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Evolution of Japanese focusers

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#1 hottr6



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Posted 27 January 2013 - 03:09 PM

I really wanted to name this thread "What were they thinking?". You'll see why if you keep reading.

I'd like to talk about the evolution of Japanese focusers from around the 1960s through the 1980s. We are all aware of the big change from 0.965" to 1.25" eyepieces, but probably not-so-aware of the change that noticeably degraded both quality of image, and 'scope handling. I'm talking about the transition from visual-only 'scopes to dual visual/AP 'scopes.

In the following photo is a 1.25" R&P off a Vixen-supplied C4.5, and a 0.965" helical focuser off a mid-60s Towa. The main difference seen is the enormous out-focus ability of the Vixen. It seems to me that around the late 1960s, Japanese manufacturers were being asked to build 'scopes for visual and AP. The Vixen in the photo could be used for AP by removing the black extension tube that was for visual-use only.

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The Towa helical focuser had no provision for the demands of in-focus made by AP. Instead, Towa used "optimized Newtonian design" by using a very low-profile focuser, and a secondary that is much smaller than in the Vixens. CO for the Vixens is 22%, but for the Towa it is 18%, meaning improved contrast. BTW, the non-rotating helical focuser is among the nicest focusers I have ever used. Pity the helical focuser had no provision for a screw lock for eyepiece security. Nevertheless, in the 6 years of almost constant use, I never had an eyepiece fall out of the focuser. [As an aside, 0.965" Orthos and Kellners do not weigh very much.]

I must confess I have an intense dislike for the decision to go dual-use visual/AP on small Newts:

Posted Image

That offset consisting of focuser out-focus, barlow and a modern super-wide eyepiece entails profound balancing issues. I also get the sense that gravity probably bends the whole Heath Robinson rig.

What were they thinking????? :foreheadslap:

#2 Joe Cepleur

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Posted 27 January 2013 - 07:17 PM

As a purel visual astronomer, I've never before known why some scopes have absurdly long focusers. Now I know.

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