Jump to content

  •  

- - - - -

DGM Optics OA-3.6


Discuss this article in our forums

Disclaimer

I have no other interest in the company than as a customer. What follows is my personal opinion only.

Introduction

I've now had my DGM Optics OA-3.6 since September of 2005 -- that's a more than half a year. I think I have enough experience with it now to do a reasonably complete review.

The OA series of off-axis Newtonian telescopes is made by DGM Optics . I've owned the OA-6.5 version for many years (see my Cloudy Nights review here ).

Much as I like my OA-6.5, there were times when I had passengers in my car and thus didn't have room in my car for the telescope as well as the people. Clearly I needed a smaller telescope! The OA-3.6 is exactly that smaller telescope.

DGM Optics (Dan Mc Shane) was a pleasure to deal with.

A little bit about the author

I'm in northern New England, living in the middle of a small city. I've been doing astronomy for three years and a bit. I like planets, planetary nebulae and galaxies.

Mechanics

Dob Mount

I got the Dob-mounted version using a PVC tube. It's about the size of a conventional Dob-mounted 6-inch f/8 standard Newtonian. It weights about thirty pounds.

There are two tension straps holding the altitude axles of the yoke to the rocker box. They tie the yoke-and-tube and the rocker box together so that you can lift the whole assembly by the tube if you like. There is a handle on the rocker box as well. It's very easy to move this little telescope around from place to place. When I'm in my back yard I move it each time I want to either put myself in a shadow or see another piece of sky. This portability is a major plus, in my opinion.

The tube slides back and inside the yoke for balance. You can also rotate the tube in the yoke to put the focuser in the most convenient position. This is another nice feature: observing is more fun if it's more comfortable.

The telescope moves easily and holds position when you let go. There's very little vibration; a tap to the tube dies out quite quickly. Because it's so small, even a stiff breeze doesn't create a lot of vibration or motion in this telescope.

The end result of the mechanics of this little telescope is that the eyepiece is almost always at a comfortable height and angle for a seated observer. One interesting consequence of the light weight of the telescope and the seated observing is that when I want to see another part of the sky, I often find that it's easier to pick up the telescope and move it around the chair while I stay seated than to stand up and move the chair. I suppose this means I should get a rotating stool!

Balancing

I use a tube extension from Astrozap to shade the interior of the tube from local lights and the Moon. I have two finders and sometimes use two-inch eyepieces. With all that weight at the top end of the tube, the tube will only balance when slid far back in the yoke. When it's that far back in the yoke, it will be so far back that the bottom end won't pivot into the bottom of the rocker box. This means you can't look at objects near the zenith.

I solved this problem by getting a pair of two-pound leg weights with Velcro straps from a sporting-goods store. They make a weight ring which I can slide up and down the bottom half of the tube for perfect balance. They stay in place due to friction.

Note that this is only a problem for me because I've put so much weight up at the top end!

Finders

The OA series as sold don't come with finders. I ordered a pair of finders from Orion and had them sent to Dan McShane so he could install them. I have a red-dot non-magnifying finder for coarse finding and an Orion right-angle correct-image 6x30 magnifying finder for fine finding.

I often don't use the 6x30 because with a long focal-length eyepiece the OA-3.6 has such a wide field of view that the telescope itself works as a finder. For example, using my 42mm GSO 2-inch eyepiece, I can see almost all of the Pleiades because my true field of view is about 2.5 degrees across!

Focuser

I got a focuser upgrade to a two-speed Feathertouch. Even though the focal length of the OA-3.6 is 1020 mm, it's nice to have that low-speed fine focus! I therefore can't review the default focuser.

PVC Tube

I got the PVC tube rather than the aluminum tube I have on my OA-6.5. While the PVC tube is about two hundred dollars cheaper, I'm going to recommend the aluminum tube over the PVC one. I make this recommendation because I have already managed to crack my PVC tube!

It was my own fault: I put the telescope on top of an equatorial platform without fastening it down, so when the platform tilted, the scope slid off and fell to the ground, cracking the bottom of the telescope tube. Luckily it didn't crack through one of the attachment points for the mirror cell.

The crack was about three inches long. I was able to fix it with glue and strapping tape, but an aluminum tube wouldn't have cracked at all.

