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Equipment buying guides

Off-axis guiders — guiding through the main telescope

A small prism intercepts light near the field edge and sends it to a guide camera. The imaging camera records the central field.

Sharing the telescope reduces separate-guide-scope flexure problems. OAGs are common at long focal lengths and with moving-primary systems.

Imaging camera · Guide camera · OAG prism at the edge of the field123
  1. 1Imaging camera
  2. 2Guide camera
  3. 3OAG prism at the edge of the field
Functional schematic, not to scale. Rays and component shapes are simplified; this is not a particular model’s optical prescription.

What is it good for?

Accurate long-exposure tracking, particularly with SCTs, RCs and other demanding setups.

Limitations and trade-offs

The OAG must fit the backfocus budget and both cameras must reach focus. A prism inserted too far can shade the main sensor.

Check before buying

  • OAG thickness and required backfocus
  • Guide-camera field and sensitivity
  • Prism placement outside the imaging sensor area

Plan the complete camera–filters–OAG–corrector train before buying individual adapters.

Matching your setup

Start with mount setup and checking play. Enter the correct guide-scope or telescope focal length and camera pixel size. Calibrate according to the software and driver documentation; do not blindly copy another setup’s settings.

Judge the main camera’s images. Excellent graphs with elongated stars can indicate flexure, field rotation or an optical issue. Dithering shifts the frame between exposures and needs coordination between capture and guiding software.

Overly aggressive or frequent corrections may chase seeing. Match exposures and settings to mechanics and conditions. A more expensive guider cannot replace rigid mounting.

Documentation and further reading
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