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Interferometrically Testing Two Celestron C14 Edge Telescopes


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Interferometrically Testing Two Celestron C14 Edge Telescopes

 

John Hayes, Ph.D., Adjunct Research Professor

College of Optical Sciences

University of Arizona

 

Introduction

I’ve wanted to interferometrically test the two C14s that I have in my shop to get some good data on their optical quality for some time but gathering all of the equipment needed for such a test isn’t easy and I was hesitant to take my scope out of action when the skies were clear.  Since it would probably be cloudy for months on end in winter, December seemed like a good time to try to get it done.  Fortunately I have a lot of friends who were willing to help with this project and I was able borrow all the equipment needed to do the test. 

 

Collecting the Equipment

For the interferometer, I arranged to use a 4D Technology PhaseCam 6000 dynamic interferometer with a 1Mpx phase sensor.  This interferometer is very similar to those used to test virtually all of the 8.4 m mirrors made at the Steward Observatory Mirror Lab, the Hi-Rise telescope now orbiting Mars and most of the major components for the James Webb Space Telescope (the primary segments and the carbon fiber backplane structure.)  The PhaseCam is a state of the art interferometer system for long path length optical testing.

 

 

Figure 1. The PhaseCam is a polarizing Twyman Green interferometer.  This allows variable signal to reference beam intensity to give a high contrast signal independent of the part reflectivity.  It also allows the use of a pixelated polarization sensor that generates the four 90-degree phase-shifted signals needed to compute optical phase using a single frame. (Click image to launch larger version in a new tab)

 

The PhaseCam is a polarizing Twyman-Green interferometer so it is very light efficient—no matter how much light is returned by the optics under test.  Figure 1 shows the layout.   Its key advantage is that it can measure optical phase using just a single frame so the exposure time can be made as short as the available light will allow.  These characteristics make the PhaseCam very insensitive to vibration.  The fringe patterns may dance all around when things vibrate; but each frame contains all of the information to compute the optical phase—so it mostly doesn’t matter.  Since data can be taken very quickly, it is possible to average out time-varying noise sources such as air turbulence.  It also does away with the need for complex and expensive vibration isolation tables and fixtures.  The references listed at the end of this article describe the phase sensing technology used in the PhaseCam in greater detail.

 

Since the PhaseCam solves the problem of mechanical vibration, the next challenge is the requirement for a full aperture, precision flat mirror needed to do a double pass test.  Locating a good quality flat that’s a bit larger than 14” isn’t always an easy task.  (Yes, this test could probably be done with a liquid flat but that presents another set of challenges that I won’t go into here.)  Fortunately, the folks in the large optics shop at the College of Optical Sciences at the University of Arizona were willing to loan me a 24” coated precision flat.  They weren’t sure exactly how accurate it is, but they guessed that it should be “pretty good” with less than 1/10 wave PV of irregularity (i.e. +/- 1/20 wv  ~ 0.010 wv rms.)   I was so happy to find a good flat that I didn’t think much about the size until it arrived on the back of a truck in a crate that must have weighed around 250 lbs.  (Oh, oh!) The flat itself was a 4” thick and it looked like it was made of Zerodur.  If that’s true, the weight computes to about 165 lbs—and that’s about what it felt like.  Without a crane, it took three of us to move it.  I had access to a 22” Unertl tip-tilt mount that would hold the flat without too much trouble.  The mount is made of steel and cast iron and it weighs about 200 lbs.  We didn’t have a crane and the flat was so heavy that even with three of us, we couldn’t safely lift it up onto the mount on the optical table!  I quietly estimated the value of that piece of glass to be around $150,000 and I sure didn’t want to even slightly risk damaging it!  So we moved the mount off the table and set up everything on the floor.  Figure 2 shows a drawing of the setup and figure 3 shows a photo. 

 

Figure 2.  A schematic diagram of the double pass test.  Outgoing light from the diverger is carefully positioned at the on-axis field point in the focal plane of the telescope. The back working distance (BWD) was set to the 146.05 mm (5.75”) specification for the Edge system.  The F/9 test beam was selected to slightly overfill the F/10.8 pupil.  The telescope expands the beam and it is reflected back through the system and into the interferometer by the return flat.

 

At one point, I looked at all the equipment I had assembled on the floor to do the test and it likely adds up to over $300k worth of stuff!  This probably isn’t a test that most amateurs could pull off very easily.  If you put aside RC Optical Systems (now out of the amateur telescope business,) I don’t think that anyone in the “small telescope business” has anything even close to this capability.  (Yes, RCOS had a PhaseCam!)

 

 

 

Figure 3.  The actual test setup showing the PhaseCam on the left and the return flat on the right.  The box under the telescope simply acts as a spacer to get everything to about the right height.  One advantage of the large flat is that the telescope axis height does not have to be set very carefully.

