Gavin R. Putland,  BE PhD

Monday, July 30, 2012 (Comment)

Diagrams for ‘Barlow-Newtonian and relay-Newtonian’

My article “Barlow-Newtonian and relay-Newtonian: Minimizing central obstruction and tube length” was posted to the Amateur Telescope Making group (atm_free) on July 15. The abstract is as follows.

The central obstruction and tube length of a Newtonian reflector can be reduced (for the same aperture and unvignetted angular field) by inserting a Barlow lens in the diagonal mirror holder, between the primary and secondary mirrors [Fig.1]. Alternatively, mounting a positive relay lens at the inner end of the focusing tube [Fig.2] allows a greater reduction in the central obstruction, elimination of vignetting due to the binoviewer (if there is one), and the option of a sub-aperture Schmidt corrector; but a relay lens introduces more chromatic aberration than a Barlow lens.

Because Fig.1 and Fig.2 appeared with reduced resolution on the group archive, I republish them below.

Fig. 1:

Fig. 2:
Relay-Newtonian. On-axis rays in blue. Off-axis rays in red.

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