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Orion XT-6i Dobsonian mounted Newtonian

Posted by John MacDonough   12/23/2012 12:00AM

First, I realize that this is a little late. Orion no longer sells the XT-6i. A pity, it’s been my default recommendation for anyone who’s asked me, “what scope should I buy” since my son and I got ours. I’ve “upgraded” my old recommendation from a 4” telescope to a 6” telescope because a 6” shows so much more than a 4”, and specifically because of the XT-6i. It was a bargain for a 6” telescope with an accurate computer. I have some experience with “go-to” telescopes available at discount and hobby stores. One of those almost made my daughter quit astronomy out of frustration. This is not one of those scopes. It is a proper observing tool. I would consider this telescope to be a real find on the used market. One of our friends has the StarBlast 6i. When I finally get to try it, I hope I find it as good as the XT-6i. Then I’ll have a recommendation again. I hope most of what follows will be applicable to the XT classics and the larger XTi telescopes.

Perhaps only parents will find this funny: My five-year-old son was invited to climb a ladder and view the Orion nebula through a 36” Newtonian at our club’s dark sky site. For the next two years I was treated to regular serenades of, “I want a big Dob”. This from a boy who, like all other boys, couldn’t manage to put his shoes away. While it did get tiresome, the constant asking did keep the thought of a Dob in my mind. Imagine my excitement when I saw an ad on Astromart for, “Trade: 6” Orion Dob for small refractor”. I happened to have a William Optics Zenith Star 66 sitting unused in the closet. A couple of e-mails turned into a couple of phone calls and the Orion owner and I met at a mutually convenient place and traded. That’s how my son and I added a Dob to our collection. We’ve had the scope for over a year now. Sad to say, I use it more than my son does, but at least he knows it’s available when his interest strikes again. So that’s how my family came to have this particular telescope.

Briefly, here’s what the 6” Newtonian will help you see under the stars. It’s a 6” f/8 (1200mm focal length) telescope. It’s usually the perfect height for me to sit in a patio chair and lean into the eye piece while observing. It’s also just right for my son to stand while observing. With a 40mm Plossl eye piece you can expect a 1.3° true field of view at 30x (5mm exit pupil), enough for both halves of the Double Cluster in Perseus, and a good portion of the Pleiades in Taurus. A 6mm Plossl will yield 200x. For me, 200x- 240x shows great detail on planets and the moon, and makes it possible to split double stars separated by 1.5”- 2” (arc seconds, not inches). If the sky is really steady, a 4mm Plossl will give 300x. I haven’t used a 4mm eye piece very often. Skies steady enough for that magnification are rare where I live and objects rotate out of the field of view really fast at this magnification. I find that I spend too much time moving the scope around and tweaking the focus at 300x.

Here are the things I like about the stock Orion scope. I say “stock” because ours is no longer stock. More about that later. The wood base is easy to put together and is sturdy enough for the scope. Even though I’m not the original owner, I’ve taken the base apart a couple of times while doing modifications. The base does shake a bit after the scope has been moved, but holding my face lightly against the eye piece cup is enough to get the shakes to stop within a second or two.

The metal tube is strong and painted well. It’s also fairly well balanced on the base. The paint on the inside is quite black and quite flat. Orion’s slewing knob is a useful tool that really does make it easier to move the telescope. I use it more for keeping objects centered while observing than I do for moving from one object to another.

The CorrecTension system works well in altitude. It takes some fiddling to get it right, but the big hand-knobs make adjusting it easy. The big knob also makes it easy to make small adjustments to the friction in the altitude bearing, which is necessary when you get close to where you want it. It does take a little practice to get the bearing adjusted so that there is enough friction to hold the scope still when you’re not moving it on purpose, but not so much that “sticktion” makes it hard to move the telescope smoothly.

The IntelliScope digital setting circle system is as accurate as I remember it from when I had one on my SkyView Pro mount. It’s also easier to set up: only a two-star alignment. I like this system, it’s very clever. I only wish it was still available for the equatorial mounts. Apparently, Orion had too many complaints from equatorial mount owners who couldn’t get it to work properly. My installation was really, really accurate. Until I ruined it taking the mount apart for cleaning, that is.

What you’ve heard about Dobs, especially smallish Dobs, it true: they are incredibly easy to set up for observing. I can take my son’s out in one piece, set it gently on the driveway, and have it focused on the moon or a planet within three minutes. While I won’t be taking this scope on a backpacking trip, its picture is next to the Webster’s definition of grab-and-go, at least for me.

Best of all, the optics are better than “good”. I was shocked by how good the images could be in this telescope. So you’ll know my frame of reference, I regularly use my 6” D&G refractor and C-11 SCT. I also get to look through a Telekit Dob, Meade 12” and occasionally a Mewlon 210 at our club’s dark sky site.

Unfortunately, the bad; in no particular order (yet). The azimuth bearing has too much “sticktion”. It was really frustrating, for me at least, to move the scope. Just when I’d used enough force to get the telescope to move the “sticktion” would break free and I’d have rocketed the telescope way past what I was trying to center.

The finder: I can understand a right angle correct image (RACI) finder for star hopping with a telescope without a computer. However, the only thing I use this finder for is getting alignment stars within the field of view of the telescope, and it’s much easier to do that with a straight-through finder. Also, while I know that this is an inexpensive telescope, a 40mm finder would be much more useful than the supplied 30mm.

