DGM Optics OA-4.0


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Brand and Model:DGM Optics OA-4.0
Price ($USD):$745.00
Type:Unobstructed Newtonian
Attributes: un-checked Go-To un-checked PEC
Aperture:98mm (3.9")
f Ratio:f/10.3
Focal Length:1010mm
Finder:not included
Electric Power:n/a
Mount:dobsonian
Tripod:dobsonian
Weight (lbs):22 lbs.
Dimensions (w/h/d):OTA= 43.5 x 6.25"
Description:Unobstructed reflector on a dobsonian mount. Mount features a hinged, split wood, yoke that can be quickly released and rotated and balanced for a comfortable view of any portion of the sky.

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DGM Optics OA-4.0
For those who are not already aware, the DGM 'scopes are Newtonian 'scopes that use off-axis parabolic mirrors whose focal axis is not coincident with center axis of the primary mirror, permitting the secondary to be placed out of the incoming light path. This yields the same unobstructed image as a refractor, is inherently apochromatic (as all Newts are), and is much cheaper to produce than apochromatic lens systems. In short, you get all the performance of an apochromat at a fraction of the cost. (Nearly all because good refractor lenses tend to transmit more light, while mirrors scatter a bitÖ)

Firstly, let it be said that Dan McShane offers first-rate customer service. He is very helpful, and very quick to respond to questions both before and after purchase, both by e-mail and by phone.

I purchased my OA-4 in late October 2001, and received the 'scope December 16 2001, a little quicker than normal, I hear.

The OTA and mount are first rate in craftsmanship. The split-yoke mount is very simple and extremely effective in allowing balance and rotation of the OTA. It's shocking that no other tube-dob makers use the same idea.

The primary mirror mount is very simple, easily installed/removed. and collimates with a wonderful buttery-smooth feel, due to its nylon construction. The secondary is collimateable, but is more or less permanently placed, negating the need for collimation, and making it difficult if it does end up needing tweeking (you have to bend the support bracket with pliersÖ mine didn't require collimation of the secondary). The focuser on my OTA is a Lumicon 1.25" helical, functions wonderfully, and has very tight tolerances on the threading.. I wasn't sure I'd like a helical focuser, but found the accuracy of the focus adjustment to be superior to rack and pinion and most Crayford focusers I've tried. (Hint: slide eyepiece barrel in and out of the focuser for coarse focus, tighten the eyepiece set screw, then use the helical focuser threads for fine focusÖ) The PVC OTA is very black inside, and painted very nicely (red, in my case) on the outside.

The 'scope and mount are easily transported, owing to the 4' tube length. The mount is a little taller than many other dob mounts, as the center of gravity of the tube is very near the middle, requiring extra rocker height, while negating the need for springs or counter weights. While this does make the mount a little heavier and bulkier than other dob mounts, it is superior to conterweighting, in my estimation. Altitude and
Azimuth motions are the smoothest of any dob I've tried (I've tried a lot!). Tracking is a pleasure. Anti-vibration pads make an already steady mount rock-steady.

Daylight collimation is very easy. The final star-collimation is a touch tedious, but absolutely necessary for the 'scope to deliver worthwhile images. It takes a bit of practice, but is very easy once the process is understood and repeated a few times. (Hint-- Use Polaris!)

I find the 'scope takes 20-30 minutes to equalize before delivering sharp images (in freezing weather, going from my warm apartment), and performs relatively poorly until the tube currents disappear (the converging off axis light cone hugs the side of the tube, where the warm air flows the most). The final star collimation can not be done until the currents are gone, as the diffraction rings are too distorted until such time. Touching the side of the tube radiates heat through the PVC and into the light cone. This is evident when focusing in cold weather, but clears immediately when the hand is removed. Most of the thermal issues should nearly disappear in warmer weather.