 

Alignment and Testing

I set up the telescope to measure the on-axis field point.  First, I set the tilt of the flat mirror to null out the fringe pattern from the collimated output beam from the interferometer.  Then I inserted the telescope into the test cavity.  I put in a spacer (a 1 ¼” PVC plumbing fixture) on the back of the scope with an edge at the required back working distance of 146.05 mm (5.75”.)  With a piece of clear tape over the rear edge of the spacer tube, I could easily position the scope so that the focused spot was within less than 1 mm of the field center.  By tipping and tilting the scope, I could get the return beam to reflect back along the input path.  Sliding the telescope back and forth along the test axis brought the beam into focus.  The focal point was positioned within about +/- 2 mm from the correct back working distance spec.  The design tolerance for the on-axis focal position is quite low so the effect of such minor lateral positioning errors will be insignificant.  Once it was aligned, I removed the tape for final (minor) adjustments of the return mirror tilt to null the fringes.

 

I used the minimum exposure of 34 milliseconds allowed by the camera and it worked quite well in spite of a little vibration due to traffic on a nearby road.   In order to get the best results, I let both scopes thermally stabilize for a day (at about 20 C) before taking data.  I also used signal averaging to reduce errors due to air turbulence in the test path.  Averaging 64 phase frames produced frame to frame repeatability of about 5 milli-waves rms (<~1/40 wave PV.)  The interferometer/diverger combination typically has an absolute accuracy of better than +/- 0.05 waves PV over the full beam and I didn’t go to any special effort to calibrate out any residual errors in the system.

 

 

Figure 4. Centering the test beam at the correct back working distance.  The spot could be centered to within about 1 mm and set to the proper 5.75” BWD to within +/- 2 mm.  The clear tape allowed the beam to be precisely focused on the tape and made it easy to see the return beam from the flat as the telescope was aligned.  Look carefully and you can see the two spots.  The tape was removed before taking data.

 

 

 

Figure 5.  The difference between two time averaged measurements with the return flat at 0-degrees and rotated by 90-degrees.  The test 14” test beam is roughly 2-3” off-center on the 24” flat surface.  This data gives an indication of the quality of the return flat over the 14” aperture used for testing.  The PV of the difference is about 1/20 wave and the rms is a bit less than 0.01 waves. This appears to be an excellent flat over the 14” diameter test pupil. (Click image to launch larger version in a new tab)

 

Estimated Accuracy

Without performing a calibration against an absolute standard and lacking firm specs on the flat, it’s hard to give a firm statement of the absolute accuracy for this test set up.  In order to estimate the quality of the flat, I subtracted two wavefront measurements using an off-center region on the flat with a 90-degree rotation and found a little less than 1/20 wave PV and 0.01 waves rms difference between the two measurements—as shown in Figure 5.  So the flat is certainly quite good over the 14” pupil.  The PhaseCam/diverger combination is generally supplied with an accuracy of ~0.013 waves rms.  Since a certified calibration sphere was not available to calibrate the diverger optics, I measured the same system twice using two different diverger lenses as shown in Figure 6 to determine that the diverger lenses should be good to about 0.011 waves rms over the pupil.  Using these numbers, the estimated accuracy of these measurement is somewhere in the range of 0.015 - 0.020 waves rms, which is certainly good enough for this type of application. 

 

 

Figure 6.  Data taken of the first telescope with two different diverger lenses (and two different setups) to show the effect of the diverger optics on the measurement.  Wavefront A is the first data set taken with a long efl diverger.  Wavefront B is the same telescope measured with a shorter efl diverger after the telescope was repositioned in the test beam.  C shows the difference between the two measurements after resizing and aligning the data.  Edge mismatch phase errors have been masked (shown in gray.)  In this case, the field point was adjusted a little, which explains the tiny bit of coma (0.023 waves) that is evident in the C data.  In this case, coma has been introduced only because of an alignment difference so it can be subtracted as shown in D to get an estimate of the rms contribution due only to the difference in diverger optics.   This measurement shows good agreement between the two configurations with an upper bound of 0.011 waves rms agreement (0.035 waves PVq(99%). (Click image to launch larger version in a new tab)

 

Results for the First Telescope

The first C14 Edge HD system is the telescope that I most often use for imaging, which was produced early in 2015.  The optics are stock but the system has undergone numerous modifications to address alignment and mechanical issues.  The test data shown in figure 7 was taken after the secondary was carefully aligned.  With tilt and power removed (the alignment induced errors,) the single pass wavefront accuracy is 0.065 wv rms and 0.322 wv PVq(99%).  PVq(99%) is a PV estimator that gives the range of values containing 99% of the data.  The Strehl ratio measures at 0.845, which exceeds the minimum Maréchal criterion of 0.8 for a diffraction-limited system at the test wavelength of 632.8 nm.

 

 

 

Figure 7.  The single pass wavefront for the first C14 Edge showing Strehl performance at 0.845.   This data shows that the system exceeds the requirements for diffraction-limited performance.  Astigmatic error is largest contributor at 0.125 wv (not shown on the plot.)  The fringes (in the lower plot) also show that the secondary may be very slightly out of alignment; though the residual coma term is only 0.048 waves (Click image to launch larger version in a new tab)

 

In case anyone is bothered by how the results relate to the Rayleigh ¼ wave PV criteria, it is important to understand a couple of things.  First, the Rayleigh criterion was originally derived from 3rd order spherical aberration and it has been extrapolated as a rough approximation for balanced 3rd order terms.  Second, when we look at real-world irregularity, it is entirely possible for the OPD (optical path difference) between two regions to exceed the Rayleigh criteria and still have a diffraction-limited system.  As long as the area is small, slightly high or low regions will simply redistribute the light a bit without seriously impacting the overall imaging performance.  In this case, only about 3.5% of the data lies outside of the 0.25 wave PV limit, which is minor in the face of normal atmospheric turbulence.  While interferometric data is quite precise, it can sometimes be susceptible to coherent noise, which may corrupt the PV value.  That’s why estimators such as PVq(99%) are used to reduce the likelihood of  noise variations producing a wildly incorrect PV value.  There are a number of other PV estimators that have been developed besides PVq, (such as robust PV which is called PVr) but that’s beyond the scope of this article.

