When you have one telescope on one mount, there are several major headaches for the amateur astronomer waiting for the day you get a second scope with the idea of putting it on the same mount as the first one, and suddenly it becomes apparent the two are not directly compatible. You will need to find the right tube rings, and bolt and unbolt mounting parts to take off the other telescope's tube rings and put on the new ones (and watch those holes- they may not be compatible) before putting in the new optical tube. At that moment, it will seem there should be a better way. Well, there is, if you are willing to become a home machinist: The Dovetail Bar.
Background
Since the beginning, there has been a question of the best way to attach a telescope to its mount. Simply put, the requirement for fine smooth tracking means you want the built up assembly to be balanced so it isn't trying to move on its own. Unlike camera lenses, telescopes are built to be as flexible as possible with their output, so what gets attached at that point can be anything from a light eyepiece to a 4 lb. camera or binocular viewer.
Since adding counterweights causes obvious problems, tube ring clamps on the telescope became fairly common as a way to let the user move the tube forward and back to restore balance. The problem is these are custom parts for that one tube, and bolting them on the mount directly means the optical tube has to come off, the rings has to be unbolted, a new set bolted on in their place, and then the new tube put on then balanced to put in a new telescope. Not only is this time consuming, but it also puts a lot of wear and tear on the hardware, scuffs and dings on the telescope, and incurs the hazard of dropping something if you try to do this in the dark.
Vixen Optics of Japan came up with an elegant and very simple solution to this problem in the late 1980s with their Great Polaris mount: instead of a couple of mounting tabs, it had a single clamp for grabbing a 1.75" wide dovetail bar. A dovetail bar is made so the bottom end is wider than the top, so clamping the sides wedges in against the mount so it can't move. This feature meant a telescope could have its tube rings attached to it and get bolted to a dovetail bar, and then when you wanted to put it on the mount, all you had to do was slide the bar into the clamp and tighten it. A telescope could even have the dovetail bar permanently attached to its optical tube and the tube rings simply would not be needed.
This dovetail system has quickly become very popular, and has been adopted by several manufacturers who are currently making mounts compatible with the Vixen Dovetail. This systems is currently the most widespread dovetail with a large number of different mounts compatible with it:
Celestron: CG-4 mount (late models, Onyx series), CG-5 mount (Standard and GoTo, including Advanced series) C5 single arm fork, NexStar SE series including 4, 5, 6, and 8 inch scopes. NexStar SLT series including 60, 70, 80, 90, 102, 114, and 130mm scopes.
Konus: Larger MotorMax series on CG-5 type mounts including Refractor, Newtonian, and Maksutov telescopes
Meade: LXD-75 series, including Refractor and Schmidt Newtonian scopes.
Orion: SkyView Pro (SVP), Sirius, Atlas, these also come bundled with Orion's large array of Refractor, Newtonian, Maksutov, and Schmidt Cassegrain scopes.
SkyWatcher: EQ-5 (CG-5 compatible) and EQ-6 (Orion Sirius Compatible) mounts and Refractor, Newtonian, and Maksutov telescopes bundled with them (sort of the Canadian Orion, eh).
Vixen (the original): Super Polaris (late model), Great Polaris, GP-2, GP-D2, Sphinx, also bundled with Vixen's large assortment of high quality Refractor, Newtonian, and Cassegrain telescopes.
Other: Chinese made CG-5/ Great Polaris Compatibles
Other dovetails NOT compatible with Vixen clamps:
Celestron CI-700, CGE Losmandy Astro-Physics
Now, before you start thinking Vixen has been ripped off in the worst way, please note they post the geometry of their clamp on the web at www.vixenamerica.com. These are forward thinking people, and they know being the source of the standard is not a bad thing (they are a sort of anti-Sony in this regard). Also please note the real Vixen mounts have some of the best tracking out there, and I encourage you to get a good look if you run across one since this compatibility means you will be able to attach your existing scope to one if it is under 30 lbs.
Description and Usage
The Orion 8 inch and 13 inch dovetail bars are actually telescope parts being sold separately for adapting other devices to their mounts. The overall shape is an aluminum extrusion with two sides angled at 15 degrees and a center buildup to touch down when the dovetail is compressed by a clamp. The shape is lighter than a solid bar would be, and the extrusion is extremely uniform in shape, so it will slide straight in a dovetail clamp.
The ends of the dovetail have holes for 1/4 inch bolts with a secondary hole on each side threaded for a #10X32 screw. The bard comes with four chrome cap screws for these holes. To attach to anything else, you will need to drill holes and adapt to your piece of equipment. This is where it gets to be a little more interesting. Here are the best tips I have on this:
Needed Tools and materials:
Safety glasses (sharp metal pieces will fly) Drill Press Drill Bits Vise X-Acto knife Tap and Die Kit (if you want tapped holes) 1/4 in. Thick X 1 inch wide aluminum bar stock.
The first thing to know is the dovetail bar must have a clean outer profile to go in the clamp, so any fasteners attaching through it need to be countersunk or recessed.
The second thing to notice is the dovetail extrusion does not touch on the optical tube except at the sides. Some mount clamps will interfere if this feature is bolted to a flat surface, like the tripod block at the bottom of many optical tubes, or the strange partial-dovetail the bottom of the C80ED collar has. You can deal with this by using a piece of 1/4 inch thick aluminum 1 inch wide, which I found at the hardware store. This will stand off a flat from the dovetail as a spacer.
Drilling with a hand drill is a mistake. A human being is not strong enough to hold on to an aluminum bar or drill while drilling holes, and the result will be non-round holes, and possibly cuts and bruises.
The Orion dovetail bar has a high quality hard anodized finish, so any holes you make will be breaking through this. So carefully use an X-Acto knife to trim burrs off of holes and cut edges if you are trimming a bar down.
The center area of the dovetail bar is wide enough to let a 1/4" diameter cap head screw get drilled in and countersunk. I strongly recommend figuring out the hole spacing needed to get at least two bolts holding on to the scope, and then repeating the spacing down the dovetail bar. You may find the most useful balance point is towards one end of what you are mounting to, and this will leave you ready to adjust in a few minutes instead of having to go back to cutting.
Remember, using some oil when drilling will make the bits last longer and produces better holes.
Last, but not least, Orion's end holes are a great place to put in a through bolt to make sure the dovetail doesn't accidentally slide out of the mount.
As brutal as this process sounds, there are some real payoffs. The first is it will make swapping tubes on one mount quick, easy, and will preserve the equatorial alignment of the mount. And, not to put a fine point on it, but getting one mount to the point where you understand it well enough to get the best use from it, you'll want to be able to put other telescopes on it.
Conclusion
OK, the fact I am writing about techniques on how to machine your own telescope parts as the easy way to deal with mounting the things says there is a problem. And, there is. There are a bunch of scopes around with odd mounting features. Telescopes like the AT-66ED, has a dovetail which doesn't correspond to any standard as its base, so adapting the Orion dovetail bar to it makes a handicap disappear. So, while the fact this part even exists points to there being a problem, it at least can fix the problem.
Dovetail mounting plates provide a platform for coupling telescope optical tubes to SkyView Pro, Atlas, Sirius, AstroView and VersaGo equatorial mount...More at Orion Telescopes & Binocu
Provides a platform for coupling telescope optical tubes to SkyView Pro and Atlas equatorial mounts. Dovetail plates are made of extruded, anodized al...More at Amazon Marketplace
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