Once cooled and well collimated, the 'scope delivers absolutely crisp images, very easily to 200x, more on nights of better seeing. Contrast is outstanding. Star testing shows the best intra and extra focal patterns I've ever seen. I think the test patterns indicate a very, very, very slight amount of under-correction (just a very, very subtle dimming of the extra-focal rings), and Dan told me the mirror had a slight zone, though I was not able to detect this. Still, it's nearly the best diffraction pattern I've seen, and I test every 'scope I look through (only the 5" Starfire had slightly better patternsÖ). Jovian moons consistently resolve into little disks of varying size (though I must admit they are probably not resolving into the actual disks as much as Airy disks of varying size), Jovain banding nearly always shows festoons, and lots of layering. Saturn always shows the Casini division, the C ring (faintly), banding, and several moons cast about on a deep black sky. Zeta Orionis splits quite easily and cleanly, each star getting it's own disk and diffraction rings, while showing the blue-white and yellow of the respective components very well. The trapezium shows as 4 distinct stars even at low powers, and another OA-4 owner mentioned resolving all six of the brightest components, though I have not done so myself. Open clusters are a joy to behold; saphires on black velvet, with the occasional red or yellow gem thrown in for good measure! M81 and M82 showed surprising detail for the aperture.

At first glance, $750 (mounted) seems like too much for a 4" Newt. Look through one, and you won't feel that way, especially when you pit the price/performance against the cost of an apochromat. I can offer no negatives on the 'scope, and can recommend it without hesitation, though I'd have to say I think the OA-5.5 or 7 would be a superlative purchase if finances permit!

Overall Rating: 10
Optics:10 Mount:10 Ease of Use:10 Value:10
Weight: 10 (Trustworthy Vote)
Date:
By: YrrekR
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=75691


DGM Optics OA-4.0
OK here we go. This is only the third scope I have ever looked through and I did alot of reading before I got this scope. It seem to be the best value I could find. This was important for me since I am a single father of three going back to school at 36. I ordered the DGM 4.0 over the internet and mailed a check to Dan McShane. Later I got a call on my cell phone from Dan himself asking if I wanted to buy a different configuration and generally just talking about what I would be using the scope for and my interest. This was very nice and helped me get over the reservations I had about buying a scope before I could look through it and compare it with other scopes. After about 2 months he emailed me telling me that the scope was finished and about 2 weeks after that I mailed him the rest of the money. The first thing to arrive was the OTA minus the primary. The secondary was mounted in the very black interior of the PVC tube and it was very loose on the verge of falling out of it's mount. This caused me considerable trouble when doing my initial alignment the next week when the mount and primary arrived. The reason was that proper alignment of the secondary is very dependant upon the distance of the secondary from the primary and normally it is set permanently before it arrives. I tightened it down without thinking much about it until later when doing star test and getting some false color from veiwing bright planets. This was not the fault of the scope however but UPS and Dan was very helpful over the phone and email. I have since worked out most of the problems and am a better astronomer because of it. The only other problem I had was that the tube cradle would slowly slip as I was observing near zenith. Dan was very concerned about this report and was willing to replace the mount but this was not a big deal to me as I wrapped a peice of thin poster board around the tube where the cradle held it and all was well.
Now to the good stuff. The new friction band on the altitude wheels allows me to use any combination of eyepeices and barlow and change them at very high magnification with out worry of the tube moving. Balance is easy to change and the movement is silky smooth in all directions. I am basically a novice and I could track Saturn at 510x, yes 510x, with this scope by nudging the planet slightly out of the feild of view and enjoying the jumbo sized image pass across my feild of view. The damping time is just under 3 seconds and actually comes in handy when jiggling the view for close doubles ect. On the first night of good seeing I was doing things as a novice with this scope that I didn't think I could. I split Rigel very easily mostly because there are no difraction spikes. The reflection nebuli around some of the stars in the Pleadies were strikingly apparent, and the three components of Castor were just easy. Saturn was all there with 4 or 5 moons. The Moon was better than any picture I have ever seen at 70x very 3d and absolutely no false color...creamy white with charcoal black shadows. I also cranked to power up to 510x with a shorty barlow and sure it was fuzzy and hard to focus but not all of the time! every once in a while the casinni division would pop into view and so would cloud bands and ring shadows any fault lies with the atmosphere more than the optics.
I use a fold out canvas camping chair to observe on and I can carry it and the scope with mount at the same time over short distances. Once you align this scope it is just a matter of learning how to starhop to use it. If you are new and you want to get into amature astronomy without learning to starhop well......after playing with this scope I think that is alot like wanting to swim without wanting to get wet. With a low power 30x eyepeice I can just point this scope at a random spot in a dark sky and just go wow. Less than a minute to set up and 30min to come to ambient temperature I leave it in the garage so 0min for me. I think maybe my optics are a bit spherical because I still have a bit of color on Jupiter but this might be my eyepeices because I don't have the most expensive lot and the color seems to be linked to where the image is in the feild of view. However the color is very slight and only apparent on Jupiter. When I read post about people having trouble finding Rigels companion in 8" scopes of lesser quality I wonder if maybe I am just a bit too picky about optics having not seen too many examples, for Rigels companion was very obvious to me wiht this scope at 80x. The workmanship is top notch, very nice woodwork. I like the helical focuser especially for star testing, good idea to put denser lube on it after some use though.
Accessories needed: You will want a cover for the end of the tube to keep dust off the primary, get a nice spotting scope, and high quality 35mm or so wide feild occular, and nice 12.5mm or so and get a UO 7-4mm ortho for the planets and for star tests. If you want a barlow make it a shorty because the secondary is right next to the focuser. My favorite targets for this scope are
Low power open star clusters...... WOW
Globluar clusters...................very nice
The Moon better than a 3d video game.
M42 ........................................cool.
The planets ........................very nice.
Binaries performs to theoritical limits.
Things I still need to work on
Galixies and dim fuzzies......maybe a bit much for 4" they are there but not over whelming..next scope big Dob! Glad I got this one first though.
Clear Skies from Oklahoma.