 

For a “pretty good” system the Strehl value can be approximated from the variance of the wavefront (s2,), which is computed over all the data points (not just two) and that makes it a much better way to assess imaging performance than the Rayleigh criteria.  For Strehl values as low as about 0.3 (Mahajan, 1982), the geometric relationship between rms (s) wavefront irregularity and the Strehl ratio for an unobstructed circular pupil can be approximated by:

                                                   [1]

This relationship is what sets an upper limit of about 1/14 wave rms on wavefront irregularity for any diffraction-limited system.

 

 

Figure 8.  Fizeau testing a C14 secondary mirror on a WYKO 6000 interferometer requires a large transmission sphere, a beam attenuator, and a precision positioning mount.

 

In the Edge design, secondary misalignment (in tilt) introduces field independent coma, but coma can come from both secondary misalignment and surface irregularity.  I stopped adjusting the secondary when the 3rd order coma value fell below 0.05 wv (0.048 wv.)  After taking a second look at this data, it’s clear from the fringe pattern that adjusting the secondary a bit more may have reduced the coma a little further.  If that’s the case, the Strehl in this system might have been slightly improved to 0.855, which isn’t much.

 

I found that the wavefront is quite sensitive to secondary tilt, which needs to be aligned to within about 1/50 of a turn of the alignment screws to minimize the on-axis rms wavefront errors.  That represents a tilt of about +/- 15 arc-minutes.  This is a very tight tolerance that is difficult to achieve without a lot of care and a good alignment signal.  Achieving this level of accuracy with a star test requires very steady seeing, a very sharp eye, and a bit of practice.

 

One of the main reasons that I wanted to test this telescope is that I’ve disassembled it many times and I’ve been slightly concerned that the main mirror may have been stressed a little the last time it was reassembled.  Indeed, the dominant wavefront aberration is astigmatism at 0.125 wave, which is an amount that could easily be due to mechanical deformation.  Keep in mind that it would only take 1/16 wave of astigmatic error on the mirror surface to produce this result and that’s not very much!  Errors of this magnitude could also be introduced during the fabrication of the primary mirror, the secondary mirror, or the corrector plate.  I’ve previously tested the surface figure of the secondary surface on a WYKO 6000 phase shifting interferometer.  Figure 8 shows the test set up and figure 9 shows the results.  This secondary has a very high quality spherical surface so it can be ruled out as a source for any of the astigmatic error.

 

 

 

Figure 9.  Test data measured by a WYKO 6000 interferometer showing the surface accuracy for the secondary mirror from the first telescope.  This is an excellent spherical surface with only very minor zonal circular zones. (Click image to launch larger version in a new tab)

 

Since both telescopes ultimately displayed similar astigmatic error oriented in the same direction, rotating the flat by 90 degrees made certain that the error was not in the flat.  There was less than 1/20 wave PV and 0.01 wave rms difference after rotation (as shown in figure 5).  Without removing the primary mirror for measurement, it’s hard to say if the error is in the primary or in the corrector plate.   Some folks have been successful at reducing the total astigmatic error in the wavefront by rotating the corrector relative to the primary to minimize the aberration but I didn’t try to make that adjustment.

 

Remember that the imaging performance of any system with a Strehl ratio of 0.8 is generally limited more by the diffraction of light than by optical aberration.  If the all of the astigmatism could be removed, the Strehl could theoretically climb to 0.946 or 0.957 (with the secondary perfectly aligned.)   As it is, the wavefront is fairly smooth and well corrected so the imaging performance of this system should be quite good.    This conclusion agrees with the excellent imaging performance that I have seen with this system both before and after this test.  Figure 10 shows the PSF performance of the system.

 

 

Figure 10.   The diffraction PSF performance for the first system computed from the measured wavefront data. A) The Airy disk displayed as I=Log(1+PSF) along with image scale circles. Keep in mind that even with good seeing, the long exposure FWHM blur size is only rarely smaller than about 1.5 arc-seconds.  B) The PSF displayed as I=log(1+k*PSF), where k =1000 to amplify the low intensity outer ring structure.  C) The same data as middle figure with the aberration errors scaled to zero to demonstrate a “perfect” pupil similarly sampled.  The bright second ring and low intensity of the third diffraction ring is due to the size of the central obscuration.  The slight irregularity in the rings is simply due to a sampling artifact.  80% of the total energy is contained within a 0.75 arc-sec aperture and 90% of the total energy fits within a 1.45 arc-sec aperture.

