1/6 or 1/8 wave

I've seen a couple of Intes MK66's advertised - one 1/6 wave and the other
1/8. The 1/8th should be better, but could someone give me an idea of what
the visual difference between 1/6 and 1/8 might be? Is there an obvious
difference in image quality? And would either of these be all that different
from, say, a Celestron or Meade 8" SCT or 7" MAK or even the Orion 150mm
MAK?

"khobar" wrote in message
...
I've seen a couple of Intes MK66's advertised - one 1/6 wave and the other
1/8. The 1/8th should be better, but could someone give me an idea of what
the visual difference between 1/6 and 1/8 might be? Is there an obvious
difference in image quality? And would either of these be all that
different
from, say, a Celestron or Meade 8" SCT or 7" MAK or even the Orion 150mm
MAK?

A true 1/6 wavefront PTV optic, if it is nice and smooth (perhaps 1/30
wavefront RMS) is a superb instrument. Yes, a 1/8 wavefront PTV that is
also nice and smooth (1/40 wavefront RMS), would do a bit better, but the
difference would not be huge, but might be apprecaited on those rare nights
when the skies are exceptionally steady.

Unfortunately, the PTV measurements do not tell the entire story, and the
makers often do not provide the RMS measurements.

The SCTs I've seen test results for have not been 1/6 wavefront PTV, but
generally in the 1/2 to 1/4 wave range. I haven't seen any interferometric
results from either of the Maks.

Clear skies, Alan

Hi:

Intes advertised the MK66 standard model as "1/4 wave." They tended to
test out a bit better than that, however. The Deluxe was "guaranteed"
to be at 1/8 wave, and the few I've looked at tend to approach that.

What's the visual difference between 1/6 wave and 1/8 wave? Very little
to the point of undetectable for most observers under most condtions.

Uncle Rod


khobar wrote:
I've seen a couple of Intes MK66's advertised - one 1/6 wave and the other
1/8. The 1/8th should be better, but could someone give me an idea of what
the visual difference between 1/6 and 1/8 might be? Is there an obvious
difference in image quality? And would either of these be all that different
from, say, a Celestron or Meade 8" SCT or 7" MAK or even the Orion 150mm
MAK?

khobar wrote:
I've seen a couple of Intes MK66's advertised - one 1/6 wave and the other
1/8. The 1/8th should be better, but could someone give me an idea of what
the visual difference between 1/6 and 1/8 might be? Is there an obvious
difference in image quality? And would either of these be all that different
from, say, a Celestron or Meade 8" SCT or 7" MAK or even the Orion 150mm
MAK?

One-twenty-fourth of a wave. :-o

In practice, the difference is mostly negligible. Some years ago, there
was a vogue of manufacturers claiming (and claiming to guarantee) that
their mirrors were no worse than 1/4 wave, or 1/6 wave, or 1/8 wave, or
1/N wave for large N. This was followed shortly thereafter by a wave of
amateur astronomers claiming that they could distinguish between 1/4 and
1/6 wave, or between 1/6 and 1/8, or between 1/8 and 1/N, etc., or that
they could rate a mirror by means of the star test to a precision of a
few hundredths of a wave. (Note that the difference between 1/6 and 1/8
wave is merely 4/100 of a wave. A very small variation indeed!)

The fact of the matter is that a true 1/4 wave mirror is really rather
good, and will show you significant detail on the planets. There is no
question that a more accurate mirror surface will reveal somewhat more
detail, but the more money is at stake, the more one emphasizes small
differences. That doesn't make the small difference a large one.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Paul Jones of Star Instruments states unequivocally on his web site
that Pyrex mirrors will hold no better than 1/4 wave because of
"expansion issues".

Sounds like false advertising to me. Lets sue em!

And even if you had 1/8 wave at the mirror surface, you would have far
less at the eyepiece, right??


RMOLLISE wrote:
Hi:

Intes advertised the MK66 standard model as "1/4 wave." They tended to
test out a bit better than that, however. The Deluxe was "guaranteed"
to be at 1/8 wave, and the few I've looked at tend to approach that.

What's the visual difference between 1/6 wave and 1/8 wave? Very little
to the point of undetectable for most observers under most condtions.

Uncle Rod


khobar wrote:
I've seen a couple of Intes MK66's advertised - one 1/6 wave and the other
1/8. The 1/8th should be better, but could someone give me an idea of what
the visual difference between 1/6 and 1/8 might be? Is there an obvious
difference in image quality? And would either of these be all that different
from, say, a Celestron or Meade 8" SCT or 7" MAK or even the Orion 150mm
MAK?

I've seen a couple of Intes MK66's advertised - one 1/6 wave and the other
1/8. The 1/8th should be better, but could someone give me an idea of what
the visual difference between 1/6 and 1/8 might be? Is there an obvious
difference in image quality? And would either of these be all that
different from, say, a Celestron or Meade 8" SCT or 7" MAK or even the
Orion 150mm MAK?

