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DGM OA-6.5 (Off-axis Newtonian



I thought about getting an 8-inch or a 10-inch from Orion, or a bigger truss-tube scope, and so I did a lot of reading of reviews, including here and on Ed Ting’s site.

Partly because I like technical neatness, and partly because of Ed’s positive comments, I found myself intrigued by the off-axis Newtownians from DGM Optics, despite their smaller apertures. After looking at the DGM Optics web site I ordered an OA-6.5 in September of 2002. The cost was about $2400, but prices have since gone up.

My relation to DGM Optics is purely as a customer – I have no other interest in the company.

I ordered my finders and an upgrade to the usual focuser and had then delivered to him for installation. He was willing to install them without any extra charges. My telescope was ready in January of 2003.

Because of the updgrades, what I have is a bit different from the base OA: it has a Feathertouch focuser rather than the stock JMI DX-3, an Orion red-dot finder and an Orion 9x50 correct-image finder-scope. Dan had worked with Orion on a power-up issue with the red-dot finder and got it resolved without invoving me. He also had to install some weights in the base of the tube to compensate for the extra weight from the two finders. This made me quite thankful that I’d sent him the finders while he was building the scope, as otherwise I’d have had difficulty balancing the tube when I got it and installed them!

My main observing site is my in-town back yard; on a typical clear night stars are visible only to magitude 3; on good nights I’ve seen down to magnitude 5. I have other sites I go to less often which are better. My own vision is only fair: I can only see four of the Pleiades, even in the best conditions. Keep that in mind when you read the “what I could see” section!

The Mount

The base is nicely made of high quality plywood, and the tube is aluminum with an auto-body-type paint (white in my case; I’d asked for canary yellow, but his supplier couldn’t do it). There’s a “DGM Optics” sticker on the tube and an embossed “DGM Optics” label on the rocker box. The logos have a schematic layout of the telescope’s optical path, which is neat.

The tube sits in a solid laminated-plywood yoke which can be opened to mount the tube and loosened while on the rocker box to let you move the tube up and down in the yoke to adjust the balance point for the current eyepieces; you can also rotate the tube to put the eyepiece in a position you find comfortable. The yoke has the altitude bearings, and comes off completely for storage. The fastener for tightening the yoke is adjustable. I was a bit surprised that there was only one fastener – two would have seemed more secure (I like redundancy!). The yoke’s altitude bearings are five-inch rounds of UHMW plastic – a slick, white, teflon-ish material. The fastener is adjustable, which is good, because the tube expands and contracts enough with temperature that what was a snug fit could become loose.

The yoke sits in a tall rocker box. I was surprised to find that the yoke was too narrow (or the box too wide) for the bearings to be fully seated. If I seat the left yoke bearing so that its full width is on top of the left rocker box bearing, then the right yoke bearing is partly on and partly off the right rocker box bearing: only about a quarter-inch is supported. Dan pointed out that the yoke can’t fall through, and didn’t seem to think it a point to worry about. Perhaps when summer comes the swelling of the tube and the wood will show me that this gap was necessary, but I don’t like it (redundancy again!).

The ground board is large and has three feet set wide at the edges. The feet are made of approximately one-inch wide, three-inch long cylinders of the same UHMW plastic used on the yoke’s altitude bearings. The ends are cut at an angle, in a shape like a blunt chisel. In the snow or on grass, this means that the ground board feet dig a bit into the surface, so the board is very solidly on the ground. On asphalt, the feet stick almost as well. Because the ground board is large and the feet stick out sideways, the support triangle is quite wide and the base is very stable.

The whole thing looks really nice; it’s clear that the materials are good and that the workmanship is done with care. If I understand correctly, the mount is made by a furniture-maker Dan knows. I think that accounts for the fine finish, but also for some “furniture-ish” features which could be “astronomy-ized”. For example, I think the mount would benefit if some large circular holes were cut in the sides and the front. This would lighten the mount by several pounds without significantly weakening the structure.