 

Results for the Second Telescope

The second system was likely produced in the first half of 2014 and acquired in August of that year.  The test results shown in figure 11 were taken after carefully aligning the secondary tilt.  This time, I simply looked at the 3rd order coma term and dialed it down to a minimum in each axis.  The values were very repeatable and it was pretty easy to minimize the value to within 0.010 waves with very tiny adjustments.  It was clear that this strategy quickly produced the straightest fringes in each direction to minimize the total wavefront error.  Once aligned, this telescope did not perform quite as well as the other scope with a PVq(99%) of 0.406 wv, an rms of 0.087 wv, and a Strehl of 0.741 so it falls short of the Maréchal criteria for a diffraction-limited system.  At 0.151 wv, astigmatic error is the largest contributor with spherical aberration a close second at 0.117 wv.  A previous test of the secondary mirror in this system using a WYKO 6000 (shown in Figure 12) revealed zones with a relatively large amount of correction so it appears that spherical aberration is not as well corrected as in the first system.  Still, simply correcting the astigmatism would bring the Strehl ratio up to 0.874.  Given most common seeing conditions, this system will certainly work just fine for long exposure imaging; however, under ideal conditions a sharp-eyed observer might notice a small difference in crispness in the finest image detail.  Figure 13 shows the PSF performance for the second system.

 

I experimented a bit with the focusing system, which introduces so much tilt that reversing focus direction introduces so many fringes that they can’t be resolved by the system.  Keep in mind that it’s a double pass test so the beam reflects from the primary twice, which amplifies any tilt by a factor of two.  Still, it appeared that the Strehl could be changed by as much as 0.1 by simply tilting the primary via the focus knob.  This simply confirms the common knowledge that these systems should always be aligned and focused by turning the focus knob in the same direction (CCW.)

 

 

Figure 11.  The single-pass wavefront for the second C14 Edge showing Strehl performance at 0.741.   This data shows that the system does not meet the requirements for diffraction-limited performance.  Astigmatic error is largest contributor at 0.151 wvs (not shown on the plot.)  The other issue is spherical aberration that fits to 0.117 wv, which likely indicates a bit less than perfect wavefront correction by the corrector plate. (Click image to launch larger version in a new tab)

 

It’s relevant to understand the required manufacturing tolerance on optical surface accuracy in a SCT to achieve diffraction-limited performance.  In order to achieve a Strehl ratio of 0.8, the rms wavefront error must be at least 1/14 wave.   Surface errors on the mirrors must be doubled to get the effect on the wavefront.  Since the telescope is a two-mirror system, the rms surface errors add in quadrature to get the total error (equal to half of the wavefront error.)  If we assume that the surface errors are split evenly between the primary and secondary mirrors, we get the total wavefront rms error as a function of the rms surface errors:

 

                                                      [2]

 

Solving for the rms limit on each surface:

                                                                                                                                                          [3]

With 1/14 wave rms as the target wavefront error, the irregularity on each surface must be no more than 0.025 waves rms.  If we assume a ~4x relationship between PV and rms, each surface must have a PV surface accuracy of at least roughly 1/10 wave PV or better, which isn’t trivial at this diameter. 

 

 

Figure 12.  Surface accuracy for the secondary mirror from the second telescope measured by a WYKO 6000 interferometer.  Large zonal corrections are evident. (Click image to launch larger version in a new tab)

 

 

Figure 13.   Plots showing the calculated diffraction PSF performance for the second C14.  A) The Airy disk displayed as I=Log(1+PSF).  B) Shows the PSF displayed as I=log(1+k*PSF), where k =1000 to greatly amplify the low intensity outer diffraction ring structure.  C) Shows the same data as middle figure with the aberration errors scaled to zero to demonstrate a “perfect” pupil similarly sampled.  80% of the total energy is contained within a 0.75 arc-sec aperture and 90% of the total energy fits within a 1.44 arc-sec aperture, which is virtually identical to the first system in spite of the slightly reduced Strehl performance.

 

 

Additional Discussion

1)     Neither of these scopes had been very carefully aligned under the stars before testing.  However, the first system had been aligned under the sky using the imaging camera.  I had simply centered the secondary shadow at high magnification and confirmed good centration on either side of focus.  The seeing was so poor that it was difficult to see what was going on really close to focus, which turned out to be a problem.  The system initially measured at a Strehl of only 0.61 and it was obviously out of alignment on the interferometer.  I did some imaging with the system in this state with pretty good success; though it was difficult to tell if the image quality was limited more by poor seeing, vibration from the wind, or the mediocre secondary alignment.  Still, it demonstrates the inherent difficulty of aligning the secondary under the stars with poor seeing.

2)     Secondary alignment is traditionally accomplished by adjusting secondary tilt to center the secondary shadow through focus and to make the Airy disk look as “good” as possible (small, round and uniform.)  That approach works because the Airy disk will always be optimized when errors in the wavefront due to misalignment are minimized.  Using the interferometer to align the secondary mirror accomplishes that goal by directly measuring the shape of the wavefront in the exit pupil.  Both methods accomplish the same thing but the digital interferometer greatly reduces the role of judgment in the process.  Numbers show unambiguously when the wavefront errors are minimized.  Unfortunately, the expense of this technique makes it a one-time affair-for me and probably a “never-going-to-happen” for most everyone else.  The only consolation is that even though seeing may limit how well a system can be aligned visually under the sky, seeing effects may also limit any additional benefit gained from “perfect” alignment.