Post-purchase Modifications

I used black fuzzy flocking paper to line the inside of the tube near the secondary. I have a baffle covered with black fuzzy flocking paper for the bottom of the tube (it has air holes under the mirror for ventilation). When the tube extension is added, the result is a tube interior which is very dark: if you look in the focuser during the day without an eyepiece, you don't see any lit bits of tube nor do you see more than a few slivers of light at the bottom of the tube. At night, this baffling increases contrast and makes those faint fuzzies just a bit less faint.

Here's a look into the focuser with the end baffle in place taken in daylight with the tube aimed at the sky (far away from the Sun, of course!). The out-of-focus lit area is the secondary and the inside of the tube lit by the bright sky. All you see in the secondary itself is the primary and some thin slits at the edge of the baffle, where the baffle doesn't quite meet the tube:

I sometimes use my scope on an Osypowski equatorial platform. I've cut little notches in the platform to hold the feet of the Dob base, so it won't slide off again. Since the feet aren't exactly symmetrically located, I marked one as the "South" foot.

A Dob on a platform is a great combination: planets just sit there in your field of view and you can adjust eyepieces and filters to get all the detail that's visible.

Here's a picture of the OA-3.6 with all the fixings (you can see the strapping tape used to fix the crack at the bottom of the tube and the "South" foot):

Price

The whole set-up cost a bit over a thousand dollars when you include the focuser upgrade and the finders.

Optics

The DGM Optics web site has diagrams of the off-axis design. The OA-3.6 mirror is an off-axis circular section of a bigger "parent" parabolic mirror. It is not a tilted mirror design. The secondary is over the virtual center of the "parent" mirror which puts it on the side of the tube:



The mirror diameter is 92mm and the focal length is 1020mm giving an f-number of f/11. Since the secondary mirror is not in the optical path, it can be big (1.3 inches , 27mm) for good illumination of the fal plane.

A focal ratio of f/11 means easy collimation and easy focusing. Collimation is actually easier with the OA-series telescopes as there's no need to adjust the secondary. I find that my OAs generally stay collimated, even if I drive them a long way. The mirror cell has three easy to use thumbscrews to collimate with:



Note that the tube is much wider than the mirror and that there's lots of open space for air. This means that the mirror cools down very quickly! You can also see that the PVC tube is a bit out of round. This isn't actually a problem, no matter how odd it looks.

I've read Web pages which claim that an unobstructed design isn't as good at it might sound, and that you get the same resolution if you just get a slightly-larger mirror in an on-axis design. I'm not going to get into that argument here.

I will mention the "rule of thumb" for comparing obstructed with unobstructed apertures. It says that an aperture of N inches obstructed by a secondary with a minor axis of S inches is equivalent to an unobstructed aperture of (N - S) inches. By this rule, the OA-3.6 is roughly equivalent to a 4.8-inch standard Newtonian having a secondary mirror of 1.2 inches (a 25% obstruction). I did a comparision with a telescope of about that size (see below); I don't think this rule captures all the difference.

The bottom line is that the OA design works for me. The 3.6-inch mirror isn't very big, but the images are great. This is an astoundingly capable telescope for its size, and it's a pleasure to use. I really appreciate the aesthetics of the image: there's no defocused image of the secondary in the middle of the field of view, nor are there diffraction spikes. Contrast is high and there's a lot of detail.

Examples

As I've mentioned in my other reviews, my own vision is only fair (I'm near-sighted in both eyes and use glasses except when reading). When I look at the night sky:

  • On good nights I can only see three of the Pleiades. I have twice seen four. With averted vision the Pleiades are a big bright blur.
  • I have only once seen Mizar and Alcor as two stars, on an exceptional night. In all other cases, I've only see one star there.
  • I perceive the full Moon to be about 25 "pixels" across.
In the examples that follow, adjust your expectations for what you'd see based on how your vision compares to mine.

At a dark-sky site (mag 5-plus, in the "green" zone when looking at the "Clear Sky Clock" maps), on a night with good seeing:

  • With averted vision and tapping the tube I could see spiral arms in M33.
  • M110 was bright.
  • M31 had a dark lane (i.e., you could see the arm outside the dark lane). Most of the center of the galaxy fit in the field of view with a big 2-inch eyepiece!
  • M101 was visible with vague hints of structure.
  • M13 didn't quite resolve the outer stars.
  • Colored stars had great color.

