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Post Script: Improving the wavefront quality of the C14 Edge

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#76 Benni123456

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Posted 22 August 2019 - 06:28 AM

company 7 also has an interferometer and provides fee based testing for telescopes they sell. they are writing on their homepage

 

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

 

Testing is usually performed at 632.8nm although testing at other wavelengths is available by special request. The testing can be performed at partial or full aperture, with the telescope optically aligned and set to nominal infinity focus. The Zygo GPI with standard camera provides 307,000 data points, and the testing can provide information requested including:

a) Interferogram diagram
b) Measure of System Peak to Valley
c) Measure of System RMS
d) Measure of System MTF
e) Measure of System PSF - energy concentration at focus
f) Surface / Wave front diagram

 

As a theorist, i do of course not know whether their device is good enough or whether they have the expertise to read the data. I will leave that to the experimentalists. But since they say they also deliver their own custom made telescopes to NASA, i guess they may know what they do.

 

If I next buy an edge with reducer, perhaps when i upgrade do 9.25 or so, I think I will strongly consider vendors that have such possibilities.


Edited by Benni123456, 22 August 2019 - 06:36 AM.


#77 jhayes_tucson

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Posted 22 August 2019 - 10:46 AM

company 7 also has an interferometer and provides fee based testing for telescopes they sell. they are writing on their homepage

 

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

 

As a theorist, i do of course not know whether their device is good enough or whether they have the expertise to read the data. I will leave that to the experimentalists. But since they say they also deliver their own custom made telescopes to NASA, i guess they may know what they do.

 

If I next buy an edge with reducer, perhaps when i upgrade do 9.25 or so, I think I will strongly consider vendors that have such possibilities.

 

Yeah, I have a Zygo GPI in my shop as well (which is somewhat ironic given my history with WYKO and 4D Technology.)  Unfortunately, I don't have a vibration isolation table, a large precision return flat, or the mounts needed to do double pass testing.  A GPI or any other temporal phase-shifting system can be used to do the kind of testing that I did, but it's a potentially time consuming process relative to using a dynamic interferometer like the PhaseCam.  The PhaseCam allows real-time display of 3rd order coma as you align the secondary, but the most important difference is its ability to make precision measurements in the presence of vibration and air turbulence.  It actually helps to use fans to stir the air (to reduce the coherence time) so that signal averaging can be used to reduce the effects of air turbulence to arbitrarily small values.  Averaging 160 phase frames only takes a couple of minutes and can reduce uncertainty due to air turbulence to well below 10 milli-waves.  With the Zygo, you have to be very careful to vibration isolate the system and to closely control air turbulence with shrouds.  The repeatability of the measurement will generally be limited by air turbulence and it can require a lot of effort to reduce turbulence noise below 100 milli-waves--depending on the air-path.  It's a very doable measurement but there's more to it than simply having the right equipment.  Making sure that the test components are thermally stable and that minimizing alignment induced errors is critical.  With the right equipment, a GPI could definitely be used to align the corrector plate on a SCT but it would be considerably more difficult.

 

I'm aware of company 7 but I don't know much about their level of expertise.  Anybody can make interferometer measurements but getting a valid result requires a bit more than just getting light through the system and pressing a button.

 

John



#78 Benni123456

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Posted 22 August 2019 - 08:04 PM

i guess thats how an experimentalist talks John....

 

By the way, i think one could automatize the collimation process somewhat with the help of kuka robots.

 

One needs a thing like this hotech advanced collimator and change the visual back of the scts such that it can be tilted.

 

The robots then fix the scope on its prism rail, remove the secondary, insert a reflector mirror in the visual back and align the target plate of the hotech like mechanism and the visual back of the scope. They do this grabbing the necessary tilting screws of the visual back and the tilting screws of the target plate.

 

The system could  measure alignment by some ccd measurement of the laser positions of the hotech in position 1 (the crosses at the target and visual back should be centered. of course the lasers should be more precise than in this hotech for that).

 

Then the lasers of the hotech are set to position 2. The robots insert the secondary and rotate it. Meanwhile the robots manipulate the collimation screws of the secondary until the laser dot in the visual back does not wiggle around during rotation. That way the robots  "zero out the knobs" of the secondary.

 

Then the robots center the corrector plate until the lasers dot in the visual back is in the the center.

 

Finally the robots fix everything.

 

That way the secondary, visual back and primary are aligned, but in a way that every surface is horizontal on each other and the thing is centered.

 

In a second step, the telescope is brought to a phase cam interferometer and it is aligned. this alignment to the interferometer may also work with the help of robots via lasers.

 

Then the robots grab the secondary and corrector plate and rotate them a bit. After the rotation, they fix everything and move their arms away. Now one makes an image with the phase cam.

 

Then the robots loose the screws and rotate the secondary and the  corrector plate again and one makes another measurement. Since the robots have zeroed out the knobs of the secondary, the scope remains in collimation if one rotates the secondary and corrector.

 

If the correct angle of the corrector and secondary is found after a few rotations, the robots fix all the screws with locktite and the scope goes to a short visual inspection by a human.

 

Would this work as an automated process like this?

 

If so, then one would only need a few optics engineers who make a last visual inspection and one would need people who look after the robots...

 

At least this is how car motors are assembled in germany. This process also has few tolerances and there is not much to be done by imprecise humans.

 

I do however, not know if that would work to assemble a Celestron telescope automatically....


Edited by Benni123456, 22 August 2019 - 08:32 PM.


#79 SandyHouTex

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Posted 25 August 2019 - 01:47 PM

My new edge hd 800 came in the following condition

 

1) Haze on the corrector, primary and secondary, showing a visible hand print.

2) the tube had strange crystals that seemed to glue on the inner tobe wall on it all over the place

3) when i put a reducer on it, it needs a different centering.

 

This suggests (especially the crystals that were adherent on the inner tube wall and the haze on every component) that they do not even look into the scope with an eye inspection when they assemble them.

 

They also do probably not even a flashlight test. How else would a telescope with hand prints on the primary and fog and particles on the tube wall leave the factory.

 

This looks like assembling by completely untrained persons who do not care.

Maybe you should buy a Meade.  Oh wait, they’re even worse.

 

Of all the Celestron SCTs in my signature, I only had to return one of them, the first C14 they sent me had a thumb print on the front of the primary near the periphery.  Optically the are all diffraction limited (what Celestron promises), with most being a 1/6 to an 1/8 wave “for the entire system”.  That’s pretty darn good.  I did have to clean the back of the 8 inches corrector due to outgassing, but other than that, nothing but initial collimation.  Almost everyone who has the Edges say they’re excellent.  I’d say Celestron is doing a pretty good job.



#80 Benni123456

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Posted 25 August 2019 - 01:58 PM

After cleaning it, I would also say that the edge itself is good. Unless I add the reducer..

Then i need to recollimate and still get color errors.

 

So it may depend on what it is used. For deep sky f10 is a bit dark. I do not know what the reducer does to the scope.

 

What i initially found on my scope did not look like outgassing. More like somebody accidentially touched the baffle with a cleaning cloth and then tried to clean the optics with that greazy baffle. With acetone, the haze went away and did never come back.

 

So the scope itself is, i would say quite good.

 

With the reducer, they may have made a wrong optics calculation. Or perhaps it is too intolerant with tilting or maybe I have been sent a defect one or whatever. I think the errors i see with the reducer are by design. I do not see the spaerochromatism in the pure edge.


Edited by Benni123456, 25 August 2019 - 02:00 PM.



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