William Optics 45° Erecting Prism - 1.25"

Astronomical telescopes naturally produce an image that is upside-down and backward. While this doesn't matter for stars, it makes viewing terrestrial objects impossible to follow. The William Optics 45° Erecting Prism corrects the image to be both upright and normal left-to-right, making your telescope function like a high-power spotting scope.

This prism is primarily designed for refractors and Maksutov-Cassegrain telescopes. While it will fit in the 1.25" focuser of a Schmidt-Cassegrain (SCT) or Newtonian reflector, you may not have enough focuser travel to reach a sharp image. Refractors are generally the best match for this type of diagonal.

There are two key differences:

• Image Orientation: A 90° star diagonal corrects the up/down orientation but leaves the image mirrored left-to-right. This 45° prism corrects both axes, providing a fully correct image.
• Viewing Angle: The 45° angle is more comfortable for looking at objects on the horizon. The 90° angle is better for looking high into the night sky.

The brass compression ring is a soft metal ring inside the eyepiece holder that tightens evenly around your eyepiece barrel when you turn the thumbscrew. This provides a secure, centered grip without scratching or marring the eyepiece barrel, unlike a simple set screw that digs into one point.

Absolutely. An 80mm refractor paired with this erecting prism makes an excellent, high-magnification setup for birdwatching. The fully corrected image allows you to easily track birds, and the optical quality of the BAK7 prism ensures you'll see fine feather detail with bright, clear views.

Yes, all diagonals change the focus point of a telescope. The William Optics 45° prism will require you to move the focuser "out" (away from the telescope tube) to reach focus compared to viewing without a diagonal. Most refractors have ample focuser travel to accommodate this without any issue.