On a different night, with medium seeing in a dark sky:

  • The two Leo galaxies (M65, M66) were clear spindles.
  • M35 and M41 were bright clusters.
  • Saturn showed that the seeing was only fair (too much wind, I guess; anyway, Cassini's Division wasn't visible)
  • I could see the Trapezium as four stars in M42.
  • M51 was a peanut, with one lobe smaller than the other and hints of arms in the bigger lobe.

In a little park near my house where there are several near-by houses and street-lights (mag 3-plus at best, in the "red" zone on the map) on a night with good seeing:

  • No difficulty splitting the near pair in iota Cas. and the two pairs in eta Lyr. (the "Double-double") at 80x or so.
  • I typically see two or three moons of Saturn and get hints of Cassini's Division. It takes a clear night with good seeing to see the division unambiguously, but I have seen it clearly in the OA-3.6, as well as banding on the planet itself.
  • During the opposition of fall 2005, Mars was great! I typically could see significant amounts of detail. Two examples from my notes:
    • October 2, 2005
      Mars was about 40 degrees up. It showed a lot of detail but you could see it waver as the air wiggled. It wasn't the "from orbit" view of perfect seeing. There was a single flash of light from the spot in Amazonis. Mare Sirenum showed up first, then the split details of Mare Erythraeum and Solis Lacus. Part of Mare Acicalium was visible on the preceding side and a tiny bit of Trivium Charontis on the followings side.
    • October 17, 2005
      I could go all the way up to 212x (12mm and 2.5 Powermate) and still get a good image (though the details were easier to see at lower powers). Color was visible in the planet up to 148x. Best of all, Mars just floated in my field of view because the platform did my tracking for me, so I could swap eyepieces and filters to squeeze the last little bit of detail out. I saw Syrtis Major first, then the arc from Sinus Meridiani to Mare Erythraeum. Utopia and Acidalium were visible in the "bottom" half (though Acidalium was faint). There was a small white South Polar Cap. The gap between Syrtis and Sinus was very sharp, as was the bottom edge just "above" Chryse. Mars did not look oblate tonight.

Head-to-Head Against Other Telescopes

I have compared the OA-3.6 side-by-side with three other telescopes:

  • A standard Newtonian of slightly greater aperture;
  • An APO of slightly smaller aperture;
  • An OA of much larger aperture.

Compared to a Newtonian of Similar Aperture

I compared my OA-3.6 to my 4.25-inch f/7.6 Newtonian (from "Stargazer Steve", and quite a good little telescope in my opinion: see my Cloudy Nights review here). They give about the same image brightness, but the OA doesn't have diffraction spikes, which is a definite plus point for it. Neither does it have a dim gray unfocused image of the secondary in the center of the Moon.

I also did some resolution comparisons on the crescent Moon, and the two showed about the same level of detail, though I think the images were nicer in the OA, if only because the two-speed focuser and the longer focal length allowed easier focusing and let me achieve a better focus. Being able to sit down while observing was nice, too.

The rule of thumb says a 4.25 with a 1-inch secondary is equivalent to a 3.25-inch unobstructed telescope, so the 3.6 would be expected to outperform it by a slight margin. I felt the OA had more than just a slight margin over the Newtonian.

The "Stargazer Steve" is considerably cheaper, as it is about one-third the price of the OA-3.6.

Compared to an APO of Similar Aperture

One question I've been asked is "Are the views really refractor-like?" They are.

A friend of mine has a TeleVue 85 on a good tripod. I've had the chance to do a head-to-head with the TV-85 twice, for about half an hour each time.

I believe the OA edged out the TV-85 with better resolution, brightness and color. Even the owner of the TV-85 is willing to say the OA does as well as the TV-85!

Ergonomically it's far superior: there's no tripod to set up and take down and the Dob doesn't wiggle in the wind the way the TV-85 did. While the TV-85's tube is shorter and thinner, the "whole kit" isn't any smaller once you add in the tripod and head.

Given that the complete OA set-up is half the price of the TV-85 optical tube alone, price-wise the OA gives you more bang for the buck.

Compared to a Bigger OA

I still have my OA-6.5. It's superior, as you'd expect. For example, I looked at Mars and Saturn in both telescopes. The 6.5 image was much brighter, sharper and more detailed. That night, Cassini's Division was obvious in the OA-6.5 and only suspected in the OA-3.6. Colors were better in the OA-6.5.