Overall Rating: 10
Optics:10 Mount:9 Ease of Use:10 Value:10
Weight: 6 (Veritable Vote)
Date:
By: shane_sweeney
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=175329

Articles like this make life so much sipmelr.
>OK here we go.......agian.
This is a follow up. I have gotten to know my scope now and have learned its strenghts and weaknesses. Now that I have gotten good at aligning this thing the veiws are incredible. In the last year I have learned how to "see" and how to tweak my optics. I have resigned to the fact that it is nearly impossible to keep your optics clean and that it really doesn't matter very much as long as your secondary mirror has been wiped down with alcohol and cotton.

This scope can do anything a apo refractor can do and the color is perfect. It scatters the light a very very slight bit more than a refractor so you lose .1 magnitude off of theoritical. Resolution on doubles is to the limits. 

Strenghts
Planetary views. Cloud turbulence on Saturn 5 moons visable.  
Double and multiple systems. Rigel very easy. "E" component of Theta Ori. Zeta Ori wide and clear at x252.
Globular Clusters.
High power airy discs are text book   "o" dim stellar components pixey dust. 

Weaknesses
Delicate.
High maintainence. but hey I like to tweak...

DGM Optics OA-4.0
I readily rate the OA-4 all tens. You won't get better marks than that at an Olympic skating scandal! I wish there was an entry for service, too (rating: ten). DGM provides outstanding personal attention and quality service, a rarity in this day and age. Considering the OA-4's modest price, the total package is a steal and, as the DGM website might claim, must constitute as one of the best values available in amateur astronomy today.

True, you could buy a large dob from one of the mass manufacturers for about the same price, but this scope is geared more to aficionados of small, high performance telescopes with truly flawless optics. The light weight and ease of use promote frequent use at a moment's notice, while the quality optics readily satisfy the senses.

If you are looking for a small scope in the 4" to 6" range where ease of use and quality optics are paramount, from what I've read the OA-4 is competitive with anything out there. If you are considering any scope for under a thousand dollars, the OA-4 should figure at the top of your list, even if you have to adjust some of your priorities. This thing is special.

Money being no object, I'd plunk down thousands of dollars for a premium 4" APO with mount just for recreational viewing. (I'd also tool up a 12.5" dob for the nights when I want to get really serious.) Having other priorities at this point in my life (like food, warmth, and a 401K), I looked for more affordable alternatives.

Reentering the hobby after many years of absence, I considered my options carefully. I decided I wanted a smaller, quality scope to promote ready use rather than give in to aperture feaver. If my interest sticks and grows, I'll buy a second, more specialized scope for whatever my primary interest turns out to be. Nevertheless, I'll still keep the smaller scope for quick set up nights.

In fact, I've noticed that most experienced observers have a second, smaller telescope on hand regardless of the size and expense of their main instrument. Often enough, this second (or third) telescope is the first one they reach for when they head out the door. So, I figured it made sense to buy the second scope first and worry about the details later.

