Quote:
Originally Posted by PhilW
One reason I'm aware of is that tubes need to be oversized for thermal reasons, so that currents along the edge of the tube are out of the optical path. This has the unintented consequence of moving the focuser further away from the secondary. For example, a 10" dob needs a 12" tube. With a truss you don't have this thermal problem, so the UTA can be just slightly wider than the mirror
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Phil is 100% correct.
Because of the thermal issues the tube needs to be a larger diameter for a given aperture than is needed for the diameter of the UCA for the truss design. This means the focuser is closer to the secondary with the truss design, allowing a smaller secondary to be used.
Using the 10" scope as an example to allow proper air flow and alleviate tube currents the tube needs to be at least 12" in diameter. This is the tube size used on most factory made 10" tube scopes. With a 10" truss dob the UCA can be 10.75" in diameter. This may not sound a lot but it allows you to come down one size in secondary mirror and this becomes important in getting the central obstruction under 20%, which is the cutoff in essentially unaffected views. It becomes more critical as the F-ratio of the scope gets faster, which is the modern trend. Using a 10"/F5 newtonian as the example, it is exceedingly difficult to design the scope and get away with a secondary smaller than 2.14" for a 21.4% obstruction. A 10"/F5 truss scope can easily be designed with a 1.83" secondary for an 18.3% obstruction. This is only an issue for visual observing but it is what is needed to get high quality high contrast views from a newtonian. FWIW the Chinese/Taiwanese 10"/F5 dobs have about a 25% central obstruction, which will reduce contrast compared to a premium truss dob with a secondary obstruction under 20%. For photography it doesn't matter, nor does the thickness of the spider vanes.
Cheers,
John B