Planewave 20" Ritchey Chretien OTA with Fused Silica Upgrade

  • Telescope tube weight: 140 lbs.
  • Focal length: 3454 mm (136 inches)
  • Focal ratio: f/6.8
  • Aperture: 508 mm (20 inches)
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  • Regular price $50,500.00
  • $48,500.00
  • You save: $2,000.00 (3%)

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Planewave 20" Ritchey Chretien OTA with Fused Silica Upgrade

The PlaneWave Instruments PW20-RC is a 20 inch (0.51 m) f/6.8 Ritchey Chretien (RC) Astrograph telescope OTA. The PW20-RC has a dual carbon fiber truss design, with 3 cooling fans ejecting air from the back of the telescope to quickly cool the primary mirror and keep its temperature stabilized. The instrument weight is 140 lb (64 kg) and it comes standard with a back plate retaining ring ready to accept the focuser of your choice.

PlaneWave's astrographs have become the telescopes of choice for universities, industry, and astrophotographers around the world, and now PlaneWave offers the PW-RC series to meet the custom and critical performance needs of the scientific community.

In certain applications such as photometry and spectroscopy, refractive lenses could easily influence scientific data results by altering the spectral content of the acquired data. This could make it difficult to quantify the data. The Planewave 20 inch RC as offered here in its native format (no field flattening lenses are used) uses only a primary and secondary mirror with reflective coatings. This can be useful when making scientific observations primarily on-axis as is the case in photometry and spectroscopy, and when wide field imaging is not a concern.

This model comes with the Fused Silica Mirror upgrade. Fused silica, also known as fused quartz, is a synthetic amorphous silica glass of the highest purity and one of the most transparent glasses made. The optical and thermal properties of fused silica are superior to other types of glass due to its purity. Its transmission and homogeneity exceed those of crystalline quartz without the problems of temperature instability inherent in the crystalline form. Fused Silica has a coefficient of thermal expansion six times lower than Borosilicate (Pyrex) glass, which means that as fused silica cools down, it preserves its shape to a high degree of accuracy. This translates into consistent optical performance and unchanging focus over temperature changes. With its high melting temperature (about 1600 degrees Celsius), a very low coefficient of thermal expansion and resistance to thermal shock, fused silica is the material of choice for professional observatories as well as various scientific applications.

Ritchey Chretien or RC optics made by PlaneWave will provide excellent on-axis performance using only a hyperbolic primary and secondary mirror with no refractive optics in the light path. (An optional field correcting lens group can be added to improve off-axis performance should this feature be desired.)

The carbon fiber truss design of the PW20-RC Ritchey Chretien (RC) Astrograph telescope by PlaneWave will minimize problems of thermal expansion to prevent focus shift with changes in ambient temperature.

Special dovetail expansion joints accommodate the difference in thermal expansion between carbon fiber and aluminum materials. The expansion joint allows the aluminum dovetail to expand and contract without stressing the carbon fiber lower truss.

Three cooling fans in the PlaneWave Instruments PW20-RC 20 inch (0.51 m) f/6.8 Ritchey Chretien Astrograph blow air out the back of the optical tube, pulling air through the telescope and past the primary mirror. This helps the telescope to reach thermal equilibrium quickly. The fans are controlled by a computer if the optional Electronic Focus Accessory (EFA Kit) is purchased. The PW20-RC RC Astrograph Optical Tube Assembly includes a Ronchi Ocular and Ronchi Spacer. These devices are used to set the correct primary to secondary spacing. The telescope also includes a primary mirror cover.

This Ritchey Chretien Astrograph model PW20-RC by Planewave is the perfect choice of telescope for a college or university observatory seeking an economical yet high-performance optical system for teaching astronomy courses while also doing publishable astronomical research work. It is also an ideal scope for the avid amateur astronomer who will be doing research such as photometry or spectroscopy, or astrometry over a narrow field of view. It will be great for visual astronomy too because of its high-quality imagery for nearly on-axis objects.

Note: This product is an OTA only and does not include any mount or other accessories such as a finder or eyepieces. The product does include a special Ronchi Ocular and Ronchi Spacer.

PlaneWave 20 inch Ritchey Chretien Astrograph OTA Specifications:

  • Optical Design: Ritchey Chretien Reflector
  • Central Obstruction: 39% of the Primary Mirror Diameter
  • Back Focus from Racked in Focuser: 5.8 inch(147 mm)
  • OTA Length: 47 inch (1,194 mm)
  • Upper Cage: Carbon Fiber Truss
  • Lower Cage: Carbon Fiber Truss with Carbon Fiber Light Shroud
  • Primary Mirror Aperture: 20 inch (508 mm)
  • Primary Mirror Focal ratio: f/3
  • Primary Mounting: Laser Collimated and Permanently Fixed
  • Primary Material: Precision Annealed Borosilicate
  • Primary Shape: Hyperbolic
  • Primary Coating: Aluminum with Broadband AR Coatings (less than 0.5% reflected from 400 nm to 700 nm)
  • Secondary Mirror Diameter: 7.5 inches (191 mm)
  • Secondary Mirror Material: Precision Annealed Borosilicate
  • Secondary Mirror Shape: Hyperbolic
  • Secondary Mirror Coating: Aluminum with Broadband AR Coatings


Shipping Information for PlaneWave Instruments PW20-RC 20 inch Ritchey Chretien Astrograph

  • Crated Shipping Weight: 291 lb (132.0 kg)
  • Crate Width: 32 inches (813 mm)
  • Crate Height: 36 inches (914 mm)
  • Crate Length: 59 inches (1,499 mm)


A member of our staff will contact you regarding shipping options. Current wait time for this telescope is approximately 4-5 months.


Aperture508 mm (20")
Back Focus147mm
Dawes Limit0.03 arcseconds
Focal Length3454 mm
Focal Ratiof/6.8
Highest Magnification1016x
Light Gathering Power5267x
Limiting Magnitude17.3
Optical DesignCorrected Dall Kirkham
Tube Length1194mm
Tube Weight140 lbs