Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Post Script: Improving the wavefront quality of the C14 Edge

  • Please log in to reply
79 replies to this topic

#51 peleuba

peleuba

    Surveyor 1

  • *****
  • Posts: 1777
  • Joined: 01 Dec 2004
  • Loc: North of Baltimore, MD

Posted 19 August 2019 - 01:02 PM

I know two dealers who have optical benches.

 

 

 

You'll not find a dealer in North America with an optical bench who would be willing to test out each telescope prior to shipping to the customer with, perhaps, one exception - Company 7 in Laurel, MD.

 

In Europe, its seems to be more common to have dealers with an optical bench.  But this is not a panacea either.  Unfortunately, on the mass produced stuff like SCT's its up to the end user to test the telescope and report any issues to the manufacture. 


Edited by peleuba, 19 August 2019 - 02:50 PM.


#52 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 01:25 PM

In fact even ts seems to have a plane mirror to test optics.

 

Here you see the owner of ts ask how to interpret the test results given that he does not seem to know exactly the characteristics of his plane mirror.

 

https://forum.astron...92/#post-109801

 

I do not know if they have tested their mirror at Carl Zeiss to get a diagram. But I wonder if this would matter much for finding the correct corrector angle...



#53 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 01:35 PM

I should add that Telescope Service apparently did not look at my edge hd before they sold it to me.....

 

 

Well,  the homepage of company 7 looks quite interesting to me:

 

http://www.company7.com/testing.html

 

The wrongly manufactured leica binocular is funny...

 

It is sad that manufacturing companies apparently use untrained workers and apply no quality tests before they ship.

 

If a 600/4 or a 400/2.8 from canikon would have such problems, no one would use this.


Edited by Benni123456, 19 August 2019 - 01:35 PM.


#54 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 01:43 PM

lol their comment to Celestron http://www.company7....tron/index.html

 

how could you send a telescope to Company Seven that you never tested even on prisoners!"

 

But yes, i guess Synta and Celestron ask a chinese optical company to make the lenses and mirrors. And they then come, as expected, probably with certificate....

 

And then they perhaps use some cheap shop to assemble these....

 

Foxconn is known to use child workers from tech schools to make components

https://www.cnbc.com...xa-devices.html

 

Some routers are sometimes made by chinese prisoners..

 

I wonder who assembles these scopes that cheaply......

 

Look at this comment:

 

http://www.company7.com/testing.html

Above: HOW MANY MM = 14 INCHES? someone at Meade Instruments obviously forgot. Images taken by Company Seven
during failed acceptance evaluation of a new Meade 14 inch telescope S/N 107674 on 19 May 2009. This is a 14 inch aperture (355.6mm)
model with a focal length of 3,556mm. But the Corrector Lens Retaining Ring read "D=406.4mm F=4064mm". Meade shipped this but most
retailers and their intended customer would never have realized this.

 

Any company with workers who would care for the product would see this...

 

Why do these manufacturers not see such things?

 

This must be assembled by people who can literally not read english letters....


Edited by Benni123456, 19 August 2019 - 01:47 PM.


#55 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7099
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 19 August 2019 - 02:16 PM

lol their comment to Celestron http://www.company7....tron/index.html

 

But yes, i guess Synta and Celestron ask a chinese optical company to make the lenses and mirrors. And they then come, as expected, probably with certificate....

 

And then they perhaps use some cheap shop to assemble these....

 

Foxconn is known to use child workers from tech schools to make components

https://www.cnbc.com...xa-devices.html

 

Some routers are sometimes made by chinese prisoners..

 

I wonder who assembles these scopes that cheaply......

 

Look at this comment:

 

http://www.company7.com/testing.html

Any company with workers who would care for the product would see this...

 

Why do these manufacturers not see such things?

 

This must be assembled by people who can literally not read english letters....

 

 

1)  I'm glad that you found a dealer with an optical bench.  They can certainly do some nice things with it but I don't see any numbers.  I've already made my point about the capabilities of the equipment that I used make this kind of alignment possible so I'm not going to rehash this with you .

 

2)  I did not start this thread as a way to bash Celestron or to encourage complaints about crummy telescopes.  The topic is about using professional-level, precision dynamic interferometry to align the corrector plate on a C14 Edge telescope to improve the transmitted wavefront quality.  If you have constructive comments or questions on that topic, I am all ears and more than happy to discuss it.  However, if you want to continue to spew about the poor quality of your telescope, please take your comments elsewhere.  Your agenda is inappropriate and you are pulling this thread way off topic.