I considered Maks (Orion StarMax 127 and INTES MK67), quality achromat reflectors (TAL-100R), and traditional reflectors (TAL-2M). The OA-4 with included dob mount had just the right combination of light weight, low cost, elegant design, and premium optics to win me over. If I eventually wanted tracking, I'd buy a nice mount like the Vixen GP or the Losmandy GM-8. (You could get away with a less expensive mount for such a light telescope, but the OA-4's optics practically demand and deserve a top quality mount.)

While the scope won't break down into a suitcase like a short focus, tripod mounted refractor, the convenience factor for everyday use might be just as good, if not better. I tuck the scope away, fully assembled, in the corner of a small closet that houses all my astronomical equipment. At maybe twenty-seven pounds including the lightweight Stellarvue finder I added, the OA-4 is effortless to pick up, move around, and position in the backyard. It's also easy to pack into a car for a quick jaunt into the country. As long as you don't require airline portability, the OA-4 is the perfect embodiment of the small, portable telescope. (While initially attracted to airline portability, I realized that the last time I actually flew to a dark sky site was--never. So, why pay the premium for even smaller size?)

The birch ply mount with brass hardware is a work of art. Its motions are silky smooth and supply a tangible, quality feel to the most mundane tracking activity. When I showed my wife pictures of the dob on the web, she was initially unimpressed (a dob scope is, after all, a telescope on a box). When she saw the scope fully assembled in our living room, she exclaimed how beautiful it was, commenting on the flawless woodwork.

The OTA is available in a range of materials, some of which you can order with your choice of tube color. Somewhat frugal, I figured the handsome, painted PVC would be good enough and it is the least expensive option. Appropriate to PVC, you'll see minor (and irrelevant) imperfections if you look hard enough. It isn't always smooth everywhere you look and the tube ends aren't absolutely, perfectly concentric (anyway, the tube is oversized to fit the off-axis secondary).

An admitted perfectionist, I was surprised that none of this bothered me. The tube's glosses reminded me of the hand stitching on a quality leather watch strap. The minor variations give it a quality, hand crafted feel. I can't entirely quantify it, but you know just from looking at this thing that it has to possesses excellent optics. (Anyway, you can always order fancier tube materials, if you want.) While the die-cast Takahashi crowd could no doubt accuse me of waxing poetic here, this modest scope is a minimalist excuse to house world-class optics with as little fuss as possible. It's simple, elegant, and handsome. I love it.

I'm not going to comment much on the most important thing of all: optics. When the mirror was ready, DGM's proprietor, Dan McShane, assured me after testing the telescope that the star test was perfect (Dan tests each scope prior to delivery, another plus to doing business with DGM). While I'm a newbie at star testing, I'll verify that the star test is indeed perfect as far as I can tell, perfectly concentric and identical circles in and out of focus. Using my starter set of four UO orthos, stars viewed are tight little balls of jeweled light at any magnification (40X to 252X). Viewing anything within reach of this diminutive telescope's aperture is a visual treat. Writing to Dan McShane, I told him that the OA-4 is like fine wine on the visual palette.

While I think the OA-4 would inherently appeal most to experienced users with an appreciation for top flight optics, I think beginners should consider this scope, too. If you are in the market for a small first scope, you might want to pick up an OA-4 just to know what perfect star images are supposed to look like. Of course, you'll never be able to tolerate mediocre optics in other, larger scopes after owning one of DGM's dobs. So, padlock your bank account if you're worried about the price of eventually upgrading to a larger scope!

You'll have to wait a few months after ordering, but like many of the finer things in life, it's worth the wait. In any case, Ed Ting, the well respected telescope reviewer, compared the OA-4 to a world-class 4" apochromatic refractor. When Mr. Ting cites that there is little to choose between the OA-4 and the spectacular Astrophysics Traveler, you can remind yourself that most folks now wait more than three years for the Traveler.

Overall Rating: 10
Optics:10 Mount:10 Ease of Use:10 Value:10
Weight: 3 (Unreliable Vote)
Date:
By: Anonymous (xxx.xxx.50.11)
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=131331


DGM Optics OA-4.0
becca - ahhhhh!! i wnetad to enter, and didn't see that is was time! my little guy needs 18 month photo's sooo bad! he didn't have 1 yr photo's. i'm awful! well congrats to the winner, and i'll have to get in touch with you to schedule a sitting!

