Re: Semi-Apo

From: Richard (rander3127_at_rogers.com)
Date: 06/17/04 

Date: 17 Jun 2004 16:25:41 -0700

"OncoBilly" <mh@yahoo.com> wrote in message news:<SukAc.7$K53.5@edtnps89>...
> > them.
> > It is true that some people will still see colour error when others won't,
> > such is the state of human vision variability. But if we standardize
> normal,
> > non-flawed eyesight as 20/20, then surely we can find a visual baseline
> from
> > which to compare the colour in a refractor?
> > IMO, based on what I've personally seen and the measurement scale being
> > purely arbitrary, I'd rate several know refractors as such for colour
> correction:
> > AP 6" EDT: 10 or visually colour free on all astronomical objects.
> > AP 6" EDF: 9 some very minor colour at high mag. on bright objects.
> > TeleVue 101: 8 Almost colour free, remarkable for it's speed.
> > Tak FS-102: 7.5 Colour cast to image and some minor fringing at highest
> power.
> > The beginnings of high power image degradation seen.
> > AP Traveler: 7 More colour than the Tak, but still very low.
> > Celestron ED 4": 6 More colour than a fluorite-based doublet.
> > Meade 5" ED: 6 Colour noticeable on bright objects, purple shadowing at
> high
> > power on lunar features.
> > TeleVue Genesis SDF: 4 Noticeable colour at all powers.
> > TeleVue Genesis: 3. Colour almost as noticeable as a long achromat.
> > Chinese 5" achro f8: 1. Colour very apparent on all objects at all
> powers.
>
>
> Great stuff! Try to rate the Japan Vixen ED's too sometime.

Same as the Celestron ED which was a Vixen.
I think the key thing is which scope has enough colour to compromise
the
image at high power so you actually lose noticeable detail. IMO,
the Meade 5" ED is the demarcation line here. Above it, you'd be hard
pressed
to see loss of planetary detail and can start concentrating on other
optical
characteristics of the scope such as the wavefront correction!
Not to ruffle feathers, but I've had three Taks and from what I can
see,
they do not match scopes like the TeleVue 101 or the APs for wavefront
correction. This DOES effect planetary detail as a side by side with
the scopes will confirm. This is NOT to say all Taks are like this,
the ones
I owned were newer models, FS series.
So, here is what I found when comparing wavefront corrections between
the scopes: No relative measurements are given, just basic rankings.
APs: All near perfect.
TeleVue 101: Near perfect but not quite as good as the APs.
TeleVue Genesis/SDF: Same as the 101.
AP Traveler: Not as good overall on planets as the TV101, for whatever
reason.
Tak FS-102: Not as good as the above scopes.
Tak FS-78: Better than the 102.
Celestron ED: Similar to the Tak, but not as good as a TV ED102 I
tried.
Meade ED 5": Figure problems, clearly visible in star tests, but for
whatever
reason they did not effect the planetary images too much.
Chinese 5" achro: Better than the Meade, not nearly as good as the
others.
PS: the Meade's Mediocre lens cell could have been the culprit for
what
I saw. Pity.
PPS: Early 1980s vintage Celestron/Vixen 100mm fluorite as good as
the
TeleVue scopes with less colour than the ED scopes. Outstanding on
planets.
Look for orange tubes.
-Rich

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