3)     I’ve centered the corrector plate on both of these telescopes and I see no evidence of significant shear errors due to the factory surface correction applied to the secondary mirrors.  That is almost certainly because the corrections applied by the factory on the secondary (ref: Celestron Edge HD White Paper) address low amplitude zonal errors with low spatial frequency variations.   I have contended for a long time that small shifts in the position of the corrector/secondary assembly are unimportant and these measurements bear that out.

4)     Overall, I’m impressed that Celestron can ship such high quality 14” systems.  Achieving the required surface accuracy isn’t trivial at this diameter and Celestron generally does a good job of shipping high quality large aperture systems, but, it’s hard to tell how many exceed the diffraction limit.  This sample of only two telescopes suggests that most are pretty close.  The newer scope is a little better than the older scope, which may indicate that Celestron is improving their process but it would take more data to say for sure.  It would certainly be interesting to measure a new system straight off the current assembly line.

5)     I often use these two telescopes on a bench to set up and align my cameras, set filter offsets, and to calibrate my auto-focusing system.  I put a 15-micron pinhole on one of the scopes to create a near diffraction-limited collimator for the second scope.  I’ve been seeing on-axis astigmatic error in the whole system and it drove me crazy—mainly because I couldn’t see any similar error when I had each scope out under the sky.  These results show that both scopes have a bit of residual astigmatic error that roughly line up and that explains why I see it so easily on the bench with both scopes and not in the sky with each scope individually.  Mystery solved. 

I’ve since made a cradle that allows the collimator scope to be rotated by 90 degrees to fix the problem on the bench and it works quite well.  Through a high-powered eyepiece, the focused spot looks almost exactly the same as the diffraction calculation prediction.  That shouldn’t be a surprise but it’s a satisfying result nevertheless.

6)     I want to point out that these results only show the on-axis wavefront quality; however, this is the best way to determine the underlying residual optical errors in the system.   Due to weather limitations, I’ve only been able to take a few test images with the first system after this alignment session and the image quality appears to be excellent with sharp, round stars all the way into the corners of a 36 x 36 mm sensor (52 mm circle at the corners.)  Figure 14 shows the imaging performance over the entire field.

 

 

Figure 14.  A calibrated, stretched, unprocessed image with a 900 second exposure showing the field performance over a 36 x 36 mm sensor (16803.)  This data was taken with the first telescope under relatively poor seeing conditions after interferometric alignment.  Each panel is 500 x 500 px sub-sample demonstrating round, sharp stars over the entire 52 mm image circle. (Click image to launch larger version in a new tab)

 

 

Conclusion

High quality digital interferometric testing is a precise, unambiguous way to measure optical quality, which is why it is so widely used to produce the components for large-scale optical systems and space optics.  While this data very precisely demonstrates the optical performance of the telescopes at 20C, at the test wavelength, in the horizontal position, it is important to understand that these results also serve to set an upper limit on optical performance.  If the telescope components slightly change position when the system is pointed up into the sky or when the temperature changes, it will not perform any better than this data shows.  When properly designed, telescope performance should not radically change with either temperature or pointing angle, but at some level nothing is perfect.  So this data may not show the exact performance of either of these telescopes under the sky depending on the thermal and mechanical stability of each system.  In my experience, the first telescope (which I use the most often) has demonstrated exceptionally mechanical stability.  The data shown in Figure 14 shows how well it performs under the sky when it is thermally stable and my expectation is that it has the capability of delivering diffraction limited performance under real world conditions.  This report has demonstrated the kind of results that high-end digital dynamic interferometric testing can produce along with just some of the performance analysis that can be extracted from the data.

 

 

 

Suggested References

Edge HD, A Flexible Imaging Platform at An Affordable Price, The Celestron Engineering Team, April, 2013.

Virendra N Mahajan, “Strehl Ratio for Primary Aberrations:  some analytical results for circular and annular pupils,” J. Opt. Soc. Am., Vol 72,  No. 9, Sept., 1982.

J.  Hayes, J. Millerd, " Dynamic Interferometry, Getting Rid of the Jitters", Photonics Handbook, H-34, 2006

N. Brock, J. Hayes, B. Kimbrough, J. Millerd, M. North-Morris, M. Novak and J. C. Wyant, "Dynamic Interferometry", Proceedings of SPIE Vol. 5875 (SPIE, Bellingham, WA), page 58750F-1, 2005

M. Novak, J. Millerd, N. Brock, M. North-Morris, J. Hayes, and J. Wyant, "Analysis of a  micropolarizer array-based  simultaneous phase-shifting  interferometer", APPLIED OPTICS, Vol. 44, page 6861, 10 November 2005

J. Millerd, N. Brock, J. Hayes, B. Kimbrough, M. Novak, M. North-Morris and J. C. Wyant, "Modern Approaches in Phase Measuring Metrology", Proceedings of SPIE Vol. 5856 (SPIE, Bellingham, WA), page 14-22, 2005

James Millerd, Neal Brock, John Hayes, Michael North-Morris, Matt Novak and James Wyant, “Pixelated Phase-Mask Dynamic Interferometer”,  Proceedings of SPIE Vol. 5531 (SPIE, Bellingham, WA), page 304-314, 2004

 

(Most of the papers about the PhaseCam (and many more) can be found on the 4D Website at:    www.4dtechnology.com/applications/reflibrary/)


  • Daniel Mounsey, davidmcgo, mark cowan and 26 others like this


85 Comments

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contedracula
Feb 25 2017 05:17 PM

WOW !