For another example, the center pivot is non-standard: it goes down from the rocker box rather than up from the ground board (this is only an issue if you want to mount the rocker box on another platform; since I’m getting an equatorial platform, it is an issue for me). I’ve recommended to Dan that he replace the current pivot with a more conventional one, or offer it as an option.

The Telescope

The first impression is that the OA 6.5 ATS is _big_. The tube is ten inches in diameter, and about five and a half feet long. The rocker box is tall: the altitude bearing centers are 34.5 inches off the ground. So despite being based around a 6.5-inch mirror, the whole thing looks and moves more like a 8-inch f/8.5 dob.

It’s much bigger than the typical commercial 8-inch f/6 or 10-inch f/4.8! When the tube is in the rocker box, the whole thing towers into the sky. I’m 5 feet 9 inches tall, and it’s winter, so I have thick-soled boots on. When I use the OA to look at objects at the zenith, the focuser is too high for me to look through while standing on the ground. Just a bit away from the zenith, however, the fact that you can rotate the tube to put the focuser on the “down-hill” side makes it usable without a step-stool. But since I take my telescope to public star parties, and want children to be able to look, I already have a step-stool.

The tube weighs seventeen pounds, the yoke six and the mount forty. With a barlow, a big eyepiece, my finders and a tube extension from Astrozap, the whole thing must weigh over 70 pounds. That’s good, because that long tube catches the wind easily. But it means that it takes two trips to move the telescope from your car to an observing site! The rocker box has a convenient handle, and if you hold one of the legs in one hand and the handle in the other hand, it’s quite easy to carry; the tube can be cradled in your arms for an easy carry as well.

Set-up is trivial: put the base on the ground, click the yoke around the tube and put the tube-and-yoke on the rocker box.

At first I found that while I could easily re-balance the tube, it didn’t stick to one orientation as much as I’d like unless it was very carefully balanced. If I changed weights at the focuser end, it often needed to be re-balanced. I tend to find objects using a low-magnification eyepiece and then swap for a high-magnification eyepiece; when I do that, the tube might tilt during the swap. I mentioned this to Dan, and he sent me the parts for a friction-strap arrangment he uses on his smaller telescopes. It was easy to install, and solved the problem.

I suspect that one reason the telescope tilted so easily in altitude is that the altitude hubs are only five inches in diameter; from what I’ve read is that the standard rule of thumb is that they should be as wide as the tube diameter.

On the other hand, the azimuth bearing is sticky. It’s just polyurethaned wood riding on three teflon-ish bearings; when I complained, Dan suggested I spray on WD-40 to make it stick less. I did that, and I also added some washers cut from plastic milk jugs to put more of the weight on the central bearing; there’s an improvement, but it’s still not as smooth as I’d expected for a premium Dob. I recently had the chance to use an 18-inch Obsession, which was not only smooth, it was smoother than I’d expected a Dob could be. So it can be done! Dan is going to send me a sheet of UHMW to attatch to the bottom of the rocker box to solve that problem.

Right now, however, this means that tracking an object within 20 degrees of the zenith with the OA is a pain: you have to reach down and move the mirror box, or hug the tube and twist. The bearing sometimes sticks and then overshoots.

The Optics

The “ATS” stands for “across the scope”: the secondary is on the far side of the tube from the focuser. That’s good, as it means you don’t have to worry about the focuser drawtube hitting the secondary (as you might with the on-the-same-side models from DGM: that’s why those have a helical focuser).

The mirror is 6.5 inches in diameter (165 mm), with a focal length of 1720 mm, for a focal ratio of f/10.4.

The main mirror is an off-axis section cut from a much larger parabolic mirror. This means my telescope has three “siblings” somewhere! The mirror is mounted so that the secondary is over the point where the center of the parent mirror was. This means the mirror is not tilted: as Dan says, it’s “decentered”: the situation is exactly parallel to a large conventional Newtonian with an off-axis mask over the mirror. It’s easy to adjust the primary: there are three spring-loaded wing-nuts, but I’ll talk about collimation more later.

