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In-line finders?

Started by yahganlang, 08/02/2005 11:58PM
Posted 08/02/2005 11:58PM Opening Post
Ok- I give up on cylindrical finders....

A couple of months back on ebay, I saw an ad for an old ATM-made Newtonian scope that had the finder on the INSIDE of the main tube. I couldn't see from the pictures how it was supposed to be looked into- whether it had its own eyepiece or somehow interacted with the main focuser (I guess with a variant of a flip mirror?).

Obviously it was a narrow aperture finder, since anything else would start getting in the way of the objective mirror. But it got me thinking that perhaps one could mound a finder on the upper side of the secondary. Someone must have thought of this before, and perhaps posted it here? This in-line finder could have an aperture as broad as the narrow axis of the secondary.

If it were just piggybacked it would still need its own eyepiece and focuser, and if it were to feed into the main focuser, then one would still need to be able to flip or turn the secondary. Or I suppose one could use a combination sled and regular in/out focuser with a secondary-piggybacked finder?


Thanks again for your indulgence.
Jess Tauber

Posted 08/03/2005 08:25AM #1
What advantage(s), if any, would such a finder have over a standard o.t.a.-mounted right-angle finder?

Mind you, that's not a rhetorical question; I may simply be "missing" something. I can envision significant design considerations &/or potential problems, but advantages are escaping me.

Cheers & best wishes.
Posted 08/03/2005 09:44AM #2

I recall seeing a design like this in some ads somewhere. There was an objective lens mounted in front of the secondary. The secondary could be rotated to divert either the primary mirror image or the finder image to the focuser. I remember thinking it would be a mechanical nightmare to get it properly aligned and keep it that way. I also didn't like the idea of having to swap eyepieces anytime I wanted a low power finder view.

The ad was not for a premium scope. I got the impression it was a marketing thing more than a really good idea.

I like an optical finder. You simply have to determine the best place and method to mount it that suits you. It shouldn't be so flimsy and in the way that it is constantly bumped out of alignment. I can't recall the last time I tweeked the finder on my 8" Newt. It stays put pretty good. If you can build a flip mirror with the needed precision to return to decent collimation everytime it gets turned, then you can mount a finder and keep it aligned (and probably do it a lot cheaper than the flippin' mirror.)

have fun,

dan hilts