[AZ-Observing] Re: The Orion 120mm ED Refractor
- From: Roger Ceragioli <rogerc@xxxxxxxxxxxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Tue, 08 Aug 2006 08:29:53 -0700
Hello, All.
There shouldn't be a yellow fringe from the objective, I think, although
I haven't looked through one of these scopes. If you center the moon in
the field and see a yellow fringe, then that is probably lateral
chromatic aberration ("lateral color") which is a different phenomenon
from axial chromatic aberration ("secondary" or "tertiary spectrum").
In such a case, the yellow fringe comes from the eyepiece, many of which
exhibit lateral color (many Naglers, for example).
The "official" definition of apochromatism is the one given earlier,
"three widely separated wavelengths brought to one focus." For a
specific pupil zone (radial ring on the objective), one should add,
since spherical aberration always varies to some extent by wavelength.
Anything else is not an apo, but this definition leaves lots of wiggle
room for color error.
Also, please understand that "white" is not a color, but the absense of
any eye/brain sensation/interpretation of spectral bias in an
image/object. Defocus any scope enough (even an achromat) on a bright
"white" star and the diffraction pattern will go white. That is to say,
"white" is a very subjective interpretation, not really an objective
judgement. After 30+ years of using scopes, and a good 16 years of
making them (both reflectors and refractors of many makes, including
"official" apos), I'd have to say that for visual observing I doubt the
intrinsic value or even intrinsic truth that an image is "white."
Subjectively you can have "white." But if a "white" image is one with
no color bias, then even reflectors don't produce that, since their
reflectivity is objectively not even across the visual spectrum. And
some types of "aluminum" coatings have an obvious yellowish cast. It's
not that you shouldn't wish for the subjective satisfaction of a "white"
image, but what looks white to one person, may not look exactly white to
another.
As for cost, there's isn't a lot of extra manufacturing expense in three
elements. The prohibitive cost in apos is the ED glass itself. FPL53
even in strip form (the cheapest form) costs about $1000/lb.
Manufacturers can charge more for triplets because of the more complete
apochromatism and better aberration control these give. So people
paying a lot of money like them better. If you're going to spend $40k
on a car, why not spend $60 and get a Mercedes?
$2k for an apo is a steal. We've grown accustomed to ultra cheap prices
in our daily lives, but these depend on ultra cheap Chinese labor and
extremely low profit margins which are possible from economies of
enormous scale. I doubt that you'll ever get such an economy in the
manufacture of a specialized instrument like an apochromatic telescope
objective. Not enough people want them, I think. It's not reasonable
to expect Roland C. or APM to produce ultra cheap ultra high quality
apos to indulge the whims of amateurs while themselves to struggle
economically. We who make and sell scopes and lenses on a small scale
need to make a reasonable profit or there's no sense in making them.
Sorry if this sounds offensive.
Cheers,
Roger Ceragioli
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