92 replies to this topic

[Bob4BVM]

Hello Darren,

 

Thanks for sharing your test experiences.

Interesting discussion for sure gents. I continue to learn from the experts here, though i am probably too old now to start pushing glass as a beginner .

 

That said if you decide that either of these mirrors in question are actually bad enough that you don't want them, allow me to be first in line to take either of them off your hands, saving you the agony of having to deal with them yourself any longer      

As an added incentive, I will not charge you anything for the trouble.

 

Cheers

Bob

Edited by Bob4BVM, 08 April 2025 - 09:33 PM.

[MKV]

MKV,  perhaps you did not understand what is being blinked.  I think it is blinking against the Bath data simulated ronchi.  I think that since the ROC number is in the upper left corner like DFTFringe dispays.  Well actually he said in his post that it was the Bath and DFTFringe.

Dale, my question was directed at the blinking Ronchi method in general, rather than the particular mirror in question. 

 

On another note, the only number I see in in the upper left corner of the DFTFringe image is 0.710. That can't possibly be the ROC.

Edited by MKV, 09 April 2025 - 12:10 AM.

[Dale Eason]

Dale, my question was directed at the blinking Ronchi method in general, rather than the particular mirror in question. 

 

On another note, the only number I see in in the upper left corner of the DFTFringe image is 0.710. That can't possibly be the ROC.

It is the ROC offset of the grid.  

 

I have never heard of the "blinking Ronchi method". 

Edited by Dale Eason, 09 April 2025 - 01:06 AM.

[MKV]

It is the ROC offset of the grid.  

I have never heard of the "blinking Ronchi method". 

I know it's the ROC offset (defocus?) but you said ROC.

 

I called it a "blinking Ronchi method" because it's two GIFs being flashed on and off for animation. Back in the days of cold cameras  and "baked" film astrophotography, we used blink comparators (a special stereo viewer) to compare two negatives of DSO images, searching for any changes  (a method used to discover novae and asteroids by the pros and ATMs alike). That was probably before your time. :o)

[Dale Eason]

MKV asked.  

Has anyone worked out a tolerance envelope (accuracy calibration) for that Ronchi blinking comparator method?

and then he said.

 

 

I called it a "blinking Ronchi method" because it's two GIFs being flashed on and off for animation. Back in the days of cold cameras  and "baked" film astrophotography, we used blink comparators (a special stereo viewer) to compare two negatives of DSO images, searching for any changes  (a method used to discover novae and asteroids by the pros and ATMs alike). That was probably before your time. :o)

So there is no commonly used test procedure called "Blinking Ronchi".  You made it up.

 

I know all about blinking star images with one another and it is a method still used today.

[MKV]

So there is no commonly used test procedure called "Blinking Ronchi".  You made it up.

Guilty as charged. :o) Human nature. I looked at the blinking ronchigrams, and that's the first name that popped up in my mind. Of course, the method exists, Mel Bartel uses something similar (maybe not blinking), but it's a way to compare things to determine how close they are to being identical.

 

But I'm in good company. Someone made up "collimation," the "Waineo" null test, my pet peeve -- "DPAC," the list goes on. The ATM cookbook is full of them. George Carlin had a list of similar popular expressions we use in our everyday life, such as a parkway (where we actually drive), a driveway (where we actually park), or even a"more perfect union," etc. They may sound awkward, but we know what was meant. More importantly, and no ill will was involved. :o)

Edited by MKV, 09 April 2025 - 03:58 PM.

[a__l]

I will offer few words of caution here and question

the methods

 

One, how good is the optical flat, how smooth

etc...? Is it truly flat or have a slight amount of 

power in fringes? This alone could what you

are seeing. With a center hole cut, it could have a

a minor defect caused by the milling.

 

 

I usually look at the totality of measurements on different mirrors. Of course, this can coincide in antiphase for good pictures. There were several mirrors from different manufacturers with a good picture, if I remember correctly. The probability of such coincidences is low. If Zambuto fell out of this row, then most likely it is so.

 

"minor defect caused by the milling."