 

Thank you.

 

John


  • EverlastingSky, kingjamez and Fatcat like this

#56 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • Posts: 20355
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 19 August 2019 - 03:56 PM

Very interesting post.  I have a couple of C14's I have been messing around with on the bench.   In DPAC these results are not so easy to see.   

 

You'll not find a dealer in North America with an optical bench who would be willing to test out each telescope prior to shipping to the customer with, perhaps, one exception - Company 7 in Laurel, MD.

 

In Europe, its seems to be more common to have dealers with an optical bench.  But this is not a panacea either.  Unfortunately, on the mass produced stuff like SCT's its up to the end user to test the telescope and report any issues to the manufacture. 

Cumberland optics in Md has a DPAC test bench.   They also have  Zygo to test optics.  They make the optics for Questar.   I do know they DPAC each optic set for Questar. 



#57 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 04:42 PM

 

If you have constructive comments or questions on that topic

Actually, my agenda is constructive.

 

With sensors that have smaller and smaller pixels, such problems in sharpness that come from a misaligned primary can be noticed in imaging.

 

I guess that, if customers do not complain, then there is no reason why a manufacturer, who seems only to be commited to cost slashing, would even buy an optical bench and do tests. 

 

I pointed out a dealer in germany who tests scopes on a bench before delivery.

 

Others have pointed out dealers from the US who test optics with interferometry before delivery.

 

I think that is constructive. 

If no one complains, then this will simply continue and they will make scopes like this:

 

http://www.company7.com/testing.html

 

JUST PLAIN DUMB someone at Meade Instruments labeled one side of the OTA correctly LX-200 ACF, the other side LX-90 ACF. Images taken by Company Seven

after failed acceptance evaluation of this new Meade 12 inch telescope S/N 107409 received 24 March 2009. Relegated to temporary display status at our showroom until
the label can be replaced; otherwise this telescope was collimated then tested just fine.

 

So it maybe, in this thread, we should start a list of vendors in Europe, Asia, and the US who test optics before they ship them...

 

Maybe we should also link to the descriptions of their test equipment...

 

Then one could discuss which vendor makes tests that are sensitive enough.

 

As for TS, i think it is a negative.

 

I know they have some bench, but they appear only to look at some refractors with it. They certainly did not look at my edge on that bench before shipping. But I guess this may change if they know about the quality differences in these sct's. So perhaps one may tell them to test them and they would do it? I do not know...


Edited by Benni123456, 19 August 2019 - 04:46 PM.


#58 Richard Whalen

Richard Whalen

    Mercury-Atlas

  • *****
  • Posts: 2842
  • Joined: 24 Sep 2007
  • Loc: Florida

Posted 19 August 2019 - 07:55 PM

John,

 

Now that you have bench tested a couple of SCT, where do you feel the manufacturers could make a improvement in optical quality (assuming proper assembly) to get the strehl up above .95? Primary, secondary or corrector? Or all 3? What do you feel is the weakest link from the samples you have tested?



#59 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 07:59 PM

they can certainly do some nice things with it but I don't see any numbers

So one question is: are numbers really necessary to align the corrector rotation?

 

I mean is a ronchi eyepiece and a 1/20 flat not enough to align the corrector?

 

Does one need an interferometer for finding the angle?

 

There are some people with interferometers who measure telescopes in Europe. But is this necessary to find the corrector angle?

 

Anyway, if people like this can afford interferometers

 

http://r2.astro-fore.../de/schwerpunkt

 

then there is no reason why synta who bought celestron can not afford this.

 

on the above site, there are many tests, in german. of course this does not imply anything about an individual scope, but it may certainly show some tendencies. A manufacturer who sells one high class optics has a lower probability that the next one from the same series is bad. Although this can happen too i guess..


Edited by Benni123456, 19 August 2019 - 08:12 PM.


#60 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 08:03 PM

here for example are some optical measurements of sc's

 

http://r2.astro-fore...astrofotografie

 

the tester describes scs in general as compromise systems of lower quality.



#61 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 08:25 PM

i find this non interferometric test of a c11 edge interesting

 

http://r2.astro-fore...is-in-die-ecken

 

they claim that the flattner introduces coloring like it can be expected from a half apo.