Because the secondary is off to one side, there is no spider and thus there are no diffraction spikes; neither is there an image of the secondary when you defocus a star: the star-test shows a circle of light, just as with a refractor. The secondary is permanently attached and aligned. It never needs to be collimated. This is another side-effect of having no spider: the secondary is connected to the tube by a short, strong “L”-shaped piece of metal and barring disaster, it’s not going to get out of alignment.

No secondary in the light path means more than no image in a star test; it means no unfocused image of the secondary in your field of view. Some people don’t mind or even notice, but if you hate having that moving grey object in the middle of the field of view, an unobstructed telescope will be a welcome improvement. For most Newtonians, this is only a problem with wide fields of view: more narrow views are all inside the bounds of the unfocused image of the secondary and so don’t distract by showing its edges. What this means for the OA is that a long focal-length eyepiece (like my 55 mm) is a joy to use, while on another telescope (like a 16-inch Dob I used recently) it’s less of a joy due to the intrusive blurry image of the secondary mirror.

The tube interior is dark and rough; the mirror cell is open and permits lots of passive ventilation. It’s open enough that you can see the ground if you look down the tube. A large part of the mirror cell is made of dense plastic, which means that there’s no big mass of metal to radiate heat. The secondary shows the whole of the primary when you look in the focuser, no matter where you put your eye.

This means that 1.25-inch eyepieces are fully illuminated. I used Mel Bartel’s on-line calculator, and it agrees. The rear is open enough that I’ve made a light-baffle for the tube bottom, to prevent light reflected off the ground from getting into the optical path when the ground is covered by snow. Using the focuser in 2-inch mode, there’s some illumination drop-off according to the on-line calculator, but it was invisible to me visually in the eyepiece.

Collimation is done in two steps, as described on Dan’s web site: first you adjust the primary while looking into the focuser, and then you use a defocused star to fine-tune it. I’m still working on learning the second step: if I understand Dan, you have to look for the diffraction rings to be out of round, and then adjust based on that. In the months I’ve had the telescope, I have only seen round patterns, and so have only done the first step. I haven’t had to re-collimate since Dan showed me how to do it: despite all the trips I’ve taken my OA on, the mirror cell has held collimation.

In focus, stars are tiny points. The sky next to bright planets and the Moon is dark black. Even with the tube extension, the Moon creates a glow when you look near it but don’t include any of the Moon itself; taking the eyepiece out reveals Moonlight on the far side of the telescope. Some extra baffling on that side would be helpful, and I may add some.

When I do a star test (which I’m only a beginner at), I see clear circular patterns on both sides of focus. Of course there’s no image of the secondary in the center! I had to use 150x to see the diffraction rings; at lower powers they weren’t visible to me.
With my “Easy Tester” from Jack Schmidling (a Ronchi grating in an eyepiece), the mirror showed even, parallel bands without any bends at the ends. This means there was no turned edge, a smooth surface and no significant under- or over-correction. It’s a very nice mirror!

The Feathertouch focuser is also nice: it’s smooth, the two levels of gearing are addictive and it feels very solid. But it does have one serious drawback: it’s too easy to put the adapter (from 2-inch to 1.25-inch) in to the focuser in the wrong orientation. There’s a small pin (actually a screw-head) which settles down into a tiny notch when it’s at the right orientation, but unless you put a flashlight on it and look, you can’t tell when the adapter is in the wrong place.

The consequence of having the adapter in the wrong place is that a 1.25-inch eyepiece won’t be held by the set-screw. The screw will feel tight because you’ve bedded the screw against the adapter. The eyepiece is still loose. And if you turn the tube so the focuser points downwards, the eyepiece will fall out. It’s a terrible shock to hear an eyepiece fall onto a hard surface!

I had ordered a 2-inch eyepiece (the TeleVue55 mm) and had it sent to Dan. Even with 2 inches of travel in the focuser, I have to clamp the eyepiece while it’s partly out of the focuser to reach focus. Another half-inch of out-travel would help. Since my 11 mm eyepiece focuses at least an inch of outward extension on the focuser, the whole focuser/secondary placement could have been set-up half an inch further up the optical path and then there’d be no problem.

