The Scruffy Astronomer

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...

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...

Occasionally, working in an optics shop, we see sad sights. This is one of them. A customer was recently setting up their telescope - a beautiful EdgeHD 925 - when it fell off the mount. In a previous life I was employed to analyse incidents like this and make recommendations as to how they might be avoided in the future. In this case, however, we didn't feel the desire to cross-examine a clearly distressed telescope owner. We took the scope into the warehouse and examined it. The corrector plate at the front of the scope was completely shattered, and some of the glass had fallen into the tube. The primary mirror didn't look damaged though, and apart from some superficial damage on the edge of the tube's front, the rest of the scope seemed - without examining it at least - to be OK. The corrector plate in a Cassegrain is not a simple piece of plain glass. It's carefully and subtly made to counteract the spherical aberration arising from the primary mirror. Beca...

A long time ago I was a bit of a Formula 1 fan. It was cool to see the things those cars could do, and I was fascinated by all the cutting-edge innovations they came up with. But one season Ferrari, so dominant for so long, had poor performance after poor performance. Eventually they got their act together, but it was a season lost. If they can be the victim of the "cascading failure", I shouldn't have been surprised when it happened to me. To quote Adam Savage, failure is always an option. I normally take my images from the the ASV's dark sky site in central Victoria. But during the lockdown, the site was closed, so I stuck at home, and also stuck with planetary imaging as the trees in my garden prevent my tracking a deep-sky target for a long time. Eight months without a deep-sky photo is cold turkey! When the place reopened, I was off like a shot. It was the time of a full moon, but that didn't fuss me. If my image wasn't great, that was OK, I was going...

Most high-end mounts attach to the telescope using a dovetail clamp. The telescope itself has a dovetail bar on it, which has a trapezium cross-section. This normally bolts to tube rings, which encircle the tube so they don't crimp it in any one spot, but some light tubes bolt directly to the dovetail. All you have to do is open the clamp on the mount, slide the telescope in so the clamp goes over the dovetail, and wind the clamp shut. Simple. The main advantage of having a clamp is that it keeps your scope parallel with the RA axis on the mount. It also doesn't scratch your dovetail bar. But are there disadvantages? I certainly never thought so. However, last time I was up at the dark sky site I was talking with far better astronomer than I, past ASV president, and general all-round good guy Russell Cockman. Russell was looking at my rig and brought up the subject of dovetail clamps. He asked me if I was scared of using one. This is my previous scope sitting on its mount...

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...

What's that madman doing this time? A few nights ago I was out in my back yard taking photos of planets. Using a monochrome camera and filters, I got a couple of images. They're not great, but that's another story. While I was out there, I noticed a problem. Jupiter had a big dark blob in front of it. The blob was about the same size as the planet, and roughly circular. Because Jupiter was a smallish image on the sensor, I was able to move my mount so that the blob didn't interfere with the image, but I suspected I had dust on one of my filters. Next morning, I looked at the filters and the camera. Try as I might, I wasn't able to see anything. Dust is a frustrating problem, especially for planetary photographers. With photos that use the whole frame, you can compensate for dust, but for tiny planets you can't do this. I decided to find it using the camera itself, by getting a "flat frame". This is a photo of what should be a blank featureles...