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Starizona introduces a new 1.25" field flattener optimized for the Skywatcher EvoGuide 50ED and other short focal length apo refractors.

Short focal length refractors like the EvoGuide 50ED inherently have very strong field curvature resulting in elongated star images off axis when used with a camera. Our field flattener uses a 2-element multicoated optical design to correct the off-axis aberrations and produce sharp stars over a large field of view. Now the EvoGuide and other small refractors can be used as portable, wide-field imaging telescopes with great results.

The above image shows the EvoGuide 50ED without (left) and with (right) the field flattener in place with a ZWO ASI294 camera (23mm diagonal sensor)

The field flattener works with sensors up to APS size (27mm diagonal). This produces a 6.2-degree image circle on the EvoGuide 50ED. The 1.25" barrel will fit into the 1.25" helical focuser of the EvoGuide 50ED, but can also fit 2" focusers using a step down ring. The field flattener is threaded for standard 1.25" filters, allowing the use of light pollution or other imaging filters.

Image of the North America Nebula with EvoGuide 50ED with Field Flattener and Atik 490EX camera and Baader 7nm H-Alpha filter

On the camera side, the flattener has standard M42 T-threads. The nominal spacing from the flattener to the camera sensor depends on the focal length of the telescope being used (see table below). With the EvoGuide scope it is 34mm. For example, using a ZWO cooled camera with a backfocus of 17.5mm, you would need a 16.5mm spacer such as the one included with the ZWO camera. Contact us for help in choosing the right spacers or adapters for your setup.

Backfocus Table

Telescope Focal Length ----- Backfocus
250mm ----- 34mm
300mm ----- 28mm
350mm ----- 25mm
400mm ----- 23mm
450mm ----- 21mm

A tolerance of +/-2mm from these nominal values maintains the optical quality.

The focal length of the telescope remains the same with the flattener in place.


  • Optics: 2 elements, fully multicoated
  • Barrel Size: 1.25"
  • Filter Size: 1.25"
  • Image Circle: 27mm (APS-C)
  • Camera Threads: M42 T-threads
  • Overall Length: 1.6"
  • Overall Diameter: 2.0"
  • Weight: 0.18 lbs (82g)

View the instruction manual for this product