<https://stargazerslounge.com/topic/82057-low-profile-focuser/#comment-800615>
<http://schema.org/text>
"I've used: 12\" mirror f/5 12.5\" from diagonal to focal plane (7\" for tube, 5\" for focuser, 0.5\" from shoulder of focuser to focal plane -- that's pretty aggressive and will e.g. prevent the use of a Paracorr) The smallest \"sane\" size is the 2.6 inch mirror, but that's really borderline, with a very small fully illuminated field. 3.1 inch is a lot more comfortable. Get a low profile focuser with 2\" less height and then the 2.6\" mirror becomes fairly comfortable, with the 2.14\" mirror decidedly borderline. If you have a truss Dob you can consider making the UTA ring smaller than 14\" and mounting your focuser closer to the axis, but the 2.14\" diagonal is still not on the safe side. If you are planning a tube scope I wouldn't go there, because a 13\" tube for a 12\" mirror gives you tube currents in the light path with an almost 100% guarantee. In other words, if you want to keep your current secondary or go to a 3.1 inch then the current focuser is fine, but if you want to go to a 2.6 inch secondary then you'd better think hard about getting a lower profile focuser. If you want to go lower, your best bet is an Orion Optics 63mm secondary (roughly 2.4\"). But that will mean a lowish (but still normal) profile focuser and a UTA/tube size that isn't too liberally oversized, perhaps accepting some front vignetting for very wide fields (not a problem, as front vignetting is usually not that severe unless you take it too far, and it helps avoid the more severe vignetting by a small secondary). If you want to go to a 2.14\" secondary, then you need something really low profile like a Kineoptics HC-2, mounted so that the draw tube fully racked in just misses the light path of the least favourable object in your widest field eyepiece. \n"
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