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Curious star test

Started by tomhole, 07/05/2003 07:12PM
Posted 07/05/2003 07:12PM Opening Post
Hello everyone,

Tonight, as the sky was darkening, I decided I wanted to do some star testing with and without the binos. My setup was the 10" 1200mm dob, TV binovues, 2x corrector, 16mm Nagler type 5's. I had the binovue and corrector inserted into a 2.5x powermate for 5x (375x). The cyclops mode was the 16 NT5 in a 5x powermate. Seeing was a 7 , transparency 4 in haze.

The curiousity: the star test with the binos looked better than the star test cyclops. I've always thought there was some correction error on this mirror, but haven't bought Suiter's book to confirm this. But the star test with the binos looked better than cyclops. The diffraction rings and airy disc in focus were much easier to see (not sure why that is) and the diffraction rings inside and outside focus matched up a little better. I didn't get a chance to try a star test with just the 2x barlow as the corrector as a high overcast rolled in. I'm also going to try this with the Denk 2" OCS when it gets back.

My guess is that my XT10, TV binos and 2x corrector just get along very well together. I did a star test with the Denk standards before I shipped them to some friends for evaluation, but I can't remember exactly what I saw. When I get them back, I'll try this with them.

Again, more useless data, but data none the less.

Clear skies,

Tom
Posted 07/05/2003 09:32PM #1
Hi, Tom.

You may well be seeing true improvement. All prisms (plano-parallel optics) add a little overcorrection to the mix. (They also add spherochromatism, too, but that's a non-factor in this case.) The larger the prism (or the "thicker" a section of light-transmitting glass), the more overcorrection will be induced.

If your XT10 is inherently *undercorrected,* what you're seeing would make perfect sense.

Best wishes, and enjoy that curious apparent correction you're seeing!
-Dan