The Scruffy Astronomer

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...

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 ...

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...

Telescope mirrors are delicate, right? Dust degrades your image, right? Can a bit of dust damage your mirror? So what happens when you shoot one with a gun? Reflector telescopes are a great way of getting some serious aperture onto a deep-space object like a nebula or galaxy. The 10-inch Dobsonians we have give a relatively inexpensive, no-nonsense and, frankly stunning view of those hard-to-see, dim fuzzies, especially from a dark sky location. New owners of reflector telescopes always ask me how often do they need to clean the mirror. They seem surprised when I tell them that with care, they probably won't ever have to. Mirrors, particularly large ones, are very forgiving. How's this for a case in point? The Harlan J Smith Telescope in Texas is a 107 inch Cassegrain (with a Caudé option as well, for the technical). To put that into perspective, the ASV's monster in Central Victoria is "only" 40 inches in diameter. On commissioning in 1968, it was the t...

It's lockdown in Melbourne (still) and I'm looking for something to point a telescope at. I've sworn off deep sky targets for the time being, as the light pollution in the city here makes anything I produce - even narrowband - look blah in comparison to images taken from the dark sky site. I've also been taking photos of planets, but I left the Celestron 8 at work. Besides, (and I might be getting a little controversial now) there's a difference between nebulas and planets. Once you've taken a great image of a planet (not that I have yet) it gets a bit repetitive. There's not that many of them (sorry, Pluto). But wait - there's the sun! I pulled out my trusty solar telescope, a Coronado PST. This type of solar telescope doesn't work in the same way as a solar film filter. A solar film is an example of a neutral density filter, which turns down all the colours (frequencies) of light to more or less the same extent.  A solar telescope cuts out all ...

I get asked a lot of questions from a lot of people. All of these questions are good, but some are so good they get asked a lot. This is one of those questions. The magnification of a telescope is how much bigger your target looks compared to looking at it without the telescope. If you like, it's how much closer the target is going to look. Magnification is expressed in multiples, so if your telescope has a magnification of "50 times", it means that whatever you're looking at will look 50 times closer, or will appear 50 times bigger than without the scope. Remember that your telescope can have several eyepieces. Changing eyepieces changes the magnification. So, for watching the Moon, your 25mm eyepiece might give you just the view you want. However, if you want to look at Jupiter, all you get with a 25 is a dot, so you need more magnification. The 10mm eyepiece will give you a much closer image. But to calculate it, there's some mathematics involved. Your telescop...

The other day I was contacted by James in Philadelphia who has just got himself a Celestron AVX800. A fine scope on a fine mount. But he’s having trouble with the 9x50 right angle correct image (RACI) finderscope. I had a look at the "straight through" one on our display model and yes, it’s fiddly. When you set up the scope each night, you have to true up the finderscope so that it's parallel with the main scope. To do this, you use the adjustment screws at the back of the bracket. This design is the "two screws" type - at 12 o'clock and 3 o'clock (see my side-by-side photo below). They work against a spring-loaded pin at 7:30 (which I've circled in the finst photo). Other finderscope designs use three screws spaced equally around the finderscope. To adjust the spring-type finderscope, all you have to do is twiddle the screws at 12 and 3 o'clock. You don't have to back off one screw in order to advance another, which you have to do with the t...

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...

What’s this weird looking thing?   Every now and again, we get waifs and strays in the shop. The other day this one came in. Once upon a time it was a 90mm Maksutov Cassegrain spotting scope.  I find this sort of thing fascinating. When they’re pulled apart, you can see how the focus mechanism on a Cassegrain works. When you rotate the focus knob, the whole primary mirror moves away from or closer to the eyepiece. Here's a second photo of the mirror extended as far as it goes. With Cassegrains, it’s very important, when the focuser moves, that the mirror stays perpendicular to the light rays, or the image gets distorted. This is generally known a “mirror flop”. I’ve had a look at this one, and the mirror seems very stable. I haven’t been able to wobble it, even though it’s on a single shaft. I don’t want to mangle the scope any further, but I probably will later on. Larger Cassegrain telescopes often have locking mechanisms on the primary mirrors to stop the mirror movin...

Last month I went to the ASV’s Star-be-cue. The weather didn’t really play the game, and for most of the evening we were dodging clouds. But star parties are mostly social occasions, and a chance to meet up with other astronomers. They're also a good opportunity to see other people’s equipment and how they use it. While I was there, I got talking with a couple of people with a saxon Novo 909AZ3. They were watching M42 - the Great Nebula in Orion. Nearby, another group of people were also on M42 but through a saxon 8” Dobsonian. I had a saxon Hyperion 1021EQ3, so it was a great opportunity to compare scopes. The 909 and the 1021 produced very similar images. Although M42 was visible, it wasn’t very extensive. However, because these are both refractor telescopes, the “trapezium”, those four baby stars in the middle were sharp and bright, clearly separated from each other. The Dob gave a very different view. M42 was wide and very distinct. You could see wisps of it extending on b...

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...

Ever wondered how refractor telescopes deal with colour fringing? The other day I pulled apart a telescope lens cell. Here's what I found. Because we service telescopes, occasionally we get patients which are, sadly, unsalvageable. This one was one of those, an entry-level refractor with a terminally mangled focuser. I took the opportunity to pull the front lens off and see how it worked. The telescope is (was?) a doublet-type refractor, meaning there are actually two lenses at the front, built especially to refract blue and red light to the same point. Single lenses refract blue closest to the lens, green in the middle and red furthest away. This is the dreaded "chromatic aberration" that refractors produce. Visually, it's not much of a problem, but taking a photo with this type of lens looks ugly. Check out my photo of the Southern Cross. See the blueish purple fringes around the brighter stars? That's what happens when most of the light is in focus, but th...

24 October 2019 Ever wondered what’s inside your binoculars? Take it from me, there’s some complicated stuff in there. The most important part of the binocular is the pair of prisms between the lenses. Without these, you’d be seeing upside-down. Design and quality of prisms can make a great difference to the quality of the image you see through the binoculars. They can also make a big difference to the look of the binoculars themselves. The older-style “porro” binoculars, with their characteristic zig-zag shape, have largely been displaced by “roof prism” binoculars, which have a straight-through look. This may or may not be a good thing, as while roof prism binoculars are more compact, more robust and easier to waterproof, most of them don’t provide as bright an image as their predecessors. Thanks to our friends at saxon, I got to pull a pair of binoculars apart and investigate the prisms inside them. I’ve written a blog about it (at http://www.opticscentral.com.au/blog/bi...