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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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A "discussion" with a gentleman over the sensitivity & accuracy of the standard knife edge test, known as Foucault led me to believe he was in gross error. I recently completed a 6" F/13.63 mirror, that I tested thoroughly 4 times with naked eye, and 4.5x50mm scope attached to the test stand. More of this later.
The disagreement was, he insisted that Foucault was not accurate to closer than .010" accuracy at the ROC with reads. I felt that the consensus was this equated to about 1/10th wave as approximate (swa) guess.
I knew this was, if not grossly incorrect, was too fast a reply. F/D plays a large part in assessing tolerance of mirrors. Also, .010" was, and has been bantered around both in literature and in various places as "accepted norm". This may appear to be true if some details are accepted as "good enough". Those being-
Construction of toy testers as wooden blocks sliding on rods, with poor or springy KE arms with various loose tolerances.
Testers as a unit, being fairly light, and too easily moved by careless testing.
Accepting everything as read, being"the final word". I call this "the earth is flat" syndrome. I also believe that various pieces quoted as sources, are "cross referenced" and so have an inevitable bias as to what's possible. In particular, the .010" accuracy limit seems to be 5th hand and is repeated ad nauseum.
***********************
On to the test of that long mirror. Through data obtained on hundreds of mirrors, I have the following range of accuracy of readings with -MY- tester.( It consists of a lathe cross slide complete with adjustable gibs, on a flat steel top, which is welded to a column affixed to a transmission bell housing. To that, there is welded a tripod. Total weight around 100# or more. It doesn't move easily, nor flex when pushed on hard)
ROC range Tolerance (max + - value, typically -within- said value)
to 60" .001"
60-120" .002"
120-160" .003"
200-300" .005-.015" respectively
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The "nominal" test of this mirror revealed the efforts at making predominantly a sphere from the start. There exists a roll off at the edge, which equates to +.002" at the 4th zone, and .012" at the outer zone. Approximately 3/4" ~ of the outer part of the mirror is outside of the nominal sphere in the central 4.5"
It has to be remembered that this spherical section is a null with the KE test, and has all the inherent characteristics of a null. You see all the surface at once, astigmatism is glaringly obvious if present, and I had repeatability in this area down to .001" as long as I stuck with either eye, or scope throughout the test sequence.
The outer 3/4" of mirror, I bagged as +.002", and +.012" respectively in the outer 2 zones. Repeatability was within .0025" when average of all 4 tests were reduced.
Shadowing was very delicate and with just a cursory glance, the mirror appeared to be a perfect sphere. Only under close scrutiny over the period of an hour and a half could all the finer points outlined above be ascertained. Data on following post later.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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Test data as follows:
Spherical section comprising central 3 zones taken as .026" The 4th zone showed a very slight increase of ROC to the tune of registering +.002"~ average on the tester. The outer zone was reading +.012" average throughout the tests Tabulated as follows:
1 2 3 4 5
.026 .026 .026 .028 .038 (PVW= 1/37th, 1.00 St., max RTA=.1837)
Of course, a sphere being relatively "simple" to make, reduces as follows on my spreadsheet.
PVW Strehl max rta
1/17 1.00 .3216 (this all without refocusing from above nominal data)
When refocused -.0015", results are as follows:
1/25 1.00 .2902
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I'd like to cover the nominal measurements of this mirror as tested with the spherical information.
When taking off the correction at the outer zone equates to a change of the wavefront error, magnitude being 1/17-1/37 = .05882-.02703, = .03179(1/31.5 additional error across the .012 delta change. No refocusing.
The refocused sphere amounts to adding smaller error, equating to:
1/25 - 1/37 = .04000-.02703, = .01297(1/77)
Taking the average of these equates to about 1/44th additional error induced my retaining a spherical figure, over the range of .012" delta of the longitudinal abberation. I did this to get an average between the 2 extremes.
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Lets suppose that the addition is linear across the total delta. I know it isn't exact, but for brevity's sake to make a point when splitting hairs-
If the test is reviewed above, the tolerance found for the outer zone readings are found within .0025" (remember, this is my tester)
If we take .003" as the basis of acceptable tolerance with my test rig then approximately 1/4th of the additional error can be taken as a rough sensitivity with the setup I use.
Over that .003 total variance of my reads, the sensitivity of this KE tester exceeds 1/100th wave. This may be a moot point with this particular mirror. ML tolerance for the outer zone is + - .044". It might be argued from his point of view it's needless. But long mirrors aren't the only thing. Short mirrors exist as well.
