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A tale of two diagonals.

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#1 Binojunky

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Posted 12 April 2013 - 11:39 AM

Two 1.25" diagonals , daytime use in a newer C90 mak, one was a 10yr old Vixen, made in Japan, the cylindrical type sold years back by Celestron , Orion and Televue.
The other is a new dialectric from Taiwan, sold under numerous names,reflectivity is a claimed 99%, the Vixen was about 94% when new however after a decade its fair to assume that this is considerably less, maybe 88%?.
Both images seem to be the same brightnesss on a variety of day time test subjects ,in fact the older Vixen unit seemed to have the better contrast.
When looking down the eyepiece end of both when no eyepice was inserted and the diagonal was removed from the scope the aluminated circle on the Vixen mirror was larger than the dialectric one. On inspecting the tube that goes inside the focuser I could see why, the dialectric one was stepped down internaly,just below the filter threads.
Measuring both with an internal caliper I found the internal diameter on the dialectric to be 1", the older Vixen unit was 1:1/8" allowing more light to reach the mirror, any thoughts and comments on this are welcome, seems to me its similar to stopping down a lens?,thanks ,DA. :shrug:

#2 Jon Isaacs

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Posted 12 April 2013 - 12:16 PM

Measuring both with an internal caliper I found the internal diameter on the dialectric to be 1", the older Vixen unit was 1:1/8" allowing more light to reach the mirror, any thoughts and comments on this are welcome, seems to me its similar to stopping down a lens?,thanks ,DA.



Because it so close to the focal plane it does not stop down the aperture the same way an undersized Newtonian secondary mirror does. Rather it can result in vignetting but it would only be minimal, maybe a soft field stop...

You might check the older diagonal for collimation. I had one that was rather badly out of collimation.

Jon

#3 Binojunky

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Posted 12 April 2013 - 12:49 PM

When I last used the Vixen at night it brought stars to a nice tight point, and gave a perfectly round out of focus image when taken through the point of focus, of the two diagonals in question it seems to be equal of the newer dialectric, regarding mirror diagonals loseing their reflectivity as they age I would think their is no rule written in stone?. I store my diagonals in a sealed container with a small bag of silica gel in it,if the mirror is well overcoated then the drop off over time may be in fact of a very minor nature ?, Dave.

#4 plyscope

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Posted 12 April 2013 - 04:27 PM

When I upgraded from a WO 66SD to a TV60 a comparison of the two 1.25" dielectric diagonals showed there was a reduced aperture in the WO one compared to the TV Everbrite.

Visually it was only noticable as a softening of the field stop on a 24mm Panoptic particularly in an f6 scope. Not so noticable on longer focal ratios.

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#5 SeattleScott

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Posted 12 April 2013 - 09:20 PM

Dielectric coated mirrors hold up very well with age. They say the newer mirror coatings with enhanced coatings are expected to last around 20 years before there is noticeable drop off in performance, and that is for regular reflector mirrors, not dielectric diagonals. Dielectric coatings are much easier to clean which will make the coatings last even longer. Still it is not unreasonable to think the Vixen might have lost up to 5% or so, but can you really see a 5% difference in reflectivity at the eyepiece? Even 10% (since the Vixen starts out at 94% instead of 99%) would be difficult to spot with the naked eye.






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