PawPaw, Some information and background on your Jaegers 5" F/5 RFT:
The original article about building one of these occupied ten pages of the little Fawcett 1960 How-to book No. 454 by Lloyd Mallan. The original specifications called for a 5"O.D. aluminum tube 21" in length. This used the big 38mm (1.5") eyepiece from the WWII Sherman tank main gun aiming scope (M71). Mallan describes cementing the heavy eyepiece onto a cardboard (gasp!) tube to slide into the scope. The cardboard appears to be cemented onto a modified (enlarged) 1.25" adapter for the big Erfle! This eyepiece was available from Jaegers early on as No. 1E1405. The eyepiece was just as removed from the tank scope and included a multiple start threaded helical diopter focus with about 11 mm of focus travel. This appears to be the eyepiece you have. See 1967 catalog photo of this. Later, Jaegers remounted the same surplus eyepiece lenses in a non-focusing anodized housing, that one is No. 1E3205.
Both versions of that eyepiece can be used with Jaegers 2.7" R&P No. 36C3081"Super Eyepiece Mount". That is the focuser you have. Since the eyepiece was from WWII, I suspect the focuser was designed specifically to accommodate the 2.7" diameter of the older M71 eyepiece attachment section. This section of the eyepiece was threaded 32 TPI, but is just a slip fit into the recessed top of the "Super Focuser" tube, where it is secured with the focuser thumb screw. The later, remounted anodized non-focusable version of this eyepiece was kept the same diameter, but slipped deeper into the Super Focuser by about 5/8", so would require an even longer 5" main tube to reach infinity focus than yours does.
My first 5" F/5 RFT build in the early 1970's used the super focuser, but I cut the tube to 20" so a home-made diagonal using a large RA prism could be used for viewing near zenith.
I took some time today to make measurements on another Jaegers 5" F/5 RFT focused at infinity that may help out. It has a tube length of 19", including .8" pushed into the back of the Jaegers objective cell. I measured the distance from the plane of the end of the tube (butted against the focuser flange) to the end of the 2.7" R&P tube holding the eyepiece. With the original military diopter housing set to its lowest, the back of the focuser tube was just 6" from the plane of the back end of the 5" tube. Repeating with the remounted No. 1E3205 1.5" Erfle (which slides 5/8" deeper into the focuser tube), the focuser end to plane of the back of the 5" tube was an unsurprising 6 5/8".
By the way, this scope was never available finished from Jaegers or even as a "kit" of the nature of the U.O 80 mm Kit Refractor. Rather, Jaegers took a cue from the listing in the Mallan article and specified in the catalog the part names, individual part numbers, and prices so the ATM could order those as needed. Additional parts like screws and glare stops were not available from Jaegers, but needed to be sourced or built from materials available locally. I'll include photos of those from 1967 and 1970 catalogs- note change in eyepiece designation and addition of the "Super Focuser.
Though I cannot find a "No. 2754" printed on the Jaegers provided instruction booklet I have, I think what Jaegers provided was just a Mallan reprint (My copy says "Reprinted through the courtesy of AMATEUR ASTRONOMY HANDBOOK Copyright 1960, Fawcett Publications, Inc.) augmented with a dimensioned diagram and listing of eyepieces and accessories that will work with this scope. I'll attach those latter two. Note that they are the "revised" version, likely referring to their use of the 2.7" "Super Focuser" rather than earlier focus schemes.
Since the Milky Way is at zenith in the summer, I personally find having a right angle diagonal view is important. Another good use of the Jaegers 5" F/5 is for nice terrestrial views under twilight conditions, so an erect and correct image can be an advantage. (Better for North America Nebula, too!) Shortening the tube to allow for a diagonal mirror or Amici prism is worthwhile, though not very traditional. To gain this convenience and close focus versatility without focuser tube vignetting, my most recent Jaegers 5" F/5 builds have been with the LSDF style focuser using a sliding built-in-the-tube big diagonal mirror or Amici prism. That style is unusual looking and a bit labor intensive to build, but can pretty well come to focus with any eyepiece you want to use, even at fairly close distances. -Bill
Edited by BillB9430, 11 February 2022 - 07:58 AM.