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Walking on the Moon

Refractor focus point

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Bit of a dumb question but I just can't grt my head around it. I have a cheap 70/400mm refractor and I can only get focus using a diagonal. I want to use it as a guide scope so will need focus without the diagonal. I am planning to replace the main tube, so will I have to increase the length of the tube to get correct focus?

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You probably have something like a Travel Scope 70? Weak focuser that you don't want to stress much? The solution is to use a 1.25" reducer. It will make the scope faster (great for guiding) and will bring the focus point closer so you won't need an extension.

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Example 1.25" reducer that I've tried: https://www.amazon.co.uk/Solomark-Photo-Visual-Reducer-Eyepieces-Nosepieces/dp/B016AN6HPM/

It is 0.5x at about 2.5cm from the sensor, less reduction (and less moving of the focal point) when closer, more when farther so you can tweak.

That review below the Amazon listing is actually mine ;)

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