Overall Rating: 8
Optics:3 Mount:9 Ease of Use:0 Value:2
Weight: 1 (Unreliable Vote)
Date:
By: Anonymous (xxx.xxx.1.28)
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=600583


DGM Optics OA-4.0
My OA-4 arrived after waiting three and a half month. Mr.Dan Mcshane, owner of DGM Optics, gave me a discount of $30 because its aperture is 95.5mm(2.5mm less than stated on his web page spec). The shipping to Japan was $200.
The seeing in Japanese winter is so terrible because of the constant strong west wind, but I find myself on the porch with my OA-4 whenever it clears up at night. This is my first newtonian reflector scope, and I've been into astronomy after nearly 30 years of absence, so I at first expected I would have some difficulty in collimating it. Fortunately, the expectation has turned out to be the opposite. Reading the "Use and Care Manual" on his web page, the assembly and collimation took me just half an hour. With a 4mm Ortho(yealding 255X), I noticed color differences inside the NEB and SEB on Jupiter. Even though Saturn was relatively low in the west I was able to see the Cassini Division. I'm amazed that even at this magnitude, which is claimed as the maximum on their web site, the image shows no breakdown at all. Lunar work is beyond description. even my wife, who usually shows little interest in stars, is amazed and lost for words.
I live in a light-polluted area and I rarely have chances to enjoy other than planets and the moon. However, with comparatively low magnitude, the stars are pinpoint sharp to the edge, and show contrasty images. No comparison with my former scope, a 90mm Mak Cassegrain.
I really look forward to Mars opposition in August.

Overall Rating: 10
Optics:10 Mount:10 Ease of Use:10 Value:10
Weight: 1 (Unreliable Vote)
Date:
By: Anonymous (xxx.xxx.188.91)
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=198186


DGM Optics OA-4.0
For visual use, this scope is equivalent or slightly superior to a 4" APO refractor.

Overall Rating: 10
Optics:10 Mount:9 Ease of Use:10 Value:10
Weight: 1 (Unreliable Vote)
Date:
By: Anonymous (xxx.xxx.104.164)
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=57204


DGM Optics OA-4.0
I have owned the DGM Optics OA-4.0 unobstructed reflector OTA for several months. I wanted to wait to take it through the paces before writing a satisfactory review. First, let me say that Dan McShane provides first-rate products, service, and technical support. Also, this scope is everything Dan claims it to be on his website. This, however, is my personal view and opinion on the scope.