 

ps I'll also explain, if there is a defect on the plane, then there should be the same anti-defect on the mirrors with a good picture. That is, Lightholder and Pegasus grate the same anti-defects. Which seems unlikely.

Edited by a__l, 10 April 2025 - 06:22 PM.

[a__l]

Has anyone worked out a tolerance envelope (accuracy calibration) for that Ronchi blinking comparator method? Although the patterns look remarkably similar to the naked eye, they are not identical. And how does that method rate when testing, say, a large and slow mirror vs large and fast one? Eyeballing is not the best method of setting up optics for testing, and judging quality of the product.

 

Again, a Bath interferometer is cheaper (currently around $150) than any large optical flat you can find on eBay, especially if the flat is certified (as it should be). A Bath IF doesn't need precision optics; commercial quality will suffice. 

In fact, these are different Ronchi from different independent methods. Of course, there is no perfect match, but integrally they are identical.
In addition, all this (Ronchi, interferometry) began after StarTest, which I did not like....
StarTest can be done by any amateur. This is the cheapest testing method. But there is practically nothing of this on the forum (from testing mirrors of famous optics, unfortunately, there are only Chinese mirrors).

Edited by a__l, 10 April 2025 - 06:38 PM.

[Pinbout]

In fact, these are different Ronchi from different independent methods. Of course, there is no perfect match, but integrally they are identical.
In addition, all this (Ronchi, interferometry) began after StarTest, which I did not like....
StarTest can be done by any amateur. This is the cheapest testing method. But there is practically nothing of this on the forum (from testing mirrors of famous optics, unfortunately, there are only Chinese mirrors).

It’s easy enough to make the program ronchi match the real ronchi if the mirror is smooth. 
the tolerance can be very tight 

 

the ronchi is the best test while working on the mirror

test the ronchi against a flat or a Ross lens is just as quick and better than 1/10 ~

 

but making an animated blinking vid is just a demo that if not perfect just tells ya get back to figuring the mirror 
 

Edited by Pinbout, 11 April 2025 - 10:29 AM.

[Mike Lockwood]

Darren, that Zambuto mirror's wavefront is not an optical null -- your overexposure of the mirror in the article bleaches out figure 16inf4_zambuto.jpgirregularities. With adjusted exposure, the mirror sides appear to bulge and the central region is deep and steep.

John Pratte says what people are claiming are defects are far more subtle to the eye than they appear in the photos.  So, once again, people are critiquing optics they have never personally seen based on processed cell phone photos.  You rightly point out some of the issues with the images, they are not in perfect focus because these are PHONE PHOTOS that are automatically CONTRAST ENHANCED by the phone.
 
John also verified that his knife edge enters from the left, but he uses a small reversing telescope to view the image (like I do with Foucault), MKV, so for the images the knife edge enters from the RIGHT.  Thus, for both mirrors, the knife position is just outside of focus for the outer zones and inside of focus for the center.  The outer zones focus shorter than the center.  This is slight undercorrection, not overcorrection, so the outer zone and center are HIGH.

 

You have the shape of the mirror INVERTED.
 
SO, you've posted a photo that is not your own that has the optician's name on it, and you have labeled a "TDE" where there is none!!!

 

If I were Lightholder and saw this, I'd be displeased.  It should be removed from the thread and corrected, at minimum.

So here are some facts that everyone should understand:

1)  A "turned down edge" or TDE is a very precipitous drop-off at the very edge of the mirror.  It is typically narrow, say 1/8" wide or so, but sometimes it is narrower or wider.  Narrow ones can be beveled away, wider ones require re-polishing.  Nothing resembling this is in the images.
 
2)  A slightly overcorrected outer zone of a mirror is where the slope error goes downward near the edge, but the slope error is relatively small, and there is no dramatic increase in the slope near the edge.  This in itself is not a significant defect in the mirror, AND IT IS NOT A TDE.
 
3)  As a mirror cools, the glass shrinks faster at the edge due to increased surface area.  This causes the outer part of the mirror to focus longer (overcorrection).  Thus, a perfect mirror will appear overcorrected while cooling, particularly for Pyrex or Supremax.  The amount is unpredictable because the weather-dependent cooling rate varies.  The error caused by a mirror cell rear support also bends the mirror in a way that very slightly adds correction.  The effect of a heated or cooling flat is somewhat unpredictable.
 