The focuser is really bad. It’s a rack and pinion unit, Orion’s least expensive. At first, I thought I could live with it. The adjustment knob is really tight and requires a lot of force to move the draw tube, which of course gives the telescope the shakes. You can’t tell if an image is focused if the telescope is shaking. I also discovered that the slop in the focuser travel was fouling up the telescope’s collimation. That made it impossible to view at high power (200x-300x) because the image would get fuzzy every time I moved the focuser. It’s OK for low power views.

So here’s what I’ve tinkered with over the past year. The previous owner had installed some Protostar flocking on the inside of the main tube opposite the focuser. I really like the Protostar flocking. I may eventually add more at the primary mirror end to keep off-axis light from getting to the primary. I’ve since lined the focuser draw tube with Protostar flocking paper, too.

The first thing I did was relocate the Telrad that the previous owner provided. Since I don’t like the RACI finder, the Telrad has been a good addition; besides, my son loves it, perhaps more than the telescope. I had to relocate it because its base interfered with removing the finder, which I do when packing the scope for trips to the dark sky site.

I then tackled the “sticktion” in the azimuth axis. I purchased a suitably sized lazy Susan bearing from McMaster-Carr and installed it between the ground board and rocker box. The lazy Susan bearing I picked keeps the ground board and rocker box just far enough apart that the PTFE blocks that used to support the rocker box are just lifted off the ground board and are thus disengaged. Now the telescope moved effortlessly in azimuth. Unfortunately, I learned the hard way what most Dob users already knew: you don’t want your scope to move effortlessly. It won’t stay put when you want to view an object. It moves in any breeze. It moves when you let go unless you’ve set the telescope up on a perfectly level site. Et cetera.

So, I was off to Home Depot for some parts, so I could install what I call a drag brake. I installed a threaded insert through the bottom of the rocker box. A friend at work welded a nut onto a fender washer for me and I stuck a felt pad onto the fender washer. I threaded an eye bolt through the insert in the rocker box, screwed the nut/fender washer/felt pad onto the end of the eye bolt and reassembled the rocker box to the ground board. I can now adjust the amount of drag on the azimuth bearing. I have the best of both worlds: buttery smooth movement, almost no stiction, and just enough friction to keep the telescope in place when I want it to stay put.

My latest change has rescued the focuser from upgrade oblivion. The good news is the focuser is made of metal, not plastic. I decided to attempt improving it before replacing it. I took the focuser apart and found “grease” that could have been better described as uncured glue. A bit of mineral spirits took care of that. When I put the focuser back together, I was careful to leave the four screws that hold the retainer plate on the bottom of the focuser looser than I found them. That reduced the pressure exerted by the leaf spring that keeps force on the focuser thumb wheel shaft. That adjustment allows the drawtube to move freely within the focuser. I also backed off the thumbscrew (used to lock the focuser in place) and two small set screws (used to provide drag on the draw tube) located on the top of the focuser. These adjustments made the focuser move with less effort, thus reducing the amount of shaking resulting from focusing. But, the collimation shifting was still there.

I took the focuser apart again and measured the outside diameter of the draw tube and the inside diameter of the focuser. I then called my local hobby store and later went and bought some plastic sheet. I made a couple of sleeves. I tested both and used super glue to fix the sleeve I liked best in the focuser. I had to sand the inside of the sleeve with 400 grit sand paper to debur it, but now the draw tube fits inside the focuser pretty snugly. I used a small amount of Mobil 1 grease, too, to make everything slick.

So there you have it. I really like our Orion XT-6i Dobsonian telescope. In its stock form, it was a good introduction to observing for my son. I couldn’t help but tinker with it, though, and I’m happy with the results. The telescope now performs like a giant killer. Here are the good parts that come from Orion:

1: The mirrors are capable of giving good images. Keep your scope collimated.

2: The IntelliScope digital setting circle system is very good.

3: The basic “structure” of the telescope and Dobsonian mount make for a sturdy and stable platform.

Here are the things I changed. I’ve put them into order, from what I think is most important to what I think is least important for improving the performance of the telescope:

1: Remove the sticky “grease” from the focuser. When reassembling the focuser, keep the four screws that hold the bottom retainer plate looser than they were when you bought the telescope. This will make the focuser easier to move, reducing shaking in the telescope tube during focusing. These changes are just too simple not to do.

2: Fix the focuser slop. You’ll never get the most out of the good mirrors Orion provides until the focuser slop is fixed. It cost me about $8 in parts and ninety minutes of labor to fix the focuser.

3: Fix the azimuth bearing. I used a lazy Susan and “drag brake”, but I’ve heard from other Dobsonian owners that reducing the contact area between the rocker box and the ground board is also an effective way to reduce “sticktion”. So, perhaps reducing the diameter of the PTFE pads is also a good option.

4: If I’d bought this new and it didn’t already have a Telrad, I’d have replaced the finder with a 40mm or 50mm straight through finder. Perhaps I’d have splurged and bought one with illuminated cross hairs, too.

5: Flock the main tube opposite the focuser and the focuser draw tube. It may not make any difference you can see, but it can’t hurt, and you might be able to tell the difference.