  The Benefits of a Large Dobsonian Finding Objects A Demanding Telescope Dobsonian Limitations Baffling a Truss Tube Dob
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The Benefits of a Large Dobsonian
There are two words that summarize the benefits of a large Dobsonian: simplicity and aperture. Simplicity because there's nothing that's quite as easy as hand-pushing a Dobsonian around the sky at your leisure, and aperture because these scopes are big enough so that there's no trouble seeing what you want to see. Want to see the Ring Nebula? No problem, even the central star is readily observable at a dark site. The shape of the Swan Nebula is... well, like a swan! You will get lost observing in the Realm of the Galaxies because there are more galaxies visible through your telescope than are plotted on Uranometria!
In addition to ease of use, the Dobsonian can easily break down. This is especially true if it is a truss tube telescope (see above picture). It will break down into the easily manageable components you see here. You will likely end up with an upper cage or ring, a mirror box, the rocker box, several trusses and a ladder if you need it. This telescope fits in the back of my Mazda B2300 pickup truck (read budget pick-up) easily with enough
room for all my camping gear to go to my favorite star party. I can even pick up all the components, although for a telescope this size you will probably want to add wheelbarrow handles & wheels for ease of moving.
Large Dobs give inferior planetary images This is absolutely not true. Here's a comment from a friend after a weekend observing with the 20 inch: "On the second night at 3am the seeing got real good, right when the red spot was just east (Jupiterwest) of center. And Ganymede's shadow was RIGHT on the corner of the eye of the spot. THREE concentric rings of clouds were visible WITHIN the red spot (just like you see in the very best Sky&Tel CCD images)." This was at full aperture. There are some things to keep in mind: 1) Quality of optics It all starts with the basics. Every optical component must be of excellent quality to obtain an excellent image. 2) Collimation The optics must be well collimated. In faster scopes (which large Dobs typically are) this is even more important. 3) Local Seeing The atmosphere is the big unknown in observing. A larger telescope sees a large "column" of air, so turbulence in the atmosphere is more obvious than in smaller telescopes. 4) Temperature Equalibrium It is important for your optics to be at the same temperature as the outside air. Columns of heat plume from your equipment if it is not close in temperature. This really spoils the view. I recommend a fan that blows as much air as possible on the your mirror. This will cool it fairly quickly and allow those great views earlier in the evening.
Large Dobs are not portable. They are too difficult to set up. Dobsonian technology has come a long way. My 20 inch is always assembled and rolls out of a walk-out basement in less than a minute. I can set it up faster than my 8" mounted on a GEM. For almost two years I had to set it up and brake it down on each observing session. It was inconvenient, but not any more work than setting up a SCT of about 8-10 inches aperture. The point is you can probably make your setup quick & easy. Add wheel-barrow handles & good wheels. This allows you to roll your rocker & mirror boxes. If you are capable of building your own, make an ultra-light version.
When you have a large aperture telescope, you have to do a little more than you may with your 80 mm ShortTube. Here's some of the things that I have noted that require my attention:
Collimation - Any telescope requires collimation, but because I don't have a permanent home for this telescope I have to bring it in & out for observing sessions. I also travel regularly with this telescope. This makes the alignment get out of whack to a degree, but is easily remedied by spending a few minutes during setup and tweaking it on occasion through the night.
Care in handling - This 20" is just big enough to be a little heavy. You need to be careful not to smash your fingers, or hit a door jam since the mass of the larger components may win a small battle with your cheap door trim. Take your time and give yourself enough leeway before hopping in the car to be off on an observing session. I've thrown everything in my truck, and then the camping gear, and then the food and by the time I was finished I didn't feel like going anymore. It's supposed to be fun, so don't rush it and be cautious with your equipment... it's not cheap!
A safe place to rest - While a 20"telescope can be made compact, the footprint of this thing is still at least a few feet square. I don't know about your situation, but my wife doesn't consider it the best showpiece of the house, so it's important (for me anyway) to have a place to store it. A walkout basement works for me. You can also use your garage, shed or observatory.
A good ladder - So here's the deal, pretty much wherever you go with this thing you're going to need a way to reach the eyepiece. I'll tell you what I did at first; I bought a $20 "light-weight" wooden ladder. That worked fine until I took it to a couple of star parties. After 30 or 40 dozen climbs (many by people who exceeded the recommended max weight of 200 lbs.) the ladder was not quite so sturdy. I consider that a feature, however, and not a bug. Some people refuse to climb the ladder anymore and I get more observing time :) Anyway, you need a good ladder. Just remember fiberglass is heavy & pricey, and aluminum gets cold in the winter... I stuck with wood.
User Time - The biggest thing my telescope needs is for someone to peer through the eyepiece. The reason is that it's the only way to justify the expense & energy of owning such a large telescope. If you don't plan on spending time observing, the telescope is not for you. Make sure you can swing it with your family and commitments before you take the plunge... and if it works out then you will not be disappointed. Personally, I find it's easiest to set it up at sundown... sneak a peek if the evening allows but go to sleep until about 1:00. I can then wake up and observe from about 1:00 a.m. - 4:00 a.m. with no inturuptions, and I still get about 5 hours sleep in two shifts. I can deal with that a few times a month.
