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So, it was kinda in the back of my mind that a prism might be helpful with certain ED refractors. The decision to try a prism in the Megrez 110ED was really a fluke. I thought it would result in something awful, since "conventional wisdom" has always suggested prisms and fast optics don't make a good marriage. When I looked into the eyepiece, I expected to see a mess. When the actual image looked significantly better than what I'd seen using a fine quality mirror diagonal, it was a bit of a "Eureka!!" moment...

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It used to be that way, as the convential wisdom was to
correct the red first (as this was most important to
best planetary images) and let the blue/violet falloff
(which Valery indicated correctly as being less important
than good red correction for the planets). In that
scenario, the prism will hurt, as it will intensify the
blue/violet error. But the issue is that blue/violet error
has become the standard for color correction (or the lack of
it). Based on traditional correction (with good red
correction as essential), less blue/violet means better CA
correction period, as the red is not compromised. But
what has appeared to have happened, and Valery has alluded
to it, is that manufacturers, in an effort to sell scopes,
have compromised the red correction to handle the amount of
blue/violet seen. I'm glad you posted this, because it has
brought into the open the issue, and a simple workaround to
it (just use a prism diagonal to get back to where you
should have been in the firstplace).

I just would like to suggest when anyone does a test of a
scope for CA nowadays, don't just use Vega as the standard,
but also test Arcturus, Antares (if you live in a southern
location), Betelgeuse or another yellow/red star. Test
Izar as you did. Adding the reddish stars to the test will
show whether the color shift game has been played. Maybe
if this were to be done, manufacturers might be less tempted
to try to borrow too much red correction in their designs,
with a better overall scope to the consumer.

In the meantime, I do agree that a prism diagonal can make
these instruments quite useful. Nice moon pictures, by the
way. Looks as if you have a winner with the prism diagonal.

Tom

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