Thanks John for sharing  :waytogo:  :waytogo:  :waytogo:

    • Daniel Mounsey and impartial like this
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rockstarbill
Feb 25 2017 05:25 PM

Great data John!  :bow:

    • Daniel Mounsey and impartial like this
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Daniel Mounsey
Feb 25 2017 05:35 PM

Wonderful work, good heavens John!

    • impartial likes this

That was very interesting reading.  

    • impartial likes this

Simply brilliant...truly impressed!

  Definitely a good test for Celestron's Edge! I'm truly impressed. Too bad you were "Weather limited" on the under- the- stars test, but I think you made your point. Let me know if you would like to test a small RC and I'll send you mine! (Just kidding:). I'd have the satisfaction of knowing it would be done precise and professionally. Great job, John! Ralph, Grants Pass, Oregon

Hi, John. Thanx for the very thorough measurement, eval and analysis. I worked aerospace for decades and was always taking astro-scopes and components into work to test (Zygo, Wyko, TWG etc. interferometers). I agree that AC double-pass off a superb flat is the premiere test. As you point out, calibration of the cavity is important/crucial. Art Jensen came up with a legitimate absolute cal, can be found in the literature. Otherwise, commercial interferometers are "only" certified to about 1/20 wave PV HeNe single pass wavefront. But, if calibrated in-house, the noise floor can easily be reduced to 1/100 PV. Anyway, I find your results consistent with what I had found on some stock telescopes: The B&L Criterion 4000 was terrible; Two Favorite Commercial SCTs were kinda disappointing; only the Questar (7-inch) was exquisite, right out of the box. Large catoptric scopes MUST be tested pointing up at the flat, otherwise gravity-induced aberrations will dominate. Tom Dey

    • wheelhouse likes this
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jhayes_tucson
Feb 26 2017 10:17 AM

Thank you everyone for your comments.

 

Tom,

Gravity-induced aberrations can refer to either 1) gravity induced "sag" of an optical surface or 2) gravity induced aberrations due to component misalignment.  In the first case, the problem of gravity induced changes of an optical surface is a problem common with ultra light-weighted mirrors designed for use in micro-gravity.  The Celestron systems use components that are quite thick and any changes in the shape of the components will insignificant and much smaller than the resolution of this particular measurement.  I commented on the second issue in the article.  It is true that any commercial SCT (Celestron, Meade) as well as many other Cassegrain type systems including Vixen and others may slightly change alignment with pointing angle.  I've made some internal modifications to the first telescope to minimize this issue and the image that I've supplied demonstrates excellent imaging characteristics when the telescope is pointed at the sky.  

 

Regardless, this kind of test is valuable to establish the inherent quality of the system and the ultimate optical performance that it is capable of producing.  Mechanical stress, alignment, thermal issues, tube currents, and atmospheric seeing may all contribute to reducing the performance of the system in actual use.  In my view, any well designed system for terrestrial use will show very little change in performance when the system is pointed at different angles.

 

John

 

 

(PS  I'm trying to remember if and where we might have met.  Your name is very familiar but I think I'm losing my memory as I get older!  Were you at OSC for a while?)

    • Daniel Mounsey and Gene@CN like this

I tested a c14 against a smooth 16"flat. 

 

Although it was still acclimating, you can see it was zoney as ****.

 

What surface?  I'm told its usually the corrector.

 

Visually it was easy to see the entire surface.

To photograph, I should shoot thru a beam splitter so the return beam isn't offset and avoid any vignetting.

 

Contrast can be controlled photographically, but for visual abserving...blaaaa.

 

https://youtu.be/BqnP3a3X07s

Photo
austin.grant
Feb 26 2017 03:18 PM

What an excellent writeup, John! Thanks for the time and effort you put into this, and for sharing it with us!

    • bsavoie likes this
John, awesome test and write up. We are lucky to have you here in CN. 👍

Sent from my SAMSUNG-SM-G870A using Tapatalk
    • Gene@CN likes this

John I think you might want to look more closely at the gravity-induced surface deformations of the primary as the source of some of the astigmatism.  I have done enough FEA work on conicals to know that there is always significant astigmatism at horizon pointing.

In addition, the basic design of the bonded hub mount used on the C14 is not really adequate for an optic of those proportions. A backplate is required but was not used. There is also an athermalisation issue with use of elastomer between the hub and the bore of the perforation in the optic. Check out the sums and see for yourself what happens at temperatures significantly different from the assembly and cure temperature of the potting...

