ASA 0.72X 4" Cassegrain Reducer Corrector

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ASA 0.72X 4" Cassegrain Reducer Corrector
Item # A2-4CAKORRR
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  • The ASA 4" Cassegrain Reducer Corrector - 0.72X corrects for both coma and curvature of field.
  • This corrector provides a 4" aperture for use on a Cassegrain scope imaging system.
  • Optimized for CCD arrays which will fit within a circle of diameter 60mm. This includes (but is not limited to) the Kodak KAF-16803 format.
  • Optimized for a bandwidth of 400nm to >700nm.
  • OAL (over-all length) = approximately 90mm.
  • OD of lens cell = 122mm (4.8")
  • FFD = approximately 94mm = The location of CCD focal plane behind rear surface of cell.
  • Proprietary design by ASA physicist Philipp Keller.
  • A valuable asset for doing wide-field astrophotos with your classical Cassegrain scope.


ASA 0.72X 4" Cassegrain Reducer Corrector

The ASA 4" 0.72X Cassegrain Reducer Corrector will improve your images from your Cassegrain telescope, by eliminating both coma and field curvature problems! This corrector lens is designed for use on classical Cassegrain telescopes having an aperture of 0.4meter (15.75") or larger, and an f/number of f/8 or slower.

This Reducer Corrector provides a 4" aperture and it's optimized for full-format CCD arrays up to 36.8mm x 36.8mm (Kodak KAF-16803 CCD). This corrector can be used with detector arrays requiring somewhat larger circular coverage, however one would expect some minor loss of performance, and some vignetting at the edges.

Without a good corrector, any classical Cassegrain telescope will show substantial focal plane errors. This lens is optimized for a bandwidth of 400nm to >700nm, and every lens used in the corrector is measured to verify its surface radius & thickness, before it is used in an ASA corrector system. The results are pinpoint stars from the side all the way into the center of the image.

The 4CAKORRR reducer lens will reduce the inherent focal length by a factor of 0.72X. This lens has a field coverage of 60mm, which will more than cover the entire array of a Kodak KAF-16803 CCD detector. The back focus of approx. 90mm is sufficient to allow for using an off-axis guider or filter wheel with your detector system. Designed by ASA physicist Philipp Keller.

The pertinent dimensions for the 4" 0.72X Cassegrain Reducer Corrector by ASA are:

  • OD of lens cell barrel = 122mm (4.8")
  • Entrance aperture = 4"; suitable for use on an f/8 system or slower.
  • No rear flange is used in this design.
  • OAL = approximately 90mm
  • FFD = approximately 94mm
  • FFD (Flange focal distance) = Location of CCD plane behind "flange," in this case, the rear surface of lens cell.

Customer Note: This lens is a special-order item.

Additional Information

Additional Information

Manufacturer ASA
Barrel Size N/A
Warranty 2 Year Warranty
Included Items

Included Items

  • ASA 0.72X 4" Reducer Corrector for Cassegrain Telescope
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