I'm seeking comments, advice, thoughts on the following prospective equipment pairing:
TeleVue TV-85 refractor together with APM 2-inch Amici prism 90-deg diagonal, *possibly* using a Vernonscope Brandon Dakin Barlow lens (positioned between the telescope and the prism diagonal).
I presently own the APM 2-inch Amici prism diagonal, which is specified as:
--46mm clear aperture --113mm focal length
I'm interested in possibly buying the 2-inch 2.4X Dakin Barlow most specifically to extend the utility/magnification options for three new 2-inch Masuyama eyepieces I have obtained. all are 85 degree AFOV:
32mm (20mm eye-relief), 26mm (15mm eye-relief), 20mm (12mm eye-relief)
I have considered simply utilizing a TeleVue 2-inch 2X Powermate between the diagonal and the eyepieces, but like the idea of going "minimal glass" with a simple 2-element Barlow such as the Dakin, and also like the idea of having the Barlow between the scope and the diagonal, thereby not having a big Powermate and 2" eyepiece towering up vertically from the diagonal.
Here are my concerns and questions:
(1) might the TV-85 have trouble mechanically reaching focus with the 2-inch Dakin Barlow between the scope and the prism diagonal?
(2) should the 2-inch Dakin Barlow work adequately
/properly feeding into my 2" APM Amici diagonal?
(3) Will the Dakin Barlow likely have adequate clear aperture to avoid Vignetting in my longer focal length 85-Degree AFOV 2-inch Masuyama eyepieces?
(4) I'm assuming that the expected increase in eye-relief via Barlow usage will likely not be a problem, given that my longest focal length Masuyama eyepiece is 32mm (with a 20mm nominal eye relief specification). Am I on track here?
(5) I assume the Brandon 2-inch 2.4X Dakin Barlow is rated at 2.4X when used in the position between the scope and diagonal? ( The documentation for this Barlow online is minimal.)
So... any input or thoughts would be much appreciated, particularly from anyone who has experience with using a Brandon 2-inch 2.4X Dakin Barlow in a TV-85 (F/7) or similar focal F-Ratio small APO scope.
If the Dakin appears likely problematic for my application, I may just end up going with a 2" 2X TV Powermate, as I'm fairly confident I'd have no trouble reaching focus, or avoiding vignetting with my 85 degree AFOV 2-inch eyepieces.
Edited by Thomas_M44, 18 February 2020 - 02:11 PM.