The C14 is a diligently designed OTA that routinely produce remarkable performance in high resolution imaging but as you well know has lots of scope for improvement. In your case it may be that re-bonding and realigning the primary onto a better designed hub mount would significantly reduce your astigmatism and make this a top notch planetary imaging scope too. Propositions like this separate scientists from engineers!

What a fantastic write up. Thanks sir!

Photo
jhayes_tucson
Feb 26 2017 09:51 PM

John I think you might want to look more closely at the gravity-induced surface deformations of the primary as the source of some of the astigmatism.  I have done enough FEA work on conicals to know that there is always significant astigmatism at horizon pointing.

In addition, the basic design of the bonded hub mount used on the C14 is not really adequate for an optic of those proportions. A backplate is required but was not used. There is also an athermalisation issue with use of elastomer between the hub and the bore of the perforation in the optic. Check out the sums and see for yourself what happens at temperatures significantly different from the assembly and cure temperature of the potting...

The C14 is a diligently designed OTA that routinely produce remarkable performance in high resolution imaging but as you well know has lots of scope for improvement. In your case it may be that re-bonding and realigning the primary onto a better designed hub mount would significantly reduce your astigmatism and make this a top notch planetary imaging scope too. Propositions like this separate scientists from engineers!

Tony,

Maybe you are right but good engineering is driven by modeling and by data.  You've got a theory but in order to convince me that gravity loading on the C14 mirror will deform the mirror by more than say 1/20 wave, I'd need to see a model (FEA is fine) and accurate measurements to back up the results.  I spent a lot of years designing and building interferometers and I've seen a lot of crazy things that can distort optical components (as seen in actual measurements;) but, in my professional opinion, the structure of the C14 mirror is more than sufficient to maintain it's shape to within (at least) 1/20 wave in any orientation.  If I were to guess, I'd go further and bet that the C14 mirror maintains it's shape to within 1/40 wave or better at any pointing angle.  Send me a model and some measurement data and then we can discuss it further.

 

I may try to do this test again in the future when I switch the tube to CF.  So, the next time I make the measurement,  I'll simply rotate the system to see if the astigmatism rotates with the system or if remains constant but I will bet you a beer that it will rotate with the telescope.  (Note that I already tried this with both telescopes on my bench (as described in the article) and saw a decrease in the total astigmatism, which would not happen if gravity were the primary source of astigmatism.)  

 

John

    • bilgebay, Gene@CN and bsavoie like this
Photo
jhayes_tucson
Feb 26 2017 09:58 PM

I tested a c14 against a smooth 16"flat. 

 

Although it was still acclimating, you can see it was zoney as ****.

 

What surface?  I'm told its usually the corrector.

 

Visually it was easy to see the entire surface.

To photograph, I should shoot thru a beam splitter so the return beam isn't offset and avoid any vignetting.

 

Contrast can be controlled photographically, but for visual abserving...blaaaa.

 

https://youtu.be/BqnP3a3X07s

 

Danny,

You do indeed have zones, but a Ronchi test is not really able to tell you much about how the telescope will actually perform.  It is a test sensitive only to slope errors and it can't tell you much about the wavefront accuracy, which is what really counts.  I'm not trying to tell you that zonal errors are acceptable; however, it is important to understand that it's the magnitude of the zones that count and you can't get that from your test.

John

Well it is a null so if the lines are perfectly straight it would be better than 1/10.

 

From my experience from testing dpac, the only magnitude I need to know about these zones are they are not acceptable.

Photo
jhayes_tucson
Feb 26 2017 11:07 PM

No, that's not right.  A Ronchi test is not directly sensitive to wavefront errors.  It is a slope test.  You might want to look at Malacara's book, "Optical Shop Testing."

 

John

    • bilgebay likes this

No, that's not right.  A Ronchi test is not directly sensitive to wavefront errors.  It is a slope test.  You might want to look at Malacara's book, "Optical Shop Testing."

 

John

its not a ronchi at RoC but at focus.

 

its a slope test, testing a spherical wavefront, whose errors are doubled because of the double pass with the flat.

 

I guess atm's have it wrong this entire time.

 

http://mirrorworksho...ollimation.html

 

 

 

and to fully develop a optical systems profile, I find it funny you did record a visual record of the system, either ke or ronchi to see roughness and to visually see the zones.

 

which lends itself to stories we see on ZOC website under critireon #4

 

http://zambutomirror...oopticalce.html

 

Photo
jhayes_tucson
Feb 27 2017 10:26 AM

I can't see your workshop site (it requires some kind of plug-in.)  Regardless, it doesn't matter where you put the grating, a Ronchi test is a slope test--not a direct wavefront test.  Doing a double pass Ronchi test does indeed double the sensitivity.  

 

Wavefront data contains the slope information.  Because the interferometer is a coherent system, it shows not only the wavefront but it may also includes a little coherent noise superimposed on top of the data.  Even though the coherent noise is small, it may have large slopes so an interferometer test will generally show background roughness that is artificially high.  Consequently, a simple, white-light knife-edge tester is probably a better way to look at slopes.  I could show you the slope data for the first telescope but I can't attach anything here.  There are minor zones visible but the amplitude is small enough that they don't significantly influence the Strehl performance.  High frequency micro-roughness contributes to scattering and isn't a direct contributor to Strehl performance either.  As for Zambuto, I'm sure that they make some fine optics but what they clearly do better than anyone else is marketing.

