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Contact flat testing - keeping things clean

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#1 precaud

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Posted 20 April 2019 - 06:34 PM

I'm setting up to do fringe tests of some secondaries and mirrored diagonals tonight. Every time I do this, the aspect that I feel the most uncertainty about is cleanliness. And understandably so, given that I live in such a dust-laden environment (airborne particles are NM's #1 export).

 

What are your rituals/procedures for cleaning the reference flat and keeping it clean?

And then for the coated diagonals/mirrors?

 

I've read some silly things, like brushing then with the hairy backside of the hand before placing the contact flat down. Yikes.

 

What are "best practices" to follow before placing the ref flat onto the one being tested?

 



#2 BGRE

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Posted 20 April 2019 - 06:52 PM

A Fizeau interferometer with an airgap of a few mm  has an advantage over contact testing in avoiding scratches due to dust. However when testing highly reflecting flats against an uncoated reference surface the fring contrast can be low. 

 

The Rayleigh water test is also somewhat immune to scratching the test surface when dust is present.

 

Ultrasonic cleaning can be effective provided it doesn't damage the coating.



#3 precaud

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Posted 20 April 2019 - 07:00 PM

Yeah, but all of that is irrelevant to the question at hand. Please... can we talk about the subject?



#4 Pinbout

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Posted 20 April 2019 - 07:09 PM

I don’t really do much except for wiping down with a paper towel. If I get fringes I don’t worry. If I’m not getting fringes I yell at the moon. Howling at the moon.  lol.gif

 

AC5F2080-0D96-4BF4-A8CF-2D60118B0185.jpeg



#5 Mark Harry

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Posted 20 April 2019 - 07:19 PM

Clean off both surfaces with acetone, and a soft kleenex.
Place a new kleenex between the glass surfaces that are gonna be used.
Hold the top piece as you draw the kleenex out smoothly.

There should be bands there.


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#6 jelloptic

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Posted 20 April 2019 - 07:23 PM

John,

Clean as best you can by gentle wipe with tissue & solvent (methanol, alcohol) or blow-off with canned compressed air (NOT shop air).

Then lower test glass onto workpiece. Do no lateral translations.  If fringes are too numerous, then you're hung up on something.  Just separate and try/clean again.  One can use modest pressure to see which way the fringes move (and hence the air-wedge direction) but again, do not move one across the other.

Do not agonize over cleanliness; the reality is there are almost always something between to create an air gap.  Should you succeed in thorough cleanliness, you may put the optics into optical contact.  ...surface reflections disappear, the interface becomes transparent and the pieces are truly glass-to-glass in contact...  and stuck.

Been there, done that.

 

regards,  kbl


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#7 jelloptic

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Posted 20 April 2019 - 07:32 PM

Danny,

Nice photography in post #4.  Question: would the fringes show up better if you used a black background?

Q2:  are the fringes indicating concavity or convexity?   Not evident from just a still pic; only the operator poking the optic to see which way the fringes move can answer that.

 

kbl



#8 precaud

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Posted 20 April 2019 - 07:44 PM

Clean off both surfaces with acetone, and a soft kleenex.
Place a new kleenex between the glass surfaces that are gonna be used.
Hold the top piece as you draw the kleenex out smoothly.

There should be bands there.

 

Sounds so simple - I'll try it. Thanks Mark.

 

 

John,

Clean as best you can by gentle wipe with tissue & solvent (methanol, alcohol) or blow-off with canned compressed air (NOT shop air).

Then lower test glass onto workpiece. Do no lateral translations.  If fringes are too numerous, then you're hung up on something.  Just separate and try/clean again.  One can use modest pressure to see which way the fringes move (and hence the air-wedge direction) but again, do not move one across the other.

Do not agonize over cleanliness; the reality is there are almost always something between to create an air gap.  Should you succeed in thorough cleanliness, you may put the optics into optical contact.  ...surface reflections disappear, the interface becomes transparent and the pieces are truly glass-to-glass in contact...  and stuck.

Been there, done that.

 

regards,  kbl

 

That is pretty much what I have been doing, though not with canned compressed air (just a Giotto hand blower), and I have made side-to-side movements to try to reduce the air gap sometimes. I know I shouldn't, but I have...  must... stop.... doing... it....



#9 precaud

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Posted 20 April 2019 - 08:25 PM

I don’t really do much except for wiping down with a paper towel. If I get fringes I don’t worry. If I’m not getting fringes I yell at the moon. Howling at the moon.  lol.gif

 

Yeah, you just breathe on it and the fringes pop out... make it look so easy...