4)  So here's a big secret of opticians - we know that amateurs will claim a perfect mirror is overcorrected while it cools (for most of the night) if they are doing star testing (which is NOT a lab test), so we have to make sure our mirrors are a little bit undercorrected, or people complain.  Yes, that's right, and this is exactly what you are seeing in these mirrors.  It's been done for a long time.  This is why I always point out that star testing is dependent on variables other than the optics' shapes AT EQUILIBRIUM, and thus it is often inaccurate.
 
5)  In the past, some mirrors were made using star testing done in a fast cooling environment.  To get a good star test, significant overall undercorrection and large undercorrection in the outer zone were required.  The particular shape was highly consistent across the many examples that I saw, so it was not a fluke.  As a result, these mirrors performed poorly near equilibrium or in slow-cooling conditions due to undercorrection and large slope error.

6)  As for the double-pass testing, there is no calibration data for the flat involved, there is no test for how it is bending in its roller-chain sling, and there is no knowledge of how the mirrors are bending in the test stand (which can affect correction).  Yes, air currents can affect the test, as can air stratification.  In my experience, air currents are not a major issue in John's test space.  The other unknowns remain.  The results are not easily quantifiable, as others point out.

 

I hope that clarifies some things related to optical testing.  It's complicated and easy to miss things.

Edited by Mike Lockwood, 11 April 2025 - 02:17 PM.

[duck]

A lot of the points made by Lockwood are why I invested in a Cervolo Fizeau IF and fused quartz optics.  The first iteration of my 12 1/2" F/4 Pyrex paraboloid had a turned up edge, which I saw in the Ross nulled Foucault.  I left it, thinking it's going to shrink during the most favorable planetary viewing right after sunset.  It didn't.  It's an obvious defect in the star test.  I'd say a 1/4 wave defect in the outer zone is going to be obvious in a star test.

 

F-o-grams, nulled F-o-grams, Ronchi-grams, all good for figuring.  If you want wavefront errors, an IF seems necessary.  The coup de grace would be to show the AC f-o-grams with a complete (>80 igrams) IF derived wavefront map.  I'm not quite there since my little board level video camera won't take an f-o-gram.

[Darren Drake]

When I originally posted this thread my intent was to provide what I see as rarely seen and revealing DPAC pics of a few mirrors of note.  There is a thread on the refractor forum that shows many dozens of DPAC pics of very high end refractors.  There's no real equivalent here on CN for mirrors and so I thought I'd post a few so those here could see what high quality mirrors look like.  The Rohr website is the only repository of many mirrors but few here ever refer to the site. 

 

After I posted the pics and got some likes I thought this thread would serve it's purpose and die off.  Then there began some high scrutiny of a couple of the mirrors and I began to regret starting the post.  The criticisms were inaccurate and unwarranted.   

 

I wonder how many here are truly experienced observers who deal with real world observing conditions.   As I said earlier, the miniscule errors shown are of no real consequence.  My main intentions were to show the amazing smoothness of the surfaces that interferometers don't show.  The first pic shows a screenshot of other mirrors of varying qualities and how surface roughness is very common in non premium mirrors. 

 

I hope Mike's clarification ends any further negative comments about these mirrors and that those here see and appreciate the qualities these mirrors possess without further inaccurate criticisms.  

Edited by Darren Drake, 11 April 2025 - 06:54 PM.

[EJN]

After I posted the pics and got some likes I thought this thread would serve it's purpose and die off.  Then there began some high scrutiny of a couple of the mirrors and I began to regret starting the post.

You should have posted in the reflector forum, unfortunately in this forum there are some who will debate how many angels can dance on a focogram.

Edited by EJN, 11 April 2025 - 06:07 PM.

[Oregon-raybender]

I see nothing wrong with this post, yes it does offer the

challenges that Mike has so kindly pointed out. 

 

This is what happens in the real world of testing outside

of the manufacturer, not a fan of doing this, but folks

want to know how good is mirror.....