Telrad: Need I say more? With a Telrad you can see where you're pointing, and then just peer through the eyepiece. If you mount it near the eyepiece, it may tower over you when you're on the ground. But that's not a problem. The Telrad allows you to use it even if you're several feet away as you peer "through" the device towards your target. It's not important to be close, just as long as you can see the red circles, you can point your telescope to within 1/2 degree anywhere in the sky.
Low Power Eyepiece: I use a 40 mm wide angle eyepiece (Pentax 40mm XL) in the main telescope when I'm trying to find things. For brighter objects I can sweep, and the 20" aperture will pick up all of the Messiers, for
example, with relative ease. For more difficult objects star hopping may be required, but the low power is still very helpful when star hopping as well.
Traditional Finder: I use an 8x50 finder on mine, and I find it to be useful. I can see M33 through it on a good night in my backyard, and for at least 1/2 of the Messiers it's the ideal way to center your target. Just be sure to provide a means of keeping the dew off. In the picture on the right, you can see I have a couple of spray-paint lids rubber-banded to the finder mount. You can just pop them on and off without fear of losing them in the dark.
A Jumbo-Sized Finder: Picture this... mounted along-side or piggy back the main telescope is a jumbo finder that is as large as a standard telescope. I've got an 8" that I can mount alongside the 20", and the idea is to allow the observer to see those faint objects that only a legitimate telescope can see. During testing of the 8" mirror, when only the jumbo finder's optical tube was completed, I found I was able to scan without a mount at 27X magnification with 2 1/3 degree field of view. This is a textbook rich-field machine. Couple that with 8" of light gathering capability and you'll be able to point your large Dobsonian at anything! At the Connecticut Star Party 2000, I had the scope set up that way for the whole weekend. Someone would be observing through the 20", and someone else through the 8". It made for a fun time.
Jumbo Finder!
Star Hopping: A Dobsonian telescope may seem disadvantaged, but in many ways it is much simpler to use than a traditional equatorial mounted telescope. For example, I recently was helping an observer using a 6" Newtonian on a German Equatorial Mount (GEM.) to verify a field of view with M95, M96 and M105, three galaxies in the constellation Leo. He had something in the field of view, but it was very cumbersome turning the manual controls, first North or South, then East or West. And to be honest with you I wasn't totally sure which way I was going... Needless to say I was not as successful as usual considering I consider star-hopping old hat. You can learn these odd motions, but a Dobsonian can be easily "mastered" in an hour or less. Once you master the reversed motions, you simply nudge the scope in the direction you want to move it and it responds by moving in both axes. This makes finding objects much simpler if you are not using setting circles, so if you plan on tracking down your own objects a Dobsonian is an ideal instrument.
There are not too many limitations to Dobsonians anymore, but let's be fair and talk about what they don't do... at least easily:
Automatic Tracking of the Stars: Most Dobsonians don't track the stars, so it is required that the user keep up with the object as the earth turns. If you have never used a telescope before you may be surprised to hear that the earth spins fast enough for this to be a problem, but if you ever get a chance to look through one then you will realize it's a matter of 30 seconds or less before the object has moved or is completely out of view. The simple solution is to simply nudge the scope ever 15 seconds or so. It gets pretty easy after a while, and can even be done at powers of 500+ although it is a little tricky.
I've included a few sites that show you some options to get over this hurdle.
Mel Bartel's website - a site that shows you how to turn the average Dobsonian into a fully GOTO telescope with tracking and many other features.
Tom Osypowski sells an equatorial platform that is a perfect match for the Dobsonian telesopes. It will allow full control of both axes that's good enough for CCD & Photo work.
Viewing at the Zenith Equatorial telescopes have a tough time viewing near the North Celestial Pole (by the North Star), and Dobsonians have a problem near the zenith (straight overhead). This area is called "Dobson's hole". The reason for this problem is the telescope may need to be moved around azimuth pivot many degrees to make a small adjustment in position. The good news is with practice it's not that bad. A few degrees away from the zenith make a big difference, as do the quality of the bearings. In addition, with a large scope it's just so high! You have to be ready to climb a ladder and be sure not to drop anything.
The Problem My 20" optical tube weighs in at over 125 pounds, but a solid wooden tube would weigh even more, and a cardboard tube would simply be too big to manage. The aluminum trusses allow disassembly of the optical tube assembly into more manageable components, but the truss tube allows external light to sneak into the eyepiece.
The Light Shroud A popular way to deal with a part of the problem is to get a light shroud. This is a material (such as ripstop nylon or spandex ) "skirt" that goes around the trusses when the tube is assembled. I own one, and it is useful for blocking light. It does, however, make the entire telescope suceptible to breezy conditions so there are nights that it is not practical to utilize a shroud.
Secondary Baffle Just as important as a shroud, is a baffle on the upper side of the secondary cage. I use a piece of Kydex that gets velcroed to the secondary cage.
Focuser Baffle Another significant method of blocking off-axis light is the focuser baffle. Cut out a piece of plastic about 3 inches in diameter, and cut about 1 3/4 inch hole in its center. Align this with the center of the focuser inside the secondary cage, and figure out a way to secure it. This helps keep a lot of the stray light from entering your focuser.