I ordered the scope as an OTA only, as I wanted to mount it on my CG5 so I could have the benefit of a motor-driven equatorial system. Therefore, I cannot comment on the DGM mounting systems. The scope arrived very well packaged; tube in one box and mirror in another. Installation of the mirror is simple with three bolts. Dan provides installation and collimation instructions on the DGM Optics website. I wonít get into the procedure here, but I will say that, once you get the hang of it, itís easier to align than a conventional Newtonian. Also, once collimated it holds up well. I ordered my OTA in PVC (other tube materials are available) painted glossy black, but Dan will paint the tube just about any color the buyer wants. Finding mounting rings the right size was tough. I wound up buying a pair of Vixen 6.25Ē diameter rings (the tube diameter is a little over 6Ē). To make up the difference, I lined the inside of the rings with weather stripping foam rubber and then felt for a nice snug fit. The OTA does not come with a finder, so I mounted a Russian-made 8 X 50 which was nice optically but very large and heavy which led to some problems balance problems. The PVC is very light in weight, and the mirror is light also. The majority of the weight was up front of the tube with the focuser, eyepiece(s) and finder. I attached a weight bar to the lower end of the tube opposite the focuser and finder to offset this, but the OTA would not hold itís place when in a vertical position and would tend to fall in one direction or another due to the uneven weight distribution. This problem was no fault of the telescope, but simply due to my choice of a finder. I have since replaced the finder with a lighter 6 X 30 model that works wonderfully. I kept the weight bar, but reduced its weight to compensate. My OTA has the standard helical focuser. I believe a Crayford is optional. The focuser housing is metal, and the threaded parts are plastic. I didnít like the feel of the focuser initially, because the threads were not tight enough, and some play was evident. I coated the threads with wheel-bearing grease very lightly. Wheel bearing grease will not break down, so there is no risk of it dripping onto the secondary. This modification tightened up the threads nicely, and also made the focusing buttery smooth. The eyepiece can be raised and lowered within the focuser housing for rough focusing, and then turned clockwise or counter-clockwise for fine focusing. Iím not a huge fan of helical focusers, but this one is satisfactory. Also, this design does not allow the use of a standard rack and pinion focuser, as it must be low-profile. The OTA is simple but attractive in appearance, and at first glance looks very much like a standard 6 inch Newtonian. The optics on this scope are perfect. Period. I know something about star testing, but donít claim to be an expert. However, I have owned perhaps thirty telescopes in the last 25 years and no star test I have seen has come close to this. Some tweaking of the primary mirror adjustments needs to be done during the initial star test to remove any traces of astigmatism in the pattern, and this is explained on the DGM website. Once this is done, the star test, as far as I can tell, is perfect. Inside and outside of focus diffraction patterns are absolutely identical. There is no trace of spherical aberration. In-focus star images are tight little balls like they are supposed to be. Textbook Airy disks at high power. Star colors are impressive, as when observing Albireo. Red stars look red, orange stars like Arcturus look orange, etc. Rigelís companion is easily visible at low power. The double-double separates easily. Vega is pure white. Absolutely pure. Of course, there is no trace of chromatic aberration. The moon is incredible. Compared to a conventional 4 inch Chinese achromatic refractor, the difference is laughable. The OA-4.0 exhibits contrast and resolution that is nothing short of extraordinary, giving the lunar features a somewhat surreal three-dimensional quality. The surrounding sky is absolutely black in stark contrast to the lunar surface. My wife (never particularly interested in astronomy), when viewing the moon at 200 power through this scope, exclaimed: ďHoly sh_ _! Iíve never seen anything like this! It looks like a picture!Ē The true test of any scope is when a non-astronomer is impressed to that degree. Mars, this past spring during opposition (2001) was a disappointment to many, as it was too low in the sky for good viewing, but the scope showed the polar caps, some dark markings (maria), and the pretty and characteristic orange-red of Mars. Jupiter is another story. The moons look like moons and not stars. The bands are too numerous to count, and the color variations from brown to tan to pale white are evident. Very much like the image seen in a world-class refractor, except for zero false color. Saturn is a dream. The Cassini Division is sharp and clear and the Crepe ring is easily visible. The Enke may be visible through this scope, but probably under near-ideal conditions and high power. I wonít comment much on deep sky viewing, as it is not my primary interest. Globular clusters do look very nice and resolve well, however. Keep in mind that this is a 4-inch scope, and any four-inch scope is limited by its aperture. The OA-4.0 is not really designed to be a deep-space scope, but DGM Optics does offer larger aperture models that I would guess are outstanding for galaxies and nebulae. This scope will, in my opinion, give any other four-inch scope of any design a run for its money optically. Are there any drawbacks? A few. The scope seems somewhat susceptible to heat distortion from the observerís body. Hovering too close to the eyepiece area for too long can cause some heat waves and ripples in the viewing field. Perhaps this is inherent in the design as the secondary is very close to the eyepiece and therefore the observer. Adjusting the primary mirror while viewing will have the same effect, but this only needs to be done rarely anyway. In any event, simply backing away from the scope for a minute clears up the problem. Another drawback is that if you order one of these (all are built to your order), you are in for a bit of a wait, perhaps a few months. Good optics, however, are not easy to come by and take time to produce. I, myself, would rather wait some time for a nice instrument than have a piece of junk tomorrow. Patience, remember, is a virtue. Finally, and this is simply my personal preference, is the fact that one needs to be standing when viewing through this scope when equatorially-mounted. This would be true for any equatorially-mounted Newtonian and is in no way unique to this design. I just happen to prefer sitting when viewing; thatís why I like refractors so much. However, if you want a refractor that will give the same optical performance as this scope, youíre going to have to go the fluorite route and that translates to paying three to five times (or more) the cost of this OTA. Nobody ever said that this world of ours is perfect, and this also applies to telescopic designs when observing other, perhaps even less-perfect worlds. You just simply canít have it all in any one scope, but the OA-4.0 gives it a hell of a try. Dan McShane, the owner of DGM Optics, is a very nice fellow and is quite knowledgeable about his products. He is great to talk to and is very open and honest, and this is quite refreshing when you consider some of the astronomical equipment dealers that are out there. If you are looking for a scope thatís a little different in design, first-rate optically, and reasonable priced, I would highly recommend the OA-4.0.