 

John

Hi John et al., I'm enjoying the tech banter... lotsa good stuff in there. I used to do OSA, IES, etc. years ago, so you may have seen me there. Daniel Malacara and I did a joint paper on "Telescope Quality, How Good is Good Enough?" Other Optical Testing friends/associates were/are Rudolf Kingslake, Doug Sinclair, Jim Wyant, Chuck Spoelhof... and also... "The Community."  I'm retired, sorely miss ready-access to testing labs.  Tom Dey

    • Gene@CN likes this

Excellent. I learned something. Wahoo!

 

John I think you might want to look more closely at the gravity-induced surface deformations of the primary as the source of some of the astigmatism.  I have done enough FEA work on conicals to know that there is always significant astigmatism at horizon pointing.

In addition, the basic design of the bonded hub mount used on the C14 is not really adequate for an optic of those proportions. A backplate is required but was not used. There is also an athermalisation issue with use of elastomer between the hub and the bore of the perforation in the optic. Check out the sums and see for yourself what happens at temperatures significantly different from the assembly and cure temperature of the potting...

The C14 is a diligently designed OTA that routinely produce remarkable performance in high resolution imaging but as you well know has lots of scope for improvement. In your case it may be that re-bonding and realigning the primary onto a better designed hub mount would significantly reduce your astigmatism and make this a top notch planetary imaging scope too. Propositions like this separate scientists from engineers!

Tony,

Maybe you are right but good engineering is driven by modeling and by data.  You've got a theory but in order to convince me that gravity loading on the C14 mirror will deform the mirror by more than say 1/20 wave, I'd need to see a model (FEA is fine) and accurate measurements to back up the results.  I spent a lot of years designing and building interferometers and I've seen a lot of crazy things that can distort optical components (as seen in actual measurements;) but, in my professional opinion, the structure of the C14 mirror is more than sufficient to maintain it's shape to within (at least) 1/20 wave in any orientation.  If I were to guess, I'd go further and bet that the C14 mirror maintains it's shape to within 1/40 wave or better at any pointing angle.  Send me a model and some measurement data and then we can discuss it further.

 

I may try to do this test again in the future when I switch the tube to CF.  So, the next time I make the measurement,  I'll simply rotate the system to see if the astigmatism rotates with the system or if remains constant but I will bet you a beer that it will rotate with the telescope.  (Note that I already tried this with both telescopes on my bench (as described in the article) and saw a decrease in the total astigmatism, which would not happen if gravity were the primary source of astigmatism.)  

 

John

 

Fair enough John thats a good engineering answer! Unfortunately I don't own either a C14 or one of those hub mount parabolic primaries Synta uses the same Celestron pressed Pyrex tooling for: http://www.teleskop-...Rueckseite.html

 

BTW - If anyone reading this happens to have an accurate geometric model for one of these mirrors as a 2D drawing or 3D model plus photos of the hub arrangement I'd be happy to run some FEA sims to look at surface shape change due to gravity and temperature effects and post my results here!

 

Your observation about the effect of OTA rotation on the observed astigmatism in your Collimator/Telescope-under-test arrangement is pretty conclusive - the deformation is in the primary. But my point is that this may be due to stress caused by the bonded-in metal hub design. I've seen this problem in some of my past analysis work on hub mounted optics.

 

If I can provide sim data John I will. Please don't think I get off on inventing problems in what I know to be an established and well-made telescope. I'd just like to see you get at least one of the scopes to > 0.9 Strehl!

Photo
jhayes_tucson
Feb 27 2017 07:42 PM

Tony,

As I said in the article, I totally agree that the astigmatism could come from the way the primary is bonded.  I also pointed out that it might be possible to adjust it a little by rotating the corrector plate.  I didn't try that but I might in the future.

John

    • Gene@CN likes this
From what I can gather the C14 mirror blank has a ribbed back, a meniscus faceplate with a center cylindrical plug - one molded piece. The edge is fairly thin. I also have run FEA on mirror forms and performed trade studies for gravitational (and thermal) effects. I have not run the C14 shape, but based on my studies on similar, larger mirror forms, I would expect negligible effect of gravity on mirror figure on a C14. The challenge is the center mount, which could easily stress the mirror if not well designed and assembled correctly.

Hi John,

 

Thanks for posting your excellent write up on the testing of the two C14s.  

 

For my own edification (and hopefully others here:) ) if you can further clarify a couple points that would be great.

 

1.  Regarding the testing of the flat, are there any limitations to the method of testing the flat  by changing the orientation of the flat by 90 degrees?  In other words, if I have an excellent but non-certified 12.5" flat ( which I do), and use it to test a 10" optic with an interferometer in double pass autocollimation, if the flat is rotated 90 degrees and there is no significant change in the results (< 1/20th wave p-v) - its fair to say the flat is sufficiently good for the test?  Also,  I totally missed yor point about why the testing of the flat was done a few inches from center.

 

2.  You mentioned the tests done give info for on axis resolution.  Can you use your set up to test the Strehl for defined off axis points?

 

Best,

 

Alan



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