Meade Lightbridge Dobsonian Surrier Truss Telescopes
A dobsonian telescope consists of optics which were designed by Isaac Newton in the 1660's, on a mount popularized by John Dobson in the 1960's. They say more than half of all amateur scopes are of this type and I believe it. Many people make their own, as I have, but all of the major telescope companies provide excellent models at competitive prices. They're economical, a lot of bang for the buck.
Unlike the solid tube of most Newtonians, the Meade Lightbridge uses a surrier truss tube, which is strong but easily disassembled for transport.
Originally Meade made these scopes in 8, 10, 12, and 16 inch apertures. They quit making the 8 inch which I think is too bad. I wish I had bought one when I had the chance. Maybe I'll come across one used and scoop it up. I can understand that an 8 inch scope is so small and portable anyway you look at it, that the added portability of a surrier truss design seems a bit much. But I am guessing Meade discontinued their 8 inch for economic reasons. Not enough people bought them, is what I'm thinking. But I'm so happy with my 12, that I wish I had an 8 to use as a loaner to youngsters for example. Basically I just think the little mini-lightbridge is adorable and I want one.
I almost bought the 10 inch version but the 12 inch is a lot more scope for just little bit more money and takes up just a little bit more trunk space. It still fits nicely in the jeep with room for my observer's stool so why not go for more light gathering and more resolution, bigger is better. I briefly considered buying the 16 inch and who knows someday I might get one of those too. Hell, I'll probably wind up with one of each. For now I'm pretty happy with the 12.
The mirror is guaranteed diffraction limited (ideal parabola +/- ¼ wavelength) which essentially means that any optical problems from the mirror's quality that are detectable by humans are not as bad as those guaranteed to be provided by Mother Nature anyway. That's standard practice and you should expect as much from any commercial telescope. That's the minimum but with rigorous testing you may find that your mirror actually exceeds this standard.
For one thing, the Lightbridge scopes have oversized primary mirrors. It's an old telescope maker's trick. The mirror is a bit bigger than the actual aperture of the scope so that the extreme outer rim, as John Dobson himself once told me, "joins the rest of the Universe in not being part of your telescope." The deal is that the outer part of the mirror is where most of the problems come from. It's the most curved portion of the parabolic curve, and thus the most difficult to figure accurately. It's the most misbehaved part. The center of the mirror is the most flat, and hence the least troublesome. So, if you have a problematic mirror, just mask off the outer portion and the majority of the problem is solved. Better yet, make the mirror a too big to start with (arbitrarily) and somehow you don't feel like you are sacrificing anything by masking.
Lightbridge mirrors have a magnesium fluoride coating which helps keep it shiny for several years. Eventually the brightness will start to degrade a bit. It's not a terrible emergency or anything but someday when you are short on ways to spend money on your hobby you can have it re-aluminized and recoated.
I haven't pulled the mirror out of mine to optics-bench tornado-graph knife-edge Foucault-test it or anything. But it seems easy to collimate and gives nice black sky with hard tiny stars that make well-shaped donuts on either side of focus. And who doesn't like well-shaped donuts?
The mirror has a built-in battery powered cooling fan for dumping off the day's heat as you are waiting for the twilight to turn to dark. That can make a big difference for how soon you get to see Venus look like a boiling pot of goo as it chases the sunset into the dirt. The mirror is a heavy piece of thermal mass that can take a while to match the ambient temperature of the night air.
The focuser is advertised as a 2 inch Crayford-Style, which is a tribute to the reputation to actual Crayford focusers. You can hardly beat the Crayford, except maybe with a Crayford-Style. Either way you got style baby, smooth and fine. There's no slop like a rack and pinion. You can rock the focus without effort (move back and forth over a decreasing range to find the sweet spot). There is also an adjustable tension knob and lock down.
