GSO 1.25" 0.5x Focal Reducer

Our Price: $30.00
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Our Product ID: OFR-GS-GSRD
Manufacturer SKU: GSRD
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GSO 1.25" 0.5x Focal Reducer

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Quick Overview

  • 0.5x focal reducer effectively halves the focal length of a telescope, enabling lower magnification, a wider field of view, and shorter exposure times for astrophotography
  • Threads into eyepieces, camera adapters, and other accessories with standard 1.25" threads, and can be stacked with additional 1.25" threaded accessories
  • Fully multi-coated optics held in a black-anodized aluminum filter cell
  • Can be used with additional spacers (available separately) to achieve further focal reduction
  • Intended for use with with longer-focal-ratio telescopes and eyepieces of narrow to moderately-wide apparent fields of view

Product Details

GSO's popular 1.25" 0.5x focal reducer screws on to any accessory such as eyepieces, camera adapters, etc. that accepts standard 1.25" threaded filters and provides a nominal reduction of 0.5x. Use of this focal reducer effectively halves the focal ratio of your telescope, resulting in a wider field of view and significantly shortened exposure times for astrophotography. For example, use of this reducer will convert an f/10 telescope into an f/5 scope.

The 2-element reducer features fully multi-coated optics in a black-anodized aluminum filter cell. It also has additional 1.25" female threads so you can screw/stack additional 1.25" filters on to this reducer. This focal reducer will work well for visual use too when coupled with the right kind of eyepieces and telescopes; it is not recommended for very fast scopes or very wide angle eyepieces for visual applications.

Note that the use of any optical element like barlows and focal reducers changes the focus point of your telescope, requiring additional in-focus or out-focus travel. The exact amount of focus travel required will depend on the optics and mechanics of your specific set-up (telescope, diagonal (if used), barlow/reducer, and eyepiece). Hence, be aware that you may not be able to reach focus with all eyepieces or in all configurations with a given barlow/reducer. In our tests, when screwed into an eyepiece, this focal reducer required approximately 8mm of additional in-focus travel so please make sure that your focuser can accommodate this travel requirement.


  • Focal length (estimated) = 101 - 103mm (3.98 - 4.06")
  • Total item length (including external thread) = 13mm (0.51")
  • Total item length (excluding external thread) = 10mm (0.39")
  • Clear aperture = 22mm (0.87")
  • Weight = 0.5 oz.
  • Made in Taiwan

Varying the Reduction Factor of a Focal Reducer

The actual reduction will vary a little over or under 0.5x depending on exactly where the focal reducer is placed in the optical path. The exact focal length specification of this reducer has not been provided by the manufacturer, but we estimate it to be approximately 101-103mm. Hence, the "working distance" of this specific reducer is estimated to be 51mm, i.e., this reducer will provide a reduction of 0.5x when the center of its lens assembly is placed 51mm from the focal plane of the eyepiece or imaging device. Move the focal reducer closer to the eyepiece/camera and its reduction factor will increase. Move it away and the reduction provided will decrease.

By using additional spacers or extension tubes (sold separately), you can vary the amount of reduction you obtain with this reducer as follows:

Extra Spacer Length
Nominal Reduction Provided
0" / 0mm
0.25" / 6.4mm
0.5" / 12.7mm
0.75" / 19.1mm
1" / 25.4mm

However, do not be tempted to buy only the longest spacer tube to get the most reduction. A focal reducer will provide good results as long as it is placed "close" to its working distance. For example, it is possible that a 0.5" spacer will provide good results, a 0.75" spacer will provide acceptable results, and a 1" spacer might be unacceptable for your telescope/eyepiece combination. You will usually have to determine this by trial and error for your set-up, so you might want to consider buying more than one spacer.

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