 

My comments were based on my long experience

watching and waiting in long dark test tunnels.

Remove any and all issues, check all the boxes,

is the flat good enough (better than the mirrors)

Is it smooth, are vibrations taken care of, how are the

images taken, does the camera have an effect? 

The list is long when one is doing optical testing, remove any chaos.

 

When working with customers (NASA, Military, world class 

companies) they ask to prove it, did you dot your i's and cross

your t's !! ?  Living in the QA world, I sure did. I spend

most of time in front of interferometers, than KE testing.

We used R screen to get close, do a quick with KE

But switched to over interferometers for the final testing

DPAC or null lens. The lab tests remove or reduce any

doubt of how good the optic is. 

 

The big question is, does the lab test tell you how the

mirror will preform in the real world, having slight

undercorrection (CC -.9 to .98 ) the best solution.

Mike points that out. In our TM class, we had the students

make their mirrors under, because you don't want to over

shoot and have to go back to sphere and start over again, or

with a large mirror, re-cut the polishing lap.

 

I have done a great amount time observing, some under

idea conditions, < 2 arc seeing or better with many types

of telescopes. Do I star test, each one, not really. The

main reasons is many, is the sky prefect, the right eyepiece

what about the total system, are air currents tube taken

care or, how about your eyesight. How well does this match

the lab test system? not even close, but you hope that

does.

 

Starry Nights

[MKV]

John [Pratte] also verified that his knife edge enters from the left...

Darren Drake (OP) never mentioned that detail. He mentions John only in the context of the opening post as having gone to "John Pratte's workshop" to do some more testing.

 

In fact all of Drake's images in Post #1 look like the knife is from the right, as it usually is. Also it's customary to draw an arrow indicating from where the knife-edge cuts into the cone. That too was omitted, so if I misjudged the edge it's because the critical information was missing, and I had no reason to believe that it was.

 

Just for the record: there were other members here who didn't object to my objections, and they are no novices when it comes to working with optics and/or observing. 

 

I and others have also mentioned the omission of specifying flat's certification. In large flats (especially if not of full thickness), flexure is not uncommon, as you yourself know, and its effects should be accounted for. Also, any reference to the type of the recording device used was completely omitted in Drake's presentation, as was any mention of using additional optics that reverse left to right. Is it our fault that we weren't told about this reversal, which I would expect Darren would have brought up immediately, but didn't. 

 

He claims that his intention was to show the amazing smoothness of the surfaces -- which "interferometers don't show"[sic]. 

 

And, he is now hoping that your clarification "ends any further negative comments"..."without further inaccurate criticisms." Wow!

Edited by MKV, 11 April 2025 - 10:45 PM.

[a__l]

 The Rohr website is the only repository of many mirrors but few here ever refer to the site. 

 

 

It was discussed on other forums. Including professional opticians. He has problems with astigmatism and one should be careful with his conclusions. Be that as it may, he bought himself a mirror not from an optician who focuses on ATM, but from LOMO, which mainly makes for professional observatories and at a completely different price.

 

This is not the only storage. There is an even larger array made by Russian ATMs. But there are no American mirrors there.

Edited by a__l, 12 April 2025 - 08:41 AM.

[Sean Cunneen]

I know how much you all love to question every facet of posts like these and its exciting to see big name mirrors included. Those of you who have spent their lives in the field of optics should take a moment to consider that these images are how most amateurs are going to see their mirrors, warts with testing circumstance and all. In fact, JP's setup looks to be far better than what most will be able to manage themselves.  The first time I tested a mirror, the mirror was propped up on my stove and my testing rig was across the kitchen on a shaky table with my dogs coming and going as they please... Rather than trying to out-minutia each other on the brand-name mirrors, perhaps you can expound on those other mirrors... I doubt the owners would call those mirror dogs, in fact they were tested because the owners thought they were good enough to want to know how good. (ask me how I know!) 

 

Sean

[duck]

I think Dale made the point early in this thread that I agree entirely with.  Unless you've made lots of mirrors using the AC Foucault test and then star test them (forever, waiting for perfect seeing), how do you know when that faint shadow is insignificant?  Ceravolo said his ego forced him to figure (using an optical null) until he could see no defect.

