ASA 20" f/3.8 NG Truss Newtonian Astrograph OTA - No Focuser or Accessories- Discontinued

ASA 20" f/3.8 NG Truss Newtonian Astrograph OTA - No Focuser or Accessories- Discontinued   Discontinued
Item # A2-ASA20NGO3
  • The ASA 120NG 20" F/3.8 Astrograph OTA produces the highest-quality imagery, and it's capable of working at multiple focal lengths.
  • Features a 500mm f/3.8 flat-field low-expansion Suprax mirror, tested and guaranteed to meet high specifications.
  • The ASA 16N Astrograph OTA tube uses a true sandwiched carbon fiber tube assembly with an open truss design for lighter weight.
  • Includes an interface port for a 3" focuser; focuser unit not included with this product.
  • Supports all ASA correctors, providing you superb flexibility when it comes to focal length.
  • The 20" Astrograph model 20NG features a super-thin, sturdy double-vane spider to reduce diffraction spike, and produces the finest stellar spot sizes in currently available fast telescopes.


ASA 20" f/3.8 NG Truss Newtonian Astrograph OTA Only

This 20" F/3.8 NG Astrograph Open Truss Optical Tube Assembly (OTA) made by ASA is the perfect starting point for the serious astronomer who wishes to configure a custom system for doing astrophotography. The OTA is the basis of a fast, wide-field imaging telescope system, which will provide the highest quality imagery and be capable of working at multiple focal lengths.

This astrograph OTA has an aperture of 20", a focal length of 1800mm, a natural focal ratio of f/3.8 and comes with a Newtonian output port compatible with 3" focusers. This 500mm F/3.8 NG Astrograph OTA by ASA will accept the 3" model A2-OK3Z electronic motorized focuser, which is sold separately. This ASA OTA does not come with any accessories so that you can create a custom telescope that is perfect for your needs.

The 20" NG ASA astrograph tube assembly is manufactured of sandwiched carbon fiber. Carbon fiber technology adds to the astrograph's visual appeal, and the open truss tube design, coupled with the use of carbon fiber, makes this telescope quite a bit lighter. Thanks to the thermal stability of carbon fiber, your focus remains stable over a long period of time. The tube assembly's stability means the optics remain collimated longer. Speaking of collimation, the primary mirror of the ASA 20NG Astrograph includes a center triangle reference mark for ease of collimation. The internal surface of each tube assembly is also treated with black velvet, to reduce scattered light.

ASA astrographs all have a fast optical system (small f/#). You can achieve good signal-to-noise ratio with your CCD using short exposure times. The N-series astrographs all have a natural focal ratio of f/3.8. Their system f/# can be reduced or extended easily by the user, so the natural focal length of the device can be shortened by a factor of 0.75x or increased by a factor of 1.8x, while retaining the highest-quality imagery. You can enjoy all three telescope focal lengths, simply by using each of the three appropriate corrector lenses. For example, by installing the 3" Wynne Corrector lens you achieve a focal ratio of f/3.6. With the 2" (or 3") Reducer, a focal ratio of f/2.75 results. Using the ASA Barlow Flattener Corrector, the focal ratio will be f/6.8. The field corrector lenses will handle the largest CCD arrays currently popular, because all of the ASA correctors provide sufficient backfocus (flange focal distance).

ASA astrographs are sold in two different models: The N-series use a parabolic primary mirror, while the H-series use a hyperbolic primary. This product uses a parabolic primary mirror. ASA primary mirrors are all verified to meet stringent specs by interferometric testing, prior to their assembly into a telescope! Testing has shown they produce the finest spot sizes available in currently-available fast telescopes. Their uncompromising optical design by Philipp Keller, Dipl. Physics, (Germany), and use of the finest materials available, yields imaging performance covering a large field of view, a fast f/#, ultra-stable construction, yet they are light weight. The new "double thin spider", now supplied with the ASA N-Series astrographs, is rigid while significantly reducing diffraction spikes from bright stars, normally caused by the spider! ASA's astrograph product line is an astrophotographer's dream come true!

This ASA Newtonian Astrograph is a special order item and requires a 20% deposit when you place your order, and the balance will be required before the telescope leaves Austria. This deposit is fully refundable as long as you cancel your order before the telescope is imported. Please email OPT or give one of our Telescope experts a call at 1-800-483-6287.

Note to customer: The image shows the 20" NG Truss telescope with some optional accessories.

Additional Information

Additional Information

Manufacturer ASA
Telescope Series ASA Open Truss
Optical Design Astrograph
Mount Type None - Optical Tube Only
Warranty 2 Year Warranty
Telescope Aperture 20"
Telescope Focal Length 1800mm
Telescope Focal Ratio f/3.8
Tube Color or Finish Carbon Fiber
Dew Shield Included? No, Not Necessary
Finder Included None
Included Items

Included Items

  • ASA 20" f/3.8 20NG Newtonian Astrograph Truss OTA
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