This long mirror favors the characteristics that KE provides quite well. It maximises the accuracy potential of the test. There is -NO- other test whatsoever that can approach this precision existing today that I'm aware of with long mirrors Interferometers/AC included.
***************
But the blanket statement that KE is only good for .010" gross tolerance of readings/ accuracies no better than 1/10th wave PVW is bunk, plain and simple. That means the literature is incorrect. It all depends on how precise -YOU- want to be/ what's acceptable to you. I'm posting this to show what's possible with careful and diligent testing, good experience, and proper equipment; and what is factual first hand information. Regards, Mark
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So many projects, so little time!
Edited by Mark Harry (10/23/09 08:06 AM)
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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An old pic of my tester, with hundreds of mirrors to its credit. I have made a few changes, so I'll try to update this shortly. The battery is just in the background, not a part of the apparatus.
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So many projects, so little time!
Edited by Mark Harry (10/22/09 04:50 PM)
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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The slit has been replaced with a pinhole. There is a place for a scope and bracket to be bolted behind the blade. There is absolutely no play with the KE itself, and is operated by a floating pushrod/thd rod arrangement. Somewhat crudely executed, but very effective.
**************
One thing I must add about using either just my eye, or the scope. The deltas, and repeatability was exactly the same value wise. There was no difference. There were things I could see with only the scope, and higher contrast with just my eye.
One thing where the scope differed- The nulls, and zonal measurement readings over 161.58" ROC was .020" shorter than just my eye -AS A WHOLE-.
I also did some calculations of misalignment with the KE stage to the optic. 2 degrees amounts to an inconsequential difference in the readings (~.0001" over total correction)where it would affect error assessment. And that represents a rather HUGE amount (almost 6" at this ROC!), considering the flatness of my floor, and the initial aim of the stage itself.
I posted this as food for thought. I'm sure there are those who'd dismiss it prematurely. Their choice. It took me about 3 years to reduce my errors to the values stated with this tester. I suppose one of the wooden block testers could be built that could provide .010 repeatability in a couple evenings. I thought the effort I put into the tester paid smaller returns the longer I spent on it. But at this point, when I test with it, the results are what they are.
If I recall correctly, the ML standard varies linearly. That being so, if this mirror could be ascertained to be tested to 1/100th precision, and F/4.5 could be fabbed to 1/30th wave sensitivity easily; not taking into account the increased sensitivity at shorter ROCs.
Enjoy, Mark
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So many projects, so little time!
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 669
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HEY MARK where is the ronchi ? You showed me alot when i flew up .thanks for the info .kf
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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Scope/eye tidbits- Well, as I mentioned, the deltas were exactly the same with either eye or scope. The scope magnifies approximately 4.5x. It spreads the light over a roughly 20-fold bigger image plane by area, and contrast is drastically reduced. I feel that a magnification more like 2.5x might be about the right compromise to facilitate looking at the glass and obtaining the information needed. As it was, between both ways, The need of using a scope on a small mirror across a large distance I see as being a good idea. The scope excelled in determining width of zonal areas, and edge defects. I wish I could take photos, but my wife's camera tries to focus on the slit, so it never comes out worth a darn.
M.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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"where is the ronchi ?"
****************
(I stuck it in your flight bag when you weren't lookin'!)
M.
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So many projects, so little time!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Hi Mark,
I'm reading through this very interesting thread a bit slowly, but allow me to contribute the following:
On a well corrected smooth mirror Foucault is geometric in execution, save the very edge of the mirror where diffraction effects alter the zonal reading slightly (and this can be modeled and so corrected for).
The inherent accuracy of Foucault absent other errors typically encountered (human bias and limitations; mechanical - tester slop, inadequacy, inaccuracy; lack of thermal equilibration; inadequate support of mirrors under test) is extremely high.
For a fused silica 14.7" f/4.5 that I tested very carefully manually both visually and digitally (the latter having less bias) the agreement on the applicable areas was better than 1/40th wave PVW. The manual testing was quite repeatable, typically <.05mm on the longitudinal, fixed source, in line I think with what you're showing.
I haven't analyzed the repeatability on the digital system, but expect to as I'll be using the full-up automated version of the same technique in production soon. Here's a link to the reduced data comparison - it makes my point better than any number of words.
Now one can ask whether both of the curves are in error, and it will take further validation through a calibrated IF to answer that, or at least to pin another error bound on the data.
