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new build single pole 10" F6 dob.


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Hello All.

I have another mirror and will build a dobsonian. For a change, I am going for a single pole arm scope.

I think I always have too large a secondary, but this time it could be the opposite. The reason for the smaller secondary is it is from a 6" Newtonian.

The secondary is 42mm across, the primary 250mm F6 and the focuser will be about 100mm from the end of the primary mirror. Is this too small?

I can always change the mirror later, and I have just ordered a laser collimator, so I may try and mount it to bounce a beam to the primary mirror edge and see where it goes, always scientific.

Kind regards,


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My quick calculation suggests the secondary is a touch too small even at centre. The on-axis light cone of a 250mm F/6 mirror at 225mm from the focal point is 250x225/1500 = 44.12mm. To get the full use of the aperture of your scope at the centre of the FOV, you would need a 45-46mm at least. Just 45mm would be excellent for planetary observations, because a 18% central obstruction (by diameter = 3.24% by area) would go largely unnoticed in terms of the diffraction disk. Even a 50mm would be fine, yielding a 20% CO by diameter, 4% by area.

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