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Dual sided uncoated flats - a problem for DPAC

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#1 TG

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Posted 27 September 2021 - 01:10 PM

I have two flats, a 6" Zerodur one and a 12" fused silica one. Both are uncoated and have both sides polished flat. The problem I have is that when trying to use them for DPAC, I get reflections from both the front and rear surface. For the small 6" flat, the reflections are very close and using a red laser diode I get two airy disks, almost touching. For the larger flat, I see them separated by about 30-degrees at high power (200x). From this I can tell that the smaller flat has almost parallel sides while the large one has a convenient wedge. But even for the larger flat, when I use a Ronchi grating, the second reflection interferes (not in the wave sense) with the Ronchi pattern from the first one. Hence, I'm looking for a method to suppress the reflection from the back surface.

 

I tried spraying water onto the rear surface to optically scramble the reflection and it did dim the second Airy disk a bit, but because it was not not wetting it uniformly there was still a large area left dry between the droplets. Smearing oil seems too messy, requiring a later clean up. I also tried water-wetted lens tissues stuck to the rear and that did dim it, but not completely eliminate it.

 

Any suggestions here? Advice would be appreciated.

 

TG

 



#2 davidc135

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Posted 27 September 2021 - 01:23 PM

It's hard to think of a solution that isn't messy. Water's R.I is too low to work well. Syrup would be messy too but maybe preferable to oil when cleaning up.  David


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#3 ngc7319_20

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Posted 27 September 2021 - 01:34 PM

Similar sized piece of plate glass and some index-matched optical gel?

 

Index-matched black paint?


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#4 ccaissie

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Posted 27 September 2021 - 01:40 PM

I read where you could protect a polished surface when grinding the reverse ( I think it was a Maksutov trick) by using floor wax, then a layer of paper or film.  Something like a multilayered approach.  Something that could be cleaned up with thinner or acetone.  

 

Krylon flat black?


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#5 Aleksandr Naumov

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Posted 27 September 2021 - 01:48 PM

What if I wipe the back surface with hydrofluoric acid?

I hope it will not relieve internal stresses in the glass that can ruin the front surface.

 

P.S. I have similar 6" flat and getting ready to do DPAC, so probably I will encounter similar trouble.

 

P.P.S. Or is it better to just apply aluminum coatings to front surface?


Edited by Aleksandr Naumov, 27 September 2021 - 01:51 PM.


#6 ngc7319_20

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Posted 27 September 2021 - 02:35 PM

What if I wipe the back surface with hydrofluoric acid?

I hope it will not relieve internal stresses in the glass that can ruin the front surface.

 

P.S. I have similar 6" flat and getting ready to do DPAC, so probably I will encounter similar trouble.

 

P.P.S. Or is it better to just apply aluminum coatings to front surface?

Think I'd just grind the back surface + black paint, before I'd mess with hydrofluoric acid...

 

Yes, maybe better to just get a simple, cheap aluminum coating on the front surface...  Could remove it easily enough later on....


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#7 Aleksandr Naumov

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Posted 27 September 2021 - 03:03 PM

Grinding the back surface will relieve glass internal stresses. You may loose perfect flat on front surface.


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#8 TG

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Posted 27 September 2021 - 04:37 PM

It's hard to think of a solution that isn't messy. Water's R.I is too low to work well. Syrup would be messy too but maybe preferable to oil when cleaning up.  David

Hmm. Didn't think of syrup, but certainly worth a try and it can be washed off easily.

TG.



#9 TG

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Posted 27 September 2021 - 04:40 PM

What if I wipe the back surface with hydrofluoric acid?

I hope it will not relieve internal stresses in the glass that can ruin the front surface.

 

P.S. I have similar 6" flat and getting ready to do DPAC, so probably I will encounter similar trouble.

 

P.P.S. Or is it better to just apply aluminum coatings to front surface?

I would not touch HF with a 100-foot pole. 

 

https://en.wikipedia...uoric_acid_burn

 

Maybe you are a PhD level research chemist and can handle HF safely (if there is such a thing at all!), but even then you never know what will happen to the front surface.

 

TG



#10 TG

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Posted 27 September 2021 - 04:43 PM

Grinding the back surface will relieve glass internal stresses. You may loose perfect flat on front surface.

Yes, grinding is a no-no. Carl Zambuto says that he polishes his mirrors clear on the back before he figures the curve on the front to prevent the Twyman effect ruining it later if he did the reverse.

TG



#11 TG

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Posted 27 September 2021 - 04:48 PM

Think I'd just grind the back surface + black paint, before I'd mess with hydrofluoric acid...

 

Yes, maybe better to just get a simple, cheap aluminum coating on the front surface...  Could remove it easily enough later on....

Thanks, you just gave me an idea: an Al coating would be several hundred dollars but a silver coating would be cheaper and for a flat that sits most of the time in a sealed box, would probably be sufficient. There're good instructions available now:

https://sites.google...elescope-mirror

 

Silver coating one side of the flat would also leave the other side free for contact interference testing with a buit-in mirror on the underside (the Van Keuren flat came with a felt-spotted mirror to put underneath it to easily see contact interference fringes of opaque surfaces).

 

But this would have to wait for next summer.

 

TG



#12 ccaissie

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Posted 27 September 2021 - 05:05 PM

Even a 50% silver coat...doesn't have to be perfect,   can make a big difference in DPAC testing.  Testing an aluminized surface against a 50% coating is the way to go.  I have a flat that has a semi-coat of Titanium Dioxide for this.


Edited by ccaissie, 27 September 2021 - 05:07 PM.