#10 Pinbout

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Posted 20 April 2019 - 08:31 PM

Danny,

Nice photography in post #4.  Question: would the fringes show up better if you used a black background?

Q2:  are the fringes indicating concavity or convexity?   Not evident from just a still pic; only the operator poking the optic to see which way the fringes move can answer that.

 

kbl

Black, no. The white board reflects the Flourescent light onto the glass and makes it more visible. A gel could be used over the camera to read one color, but why bother for a quick test like this



#11 Pinbout

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Posted 20 April 2019 - 08:34 PM

Yeah, you just breathe on it and the fringes pop out... make it look so easy...

Nope no breathing on it, that would distort the test tongue2.gif

 

it it is easy, unless your tool has AR coating on it.



#12 PeteDCard81

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Posted 20 April 2019 - 09:11 PM

"I've read some silly things, like brushing then with the hairy backside of the hand before placing the contact flat down. Yikes."

 

This is the method I have used for many years to prepare reference flats and work pieces before contact testing. No issues at all. This works fine for uncoated glass and reference flats that have a protective titanium dioxide coating.

 

Obviously you cannot do this with items that have reflective coatings.

 

No lateral movement when placing the pieces together.

 

 

Mark



#13 Ed Jones

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Posted 20 April 2019 - 09:54 PM

I agree with Mark Harry except I use Kimwipes or a lint free paper.  


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#14 precaud

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Posted 21 April 2019 - 01:03 AM

The tissue trick worked great. Thanks for the tip!

 

Apart from the elliptical needing to be cleaned several times, part of the problem I was having is, I was using a different florescent light this time, a standard 18" GE linear tube (not the "cool white" type). I could barely see the fringes, and not at all sometimes. I switched back to my old 13W GE CFL lamp and voila much clearer fringes. I'll post the photos tomorrow in a separate thread in the reflector forum. I wanted to do a comparison of an elliptical, as mounted by the factory with 2-sided tape to the stalk, and then removed from the stalk. The results are quite interesting...

 

Thanks for your help, and on such short notice!   smile.gif   bow.gif


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#15 ccaissie

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Posted 21 April 2019 - 05:25 AM

I cut bits of cigarette paper and keep a small stash of them.  3 pieces on the edge and lower your workpiece on it.  press selectively to get the fringes where you want.

Uncoated silica,  I wipe with my hand and then lower carefully.



#16 Mark Harry

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Posted 21 April 2019 - 05:27 AM

if you used a black background?
******

Dunno exactly how he made the pic, looks like a mirror underneath.
But at work, we use something black all the time with green, and it makes the bands roughly 3-5x easier to see.

Has to be remembered, if you have bands like Danny has there, that represents a thin air film between the pieces- and it can be slid around some to view the very edge areas, etc without scratching. (after all, you cleaned them, right?)
However, don't try to squash the pieces together- that can cause scratches, or "contacting", mentioned earlier.

Contacted pieces can be "defeated" by placing on a warm hotplate, with a scot towel underneath with pyrex or BK7.
Can be un-nerving, for it usually lets go with a "pop" or "bang". Kelly used to do this all the time.

This takes practice, YMMV.



#17 precaud

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Posted 21 April 2019 - 12:10 PM

Here's the thread with the comparison of interference fringes on an 82mm MA elliptical, factory-mounted versus unmounted:

 

https://www.cloudyni...-mounting-woes/

 

If I've made any technical errors in the description, please let me know.



#18 MKV

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Posted 21 April 2019 - 03:02 PM

I cut bits of cigarette paper and keep a small stash of them.  3 pieces on the edge and lower your workpiece on it.  press selectively to get the fringes where you want.

Uncoated silica,  I wipe with my hand and then lower carefully.

If you use spacers, you need a collimating lens. Otherwise, your readings are off unless you observe from a long distance. A long focal length Fresnel makes a good  collimator. 


Edited by MKV, 21 April 2019 - 03:08 PM.


#19 Mark Harry

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Posted 22 April 2019 - 04:29 AM

I would have tested that secondary before removing the taped holder. Your assessment is about right, however.
I punched a hole in Antares accuracy claims a long time ago, just for your info.
Not criticizing them, just reporting it happened with a cursory check. They made good on it.



#20 BGRE

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Posted 22 April 2019 - 05:22 AM

If you use spacers, you need a collimating lens. Otherwise, your readings are off unless you observe from a long distance. A long focal length Fresnel makes a good  collimator. 

The thickness of cigarette paper is typically around 75 microns or so.

To sure high contrast with a 5mm pupil (entrance pupil in the case of a lens) the corresponding viewing distance should be greater than about 125mm (lambda = 500nm) to ensure high contrast fringes.

