Addendum - Astro Hardware

Ronchi Eypieces

updated: 2024-04-12

Ronchi eyepiece - evaluate the optical quality of your telescope with a star
By means of easily to interprete stripe patterns, you can see how good your telescope is.

  •  High quality grating in 1.25" nose piece
  •  On a plano-parallel glass substrate, 10 lines per mm are etched in. This leads to a high testing precision.
  •  Easily interpretable - if the stripes are straight and parallel, everything is okay (see picture on the left)
  •  By interpreting deviations, the optical errors can be determined
  •  Also ideal to find the focal plane
  • Helps also to correct the mirror distance as in RC-Telescopes:

Spherical Overcorrection

Perfect Optics

Spherical Undercorrection







Mirrors are too close together

Parabolic mirrors may have a slight spherical Overcorrection, as many coma correctors introduce spherical Undercorrection. Both errors might erase each other out.

Mirror distance is good

Mirrors are too far apart

Same as in Spherical Overcorrection but with changed prefix! 
Very bad contrast when observing planets!

Lots of fast optics for astrophotography show a slight spherical abberation. Due to the scale of the images that doesn't matter too much. (You're far away from the theoretical limits of your optics!) Lot's of coma correctors introduce some spherical undercorrection.

See also: Correcting the mirror distance 

Documentation:  ronchi-manual.pdf (

Suppliers:         Teleskop Express

               Ronchi Eyepiece (Gerd Neumann)

Price:                 ~ 50 EUR

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