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#13 DAVIDG

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Posted 27 September 2021 - 06:13 PM

 You can take another piece of glass like a piece of standard window glass and grind one surface. Now use a  few drops of liquid soap and "oil" the polished side of the flat to the polished side of the window glass. Now light will pass through the "oiled" surface like it is not there and then be diffused by the ground back surface of  the window glass.

 

               - Dave 


Edited by DAVIDG, 28 September 2021 - 09:43 AM.

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#14 starspangled

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Posted 28 September 2021 - 12:38 AM

I often have these problems with testing flats where there are parallel fringes interfering between front and back . I just use a small dab of petroleum jelly- called `Vaseline ', and smear it over the side you don't want reflecting . Just a small dab can be spread thin over the surface if you keep rubbing around - it only needs to be a micron thick layer to break the reflectivity of the polished surface . It will dry out a little and not become messy. You can seal the surface then with some contact plastic like you would cover books with -if you want to handle the mirror often .


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#15 TG

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Posted 28 September 2021 - 10:15 AM

I often have these problems with testing flats where there are parallel fringes interfering between front and back . I just use a small dab of petroleum jelly- called `Vaseline ', and smear it over the side you don't want reflecting . Just a small dab can be spread thin over the surface if you keep rubbing around - it only needs to be a micron thick layer to break the reflectivity of the polished surface . It will dry out a little and not become messy. You can seal the surface then with some contact plastic like you would cover books with -if you want to handle the mirror often .

Thanks, Vaseline + cling film, sealed on the edge with painter's tape might be the thing.



#16 DAVIDG

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Posted 28 September 2021 - 10:52 AM

 Hopefully the Vaseline will work but the reflection is coming from the inner surface not the outer one. That is why some of the other methods only partially work.  The water and other materials have  a partial refractive index match to the glass so some of the light is passes through  were  the  two materials meet and then get scattered but some is still reflecting off the inner surface. You need the light to pass through the rear polished surface into the other material that will scatter it. 

   So the concept you need to keep in mind is a method  to diffuse the light from the inner polished surface. That is why I  suggested "oiling" a piece of ground glass  to the back of the flat with the ground surface out and the two polished surface together.  The two polished surface when oiled together now disappear allowing all the light through to the rear surface of the ground glass. Now you have optically made a new  rear surface that will diffuse all the light.  

 

               - Dave 


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#17 TG

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Posted 28 September 2021 - 11:46 AM

 Hopefully the Vaseline will work but the reflection is coming from the inner surface not the outer one. That is why some of the other methods only partially work.  The water and other materials have  a partial refractive index match to the glass so some of the light is passes through  were  the  two materials meet and then get scattered but some is still reflecting off the inner surface. You need the light to pass through the rear polished surface into the other material that will scatter it. 

   So the concept you need to keep in mind is a method  to diffuse the light from the inner polished surface. That is why I  suggested "oiling" a piece of ground glass  to the back of the flat with the ground surface out and the two polished surface together.  The two polished surface when oiled together now disappear allowing all the light through to the rear surface of the ground glass. Now you have optically made a new  rear surface that will diffuse all the light.  

 

               - Dave 

Understood, Dave. Unfortunately, I'm not a glass pusher (yet), so getting a matching 12" think glass disk, grinding it, etc. is beyond my pay grade at this time. However, I see that the refractive index of FS (1.46) matches that of common oils like olive oil so I think a few drops or so smeared and covered with cling film and taped to the edge might offer a semi-permanent solution.

 

TG



#18 davidc135

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Posted 28 September 2021 - 01:08 PM

And if some light did manage to reflect off the shiny back of the cling film perhaps black polythene bin liner film could be substituted. It's RI is very close.

 

A glazier could cut a 12'' disc of window glass and one side roughed up with anything hard and a few teaspoons of 180 grit or iron filings etc.

 

David



#19 KLWalsh

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Posted 01 October 2021 - 12:41 PM

At work I test the reflectivity of Anti-reflection and conductive ITO coatings on filters we buy.
I have a piece of black glass (Schott IR800, iirc) that I ‘wet’ to the uncoated side of the glass using an index-matching oil. Any light that hits this wetted surface passes into the black glass and gets absorbed. Thus, only the surface I need to test creates a reflection.

A cheap (semi-permanent) way to do this, not quite as effective but probably sufficient for your needs, is to get a good quality matte black spray paint and lightly spray thin layers of black paint on the back side. Even if the index of refraction is quite different, the paint will help eliminate this reflection. (BTW, the reflection off a layer of glass, or any dielectric surface, is called a ‘Fresnel Reflection’).

The formula is R = [(n1 - n2) / (n1 + n2)]^2.
So for glass-to-air, n1=1.5, n2=1.0, the single-surface reflectance is [0.5/2.5]^2 = 0.04, or 4%.
Even if the paint index is as high as n2=2.0, the net reflectance would be [-.5/3.5]^2 = 0.02, or 2%, half that of air.
The closer the paint index is to glass, the lower the reflectance.

Edited by KLWalsh, 01 October 2021 - 12:42 PM.

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#20 starspangled

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Posted 01 October 2021 - 07:04 PM

A super thin smear of vaseline  destroys'' the surface from "front and back in my experience . Removes any sign of fixed parallel fringes forming between the two surfaces completely . Should be fine to remove reflections from the rear surface of an A/C flat .


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#21 jelloptic

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Posted 01 October 2021 - 07:21 PM

Back in the day, I would use Polaroid print coaters to suppress rear-surface reflections.  The applicators allowed one to quickly and safely "paint" a surface with the clear syrup-like fluid, which dried in a few minutes.   Readily removed with acetone.   An alternate was black Krylon spray-paint.




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