Okay, today the last of the purchased optical components for this project arrived, and it has presented a problem on this project, which may in fact be an opportunity in disguise.
Ed Jones told me last month by PM that approximate distances for my 8' f/10.6 optical design are: primary to lenses 73.25", distance between lenses 0.45", and distance from lenses to focal plane, 11.57". Remember, the secondary will go between the primary and lenses. The larger the secondary, the shorter I can fold it.
Except for the lenses, which I got from Newport Lenses, all other optical components were obtained through CN classifieds. I opted for a Meade focuser salvaged from a 90 mm refractor since it was sturdy, cheap, and appeared to have an adequately long draw tube to allow me to set up either with or without the diagonal. That length expectation was more than met:
Now this focuser is a whopping 9.25"long! The dielectric diagonal, according to its literature, is 9 cm, or just over 3.5" of optical length. Together they add up to 12.75" approximately.
With Ed's rough figures, that puts both lenses slightly INSIDE the draw tube. That's the problem.
So here's the opportunity. I could trim an inch off of the draw tube easily, and another 3/4" if I drill and tap a couple of 4-40 or 2 mm holes to move the rack (I haven't checked screw sizes yet).
I could in fact make a cell for the tilted lenses that is attached to the draw tube, much like a Jones Bird has the negative lens in the drawtube. That would make alignment, distance, AND tilt easy to set with respect to the focal plane. If the first iteration is off a little, a replacement cell could be easily built. This would ride with drawtube inside a box that has the focuser at one end and the secondary at the other.
I could build that cell or 3D print it to attach to the 1.75" approximately draw tube, and that would keep it accurately distanced from the focal plane, which has a pretty tight tolerance anyway. This would mean I would have to always use the diagonal, but I'm okay with that. It is worth it to me for the improved ability to center and set the tilt and distance of the lenses.
Thinking ahead, if someone wanted to make Chief kits for standard optics, say, with Discovery Optics 8" f/8 primaries, a standardized cell to attach to a standard refractor focuser could be made for that prescription!
That would make an 8" chief only slightly more difficult to build than a dob!
What would we call such a correcting cell?
An "ASTIGMACORR", perhaps?
Edited by jtsenghas, 14 September 2019 - 02:13 PM.