What I Could See

Because it has no obstruction from a secondary mirror, the OA tolerates long focal length eyepieces: this means that despite the “slow” f/10.4 focal ratio, I can use long focal-length eyepieces to get wide views. Using the 55 mm eyepiece, I get a true field of view of 1.6 degrees at 31x. If there were a 100 mm eyepiece, I could use it! (Though because my pupils are only 5 mm wide at most, such an eyepiece would give too low a magnification to use the light-gathering power of the whole mirror).

Even better, the “slow” f-ratio means that high magnifications don’t require super-short eyepieces with vanishing eye relief: my 11 mm eyepiece is comfortable to use and results in 156x in a 19-minute true field of view. An f-number of f/10.4 also means that focusing is low-stress and simple, especially with the low-speed gear in the Feathertouch.

Recently I’ve been running comparisons between my old 4.25-inch and the OA. Based on aperture, I expected the OA images to be 2.45 times brighter – about one magnitude – and have 1.53 times the resolution – visibly sharper, but not astoundingly so.

But the differences are astounding, especially in dark conditions with good seeing. The Sgr-4 shows Jupiter’s two main bands, while the OA at the same magnification shows two more, one to the North and one to the South. With the OA you can see that Io is orange. When the Sgr-4 shows Saturn’s “A” and “B” rings, and the Cassini division in the ansae, the OA shows the “Crepe” ring as well, and the Cassini division is visible over the whole ring.

On a recent good night in late Februrary (one which was quite clear and dark for the location) I got some astounding views of Saturn and Jupiter. I was able to use a TeleVue 11 mm eyepiece with a TeleVue 2.5x Powermate for a whopping 390x. Even with that level of magnification, it was easy to focus; five bands were visible, as was the shadow of a transiting moon and another moon (perhaps the transiting one?) just on the limb. During instants of good seeing I saw detail inside Jupiter’s bands and zones. The moons were different sizes and showed as steady dots.

Saturn showed five moons clearly and a sixth flashed in and out; I could see bands on the planet, Cassini’s division and the Crepe ring. There was a thin arc of shadow cast by Saturn on its ring. That level of magnification makes the apparent size of the planets quite large: at 390x, Jupiter had an apparent width of more than four degrees!

Due to the trickiness of tracking at that power, I also observed with just the 11 mm eyepiece, at 156x, where the planets looked significantly smaller but just as crisp.

M42 was good as well, with visible mottling; since I haven’t been able to see more than four stars in the Trapezium in other people’s 10-inch telescopes, I’m not surprised that I can’t see more than four in the OA. I’ve resolved the outer stars of M13, and seen M57 (the Ring Nebula) as a tiny circle. Open clusters are very nice in the OA, as it’s easy to tune the magnification to suit the cluster size, and the stars are tiny bright specks.

From modestly dark sites (limiting magnitude 5) it has shown me M81, M82, M97 (the Owl Nebula), the Eskimo Nebula and the like. From my back yard I’ve seen M51 on a “magnitude 3+” night, though only as a lighter patch of sky. For a six-inch, in those conditions and with my eyes, that’s not bad at all!

The Moon is wonderful in the OA. I’ve gone up to 390x on it and still seen sharp edges and black shadows.

At only 6.5 inches (165 mm), the mirror gathers about 1000 (square of 165/5 mm) times more light than my eyes, for a gain of 7.5 magnitudes. With the typical limiting magnitude in my back yard being 3.5, that means that the OA will let me see magnitude 11 stars. Extended objects are visible at magnitudes which depend on their surface brightness. The OA 6.5 is not a “light bucket”, and to see really dim objects, it needs a dark sky.

Other club members have used the OA, and had postive reactions, but no one has done a full review.