 

This is what i am seeing and photograph on my scope.

 

Chromatical abberrations, that increase much with the reducer.


Edited by Benni123456, 19 August 2019 - 08:48 PM.


#62 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 08:34 PM

here is another test.

 

they claim that the chinese only have HeNe lasers in their factory and optimize scs exclusively for this wavelength...

 

http://r2.astro-fore...ei-aelteren-scs

 

my god..  one is buying a mirror telescope because one thinks this wont have such problems the corrector has no focal length..

 

Apparently that was too simple. Some scts of Meade and Celestron are worse than half apos... even without reducer..



#63 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 08:47 PM

i guess this is an interesting meade vs celestron debate contribution

 

http://r2.astro-fore...lestron-c11-f10



#64 charles genovese

charles genovese

    Viking 1

  • *****
  • Posts: 868
  • Joined: 04 Feb 2006
  • Loc: Madisonville Louisiana

Posted 19 August 2019 - 09:40 PM

My C14 star test- comments welcome

 

https://www.facebook...213660818225979

 

Charles


  • eros312 likes this

#65 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7099
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 19 August 2019 - 10:14 PM

John,

 

Now that you have bench tested a couple of SCT, where do you feel the manufacturers could make a improvement in optical quality (assuming proper assembly) to get the strehl up above .95? Primary, secondary or corrector? Or all 3? What do you feel is the weakest link from the samples you have tested?

 

Richard,

Before I answer your question, I want to address a misunderstanding that I se

 

 

Actually, my agenda is constructive.

 

With sensors that have smaller and smaller pixels, such problems in sharpness that come from a misaligned primary can be noticed in imaging.

 

I guess that, if customers do not complain, then there is no reason why a manufacturer, who seems only to be commited to cost slashing, would even buy an optical bench and do tests. 

 

I pointed out a dealer in germany who tests scopes on a bench before delivery.

 

Others have pointed out dealers from the US who test optics with interferometry before delivery.

 

I think that is constructive. 

If no one complains, then this will simply continue and they will make scopes like this:

 

http://www.company7.com/testing.html

 

 

So it maybe, in this thread, we should start a list of vendors in Europe, Asia, and the US who test optics before they ship them...

 

Maybe we should also link to the descriptions of their test equipment...

 

Then one could discuss which vendor makes tests that are sensitive enough.

 

As for TS, i think it is a negative.

 

I know they have some bench, but they appear only to look at some refractors with it. They certainly did not look at my edge on that bench before shipping. But I guess this may change if they know about the quality differences in these sct's. So perhaps one may tell them to test them and they would do it? I do not know...

 

First, you are seriously testing my patience.  I did not start this thread to serve as a crusade to complain about the lack of testing telescopes.  I also did not start this thread to discuss the lack of dealer testing or quality control in the amateur telescope world.  Go be constructive in your own thread; otherwise, we'll have to get moderators involved and I'd rather not do that.

 

Second, I'll try one more time to explain this to you.  A key part of making or aligning precision optics is the ability to accurately measure the results.  Simple slope tests like the knife-edge or Ronchi test can be used to make excellent spheres and parabolas (including oblate spheroids.)  The problem with these tests is that you typically don't get very accurate quantitative data beyond zonal departures.  When it comes to aligning a system in double pass, slope testing can't tell you anything about the orthogonal aberration components that combine to produce the final wavefront.  These components are called Zernike polynomials and they relate directly to the Seidel aberration coefficients.  Measuring a telescope with a digital interferometer can tell you what the wavefront looks like; but, it's not as simple as simply making a quick measurement.  Without properly aligning the test optics and the optical system, the result will show the optical fabrication errors plus systematic alignment errors, along with the alignment induced errors of the test itself and the errors in the coupling optics, reference surface, and return flat. On top of that you'll have uncertainty due to air turbulence, which depending on the test setup can be a very significant fraction of the signal that you are trying to measure.

