We had some lively debate in the last digest! I would like to add a few
additional points:

* Only a comparatively small number of people overall have the visual
acuity to reliable detect the difference between 1/4 and 1/8th wave. There
is only a very slight shifting of the energy distributed between the airy
disc and the diffraction ring(s). It is no shame to say your C14 is 1/4
wave. That is the standard for diffraction limited performance.

* While one can effectively argue the merits of .8 strehl vs. .95, I must
add that there are tertiary effects. The central obstruction causes more
energy to be thrown into the rings both statically and dynamically. As
atmosphere turbulance increases, the C.O. modification to the wavefront (a
negative diffraction pattern coaxially imposed over a positive diffraction
pattern) is such that more energy is distributed into the rings (randomly)
at the discs expense. This is seen as degraded image contrast compared to
an unobstructed aperture, and as an increase in "flare" to the point source.
A telescope with a better strehl ratio will "behave" better under less than
ideal conditions, retaining a higher contrast, especially at lower spatial
frequencies. So it is not irrelevant to have a high strehl ratio even if
seeing is seldom ideal at your location.

* It is important to note that the only number which is significant is
*system performance*. Some have spoke of surface accuracy of the mirror,
etc., but what matters is what the system delivers, at the wavefront, to the
eyepiece.

* For clarification, the most common value given is Peak to Valley. This
is a very deceptive number. Two optiks may have the same P/V number but
have vastly different qualities in actual performance. More accurate is the
Root Mean Square derivative, which is calculated from a large sampling of
test points and mathematically averaged. A 1/6th wave optik may test as
1/33rd wave RMS. The RMS value is a better indicator of quality
(smoothness), and two optiks of similar RMS value can be expected to give
similar performance. The Modulation Transfer Function (MTF) is the most
demanding and revealing test, plotted as a graph pitting contrast against
spatial frequency (lowest being equal to the entire FOV, highest equal to
the system limit). However this is a very expensive test requiring costly
equipment. Only Questar routinely used the MTF in their optics for years.
The Strehl Ratio is just as accurate but simpler and cheaper to produce.
Put simply, it is the ratio of energy inside the Airy disc to the energy
displaced elsewhere in the diffraction rings, *compared* to a perfect optic
(theoretical) of the same design. A perfect optik equals 1. No telescope
ever reaches that. .996 is the best I have seen (system performance). Note
that .95 in a catadioptric (C14) is NOT the same as .95 in an APO, since the
value is relative to the particular design. A *perfect* C14 still has the
effects of the C.O. which the refractor does not, etc.

* As far as Celestrons response to Paul. . . what did you expect? They
are mass-producing these things down an assembly line. Their bottom line is
keeping their nose above water, with Meade operating the spigot!
Truthfully, I believe the optics are figured and tested using a Ronchigram.
A Ronchi is accurate to 1/4 wave which is what Celestron guarantees. This
shows that an optic set meets that lower limit, but does not mean that a
given set cannot occasionally be better! The Ronchi just will not show it.
Markus said it right: it was luck, luck, luck! :-)

* Down over the years, far more scopes have been made that were worse than
1/4 wave, than better. 1/2 wave, even 1 or 2 wave is not that uncommon.
Many people never have their optics even collimated properly. We are living
in a great era for amateurs: the general quality of optics today is better
than ever in the past! Now if we could only see the stars down to 6th or
7th magnitude in my backyard as was 25 years ago! :-\

Wayne E. Gondella, CGA
Amateur Astronomers Association of Pittsburgh

So would I. I am fortunate to live within driving distance of Kendrik
Instruments (they also sold me my C14) and I am considering buying his tent
aswell. I should have tried it when I had took my Williams Optics GT1-HD
mount down to try out the C14 on it.

-Jim

Paul,
I would like to see photo of Kendrick tent with C-14 to get some idea
of scale, can you post/e-mail (brian@...)?
How do you evaluate the K-tent - C14 combination?
Best regards,
Brian

Surely, you must have some criteria for measuring the quality of
optics that are put in your scopes. What might that be?

Paul



Paul,
We guarantee that the optics are " diffraction limited "
and for a compound instrument to be at least a 1/4 wave
at the final wave front, each of the components needs to be
much better, as it is accumulative.
Tech

Has anyone tried the SBIG Hartman Mask software? I have tested my
y2k C14 several times using a 24 hole mask. The best results so far
show that the optics are about 1/4 wave as advertised.

I am not sure just how good the test is since there are a lot of
variables that may influence the results. It would seem that things
like seeing conditions, hole size, drive and mount stability, camera
resolution, etc, etc, would serve to degrade the results. But since
the variables all point to degradation of the results, does that mean
the optics are really better than 1/4 wave?

Ken

开云体育

I contacted both Celestron and Markus Ludes. Below are the responses
I got from both regarding the Strehl Ratio:

CELESTRON:

Issue: Can you please tell me approximately what the average strehl
ratio is of most mirrors installed in the C14?

Since our telescopes are used for both visual and astrophotography
use by amateurs, this is not a specification that we carry.

Technical Support
Celestron International

***I found this response to be odd. This is information that I
would think Celestron would have. What does using the scope for both
visual and Astrophotograpy have to do with knowing the Strehl ration
of the telescope. The must have some parameter of knowing the
quality of their optics.***

MARKUS LUDES - APM TELESCOPE:

Markus,

How did a mirror of this quality get put into a C14?

luck,. luck,. luck

Was it specially made?

no

Is it a stock C14?
yes

more info: i am a Meade dealer and Meade SC quality varries as much
as celestron. Last year I have had same luck, I got a 122 Meade LX
200 with opticsd are good as AP, Aries, or TEC, but I can tel you ,
such luck you may have by such massproduced telescopes on 1 of a 1
year massproduction

best wishes

Markus

Plucked from:


"Strehl Ratio: A ratio of the amount of light focused into disk 0 of
the Airy disk by a perfect optic (wavefront error = 0.0) versus an
optic with aberrations (wavefront error > 0.0). By definition, a
diffraction limited telescope (including central obstruction) focuses
68% of the incoming light into disk 0. The maximum amount of light
that can be focused by a perfect unobstructed optic is 84% which is
equal to a Strehl ratio of 1.00. Dividing 68% by 84% yields a Strehl
ratio of 0.80. An optical system with a Strehl ratio of 0.87 focuses
73% of the light energy. The higher the Strehl ratio the better the
optic."

I have contacted Celestron asking them what the average Strehl ratio
is on mirrors installed in C14's. Will post when I get the response.

Paul Atkinson

A better choice might be to use an ornament placed a few thousand

feet away

(such as in Suiter's book) just at daybreak for an artificial star

test.

Has anyone done this with his or her C14 as per Mr. Suiter?

I do not believe the wave front that reflective optics are tested

at

is critical (as it it sin the testing of refractive optics).

WG: I am not sure your point you are making. There are many

different

testing methods and many different numbers they yield. Some

numbers are

equivalent, some are not.

Simply that a Zygo red laser can be used effectively to test a
primarily reflective system. Then aberration can then be scaled to
various wavelengths. All other factors being equal.

However, I think the most important factor is that unless the

seeing

conditions are extraordinary; you will never realize the full

potential of

this number.

As compared to what? A run of the mill C14 with a .78 Strehl? I
believe most amateurs are capable of appreciating the difference
between this and a .97 Strehl.

I agree that the difference between a .9x Strehl vs a .9y Strehl may
be negligible -- and would not pay more for such a small difference
in value. But -- are we concluding that an avg C14 has a .9x Strehl
or a .7x Strehl?

Well, what type of wavefront are your guys C14's star testing at?

WG: One must have very steady seeing because of the large C.O. I generally
average my tests as looking to be better than 1/6, but there are other
factors which cannot be clearly determined without good seeing, such as
microripple, high order SA, smoothness. I have a hard time getting good
seeing for a 6 inch but a few times a year. Frankly, when they come, it is
hard to spend such valuable time doing clinical evaluations. One does not
know if the seeing will hold for 5 hours or 5 minutes. If I take my star
test at face value, that might place my optic at around .96 strehl.
However, I do not expect that it is truly that good. .78 is 1/4 wave.
Better than 1/4 wave is diminished returns for most people. .95 is
exceptional. I find it hard to believe Celestron could produce a .97 in a
C14 even if they wanted to. Few opticians could be that good. What are the
odds of making one by accident?

A better choice might be to use an ornament placed a few thousand feet away
(such as in Suiters book) just at daybreak for an artificial star test.

I do not believe the wavefront that reflective optics are tested at
is critical (as it it sin the testing of refractive optics).

WG: I am not sure your point you are making. There are many different
testing methods and many different numbers they yield. Some numbers are
equivalent, some are not.

What has been customized? Have you refigured the mirror? What
wavefront do you estimate the optic to be corrected to?

WG: Refer you to past postings. You cannot refigure the mirror without
changing the entire system.

I am personally very skeptical of the high test number. Of
all the scopes, I think that an SCT poses the most problems for achieving
this.

WG: Yes. The SCT is not a true Schmidt. You couldn't afford it. A true
schmidt uses a forth order curve corrector (a positive lens becoming a
negative lens 78% of the way out from center, extremely difficult to
figure). Our SCT corrector is formed by bending the glass in a vacuum and
grinding a sphere. When released, it approximates the schmidt formulation.

Is the mirror .97 with the test
being done using the corrector? I doubt it. If the corrector is less

than

perfect what type of light is actually making it to the mirror in the

first

place? What is the real benefit at the EP?

WG: The test must be done with the corrector. Without it, the optics are
severely distorted. Again, many cannot see any improvement past 1/4 wave.
Far fewer past 1/8th wave. .97 strehl is around 1/12th wave, at the
wavefront.

However, I think the most important factor is that unless the seeing
conditions are extraordinary, you will never realize the full potential of
this number.

WG: Fully agree with this.

Wayne E. Gondella

?Please send in a picture of it here my email ( )

? paulatkinson22 wrote:

The Astromart post by Markus Ludes has gotten many people to ask
about the quality of the "average" C14 optic.? Many people know that
I think Celestron is lacking in many areas.? However, optics is not
one of them.? If you missed it, read my comparison of my C14 vs. my
Meade 12" several posts back.?

In May, I had my scope at the Chiefland Star Party in Florida.? Many
people who looked through my scope were impressed.?

First, a gentleman with a 20 inch Dob (Starmaster I think) that had a
Zumbato mirror with blah, blah this, and a strehl of blah blah that
asked if he could look though my Celestron.? I invited him over that
night.? When he arrived I swung around on M51.? His exact words were
quote, "WOW, I can see the bridge between the galaxies better in your
scope than mine!"? He was amazed at the image quality for a 14" SCT.
He spent the next hour viewing through my scope.

Second, my good friend has a $12,000+ Takahashi Melwon 300 Dall-
Kirham which is a modified Cassegrain design.? That's 12k without a
mount!? This scope is impressive to look at and view through on his
AP1200.? We have spent several star parties going back and forth
comparing the same objects in our scopes.? Accept for some very minor
contrast issues on some objects the C14 preforms every bit as good in
my opinion.? Granted the Mewlon is clearly giving up 2" of ap.?
However, I did not spend $12k on my scope either.

In other words, I think the "average" C14 has exceptional optics and
most will be all you could ever hope for right out of the box.? If
anyone is interested I would be happy to send you pictures of it with
the color matched Astrozap aluminum dew shield and inside my Kendrick
obeserving tent.

Paul Atkinson?


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of 2002 FIFA World Cup

I found the old articles, thanks. I wish celestron would have
addresed these screws in their documentation-

-- In C14@y..., paulatkinson22@a... wrote:

Don't take the screws out if you dont need too. They really don't

interfere

with anything. Check back on some of the previous post. This was a

topic

that was hit on for a while.

Paul

Can someone outline the proper procedure for locking the mirror down ?
Also, should the lock down screws be loosley left in place or taken
out and the holes covered ?

thanks,
Dan



--- In C14@y..., "Mark Lancaster" <mslancas@b...> wrote:

The only reason to lock the mirror down is for long exposure

photography. The mirror can "rock" over the course of time,
particularly when crossing the meridian during an exposure. Otherwise,
don't worry about it. BTW, improper locking of the mirror can bring
its own headaches if not done properly!

Mark

----- Original Message -----
From: paulatkinson22@a...
To: C14@y...
Sent: Friday, May 31, 2002 10:01 AM
Subject: Re: [C14] Locking down mirror for Optec TCF-S


I would not see the need to lock the mirror down. Why do you

think you need to lock the mirror down? You should only have to do a
rough focus with the manual knob and then fine focus with the TCF. By
the time you get to this point mirror shift (if there is any at all)
should be a mute issue as the scope is pointed at object, focused, and
everything should be settled.

Paul Atkinson

The Astromart post by Markus Ludes has gotten many people to ask
about the quality of the "average" C14 optic. Many people know that
I think Celestron is lacking in many areas. However, optics is not
one of them. If you missed it, read my comparison of my C14 vs. my
Meade 12" several posts back.

In May, I had my scope at the Chiefland Star Party in Florida. Many
people who looked through my scope were impressed.

First, a gentleman with a 20 inch Dob (Starmaster I think) that had a
Zumbato mirror with blah, blah this, and a strehl of blah blah that
asked if he could look though my Celestron. I invited him over that
night. When he arrived I swung around on M51. His exact words were
quote, "WOW, I can see the bridge between the galaxies better in your
scope than mine!" He was amazed at the image quality for a 14" SCT.
He spent the next hour viewing through my scope.

Second, my good friend has a $12,000+ Takahashi Melwon 300 Dall-
Kirham which is a modified Cassegrain design. That's 12k without a
mount! This scope is impressive to look at and view through on his
AP1200. We have spent several star parties going back and forth
comparing the same objects in our scopes. Accept for some very minor
contrast issues on some objects the C14 preforms every bit as good in
my opinion. Granted the Mewlon is clearly giving up 2" of ap.
However, I did not spend $12k on my scope either.

In other words, I think the "average" C14 has exceptional optics and
most will be all you could ever hope for right out of the box. If
anyone is interested I would be happy to send you pictures of it with
the color matched Astrozap aluminum dew shield and inside my Kendrick
obeserving tent.

Paul Atkinson

--- In C14@y..., "W. Gondella" <gondella@s...> wrote:

2). Since other C14s come not with zygos, we have no basis of

comparison as

to how much better this one is testing over the "average" C14.

Well, what type of wavefront are your guys C14's star testing at?

5). Markus does not qualify the "interferometric" test

result. . . this

(Mark II?), and at what frequency (wavelength)?

I do not believe the wavefront that reflective optics are tested at
is critical (as it it sin the testing of refractive optics).

7). If I didn't already have a custom moded C14 I love, I'd be

very tempted

What has been customized? Have you refigured the mirror? What
wavefront do you estimate the optic to be corrected to?