Regarding the 75mm for the Paracorr - you are probably right, I did not physically look at the Paracorr and was just assuming the draw tube was the limiting length, sound like the the Parcorr can be considered in isolation. In which case my math in post #54 is probably in error? Don, could you check whether I have the Paracorr intercept math correct - I do not know the correct way to take it into account, hence my motivation for starting this thread.
No I really cannot explain the Paracorr math, I am simply presenting my current attempt to get it correct. I am still not certain that it is.
Regarding Suiter, thans for reminding me, I will have to consult him. But right now I am traveling and will have to go back home to get to the book.
The 75mm length of the inserted Paracorr is longer than a drawtube with 1.5" travel, but shorter than a drawtube with 2" travel.
Since the Paracorr significantly reduces the necessity for focuser travel, a 1.5" travel drawtube is just fine.
[this also yields the ability to reach inside the tube and thread a filter onto the Paracorr without removing anything from the focuser.]
The upper assembly is 49mm long. The Paracorr is raised from setting A to setting E to put the focal plane of the Paracorred scope at the lip of the Paracorr, or 0.4" = 10.16mm.
That means the length of the Paracorr, when the focal plane is exactly at the opening, is 134.16mm without an adapter. TeleVue rounds it off to 135mm.
Incorporated into the 134.16mm Paracorr length is the focuser board inset, the focuser board thickness, the focuser height.
The Paracorr's bottom, in focus, should not intrude into the light path, which is the diameter of the mirror plus (approximately) the field stop of the lowest power eyepiece.
With an eyepiece of 41mm field stop, that would be a 20.5mm clearance around the mirror, with the bottom of the Paracorr at the edge of that (ignoring offset or myopia adjustments).
I think the point of earlier posts in the thread was that the 41mm field stop effectively becomes a 35.65mm field stop after the Paracorr, meaning the clearance between
Paracorr and mirror can be reduced by 5.35mm. Note how close that is to the allowance for myopia, and I think it's safer to just continue with the 20.5mm clearance between mirror and UTA inner diameter.
That makes the distance to the focal plane at:
mirror radius + 20.5mm + 134.16mm. For the 20" mirror, that's 408.66mm (16.09")
You might want to allow for a trace more for the offset of the secondary mirror (assuming you'll offset it), and a "fudge factor" for any sloppiness in construction.
Now, if we look at it a different way, and put the focal plane of the scope WITHOUT Paracorr at, say, 20mm above the racked in focuser (to allow for infocus of the Paracorr and myopia adjustment), then the intercept distance is:
mirror radius + 20.5mm clearance + inset of focuser board + focuser board thickness + focuser height + 20mm.
I estimate 358mm (roughly), which would indicate a smaller secondary.
It is the Paracorr which moves the focal plane outward, so the second set of dimensions will still yield the first set when the Paracorr is inserted.
However, as Tommm pointed out, it is using the new focal plane distance that is goofing you up. There is nothing wrong with using the second set of dimensions to figure the size of the secondary.
And if that says 3.8" is enough (the size of a 4" in a holder), then, by all means, go with the 4" secondary.
Edited by Starman1, 27 March 2019 - 02:39 PM.