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Screws vs Threads

Why Most OAGs Are Connected with Screws Instead of Threads

Most Off-Axis Guiders (OAGs) are designed to be bolted to a filter wheel, a filter drawer or an adapter, on the camera side rather than attached via threaded connections. This is intentional and important for several reasons:

  1. Precise Prism Alignment

    • The prism inside the OAG must be carefully positioned relative to the primary sensor, capturing light from just outside the optical axis without casting a shadow on the image.

    • Bolting the OAG ensures that the prism is always in the correct orientation relative to the primary camera sensor.

    • Threaded connections rotate the OAG as you tighten, making the final prism position unpredictable.

  2. Mechanical Stability

    • Bolt connections create a rigid, flexure-free assembly between the OAG and the camera.

    • Threaded connections can introduce slight mechanical play or loosening over time, especially as temperature changes during a night of imaging.

  3. Repeatable and Reliable Setup

    • With bolts, you can always reinstall the OAG in the exact same position after disassembly or maintenance.

    • Threading cannot guarantee repeatable alignment, leading to variations in guiding performance.

  4. Optimized Backfocus Control

    • Bolting allows the OAG to sit in the correct optical train spacing without introducing unnecessary thread length, which could impact the required backfocus distance.

    • Many camera and filter wheel manufacturers design their mounting plates specifically to bolt OAGs at the correct optical spacing.

Note:

For flexibility, some OAGs include a 20mm adapter with an internal M48x0.75 thread, allowing you to connect the OAG to a camera with a threaded interface if needed.

However, for best performance, especially when guiding precision is critical, we recommend bolting the OAG directly whenever possible.

Always check the product information to determine whether a 20mm adapter/spacer is included.