Overall Rating: 10
Weight: 1 (Unreliable Vote)
Date:
By: Anonymous (xxx.xxx.200.146)
Link to this vote: http://www.excelsis.com/1.0/displayvote.php?voteid=41381


DGM Optics OA-4.0
When I first went hunting for a new telescope, I was living in Chicago, bright lights, big city and very few stars. I wanted an instrument that was relatively portable, but I did not care if I could fit it in my back pocket. It seemed that my choice was limited to a small dobsonian, a small refractor, or something like a Celestron C5. Unfortunately, the quality of the images in small Cats and dobsonians was only fair. The huge central obstruction of the C5 (and also the ETX, which was not available in a 5inch at that time) smeared
planetary detail. Likewise the level of optical and mechanical quality of small dobsonians is not too good. Every time I looked through one of these instruments I longed for the views I had experienced while observing with apochromatic refractors at the local Astronomy club, and the school's observatory. But the tube assemblies of these instruments cost more than the car I owned at the time. So what is a poor astronomer to do? To show my hand early, look no further than the DGM OA-Series. The views are identical to those of equal aperture apos. There are only two qualifications. First, if you are interested in film-astrophotography, the F/10 instruments are not for you, second, if you drive to your observing site in a BMW station wagon, and want a short focus instrument that fits in your Louis Vitton gym bag, stick with a Takahashi or Astro-Physics

What is it like?

I don't want to belabor the technical description because the company's website offers all the information you need. Instead, let me confirm some of Dan McShane's claims about the scopes appearance and mechanical functioning. The scope arrived in two shipping boxes, and easily traveled UPS. When I unpacked the mount, I was amazed at its level of fit and finish This is not your grandma's Dob. The baltic birch and brass hardware are rugged, elegant and sturdy, and the finished product looks worlds better than the commercial fiberboard or plywood stuff that sells for roughly the same price. Unpacking and setting up the tube assembly proved equally rewarding. The tube is very black on the inside. The mirror cell is configured for easy alignment with wingnuts, and has large holes for adequate ventilation. The secondary is fixed, and never needs alignment. The alignment procedure itself could not be easier, requiring nothing more than a peep-sight made of a plastic film-can. The whole scope sets up m a minute or two. Once I had the tube on the mount, I quickly realized the virtues of the yoke which hold the tube. The tube can be balanced and rotated in an instant, and the brass tension fitting that holds the yoke together is foolproof. One of the problems with many dobsonians is that the motions are rather jerky, making any adjustments at high-power frustrating. The OA-4 I purchased was completely smooth in both altitude and azimuth. Overall, mechanical construction of this the DGM mount and tube assembly is first rate. My only minor quibble is with the helical focuser. While it is far better than the plastic rack and pinion models offered on other similarly priced units, it does not mach the silky smooth machined marvels that come with refractors in the optical class of the OA-Series. Nonetheless, the design of the telescope requires an extremely low profile unit, so even crayfords are out. I have heard that Dan now offers a wood veneer tube. Coupled with his fantastic mounting, I can easily imagine the scope at home in a living room as well as under the stars.

The Sky, at Last

Though I purchased the scope in Chicago, which allows mainly for planetary and double star observation, I soon moved to lowa, under darker skies. I have also taken my OA-4 to Nebraska My in-laws live near the site of the Nebraska Star party, where the reports of naked eye limiting magnitude of 7 are not unrealistic. I use various eyepieces in this telescope: a 6mm Unitron ortho, a 7mm Pentax SMC-ED ortho. 11, 17 mm and 32mm Televue plossls, and a 25mm Zeiss. I use the Pentax for most of my planetary observations and the l7mm and 25mm for general purposes.

The following notes and opinions have been compiled over a two year period. I have also directly compared the OA-4 to 5 and 8 inch Celestrons, a 102mm Zeiss AS, a Televue Genesis, and 6 inch Meade Starfinder. For what its worth, I have been an amateur astronomer for 15 years, so while not an old-timer, I am not a newbie either. Most reviews of telescopes would have the reader believe that there are only two types of objects in the sky, Planets, and fainter objects, like galaxies and diffusion nebulae. In the end, Sky and Telescope always concludes that aperture wins out every time, with 6 inch maksutov newtonians edging out 5inch apos, and 8inch SCT edging out both. The planets are better resolved in larger scopes, and faint objects are easier to spot, and appreciate in detail. In two ways this is only partially correct, as I hope to describe

