Do you believe in such a statement?

!/20th wave I sure as hell don't believe it, From machine made optics!
Yeah right. Hey I have this slightly used bridge up sale in Brooklin any
takers. I sell it to you cheap. Hey in the same breath I also heard 1/38th wave
surfaces from china too. This is definately the marketing department talking.

The kind of figures I read here means that on a night of really good
seeing I which happens from time to time in Arizona like last spring south of
Ariz city we did get some .5 arc second seeing. We should be able to take that
achromat all the way to 500 x that is 100x per inch of aperture on the
planets. The only scopes I have been able to get away with this on has been my
AP 155 F7 my AP 130 F6 and my 6 inch Quantum Maksutov. I still have yet to see
any chinese refractor that can come anywhere close to this most of them seem to
start petering out at around 200 to 275 x this tells me they are only 1/4 wave
RMS and probably undercorrected and that's without even looking at them!

If you really want to wave rate an optic the only way this can be done is
with a double pass interferometer under "Controlled" conditions. The person
doing the testing should also know how to use the device as well.

Looking at a star under the night sky under questionable conditions don't
cut it.

The best test that any amatuer can do realistically under a night of good
seeing is look at Jupiter under high magnification. How much detail do you see.
I really decent 6 inch recfractor should be able to reveal detail inside
Jupiter's GRS. How does Jupiter appear in the telescope next to your setup.

My favorite contest is to compare a refractor with a Newtonian with
hand figured optics with a known wavefront. The only scopes I've seen that can
pass this acid test are the AP's and Tak's I have yet to see any other that
can.

Now you have to ask me this do I believe these kinds of claims that the
manufactures of commercial grade opitcs tout. Hell know, What do you think I
just fell off the Turnup truck or something! From what I read on these various
groups I think people need a reality check. Consumerism in this country is
making everone muddleheaded. Amatuers need to learn how to start grinding and
polishing there own mirrors and learning how to really test optics with a knife
edge or Ronchi grating. Making a refractor is just one step above mirror making
but can be easily done there is no magic about it.

Clear Skies
Dwight L Bogan

"Jon Isaacs" wrote in message
...
I wonder if those values are determined with an interferometer or other
test
method(s)?

In general, I think the properly conducted interferometer test results
would
show lower values, but a bit more realistic values.

Dan McShane

Now, 1/20th lambda implies a surface precision at a single frequency. But
given that the objective in question is an achromart and about 127mm at
about
F8, I wonder how much difference such precision would make since the
design of
such an objective is quite a compromise????

In other words, ignoring the issue as to whether these really are 1/20th
lambda, is there anything to be gained by making an 127mm F8 achromart to
1/20
th Lambda???


Jon,

I had that thought too, as a post script.

Does 1/20 wave really buy you anything with an f8 127mm where the system may
be swamped (comparatively speaking) by chromatic abberation?
Still, at $200 even if it`s really *only* 1/8-1/10 PTV, (and has RMS in
line with those values) it seems like a hell of a good deal. As long as the
CA is tolerable.

Dan McShane



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.544 / Virus Database: 338 - Release Date: 11/25/2003

"Roger Hamlett" wrote in message
...

"Dan McShane" wrote in message
...
"ValeryD" wrote in message
om...
About BO's new 127mm F/8 achromatic refractor quality:

"I called BO on Monday 12:00 noon EST. Tammy answered and took a
message for Bill about my situation. Bill called me back 2 minutes
later and gave me a brief education on how the BO 1278 is made. I
enjoyed my call tremendously! Bill told me about my objective as he
had inspected and tested it himself. He told me that these objectives
are polished to 1/20th wave after the 3rd polish. That's how fine the
grit is that is used. Also that a good number of these objectives are
testing null, as well as approaching null! That explains the Star
test results with my Ronchi EP. Anyway in Bill's usual engaging
conversation he told me he had one other call that day about what
were thought to be scratches on the objectives. I can't say what the
out come of that call was. But mine I can and will."

1/20 wave? Yes? Did I understood you correctly?

Anybody else believe in such a statement?

V.D.

Greetings Valery,

I wonder if those values are determined with an interferometer or other
test
method(s)?

In general, I think the properly conducted interferometer test results
would
show lower values, but a bit more realistic values.

Dan McShane

Also, (critical one), whether these are peak, or RMS values. I have a
scope
here, where the entire optical system, tested on a Zygo interferometer to
0.026 RMS (nudging towards 1/40th wave). However the peak error was nearer
1/7th wave.

Best Wishes

Roger,

Agreed, PTV by itself is somewhat useless. We`ve had off-axis parabola`s
measure near 1/10 PTV, 1/50 RMS and but very small local errors (maybe
something like 1/8 wave) have had a more profound effect on the image than
optics with noticably less overall values but a smoother surface.

If Dr Ruth was into optics she might say; "It takes smoothness, smoothness,
smoothness"

:-)
Dan McShane


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.544 / Virus Database: 338 - Release Date: 11/25/2003

Jon,

I had that thought too, as a post script.