The OA-3.6 is astoundingly good, but the OA-6.5 clearly beats it on resolution and brightness. It's not a subtle difference. As an example, I could use the same eyepieces and get a better view in the OA-6.5. The differing focal lengths mean that the OA- 6.5's view has 70% more magnification. At the same magnification, the view is brighter and sharper in the OA-6.5.

That's what you'd expect, of course. At almost twice the mirror diameter you expect to get almost twice the resolution and nearly four times the brightness (almost two magnitudes!). When looking at faint objects, the extra light-gathering area of the OA-6.5 makes a big difference in what you can see.

On the other hand, the OA-3.6 is far easier to move around. I'd always thought of the OA-6.5 as easy to set up, but by comparison with the OA-3.6 it's big, bulky, awkward and heavy. Compared to the OA-6.5, the OA-3.6 is a true grab-n-go: it takes less time to set up than my Orion "Short-tube 80", as there's no tripod with legs to extend.

When I pick between the OA-6.5 and the OA-3.6, I pick the OA-3.6 about two-thirds of the time because I know it's easier to use.

Finally, the OA-6.5 is more than twice as expensive as the OA-3.6.

Missing Comparisons

I haven't had a chance to do a head-to-head comparison with a standard 6-inch Newtonian. The rule of thumb says it should perform at the same or slightly better level than the OA-3.6, and it would be cheaper even for the same quality of mirror and focuser.

A comparison with a 5- or 6-inch Maksutov would be interesting to do as well.

Why an OA?

The OAs are price-wise between standard Newtonians and apochromats. If you took the money an OA of a given size would cost and insted bought a standard Newtonian, you could get a much bigger telescope. Even when you apply the rule of thumb and subtract the size of the secondary mirror, in theory you'd still come out ahead.

For example, for the price of an OA-3.6, you could get an Orion XT-classic 12-inch and have change left over. Then if you really wanted unobstructed aperture, you could use a 4-inch off-axis mask on the 12-inch.

I've tried a 6-inch off-axis mask on a high-quality 16-inch (Night Sky structure, Meade mirror re-figured by Pegasus Optics, very high Strehl numbers), and I've compared it side-by-side to my OA-6.5 using Saturn as the target.

The images in the full 16-inch aperture were the brightest and the best. I suspect they would have even better if I'd magnified more or used a filter to reduce the brightness of the planet.

The next best image was in the OA. The four possible positions of the off-axis mask were not equal: three of them gave very good images, almost as good as in the OA, and the fourth gave an image with some blurring.

So why not just get a 16-inch instead of an OA-6.5, or a 10-inch instead of an OA-3.6? Here are my reasons:

  • The OA is far smaller than a masked big Newtonian. It thus requires less storage space, less car volume when being transported, is lighter when carried and easier to move. My OA-3.6 is a "grab-and-go" telescope which can be easily moved from place to place for different views. A masked 10-inch wouldn't be that moveable.
  • The smaller mirror in a relatively bigger tube cools down far faster than a big mirror. I'm in New England, and it often gets much colder as the night progresses. A big mirror might well cool so slowly that it could take several hours before the "warm" air plume from the mirror and the distortion it causes disappeared. This means the theoretical advantage of a bigger aperture would not be realized.
  • I really hate the defocused image of the secondary, which shows up as a gray blur in the middle of the field of view when you're looking at the Moon and other extended objects.
  • I don't like the diffraction spikes from the secondary spider in a standard Newtonian either.
  • The OAs are made with high-quality mirrors, beautifully-constructed bases of real wood and upgraded focusers. The correct comparison is not to a mass-market Newtonian, but to one of the top-quality Newtonians from one of the high-end vendors. When that comparison is made, much of the price differential disappears.
Don't misunderstand: I've had wonderful views through big Newtonians. I treasure the memory of a view of M51 I had some time ago in that high-quality 16-inch. It looked like one of Lord Ross's sketches. The big Dobs are very good at what they do well.

Note that the same argument could be made about high-end refractors: why not just spend the same amount of money and get a big Dob? And yet many people buy and enjoy APOs. I think some of the same considerations apply to OAs. Ultimately, it's your decision whether an APO, an OA or some other design represents value for money for you.

Conclusion

I keep saying and thinking the phrase "astoundingly good". This telescope seems to think it's a bigger aperture than it really is. It's got wonderful ergonomics as well. I like it.

If you're in the market for a grab-and-go high-quality telescope and you're considering an APO, you should also check out the OA-3.6 as well.


  • Max T and fanglei like this


0 Comments



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics