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Why Can I Sometimes See the Secondary Support Structure When Observing With a Reflecting Telescope?

-- Vince | July 19, 2020

Question:

Sorry – this maybe a dumb question – BUT – I just bought a reflective telescope – and sometimes when looking through the viewfinder i see the crosshairs or the X of the bars holding the secondary mirror in place – but other times I don’t. When i have an object fully in view – I don’t see the X, but when I’m looking to find items is when i notice it. I hope that makes sense, and sorry that this is likely wasting your time.

-- Vince

Answer:

I believe that what you are seeing is your eye sometimes focusing on the primary focal point, where your secondary mirror is located, rather than the secondary focus of your telescope. I suspect that you may notice this when you position your eye further away from the eyepiece of your telescope than when you observing normally.

-- Jeff Mangum

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