Does 1/20 wave really buy you anything with an f8 127mm where the system may
be swamped (comparatively speaking) by chromatic abberation?
Still, at $200 even if it`s really *only* 1/8-1/10 PTV, (and has RMS in
line with those values) it seems like a hell of a good deal. As long as the
CA is tolerable.

Dan McShane

I think it is a hell of a deal for anyone who does not believe in Magic. I am
afraid that some folks are believing that there is some magic way that these
objectives can perform significantly better than other achromats of similar
diameter and focal length.

I am hoping for a scope that will provide decent widefield views, split the
double-double and hopefully, on a good night, show the Cassini division. Other
similar achromats reportedly are capable of this. Currently in this
aperture/focal length range I have an Orion Space Probe 130ST which is a 130mm
F5 Newt with a parabolic mirror and an aftermarket 2 inch focuser. Does pretty
good with a Paracorr and OK without. With a 32 mm WF eyepiece it does better
than 3 degrees.

I think comparing the two will be interesting though in general I prefer
Newtonians...

jon

ValeryD wrote:
1/20 wave? Yes? Did I understood you correctly?

Anybody else believe in such a statement?


No, but this doesn't necessarily mean I believe the manufacturer is being
intentionally dishonest.

As Dan McShane pointed out, stating a surface accuracy without the details
of the test procedure isn't particularly useful. Aside from the obvious
issues like testing wavelength, rms vs. p-v, etc., you need to know how the
interferometric data reduction is performed. Take the Zygo GPI software for
example. There are user inputs like "trimmed", "noise", "filter", and
"aperture percentage". These variables have legitimate purposes, like
eliminating unused edge areas from the test, but if set wrong they can give
overly optimistic numbers. Also, some software relies on the user to mouse
click on the fringe pattern to define the shape. This is reasonably
accurate and repeatable up to a point, but not for 1/20th. I can take the
same fringe pattern and make it read wildly different numbers depending on
where, and how many, points I define.

Your point is well taken, though, Valery. 1/20th is thrown around far too
commonly.

Sincerely,
Bryan Greer

"Edward" wrote in message thlink.net...

Valery,
Good to have you back. How are the AP eyepieces coming along? 1/20th wave?
sounds like the marketing department doing the testing.

Regards,
Ed

Ed,

These eyepieces are in progress in all focal lengths. Roland Christen
strictly tested several prototypes and we improved some sides.
He tested the most critical f.l. ones and was finally satisfied.
I should say, that it was far not easy to satisfy Roland's requirements
if consider them all together.

I hope, that all eyepieces in the very first serie will be 100% as
Roland required and then we will begin a full production run.


Valery Deryuzhin.

(BllFs6) wrote in message ...
He told me that these objectives
are polished to 1/20th wave after the 3rd polish.

Well

Given that a REFRACTING surface effects the wavefront roughly 1/4 as strongly
as a mirror surface.....this means that any given refracting surface that hase
1/20 th wavefront error is roughly that of a 1/5 wavefront error mirror in
terms of absolute surface accuracy requirements.....and given moderate,
spherical surfaces.....the claims may be slightly exaggerated but the are not
neccessarily "astronomical"

Blll


Blll,

It is clear, that lenses were _polished_ to 1/20 wave (guess surface
accuracy). That mean, wave front for each lens is about 1/30 wave
P-V. Complete objective (if it is a doublet) will has about 1/25 wave
wave front P-V.

Ask any refractor manufacturers out here and there if they can make
such precision in 5" objective for... about $200 (dealer's profit included).
Ask them if they can support such quality in general.

I think, that they will only smile in answer.


Valery Deryuzhin.

"ValeryD" wrote in message
om...

Anybody else believe in such a statement?

They're made at the Hogwarts School of Opticians, using brooms instead of
pitch laps.

Clear Skies

Chuck Taylor

Looking at a star under the night sky under questionable conditions don't
cut it.

. Consumerism in this country is
making everone muddleheaded. Amatuers need to learn how to start grinding and
polishing there own mirrors

Is that what amateurs need to learn?
Bill Meyers

Ask any refractor manufacturers out here and there if they can make
such precision in 5" objective for... about $200 (dealer's profit included).
Ask them if they can support such quality in general.

I think, that they will only smile in answer.


Valery Deryuzhin.







Is that 200 dollars of USA labor rates or starving chinese peasant labor rates?

A western paid optician could probably only spend a few hours getting a good,
smooth spherical surface.....a chinese peasant could spend a WEEK or 2 on
it...and you can be a peasant and STILL be a skilled optician .....at least in
some ways....

You ASKED about possible accuracy....my answer being that 1/20th wave refractor
wavefronts ARE ALOT easier to obtain than 1/20 wave reflecting wavefronts....

labor/cost is a whole other can of worms...

take care

Blll