A computer controlled Dobsonian Telescope

I've more or less finished moving from the USA to Australia and have started playing with my telescope again. The weather in Sydney has been dismal for observing for the last few months and I've spent my time building a Cookbook CCD camera instead. (Get the book here). When the sky clears and I get my dob tracking well enough and I take some shots, I'll post them on this page.

my 12 inch DOB telescope the Greenbank radio telescope Making the side bearings
Photos of my dobsonian, the Greenbank "monster" Telescope in West Virginia and the construction of the side bearings.
Click on the boxed images for a larger view (approx 50k bytes each).

Click here to read about the unconventional bearing design I used for my Dobsonian. UPDATED 28th November, 2002

Click here for a growing description of the electronics options I've considered and the designs I've chosen. UPDATED 23rd January, 2003

Click here to see and hear about the software I'm writing. UPDATED 12th June, 2002

If you've never looked through a decent telescope, I whole heartedly encourage you to contact your local observatory or amateur astronomy group and see if you can tag along some time. Apart from the obvious thrills of being able to see the craters on the moon, the ice caps on Mars, the rings of Saturn or the moons of Jupiter (all which are relatively easy to see even with small telescopes), eyeballing a galaxy or star cluster for the first time is a moving experience. Our insignificance amongst the immensity of the universe cannot begin to be comprehended by our minds which have been attuned to mere earthly scales.

What's so good about a big telescope?

For a telescope, size is often what matters and in my case, the size is 12.5 inches. You can see that the telescope is obviously much larger than this. The 12.5 inches refers to the diameter of the primary mirror, or it's aperture. Basically, the larger you make the primary mirror, the more light your telescope is going to be able to collect and focus into your eye. The more light that enters your eye, the brighter the sky will appear. The brighter the sky appears, the dimmer the objects you can see and notice. Looking at dim objects is the whole point of having a large telescope! (Apart from being able to control it from a computer).

Just to set the record straight, a big telescope is perhaps not the best tool for observing something bright, such as the moon or one of the "inner" planets (Mercury through Saturn). These objects are so bright, the larger the aperture, the more difficult they become to look at through the eyepiece; almost to the point of pain in the case of the full moon viewed through a big telescope. Often, an observer of the moon will cover over a large proportion of their aperture using a mask or insert a light-blocking filter into the eyepiece, to reduce light levels to make the telescope more comfortable to use. By blocking out some of the light in this way, you're really making a big telescope behave like a smaller one so in other words, big telescopes are not necessarily the best tools for looking at our planetary neighbours.

When thinking of telescopes, people often imagine that their prime purpose is to help us look at stars. Another common misconception is that the reason we need telescopes is because of the magnification they provide. It is true that stars are often the target of telescopes but many people will be surprised to learn that no matter how large the telescope aperture or how great its "magnification", a star will always appear to be an infinitesimally small pin point of light. In a sense, telescopes do magnify, but rather than size, they magnify brightness. Stars which are invisible to the naked eye become dim pin points in small telescopes and dazzling beacons in much larger ones. The larger the aperture, the brighter each object will appear.

Because larger telescopes produce the brightest images, you can see many more stars with a larger device. It is truly magnificent to behold a part of the sky which is otherwise dull to the naked eye and to discover that it is bursting to the point of overcapacity with thousands of tiny pin points of light. Some of these pin points will be bright and others will be barely noticeable and will be scattered sometimes haphazardly and other times with apparent order.

Despite the apparent monotony of a shining pin point, another purpose of looking at a star through a telescope is to reveal it's often hidden partner(s). At least half of the stars in the sky have one or more close companions and it requires the help of a telescope to be able to see them each as separate objects. Magnification helps in separating close stars but higher magnification is almost always a hindrance more than a help due to perturbations in our imperfect atmosphere. It is often best to reduce magnification to get a better view of the sky and so as I said above, larger size matters, but now you know that larger magnification most certainly does not.

Most objects in the universe are far more distant than the stars you can see with your naked eye and amateur astronomy might become dull if "splitting doubles" was the only challenge for a telescope. In order to conquer the dimmer, deeper parts of the sky, big hardware rules!

Deep sky objects such as galaxies, star clusters and nebulae are not pin points like stars and even to the naked eye, some are spread out over a broad and noticeable area. For the larger of these deep sky objects, their area can be far broader than the area of the full moon but most deep sky objects are considerably smaller. Why then are most people so unfamiliar even with astronomical objects so much larger than the full moon? Because these objects are so dim!