 

The point I disagree with is about how seeing overwhelms any of these minor defects....  Seeing is stochastic, figure errors are biases.  I suppose it would be simple to model seeing as a colored noise, then add in a bias.  What you want to know is what percentage of time is the exit pupil wavefront error below some threshold value.  The bigger the figure errors, the less that percentage becomes.

[Pinbout]

I miss going to Delmarva and using Dick Parker Autocollimator 

 

so eventually I got a 16” flat from a comparator, several waves not flat but smooth, so good enough.

 

and it’s quick and simple to see the figure

 

at Delmarva I had a hard time getting my blackberry phone in the cone correctly 

 

always work to, just get the lines smooth and straight to the edge, then worry about the ke shadows kinks 

 

https://youtu.be/Ef8...CRaMHk4evdtKyu7

[MKV]

What you want to know is what percentage of time is the exit pupil wavefront error below some threshold value. The bigger the figure errors, the less that percentage becomes.

The same goes for atmospheric conditions. You want a telescope with an exit pupil OPD of about 1/10 wave or less, PV, and hope the atmosphere cooperates. 

[a__l]

and then star test them ( waiting for perfect seeing)

 

This is much related to planetary observations. The observer waits for a short moment of improvement.
This is also related to planetary photos. People have made good programs that process video streams to discard noise. So getting a good StarTest is not a big problem. Including in the form of photos. At least for not very large mirrors. The sizes discussed here are quite suitable for this.

Edited by a__l, 12 April 2025 - 05:50 PM.

[Pinbout]

The same goes for atmospheric conditions. You want a telescope with an exit pupil OPD of about 1/10 wave or less, PV, and hope the atmosphere cooperates. 

1/2 of the mirror’s surface contributing the wavefront errors at the exit pupil is from the .70 zone to the edge. So stuff inside the .70 zone can contribute less. Especially when you have a 2ndry covering over the primary around 25%
 

To get an overall wavefront reading is by judging the 2ndry shadow size differences from both sides of focus. And to do that you need 2 shims to make sure the distances from best focus is same-same

 

and You need a well corrected eyepiece the same focal length as the mirrors focal ratio

below I test my f3.45 with a 3mm eyepiece

 

testing my 8” f3.45 

https://youtu.be/QxU...d7jbsA1eJj_orRg

Edited by Pinbout, 12 April 2025 - 06:52 PM.

[MKV]

1/2 of the mirror’s surface contributing the wavefront errors at the exit pupil is from the .70 zone to the edge. So stuff inside the .70 zone can contribute less. Especially when you have a 2ndry covering over the primary around 25%

This is true only if the mirror's figure is a perfect parabola (conic constant -1.0), and all other factors that affect exit pupil wavefront are either absent or ignored. In that case, one half of the contribution of the mirrors error will be the 71% zone (square root of 0.5 = 0.7071). In the conic is other than -1.0, the value of the zone will change proportionally,  and as you mention, when the secondary covers a fraction of the inner aperture. 

 

To get an overall wavefront reading is by judging the 2ndry shadow size differences from both sides of focus. And to do that you need 2 shims to make sure the distances from best focus is same-same and You need a well corrected eyepiece

I am not sure how sensitive that method is, especially for slower mirrors, and for very fast mirrors it may not be sensitive enough. Can you tell if an F4 is a 100% parabola or 98%? I think star testing doesn't allow such precision. 

 

Far more exacting objective and consistent results are obtainable when a mirror/lens testing or an OTA in done in a controlled environment on an optical bench using an interferometer, which is why professional optics (and advanced amateurs) don't practice star testing when it comes to the final assessment.

 

___

edit: removed top portion of the second quote as it contained irrelevant text from another quote, by mistake

Edited by MKV, 14 April 2025 - 11:58 AM.

[BGRE]

The quantitative version of the star test essentially measures wavefront curvature.

[MKV]

The quantitative version of the star test essentially measures wavefront curvature.

How does Roddier stack up against interferometric results? Isn't it still subject to atmospheric variance, like any star test? How does one standardize that issue? Why is Roddier not more popular?