If you've ever built your own dobsonian then you are probably familiar with John Dobson's trick of using a vinyl phonograph album as an azimuth bearing surface for the rocker box. I've found that any of the Lawrence Welk titles from a thrift shop work rather well. Those are pretty much scratch-free because no one ever actually listened to them. Ray Conniff Orchestra, Ferrante and Teicher, they all make good low friction bearings. But the Lightbridge goes one engineering step better with real steel roller bearings. The action is seemingly "inertia-less" as they say, no jerky effort to start the motion and when you stop pushing it, it stops moving. Very satisfying.
The finder is a 1x (naked eye) red dot finder, a more modern version of the old Telrad finder. It's hard to beat the no-nonsense Telrad, but these new-fangled versions are classier looking, more like little spaceships, and less like the box they came in. What is glaringly missing on the Lightbridge is some obvious way to add a good old 7x50 finder or some such. Red dot is great, but for real star hopping? Come on. Someday, I can see myself drilling a couple of holes and attaching a proper and traditional straight-through finder. Until then, I have a 56mm eyepiece that gives almost a 2 degree true field in the 12 inch scope. It's not quite the same as a finder because there are way too many background stars showing up. That can hinder navigation. But on the other hand, if you happen onto anything NGC-fuzzy in the area, it shows up real well. Over all I'm having a good time in spite of being finder-less. It remains to be seen if I can wean myself from the compulsion to add one.
You'll want the black shroud that wraps around the truss tubes, keeping stray light from entering the optical path from the sides, thus maintaining optimum contrast. It also helps keep out dust, although the scope comes with a tight-fitting dust cover for the mirror itself. Then there is the shield that attaches to the secondary cage to keep stray light from entering the eyepiece directly, again to help maintain contrast.
Each Lightbridge telescope comes with a 26mm 2-inch 70-deg eyepiece. Unfortunately it's not a very good piece. Every review seems to agree the scope deserves better. A 26mm is the right power, a workhorse, the perfect size for a lot of what you want to do. Luckily I have a large family of Plossls, Ultrawides, and TeleVue Naglers, aka my children.
One drawback to a large eyepiece or camera is all that extra weight on the front end of the tube, unless you want to look at the trees on the horizon all night. I'm reluctant to permanently alter anything brand new and shiny, but this called for drastic action. I drilled two holes in the mirror cell and mounted bolts for adding counterweights. I have a pile of these stack-able weights but two of them, one on each stud, seem to be just right for countering heavy eyepieces, but not too much that I have to take them off when switching to lighter eyepieces. The only time I have to change them is to add more weights to counter a camera. That isn't often since the scope doesn't track. I can shoot the Moon, or terrestrial things, that's about it.
Accessories are essential elements of course, the right hat, photographer's vest, red flashlight and all. With a dobsonian it's nice to have a quick, precise, and broad daylight way to collimate the mirrors. Once it gets dark, nothing beats a star test. You might use the spectral highlight of the sun on a distance utility pole insulator, which can be almost star-like. But when you are standing on a mountain mid-afternoon, you can't beat a laser collimator for lining up the optics. I ordered a 2 inch variety but when it arrived I found it was really a 1.25 inch variety in a 2 inch adapter. I have plenty of adapters already so I could have saved some dough there.
I added an extra eyepiece holder to the rocker box. I don't always use eyepiece holders, usually one eyepiece in the focuser and one in my pocket is plenty. But at other times I have several going and it's nice having them handily available. And it looks cool and geeky.
Throw in a folding camp table for star maps to which I attached a shelf underneath, for structural strength, and to store my accessory cases out of the way while viewing. Also attached to the table, another eyepiece holder, but mostly because I realized I could drill one of the holes out to just the right size to hold my coffee travel mug. No more spilling on the maps. Not that it hurt the maps, they're laminated. But why waste coffee.
Here is a Meade page of Lightbridge specs: http://www.meade.com/lightbridge
And here are the prices: http://www.buytelescopes.com/Category/3-dobsonian.aspx
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