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?

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?

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??

RMOLLISE wrote:
Hi:

1. That's his opinon.

Hi Rod,

While you are correct that it is his opinion it is also true that in the
course of polishing a mirror heat is generated. That heat changes the
mirrors figure slightly. So it becomes problematic to get a good surface
without altering the figure. A rather decent glass pusher once told me
that he would work a rough zone/high spot by warming that spot with his
hand prior to working it.


It should also be noted that these are not spherical, or parabolic mirrors.


2, The Deluxe scopes didn't _use_ Pyrex.

Indeed.


Regards

Bill

PS: The last time I spoke to R.R. about a large mirror he couldn't
provide a zero expansion one due to problems in getting the blanks.

Hi Bill:

Thanks. I'm certainly aware of the difficulty of producing a good
figure in plate glass, but I'm a bit skeptical that this will be a huge
problem with the spheres on the Intes scopes. It should also be
rmembered that the wavefront error on these scopes is the product of
all the components.

Uncle Rod

William R. Mattil wrote:
RMOLLISE wrote:
Hi:

1. That's his opinon.

Hi Rod,

While you are correct that it is his opinion it is also true that in the
course of polishing a mirror heat is generated. That heat changes the
mirrors figure slightly. So it becomes problematic to get a good surface
without altering the figure. A rather decent glass pusher once told me
that he would work a rough zone/high spot by warming that spot with his
hand prior to working it.


It should also be noted that these are not spherical, or parabolic mirrors.


2, The Deluxe scopes didn't _use_ Pyrex.

Indeed.


Regards

Bill

PS: The last time I spoke to R.R. about a large mirror he couldn't
provide a zero expansion one due to problems in getting the blanks.

What "Deluxe scopes"? The company is defunct.

And, obviously, a 1/4 wave Pyrex primary mirror will be nowhere near
that at the eyepiece.


RMOLLISE wrote:
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??

And, obviously, a 1/4 wave Pyrex primary mirror will be nowhere near
that at the eyepiece.

Don't you think it is time to post, completely and concisely, your position
on this 1/4 wave Pyrex thing, and get it off your chest?

Dennis

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/
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