Next, the distortion incurred by the variation in angle of incidence across the flat under test, if viewed from a distance of 5 times the maximum dimension of the flat the distortion will be less than 1%

Thus as long as the size of the flat under test isn't excessive its usually possible to view the flat from an adequate distance to ensure that the distortion is sufficiently low for accurate assessment of its figure without using a collimator.



#21 ccaissie

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Posted 22 April 2019 - 05:25 AM

I cut bits of cigarette paper and keep a small stash of them.  3 pieces on the edge and lower your workpiece on it.  press selectively to get the fringes where you want.

Uncoated silica,  I wipe with my hand and then lower carefully.

Sorry....Spacers means non-contact......not what OP asked...

Right.  if using spacers...view from along way off...



#22 BGRE

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Posted 22 April 2019 - 05:45 AM

The fringe distortion aspect is somewhat dependent of the spacing and applies but can be significant even if no explicit spacer is used.

With a 10 micron airgap a fringe distortion of 1% due the variation in the angle of incidence along the fringe results in an error of 100nm or about 1/5 wave at 500nm.

With a 100 micron airgap the distortion 

 

If an object space telecentric camera lens is used then the distortion can be small.

However these tend to be expensive especially those suitable for use in testing large flats. 


Edited by BGRE, 22 April 2019 - 05:48 AM.


#23 precaud

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Posted 22 April 2019 - 07:23 AM

I would have tested that secondary before removing the taped holder. Your assessment is about right, however.
I punched a hole in Antares accuracy claims a long time ago, just for your info.
Not criticizing them, just reporting it happened with a cursory check. They made good on it.

 

What "taped holder" are you referring to?

 

I am considering returning the Antares just because it is so much thicker than he said it would be.



#24 Mark Harry

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Posted 22 April 2019 - 04:05 PM

From the other thread, it looked to me like a 3M double-sided tape. ???

I wouldn't think it would make much difference if it was, because there's a lot of give, considering expansion differences.

BGRE:
"if viewed from a distance of 5 times the maximum dimension of the flat the distortion will be less than 1%"

Who are you trying to kid???

Are you talking about POWER wise-- the distortion?
Or something else???

I never miss on power/defect error by eye with direct IF testing, so you better have all your ducks in a row on this one.
If you cannot get to at least 10x separation, your flat assessment will be in error, no matter what your experience is.
Most definitely on any radii.
Don't get upset Bruce, I'm just trying to toss the mis-information.
IF with a certified rig compared to a specified limiting distance will separate the men from the boys.
(It don't look the same!)



#25 BGRE

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Posted 22 April 2019 - 05:23 PM

You seem unaware of the fact that the effective wavelength varies as the cosine of the angle of incidence. This was established about a century ago (Tolansky, Williams and Francon and others cover this  in their books on interferometry).  Haidinger fringes (fringes of equal inclination) demonstrate this effect clearly.

Haidinger fringes are readily visible in microscope slides and cover glasses for example preferably in reflection for increased contrast. However despite some textbook claims they are also visible in transmission (at least by some).

The variation of effective wavelength with angle of incidence is important in gauge block interferometry particularly with large airgaps. NRC have shown that due to this effect the finite source diameter also has to be taken into account. 

 

The consequence is that if one has a wedge airgap bounded by a pair of perfectly plane surfaces the fringes will not be perfectly straight if the angle of incidence (and consequently its cosine) vary along the fringe. The significance depends on the actual airgap and the change in the angle of incidence along the fringe. The effect is smallest for near normal incidence. 

 

If the flat is viewed from an angle far from normal incidence the effect becomes more significant.

 

If for example the airgap were 10 microns for a particular fringe near and parallel to the edge of a square flat viewed from a distance of 10 times the length of a side of the flat being tested, then for near normal incidence the cosine of the angle of incidence varies from 0.99875 to 0.9975 from the center to the edge of the fringe. The fringe will deviate from a straight line by about 1/20 of a wave. The effect becomes worse for shorter viewing distances quadrupling to 0.2 waves when viewed from half the distance. 

 

Should the flat be viewed from and angle of incidence of 45 degrees then the effect is much larger (around 14x for the above example).

Without an accurate value for the actual airgap correction for this effect isn't possible. If one has 20 fringes across the flat then for a 500nm source wavelength at best all that is known is that (for near normal incidence) the airgap varies by 5 microns in a direction normal to the fringes. In principle measurement at several wavelengths could be used as in gauge block interferometry to determine the actual airgap.


Edited by BGRE, 22 April 2019 - 05:28 PM.



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