Tweaks

I’ve made a light baffle for the bottom of the telescope, as explained above. I’ve had a custom AstroZap dew shield made – not to protect the optics from dew, but to extend the tube past the focuser. There are local lights where I observe, and the extension keeps them from illuminating the interior of the tube and thus casting a glow over the field of view. In my opinion, the tube extension significantly improves the view. But it does throw off the balance, and I wish I’d known I was going to add the AstroZap in time to have it sent to Dan so he could adjust the balance.

Indeed, with both finders, the monster 55 mm eyepiece and the 18-inch AstroZap tube extension, if I balance the tube, it has to be pulled so far back that it won’t swing into the rocker box! Experimentation showes that to balance the tube and still permit it to swing into the rocker box, I had to add two pounds to the mirror end.

I’ll need to come up with some kind of detachable balancing system to fix this longer-term; at the moment I’m using a two-pound rubber-covered exercise weight (a mini-barbell) and some cotton rope!

I also need to cover the area near the secondary with black flocked paper to reduce Moon glare when looking just next to the Moon.

Conclusion

The OA-6.5 functions particularly well as a planetary-specialized Newtonian, but due to its lack of obstruction can give high-quality medium-wide-field views as well. It’s meant to compete with high-end refractors of about the same aperture and with slightly larger planetary-optimized Newtonians of more conventional design.

Compared to the refractors, it’s much cheaper and (because it’s a mirror-based telescope) utterly without chromatic abberation, but it’s big (though it’s not heavy compared to a big refractor) and on a Dob mount rather than an equatorial mount.

Compared to a Newtonian, it has better contrast and no diffraction or shadow effects from a secondary and its spider, but it’s slightly more expensive than the other high-end Newtonians and gathers less light than you might expect from its size.

Some days I think of it as a high-contrast Newtonian, and other days I think of it as a refractor sustitute. Either way, I’m pleased to have such a nice telescope!

Update 11/01/03

Hot wet weather did indeed change the relative position of the yoke bearings in the rocker box. But it didn't change it very much, and the yoke was still too narrow. On the other hand, nothing bad has ever happened as a result of the narrowness, so it's purely a cosmetic issue.

The tension strap on the altitude bearings continues to work well; I particularly like the fact that the tension can be adjusted with a thumbscrew. This means I can find my object with the strap loose and then tighten it while I observe.

I did cut several six-inch holes in the rocker box, which lightened it by six pounds. The holes also work as convenient handles. I plan to cut some holes in the ground board as well when I invert the pivot (so that the ground-board side of the pivot is the one which doesn't rotate) to make the telescope work better with my equatorial platform.

Dan sent me a UHMW sheet for the ground board, and I installed it and some furniture sliders. Azimuth motion is smoother and less sticky now, but it's still not "Obsession" level. I did stick black flocked paper around the secondary, which improved contrast considerably, especially in moonlight. I also made a baffle for the mirror end of the tube which blocks the direct view of the ground but doesn't block air flow. It improves
the view, too. I'm now convinced that good baffling is really worth doing: it requires only modest investments of money and time but produces big benefits.

Cats and dew lead to considerable gunk on my mirror. So now I can report that it's quite easy to remove the primary for washing. More importantly, it's easy to re-install it so that everything is still properly aligned, needing only some fine-touch collimation after it's re-installed. Dan very kindly taught me how to do the star collimation. It turns out not to be very hard: first you defocus a bright star to the "few rings" level when using a medium-power eyepiece. If it's round and centered, and the pattern looks the same when you go in and out through focus, you're done. Otherwise you move the tube to put the star image in different parts of the field of view, and inspect it in each place. One of them will be better. You then turn the collimation wing-nuts, moving the better image to the center of the field of view. Then you go to a higher-power eyepiece and repeat. It's quite easy once you've done it a few times.

Finally, I want to report that Mars totally rocked in the OA!

This telescope really works well on planets, where brightness isn't as much a consideration as are sharpness and contrast. At star parties I've looked at Mars through many other instruments (including a 4-inch Alvan Clark refractor that the owner had bought at a yard sale for fifty dollars). I don't think I'm being too much of a predjudiced owner when I say that the OA-6.5 did very well against some really nice telescopes that cost quite a bit more than it did.



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