Best,
Mark
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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I tend to agree with Mark that after making over 55 mirrors myself and testing probably over a hundred other mirrors over the past nearly 40 years, I have developed a fairly keen eye for zonal testing and can usually repeatedly call zones to within 0.003"-0.005" for mirrors up to f/8 (f/16 at COC), at least in the outer zones. For f/4 mirrors and narrow edge zones I can usually repeat outer zone readings to within 0.002". The central zones are always cranky to read and my scatter increases toward the center, but their reading tolerance increases anyway. I visited about this very thing with Carl Zambuto off and on while he was making the McDonald Observatory 18" f/8 mirrors and he's pretty much of the same opinion regarding his zonal reading repeatability.
My 20 millidollars: to repeat zone readings to below 0.01" during a long work session requires:
(1) A precision tester with ball-bushing ways or slides, real micrometer spindles, and zero backlash (springs and/or magnets)
(2) A slitless tester and nice bright LED light of your preferred color and variable brightness (I like yellow the best),
(3) A REALLY steady test path with zero turbulence, often achieved by making a tunnel with bubble wrap right up to the test stand, and
(4) Jitter-free mirror and tester stands, preferably with remote-control az/el motors on the mirror stand to eliminate the endless back/forth trips during alignment.
I don't EVER want to take away from the simple wood and bolt-based testers so many people use and are rightly proud of - I built one myself way back at my start in ~1971, then bought one of those old University Optics testers and used it for years, until I got lucky at a ham swap meet and found the 2-axis precision stages with micrometers I use now.
But to punch through the 0.01" and 1/10 wave accuracy/precision barrier does take some fancier and more precision equipment, as well as a practiced eye that comes from doing a lot of mirrors, and independently cross-validation testing them on the sky, with autocollimation and/or with an interferometer.
Mike
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John Carruthers
Skiprat
Reged: 02/02/07
Posts: 2273
Loc: Kent, UK
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I agree that 0.010" may be a fair estimate for some testers but I would hope to do better. My tester is built around a micrometer head for advancing the knife. Several (read 'many') tests improve accuracy too. Repeatability is all and it was satisfying to see my values duplicated on another test bench at a different location with a different opperator.
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Jc
ATM 10" F6.1, 1/25th wave spec (max wavefront error +/- 1/12.6 in zone 4 of 6, sodium light )
6" F7 spec
127mm F9.4 Refractor
10 x 50 bin
ETX80 (finder)
Canon 20D
PST
DSI 1
and a curious mind
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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I have a new pic showing the scope mounted
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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2 pics of the mirror test "rack". It's a massive column mounted on floor and joist bearings, made out of ash hardwood, and 2x6s, glued and screwed together. It has a hinged spindle mount so as to accomodate a mirror mounted on a backer plate directly off the machine polisher. It also has bare glass mirror rack shown in use.
When the spindle mount is used, if there's a section of the mirror I don't touch, the repeatability is -WITHIN- .001" whatever the ROC is with any subsequent reads.
Mike mentioned about taking away from the wooden block testers, and that is the last thing I intend to do. -HOWEVER-, if you want to track just what the result of figuring actions are doing to the glass, you -HAVE- to have this repeatability available to make correct assessments of the actual wear taking place. This will quickly show how scetchy and inaccurate most all figuring information is in the printed literature that's available. I can't stress this fact enough, sorry to say. If correct information was available, it would eliminate a lot of tail-chasing, IMO.
Mark
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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The spindle mount.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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Mike wrote:"and independently cross-validation testing them on the sky, with autocollimation and/or with an interferometer."
Mark C. wrote:" will take further validation through a calibrated IF to answer that,..."
*********************************
I agree with testing them in actual end use, but with the case of this mirror, where the central 3/4 of the mirror's a sphere, AC will not show a null. Also, the 4th zone defined as +.002 delta, will result in a movement of the stage of AC setup in the vicinity of half, to 1/4 thousanth of an inch; depending on the light source being fixed or moveable(generally the latter) AC might see it, but I doubt it.
IF reference optics are considered mighty fine if 1/20th surface or better. I'm not sure it could possibly reveal any additional worthwhile info with this particular mirror. Not sure -if- the calibration required is possible...? I doubt it. There would have to be a point of reference used...?
*******************
I was spoiled with IFs. I could set a block of lenses in a rack, center the IF pattern, go to the monitor and stab a button on the keyboard. The phase shift would sweep across the optics, and BANG, there were the results of error, Strehl, RMS, etc. in about 1 second.
