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in-line barlow?

Started by yahganlang, 08/08/2005 12:13PM
Posted 08/08/2005 12:13PM Opening Post
As a continuation of my exploration of what one might be able to do along the main telescope tube axis, I started wondering today whether anyone has ever heard of ATMs placing barlows or focal reducers in between the primary and secondary of a Newtonian system. Something like this might take advantage of the optical shadow already present from the secondary, and at the same time allow much easier use of binoviewers. Yet it would probably require a trap door to put it in when tube is solid. Dunno if any advantage gained in terms of light-loss versus a more traditional mounting at the focuser since intercepting the the full light cone from the primary means wider radius than the secondary- unless its mounted directly on the secondary itself to minimize this effect?

Jess Tauber
Posted 08/08/2005 01:31PM #1
One big advantage of an intermediate extender is the increased distance between it and the eyepiece, which (when properly implemented) would reduce potential focuser-travel issues, compared with much shorter extender-eyepiece distances.

As for secondary obstuction: yes, implementing an intermediate extender into an existing primary/secondary system might pose a logistics problem. However, full aperture can still be maintained by altering that system (i.e. primary-to-secondary separation, diagonal size, or combo thereof). Even if such modification isn't practical, the scope's design & intended use may very well be able to "accomodate" a larger secondary obstruction with little/no significant reduction in resolution or contrast (fast astrographs come to mind). When used for visual astronomy, of course, a larger obstruction would impact resolution/contrast. The question would then be, "Would the impact be visible &/or significant?"

I suppose an intermediate reducer could be integrated with an existing slow objective (term includes primary mirrors) to allow for a more-compact structure, thereby perhaps reducing the capacity requirements for a given mount. -Sounds like a potentially reasonable solution for certain specific/limited scenarios.

Posted 08/08/2005 03:11PM #2

An interesting wrinkle is to use a spherical primary mirror (MUCH easier to make than a paraboloid) and correct the spherical aberration using a weak (specially designed) barlow placed before the newtonian secondary. Some thirty years ago the local astronomy club ran such a project. Some twenty such 8" f/6 mirrors were ground and polished to a spherical figure by amateurs. The "barlow" corrector sets (two element achromats) were produced as a special batch by a local optical shop. From memory the corrector slowed the system to f/6.6 ish. Although a very small amount of chromatic aberration was introduced, coma was well contolled. I don't know what happened to the other parameters (like flatness of image plane).

Wellington NZ