I was recently fortunate enough to spend a few nights on a remote Australian farm property and even more fortunate to witness a truly clear and the most profoundly dark night sky of my life. Under such conditions, the sky gains a new dimension! The milky way, our galaxy, stands out as a stripe across the sky and the Large and Small Magellanic Clouds, two of our nearby galactic neighbours, shine brightly against the inky blackness of the background. These objects were so striking, when my wife Vivien joined me outside that night, her first question was to ask the identity of "the big clouds". The Large Magellanic Cloud truly dominates a dark southern sky and is about twenty times the diameter of the full moon yet from my city backyard, I've never previously seen it with my own eyes.

One of the aspects which make deep sky objects so pretty and interesting to look at is their non-uniformity. With smaller telescopes, you may be only able to see the brightest parts of these objects, the dimmer parts simply appearing too dark for your eyes to register. With a larger aperture telescope, you effectively make all features brighter because you're gathering more light and in so doing, a greater richness of detail will become apparent.

When you're face to face with your first star cluster or galaxy, you can't help but think about how we earthlings could possibly be alone in the universe. The galaxy in the eyepiece is likely to consist of billions of stars (some no doubt with solar systems similar to ours). As you ponder this wondrous sight, you understand that if you had a truly large telescope (much bigger than mine, alas), you could find millions of galaxies. With a enough time and a telescope such as the Hubble, you could find billions. They're everywhere you look around the sky, just far too dim to see with your naked eye and most even too dim for all but the largest telescopes.

Despite the tremendous amounts of energy coming from each galaxy, the reason that they appear so dim is because they're so far away and because they're so far away, our universe is incomprehensively large. Well, the magnitude of its' immensity is incomprehensible to me anyway.

Building a truss Dob

Despite its simple appearance, construction of a truss Dob requires considerable time and effort. A lot of this effort is in the planning and if you work on it every night, it will still take you at least a month before the telescope will start to be useable. If you make your own mirror, double this time estimate and treble your blisters.

If you think you'd like to build a telescope of your own, welcome to the hobby! Check out the SanFrancisco Sidewalk Astronomers web site, the original home of John Dobson and the Dobsonian telescope. This link will take you to several simple plans for cheap Dobsonian telescopes and you'll get the general idea of what will be involved.

To build a telescope like mine, a truss Dob, you could do a lot worse than buying and reading The Dobsonian Telescope by David Kriege and Richard Berry. These authors have done an excellent job in describing the process and details of designing and building a large truss telescope leaving the calculation of the dimensions and finer design decisions to you. I thoroughly recommend the book. Although you can order the book directly from the publisher, I've also seen it in specialty telescope shops, even in the remotest continent in the world: Australia.

looking at the primary mirror

Here you can see the lower part of a truss Dob, called the mirror box.

Actually the mirror box is normally balanced in something called the rocker box and the rocker box is balanced on a ground board. In order to see higher or lower in the sky, the mirror box rotates up and down like a canon being ranged. In order to point in the correct compass direction, the rocker box rotates clockwise or anticlockwise on the ground board. A picture is worth a thousand words and you can see what I mean by looking here. Often, this form of a telescope mount is called an Alt-Az (Altitude - Azimuth) mount.

Look carefully and you will see a reflection of one of the truss tubes in the mirror. The optical quality of the mirrors used in telescopes is substantially greater than anything you'd buy down at the local hardware store. One of the main differences (other than the parabolic shape) of Dobsonian primary mirrors and bathroom wall mirrors is the fact that for a telescope, the reflecting surface lies on the side of the glass closest to you whilst for a bathroom mirror, the reflecting surface is on the far side of the glass. In the bathroom, the layer of glass protects the reflecting surface from being scratched and deteriorating but for a telescope, sending your light through a piece of glass to be reflected and then travel back through the same piece of glass would be unacceptable.

looking at the secondary cage

There is another mirror in every Dobsonian telescope and it's located along the central telescope axis directly inside from where the eyepiece goes. In this photo of the "secondary cage", you can't see the mirror, but you can see the eyepiece holder. Although the primary mirror is large, the secondary is much smaller. My secondary is said to be a 2.14 inch secondary although this is only the dimension of its minor axis (thin axis). Because a secondary mirror is mounted at an angle of exactly 45 degrees, its shape needs to be that of an ellipsoid (something like a football shape) which when viewed from the eyepiece, will appear in profile as a perfect circle.

For me, the next step in the project is to get the digital setting circles code working on the Dob. Digital setting circles are of course, completely optional on a telescope intended for general use. For a computer controlled telescope on the other hand, Digital Setting Circles are at the heart of the system. To move forwards, I need to get my Linux box working again so I can edit and compile the source code. So many things to do and so little time in which to do them..... If you're interested, read what I have now and then check back every couple of weeks.

Click here to read about the unconventional bearing design I used for my Dobsonian.

Click here for pointers to descriptions of various aspects of my telescope design.

Click here to see and hear about the software I'm writing.

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