But when I came home the realisation of affording such a setup was way outside my wallet. I chose what was the best test as far as ease of use, reduction information available, least amount of reference optics, and highest sensitivity available within reason. KE made the most sense, especially when coupled with my previous polishing experience. A compromise nonetheless, but it also is fairly easy to understand conceptually and mathematically; a big plus.
All I can say, it works for me very well. I can see astig, detect anomolies well below the threshhold of other similar caliber home-brew test methods, track specific and targeted polishing regimens, and measure slope error very accurately. Might not be everybody's cup of tea, though.
********************************************
Referring to the first mentioned "discussion" I would not think it proper that my commentary be condemned on that aspect alone, nor accuracy potential doubted based solely on their particular experience and capabilities; and their use of antiquated and biased 5th hand information as references for their justification. I hope the gentleman comes to that realisation in the end.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3128
Loc: Northeast
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(sorry for the rant!)
M.
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So many projects, so little time!
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kfrederick
professor emeritus
Reged: 02/01/08
Posts: 669
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HI Mark How much more senitive is the knife edge as the distance increces .NICE setup
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RossSackett
professor emeritus
Reged: 08/17/07
Posts: 692
Loc: Memphis, TN
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Mark,
You've tramped through a lot of territory here that I have no qualification to comment on, but I will mention that I completely agree with the advantages of a heavy, solid base for the Foucault tester and smooth linear motions. I anchored my tester on a granite tile, and use an X-Y table designed for a milling machine for my motions, both for finding ROC and cutting in. I use a 0.001" micrometer head for the actual readings rather than the divisions on the handle wheels. With calm air, a dark room, and a 96" ROC mirror close to ambient my repeatability was around 0.003", not far from your experience. I also found that I was more reliable nulling by first darkening the zone, then cutting out looking for the first glimmer of light (rather than starting full-bright and cutting in for the first hint of a shadow). (I think I posted on that over the summer.)
I realize this isn't really what you were on about, though. 
Ross
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"A craftsman relies on science when the state of knowledge allows it, tradition and experience when it does not, and makes art whenever he can."
12 scopes from 4.25 to 18" and a 24" in progress. 12 ATM awards. Webpage: http://stardazed.com/ Some more scope pix at http://www.flickr.com/photos/8315630@N04/
Anagrams: Amateur astronomer = A mature moon-starer; Dobsonian maker = Debonair as monk
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Mark (et al?),
Much of this is over my head but I have a "toy tester" (I guess two if you count the AC) and probably the means to make something a little better so have a question:
How do you align the tester so that the KE stage is moving parallel to the optical axis of the particular mirror for that test session? Or do you need to? In other words so that at both extremes of its (longitudinal I think) travel it's parallel.
Quote:
but with the case of this mirror, where the central 3/4 of the mirror's a sphere, AC will not show a null.
Anyone:
This puzzles me. Why wouldn't AC show a null for the central 3/4 of the mirror, or for that matter, the entire mirror, given the focal ratio mentioned?
From my very limited AC experience it seems, with a fine screen, say 250 lpi, that it is very sensitive to edge errors - they stand out very clearly. I wonder how that resolution compares to what you can evaluate with KE, either visually or with the automated system described?
Thanks,
Gary
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Gary,
I test carriage alignment by first running the longitudinal stage in and out (axially) with the KE cut halfway in. If the KE shadow stays put during axial translation, it's aligned. If the shadow translates laterally during axial translation, that tells you which way to turn the stage. You can then turn the KE 90 deg. and repeat the process to get vertical alignment. I probably should have added good alignment to the list of 0.01"-busters I posted earlier.
I've started making an adapter for a laser pointer on my tester to try to speed the alignment up but haven't finished it. Even if that works well I'd still double check it with the process above.
Mike
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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One also needs a method to check the test procedure as a whole to determine if one has any bias in doing the test. How one person judges equally nulling of the zones can very differently from the other and one method can be correct and the other not. Without a cross check to making sure that there is no bias, one can be systematically adding errors to the results and not know it. These erors can both come from the person doing the test, the equipment and the procedure.
One common mistake is to move the knife edge close to were the ideal reading should be and then try to find a null. If you want to check your repeatablity you need to do the test and have someone else record the values. Then take the whole setup down, set it back up and do the readings again. You need to do this at least three times, ten is better and look at the standard deviation of each group of zone readings. One also should start at different zones, not just the center or the edge. In theory it shouldn't matter were you start, if the data shows it does, then you have a bias.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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