 

If you go back and read my original report, you'll see that I addressed these issues and discussed error limitations.  Furthermore, you'll see that on-axis 3rd order coma is the key aberration that determines optimal secondary alignment.  It is very easy to measure the on-axis 3rd order coma contribution using digital interferometry.  It is also very easy to extract 3rd order astigmatism from the data, which for on-axis values, shows systematic anamorphic errors.  This is an error that might be reduced by changing the clock angle of the corrector plate--if the secondary is known to have a high degree of radial symmetry.  Once you have three anamorphic components, you have to measure all three components to find the optimum alignment angles.  As I indicated above, this telescope has a nearly perfect spherical secondary so this is a system that may benefit from simple rotation of the corrector/secondary assembly.  Remember that simply rotating the assembly requires careful secondary realignment to minimize on-axis coma before assessing the result.  You can't just rotate the corrector assembly and retest without minimizing both systematic and test induced alignment errors.  Slope tests are completely insensitive to anamorphic errors and you'll have a very hard time achieving accurate secondary alignment given the sensitivity of either a Ronchi or knife-edge test.

 

Your notion that you'd automatically get a good telescope if someone provided a test report is flawed on a number of levels.  First, I could easily set up a poorly configured Ronchi test to show beautiful straight-ish fringes from a pretty crummy telescope.  Second, just because a system tests poorly doesn't necessary mean that it's a bad system.  As I've said, there are a lot of issues that can affect the wavefront that aren't related to the manufacturing errors.  Finally, there's a trap for everyone who provides telescope test data to the amateur world.  Very few amateurs understand how to properly read high end test data, understand how to confirm the validity of the data, or understand appropriate quality thresholds.  Supplying high quality test data will and does result in endless arguments over who got the better telescope--one with a Strehl of 0.91 or one with a Strehl of 0.97?  This is a lose-lose game for anyone selling amateur telescopes and that's why virtually no one supplies high-quality test data.

 

John


Edited by jhayes_tucson, 20 August 2019 - 09:15 AM.

  • 3 i Guy, EverlastingSky, kingjamez and 1 other like this

#66 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7099
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 19 August 2019 - 10:22 PM

John,

 

Now that you have bench tested a couple of SCT, where do you feel the manufacturers could make a improvement in optical quality (assuming proper assembly) to get the strehl up above .95? Primary, secondary or corrector? Or all 3? What do you feel is the weakest link from the samples you have tested?

 

Richard,

I'll give you a "weak" answer.  I haven't tested a large enough sample to give a very sweeping assessment--just three C14 Edge systems.  Of those I have tested:  Once you have a smooth and correct high order curve on the corrector, the number one problem that I've seen is anamorphic errors in the components.  They all show on-axis astigmatism to some extent and I suspect that it's because of their reliance on slope testing throughout production, which is insensitive to anamorphic errors.  They try to capture astigmatic errors using a star test and that works, but it's not super sensitive.  I think that they achieve sensitivity of around 1/4 wave in the final assembly so that's good enough but that limits the ultimate wavefront quality.

 

John



#67 Richard Whalen

Richard Whalen

    Mercury-Atlas

  • *****
  • Posts: 2842
  • Joined: 24 Sep 2007
  • Loc: Florida

Posted 19 August 2019 - 10:31 PM

Richard,

I'll give you a "weak" answer.  I haven't tested a large enough sample to give a very sweeping assessment--just three C14 Edge systems.  Of those I have tested:  Once you have a smooth and correct high order curve on the corrector, the number one problem that I've seen is anamorphic errors in the components.  They all show on-axis astigmatism to some extent and I suspect that it's because of their reliance on slope testing throughout production, which is insensitive to anamorphic errors.  They try to capture astigmatic errors using a star test and that works, but it's not super sensitive.  I think that they achieve sensitivity of around 1/4 wave in the final assembly so that's good enough but that limits the ultimate wavefront quality.

 

John

Thanks John. Interesting info, looking forward to your next report.



#68 Benni123456

Benni123456

    Vostok 1

  • -----
  • Posts: 194
  • Joined: 20 Apr 2017
  • Loc: Munich, Germany

Posted 19 August 2019 - 11:57 PM

Interesting. So one must use an interferometer to find the correct secondary and corrector rotation.

 

Have you also tested them with this Celestrom reducer on the interferometer?

 

What does this thing do with the optical properties? Especially off axis?

 

On the page of rohr, only the pure scts are tested with an interferometer. Never the sct with the reducer. I wonder since some time how the optical characteristics and which errors then change? It appears to become a somewhat different system at least. 

 

I would like to see what this reducer does in some numbers and diagrams. Do you have some figures on that?