Check out http://www.rfroyce.com/compimag.htm for an interesting comparison.

Dennis

"Dennis Woos" wrote in message
...
I've seen a couple of Intes MK66's advertised - one 1/6 wave and the
other
1/8. The 1/8th should be better, but could someone give me an idea of
what
the visual difference between 1/6 and 1/8 might be? Is there an obvious
difference in image quality? And would either of these be all that
different from, say, a Celestron or Meade 8" SCT or 7" MAK or even the
Orion 150mm MAK?

Check out http://www.rfroyce.com/compimag.htm for an interesting
comparison.


Dennis,

I have a poster from AstroQuest that shows a similar comparison through an
8" scope, with additional comparisons of various secondary obstructions.
I've done a fair of observing with good 8" and 10" scopes, and I don't think
either the web site or the poster quite convey the amount of detail that can
be seen under steady skies.

Clear skies, Alan

"khobar" wrote in message
...
I've seen a couple of Intes MK66's advertised - one 1/6 wave and the
other 1/8. The 1/8th should be better, but could someone give me an idea
of what the visual difference between 1/6 and 1/8 might be? Is there an
obvious difference in image quality? And would either of these be all
that different from, say, a Celestron or Meade 8" SCT or 7" MAK or even
the Orion 150mm MAK?
The problem is that this is just one figure.
Others have tried to explain, but without perhaps the analogy, that can
give an idea of what is meant, and going on. Visualise a sports pitch.
Because of some subsidence, it has a small dip over on one edge, falling
9" below the rest of the field, and another bit near the other end, where
there is a little hump 9" high. If this was measured as 'P-V', then a
figure of 18" would exist. Visualise next door to it, there is a field,
that the farmer has ploughed, and is covered with furrows, that also are
18" from top to bottom. This would again measure as 18" P-V, but obviously
represents a very different surface...
P-V, only gives a 'limit' to the maximum deflection from the ideal.
Generally, it does give some guide to the overall performance, since the
normal methods of mirror production, don't result in a surface covered in
furrows, but without other data, it doesn't really _prove_ how good the
shape is.
RMS, gives you a measure of how much of the time, the shape actually
spends 'away' from the ideal, so the sports field would have a very low
RMS figure (since only two tiny areas deviate from what is required),
while the ploughed field would have a large RMS value. However it again,
on it's own, does not tell the whole story (a flagpole in the middle of
the sportsfield, would only give quite a small 'RMS' figure, because it is
so small in diameter - it would though give a massive P/V figure).
So you need both measures (RMS, and P-V), to get a quantitive feeling for
the surface irregularity.
Separately, there is then the issue of 'units'. If a test is done in green
light, it'll give apparently worse results, than the same test in red
light (because of the shorter wavelength involved). It is a bit like the
sportsfield measurements being given in inches, or mm. The same field,
comes in as 18" in one measurement, and 457 in the other, yet this is the
same surface.
In fact though, because mirrors are made by the same basic grinding
process, there is a pretty good correlation between P-V, and overall
performance. The Intes optics though are pretty good as standard, and the
normal spec (1/4 wave?), is normally amost indistinguishable from the high
spec' units.

Best Wishes

Hi:

1. That's his opinon.

and

2, The Deluxe scopes didn't _use_ Pyrex.

;-)

Uncle Rod.



wrote:
Paul Jones of Star Instruments states unequivocally on his web site
that Pyrex mirrors will hold no better than 1/4 wave because of
"expansion issues".

Sounds like false advertising to me. Lets sue em!

And even if you had 1/8 wave at the mirror surface, you would have far
less at the eyepiece, right??

I have a poster from AstroQuest that shows a similar comparison through an
8" scope, with additional comparisons of various secondary obstructions.
I've done a fair of observing with good 8" and 10" scopes, and I don't
think
either the web site or the poster quite convey the amount of detail that
can
be seen under steady skies.

Clear skies, Alan


I agree with you about the amount of detail shown, but I also find it hard
to directly compare detail in an image and detail seen through an eyepiece,
especially as I have been accused of having "averted imagination" by the
more visually challenged members of our club. As an aside, these sceptics
recently clamored for an eye doctor who was a guest speaker at a club
meeting to give me a contrast sensitivity test that he had with him. To
their chagrin, I scored higher than anyone else and above the "normal"
range, and even a bit better than my older teenage son who I know has
somewhat better visual acuity.

I would get the best optics I could afford, period. I have found that
superior optics are a pleasure to observe with, and are less susceptible to
bad-seeing/bad-collimation/bad-cooldown/bad-eyepieces/bad-karma problems.

Dennis