DGM Optics Off-Axis Newtonian Telescopes
The Inspiration for the OA-Series
Thank you for your interest in DGM Optics™, OA-Series™ telescopes. The OA-Series™ telescopes are a result of 6 years of design and development with 4 years of field testing and comparison with a variety of commercial and amateur built instruments. They also represent the insights I’ve gained through 15 years of experience in the field of commercial optics as a Thin Film technician and as a Research and Development technician in the field of fiber optics.
Most astronomers are are familiar with the razor sharp, high contrast images offered by refractors, but also are aware of the very high cost of these scopes, particularly if they are the so-called apochromatic instruments. It is the freedom from obstruction in the light path that is a very significant reason for the superb resolution offered by refractors. Knowing that, the question I, and probably a million other astronomers, asked was; “Would an unobstructed reflector system have similar qualities?"
Analysis of Other Mirror Systems
There are a number of unobstructed reflector systems already in use, most notably the tilted component designs such as Schiefspieglers, Yolos, and a host of other unusual 3 and even 4 mirror systems. These designs do feature excellent images, however they can tend to be somewhat user-unfriendly, with corrective second, or even third elements which require extremely precise and often difficult alignment. They also utilize F/ ratios of 15 or greater, fine for lunar or planetary, but resulting in a very narrow field of view. Another approach is the use of an off-axis mask, but the primary mirror must be free of zones. In addition to that, the scope must be slightly realigned in order to direct the diffraction limited area to the center of the field of view. Also, this scenario still retains the big mirror thermal disadvantages.
Some the most popular designs in today's market are the compound element systems such as the Maksutovs and Schmidt-Cassegrains, and with good reason. They offer compact design and portability, which are strong selling points. However, if you do the simple transmission/reflectance equation I will present, you`ll find the typical system will have lost 15-20% of it`s effective optical throughput before it even reaches the focuser. If you own or are considering owning one of these instruments, determine the transmission or reflectance of all the optical elements in the system, then express the value(s) as decimals and multiply them. An example of the equation would be .98 x .95 x .95 x .95 = .84. So an 8-inch SCT ends up with a 16% optical throughput loss at the focal plane. This is assuming 4 elements with transmission/reflectance values as stated. Lesser values will rob the system of even more energy, and, the equation does not include losses caused by diffractive effects and central obstruction.
Why Choose The OA-Series Telescopes?
Telescope makers often claim optics that meet "diffraction limited" criteria. However, careful optical testing using laser interferometry reveals that many of these instruments do not meet that fundamental standard of optical quality.This is especially true of mass produced instruments. DGM Optics™ primary mirrors are produced by Dodgen Optical . Dodgen Optical has specilized for many years in space qualified, on and off axis, parabolic and hyperbolic optics, producing superb systems for a variety of government and commercial customers. Peak to valley surfaces are a minimum of 1/8 wave and each mirror`s figure is determined by a Zygo interferometer. Use of these highly sophisicated, extremely sensitive instruments, further ensures a smooth, consistent figure, and superb image quality from each OA-Series™ telescope. All optics are produced with fine, annealed, low expansion Pyrex glass.
Why then consider one of DGM Optics™, OA-Series™ telescopes? The answer is resolution! OA-Series™ scopes offer some of the finest optical images you will see with any telescope of any design! Images of the major planets will astound the observer in revealing the same detail as much larger instruments. Lunar detail is tack sharp, rekindling long lost interest in lunar viewing. But the OA-Series™ instruments are not limited to only superb, lunar, planetary, and double star work. The views of nebula, galaxies, and clusters are significantly enhanced due to a very dark, scatter-free background. The moderate F/ ratios (10 average) of the OA-Series™ telescopes allow the use of long focal length eyepieces and the attainable true field of view ranges from approximately 1.6 to 1.8 degrees.
Alignment and Materials Used for OA-Series Telescopes
Are the OA-Series™ telescopes difficult to align? Not at all, in fact OA-Series™ scopes are easier to align than classic Newtonians instruments and, are not tilted component designs and therefore do not require corrective secondary mirrors. Complete systems and tube assemblies are shipped with the secondary mirrors permanently aligned. The primary is then mounted in the tube assembly and is easily aligned in just a few minutes. Unlike traditional on-axis Newtonians, the secondary mirror is mounted on a rigid “L” shaped stalk and will maintain alignment even during the most abusive handling. If you need more information on alignment, Click Here. to view DGM Optic`s new online 'Use and Care"manual which contains a detailed tutorial on alignment and other detailed information about the OA-Series™ telescopes .
What about the mounted systems? The DGM Optics™ dobsonian mounts are constructed with a high quality, 5 ply Atlantic birch and triple coated with a clear polyurethane. Bearings are custom manufactured using UHMW (Ultra-High Molecular Weight Polyethylene), riding on Teflon, and feature very smooth vertical and horizontal motion. Then, to make viewing sessions as comfortable as possible, the tube of the OA-Series™ instruments are set in a hinged split yoke with an adjustable draw latch enabling the tube to be balanced and rotated for a “no neck strain” night of observation.
Tube Options
As for tube materials, DGM Optics™ offers ultra light Fiberglass tubes from Sky Valley Scopes. These beautiful tubes are available as Oak veneer wrapped Fiberglass, or painted Fiberglass with the color of your choice. These tubes are available for all apertures with the standard OA-4™ using schedule 20 PVC. The use of the these extremely light materials greatly enhances the portability and handling of the OA-Series™ instruments. Fiberglass also has a long standing reputation for durability in a variety of industrial uses.
The second tube option offered by DGM Optics™ are carbon fiber tubes by Pacific-Works Extremely light weight, these tubes also offer very high rigidity and are recommended for CCD camera work with the OA-Series™ telescopes.
DGM Optics™ also offers the OA-Series™ telescopes with tubes from California Tubular Designs You`ll find CTD tubes to be far from your ordinary run of the mill sonatubes, offering a variety of interesting and attractive cloth patterns wrapped around spiral bound wax permeated heavy wall cardboard tubing. Also, CTD tubes are availible in a hardwood wrapped veneer and, delivery time for these sturdy and surprisingly lightweight tubes is unsually under 4 weeks.
I sincerely believe the OA-Series™ telescopes are filling a long standing void in amateur astronomy by offering high resolution and contrast, a moderate field of view, without the high price tag of refractors. Factoring in the absence of a central obstruction, use of enhanced reflectance coatings, you`ll find the OA-Series™ telescopes outperform classic Newtonian instruments of considerably larger aperture. The image quality, construction, and other highlighted features of the OA-Series™ telescopes make them one of the best values available to the amateur astronomer today.
Sincerely,
Daniel McShane
President, DGM Optics
Tel. 978-874-2985 (7days a week)
P.O. Box 120 Westminster MA. 01473 USA