#69 charles genovese

charles genovese

    Viking 1

  • *****
  • Posts: 868
  • Joined: 04 Feb 2006
  • Loc: Madisonville Louisiana

Posted 21 August 2019 - 06:52 AM

John- was hoping you would comment on my C14 star test

 

Charles



#70 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7099
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 21 August 2019 - 09:26 AM

John- was hoping you would comment on my C14 star test

 

Charles

I’m not on Facebook so I can’t see it...


  • kingjamez likes this

#71 macdonjh

macdonjh

    Skylab

  • *****
  • Posts: 4206
  • Joined: 06 Mar 2006

Posted 21 August 2019 - 09:53 AM

Richard,

I'll give you a "weak" answer.  I haven't tested a large enough sample to give a very sweeping assessment--just three C14 Edge systems.  Of those I have tested:  Once you have a smooth and correct high order curve on the corrector, the number one problem that I've seen is anamorphic errors in the components.  They all show on-axis astigmatism to some extent and I suspect that it's because of their reliance on slope testing throughout production, which is insensitive to anamorphic errors.  They try to capture astigmatic errors using a star test and that works, but it's not super sensitive.  I think that they achieve sensitivity of around 1/4 wave in the final assembly so that's good enough but that limits the ultimate wavefront quality.

 

John

I really like reading your posts, so thank you.  What is the meaning of anamorphic when discussing an optical surface?  Merriam Webster's definition doesn't make sense to me in the context of this discussion.



#72 Vla

Vla

    Messenger

  • *****
  • Posts: 496
  • Joined: 08 Mar 2014

Posted 21 August 2019 - 10:54 AM

Interesting. So one must use an interferometer to find the correct secondary and corrector rotation.

 

Have you also tested them with this Celestrom reducer on the interferometer?

 

What does this thing do with the optical properties? Especially off axis?

 

On the page of rohr, only the pure scts are tested with an interferometer. Never the sct with the reducer. I wonder since some time how the optical characteristics and which errors then change? It appears to become a somewhat different system at least. 

 

I would like to see what this reducer does in some numbers and diagrams. Do you have some figures on that?

You should start a thread if you want to have discussion on that (which would be interesting). People won't comment if the OP doesn't want it.



#73 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7099
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 21 August 2019 - 01:27 PM

I really like reading your posts, so thank you.  What is the meaning of anamorphic when discussing an optical surface?  Merriam Webster's definition doesn't make sense to me in the context of this discussion.

 

An anamorphic surface is one where the base curvature depends on the azimuth angle.  For example, a pure cylindrical surface is anamorphic.  An anamorphic system will have different magnification between two orthogonal axis in the focal plane.  Light focused by an anamorphic system mimics astigmatism, which is a field dependent aberration that cannot occur on-axis.  Anamorphic systems are by definition non-radially symmetric. 

 

John


Edited by jhayes_tucson, 21 August 2019 - 01:29 PM.


#74 Jeffmar

Jeffmar

    Viking 1

  • -----
  • Posts: 605
  • Joined: 18 Mar 2012
  • Loc: salt lake city, utah

Posted 21 August 2019 - 04:53 PM

 I have been following this topic since its beginning as well as I can. I certainly don't have the background to understand telescopes the way an optical engineer would, but I get the basic ideas. What it all comes down to is whether you are happy with your scope. I have both the C11 and C8 edge models and like them both. I prefer the C11 because the optics seem to be incredibly good on nights with good seeing. The combination of aperture and good eyepieces seem to give almost apo like images. For faint fuzzies no apo that costs less than a very good car can touch it. I have also been getting better astro-images than I should given the relatively short exposure times and low quantities of frames I am using. So, do I care what the strehl number is with my scope? Not so much. 

 

It is always fun to read John's posts. I do have a background in math and some physics so everything John writes about appeals to my inner nerd. 

 

Thanks John!


  • Paul Hyndman and jhayes_tucson like this

#75 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7099
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 21 August 2019 - 05:10 PM

I was thinking about this thread today and realized that I should point out that Mike Lockwood does have a PhaseCam interferometer.  I don't personally know Mike and I don't know if he'll supply PhaseCam test data with the optics that he makes; but, the fact that Mike has this capability puts him at the top of my list the next time I want to buy telescope optics.  He has the measurement capability to quantitatively assess the quality of the components he sells with excellent accuracy.

 

John




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics