Quote:does having the front aperture smaller than customary affect the image at the focal plane
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Quote: Quote:does having the front aperture smaller than customary affect the image at the focal plane it brings it closer to the secondary. this enables the use of a smaller secondary. smaller co = better contrast.
Quote:If one is using a tight upper ring, then the off-axis illumination will be vignetted by the upper cage. One can also use a smaller secondary with larger upper ring and accept a similar amount of vignetting.
Quote:But in a scope large enough that trusses and struts are desirable, I am thinking 15 inches, contrast on the planets is almost entirely a function of seeing and thermal management.Jon
Crazy ATM 6" F9.4 Newtonian first light July 2011 8" F/5.6 Dob, first light June 2003 12.5" F/5 Dob, first light Aug 2007, as well as 9- 4" f/6 mirrors. In progress:12" f/7 on quartz, 18" F/5 on BVC, and a set of 3-8" optical flats All optics by yours truly.
Quote:What surprised me is that the amount of vignetting isn't similar (according to Highe).
Quote: Quote:What surprised me is that the amount of vignetting isn't similar (according to Highe). Making a drawing should make this quite obvious - the secondary is much smaller and more affected than the upper tube opening. Even worse is vignetting by an overly tall and narrow focuser drawtube - possible but not common I hope.So, if making the tube opening minimal saves significant space, go ahead, but at least see to it that the optical axis is centered, by offsetting the secondary properly (down the tube as always, but also away from the focuser, as often omitted if the tube is wide). Nils Olof
Quote:Check out this web page for a clear explanation. Of course, this is for truss scopes, where tube currents aren't a problem.http://www.bbastrodesigns.com/SmallerUpperCage/SmallerUpperCage.html
Quote: I will indeed do my best to address optical centering.