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Tuesday, October 21, 2008

My next telescope...

This hybrid Cassegrain design from Vixen is awesome. The VC200L uses a sixth order aspherical primary mirror and a convex secondary mirror, replacing the meniscus corrector plate with a triplet corrector lens near the end of the optical chain. Focusing is performed using a 2" Crayford focuser at the end of the optical chain, rather than moving the primary mirror and suffering mirror shifts during focus. The optical tube is baffled and the secondary is oversized. At f/9.5 it is somewhat faster than traditional Cassegrains.

For $1700, it is certainly more pricey than my $380 Newtonian. Yet, the step in optical precision is significant, and comparatively well-priced against competing and mainstream high-end designs.

Of all the optical systems I've researched, Vixen's VISAC design is the only one that addresses spherical aberration, coma aberration, and field curvature. Even chromatic aberration is far less than the exceptional (and more exceptionally expensive) flourite lens refractors. Considering that field curvature is eliminated, suggests better optical correction than even a Ritchey-Chretien...and new manufacturing methods for RC scopes are producing some much less expensive designs

From what I've read and observed from vendors and blogs, I think the VISAC design hasn't caught on simply due to technical arrogance in much of the user base and limited marketing visibility. It just isn't sexy enough and there simply are not any additional distractions or operational steps to get in your way. But, as of today, you can buy them from Orion Telescopes online!

Eliminating the corrector plate and complex mirror focusing mechanism clearly helps reduce the manufacturing costs. Baffling and Crayford mechanisms are cheap. So, the cost is likely rolled up into the improved mirror coatings and the corrector lens. A fair trade, I'd say.

All my deep sky photography relies on the reflectivity of my 6" primary mirror and oversized secondary without any loss in light transmission due to an intermediate lens configuration, though my reflectivity is somewhere in the mid 90% range (it's less than $400 bucks what do you expect). So in my view, the VC200L decrease in transmissivity by adding a triplet lens is offset by the 1.8" larger aperture and higher reflectivity from the superior Japanese mirror coatings.

Don't just take my word for it, read this article by Astronomy Technology Today. Note, the article is taken from Vixen's web site as access online requires subscriber login. I just need to save my pennies.

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