The Moon and Planets

First, the high level of contrast available from the OA-4 allows me to consistently pick out features on Jupiter that are hard to find when seeing is less than perfect, thereby acting against the larger apertures. Jupiter's 4 main moons are consistently resolved into disks under very good seeing. I have observed numerous transit shadows, and even the transits themselves. Seeing the giant planet's disk so starkly resolved against a black sky is a sight to behold. Lesser telescopes smear the edges, and suffer from glare, false color and, some fuzziness. I remember comparing the OA-4 to a C5 recently. The views of Jupiter and Saturn were worse in every way through the Cat. In other words, the Celestron rendered the images duller, less resolved, and to my surprise, dimmer. When I first got the telescope, I was able to compare it with an older Televue Genesis. It was difficult to compare the planets precisely because of the vastly different focal lengths, but a view of the limb of the Moon told the story the Genesis displayed some color, while the OA-4 was flawless.

Fuzzy objects

I shouldn't have to say that if you want to go around chasing obscure planetary nebulae, you should get a much larger telescope. On the other hand, the black background against which faint objects stand out in the OA-4 makes it easy to pick out galaxies that are challenging in a scope of this size. The high contrast also makes it possible to see a wealth of detail in brighter objects such as M31 and the Orion Nebula. During my last trip to Nebraska, the ring nebula was both a ring and oblong. The oval shape was seen by all the observers present. This is very impressive for a 4inch scope.
The view I get from a dark site consistently outperforms 6 inch reflectors, of lesser optical quality. I have never compared the OA-4 directly to a top class newtonian of larger, but close aperture, such as a Takahashi MT 130. I remember being impressed with the Takahashi, but also remember a 4 inch apo edging that scope out on the planets.

Stars, Lots of Stars

Telescope reviews seldom include globular and open clusters. If you think that a slightly larger obstructed telescope will provide better views of either of these objects, you are headed for disappointment. Under dark skies, the OA-4 resolves M13 to its core at high power. The star images in the OA series are simply so tight, that although a 4inch scope gets dim at over 100X, the stars are all apparent. M3 and M5 are also a real treat. I have found myself doing something I have never done with another telescope spending long periods at the eyepiece studying globular clusters. On the flip-side, open clusters through the 32mm plossl are splendid. The OA-4 has a wide enough field to take in the whole Double Cluster with this ep and the stars look like jewels hanging in front of black velvet. A telescope with a 30% central obstruction simply cannot pull this off. As an added bonus, the lack of chromatism insures that colored double stars are always true.

Concluding Remarks

While I thought that Ed Ting's review of the OA-5.5 was justifiably positive, I think he leaves the reader with the mistaken impression that DGM telescopes are in some way specialized instruments. I think that anyone considering a telescope under $1000, should seriously think about purchasing an OA-4. (If you want to spend more, you should probably buy one of DGM's larger instruments) Every night I use it, I continue to be impressed by its optical performance, quality construction, and incredible ease of set up. I can think of no other telescope that combines reasonable cost, astounding planetary views, wide diffraction limited field of view, and diverse capabilities as a deep-sky performer. Following is some specific advice, recreating my thoughts about what telescope to buy:

1) If you are considering an ETX or C5, ask yourself whether you want to make such tremendous sacrifices in optical quality relative to the OA-4. You do gain portability and GOTO capabilities. As for the later, who wants to hop around the sky driven by the
endless buzzing of cheap computer controlled mounts looking at mediocre images.

2) If you are thinking of a high-quality 70-80mm Apo, ask yourself whether you want to pay so much for portability and fancy construction. The OA-4 equaled and in one sense outperformed a Genesis. Do you really think that a smaller ED or Flourite instrument will fare any better?

3) If you are thinking of a 4inch Achromat, don't. The only ones that are any good are no longer available and are expensive used. The cheaper ones tend to come on almost useless equatorial mounts that add significant weight to the system. If you want an equatorially mounted telescope, buy the optical tube assembly from DGM then by or build a good mount.

4) If you are considering an 8inch dobsonian from Orion, Meade, Celestron, you need access to dark skies on a very regular basis. While it is true that less aperture is not a cure for bad skies, more mediocre aperture does not help anything. These telescopes are bigger, less well made optically and mechanically, and only provide better views of faint deep sky objects. Although I have read on occasion that an 8inch f/5 scopes outperform 4 and 5 inch apochromats on the planets, I have never actually seen this happen.

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