[Aavso-photometry] Meade f/6.3 'new style' focal reducer - large IR focus shift?

[Tom Krajci]

I've taken some BVRI image sequences of Landolt fields for calibration, 
I notice the focus for the I band is way off...not bloated stars...big 
donuts!

I'm using a 'new style' Meade f/6.3 focal reducer in the optical path.  
(45mm spacing from focal plane, not 110mm like the 'old style' Meade 
reducers.)  Anyone know about the color correction of this reducer in 
the near infrared?  (As a comparison, another rig on my hill has a 
Celestron focal reducer...and I don't have any focus shift in I band.)

No, I have not measured the filter thickness for this BVRI set, and that 
could be the culprit, but these days parfocal sets are held to decent 
thickness tolerances...and the focus shift I see is very large...as if 
the filter were not just 0.1mm thicker, but 1 - 2mm thicker!

Thanks in advance.

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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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