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Old 14-04-2008, 02:19 PM
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RickK is offline
Join Date: Apr 2008
Location: Castro Valley, California, USA
Posts: 1
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I too have been recently considering an on-axis guiding system for my relatively fast newtonians.

The use of a "hot mirror" to transmit the visible and reflect the IR to a guider is what is described on the referenced Kazuyuki web site--a prototype. Turns out that this has been done commercially, see While they seem to have abandoned the imager/guider principle for dual imagers, it shows it can be done. But at $2500 USA, about 10X too much. In any case, the RCOptical hot mirror fails to properly handle the Ha wavelength (transmits half, reflects half). And transmitted light will always be subject to aberration that could degrade the image unless corrected.

A better choice, perhaps alluded to on the Kazuyuki site, is to use a "cold mirror" to reflect the visible light to an imager and transmit the IR for guiding. The wavefront error imposed by the beam splitter will surely be much less than the coma imposed by trying to guide off axis at F 4.5 or 6, as in my case. Most commercial cold mirrors suffer the problem of splitting the Ha light at 656 nm, but at least one does not, according to their specs-- These mirrors are said to be only 1/2 wave in flatness, that may or may not be a problem for deep sky astrophotography. But they meet the other specifications!
Regards, Rick
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