he just took credit for it. In fact, the refractors we sell in the shop aren't like Galileo's refractor at all - unless you are buying a pair of opera glasses (https://www.opticscentral.com.au/binoculars/opera-theatre-binoculars.html). Galileo's refractor had a single convex lens out the front and a small concave lens for an eyepiece. It showed the image the right way up, but the field of vision was very small - it was like looking through a straw. It was Kepler who replaced the concave lens at the back with a convex lens, lengthening the scope but giving a much bigger field of view. It didn't really matter i...

One of the most common questions clients ask us is how to get more magnification from a telescope. Just about everything optical these days comes with a zoom lens, so people think nothing of zooming in and out. So how do you do this with a telescope? Short answer: you change the eyepiece. Warning: I’m about to get a bit technical. I’ll use pictures to illustrate, but I won’t be offended if you don’t read on! All telescopes work in the same basic way. The main scope bends light into a focus, and then you use an eyepiece to look at that focus. The main scope might use a lens or a mirror, but the effect is the same – all the light squashed into a focus. The distance between a lens and its focus is called the focal length. The shorter the focal length, the more powerful the lens. Have a look at my little hand-drawn diagram. I’m starting to get a reputation for this .  Magnification of a telescope is given by the focal length of the objective (main) lens or mirror divided by...