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Help Needed with Tiny Mirror crack

ATM beginner mirror making
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#26 ccaissie

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Posted 27 July 2018 - 04:43 PM

I'm having a bit of difficulty interpreting the reflection, as my mirror is an old Pyrex with the 1/4" wide raised lip on the underside.  It almost looks like this is messing with the clear reading of the edge at times.  Your thoughts?

 

Exactly,  I have a 6f9 on my tester and the reflections from the back are distracting. Not the raised rim, the smooth cast surface just inside of it... sort of an interrupted circle of light..wrecks subtle shadow and fringe interpretation.  I'm ignoring it, but a light spraycoat of paint would help.



#27 jwestervelt

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Posted 27 July 2018 - 04:48 PM

I'm having a bit of difficulty interpreting the reflection, as my mirror is an old Pyrex with the 1/4" wide raised lip on the underside.  It almost looks like this is messing with the clear reading of the edge at times.  Your thoughts?

 

Exactly,  I have a 6f9 on my tester and the reflections from the back are distracting. Not the raised rim, the smooth cast surface just inside of it... sort of an interrupted circle of light..wrecks subtle shadow and fringe interpretation.  I'm ignoring it, but a light spraycoat of paint would help.


How does spraycoating the back help?  The reflections are still going to bounce off of the polished surface under the paint... unless I'm misinterpreting your method.  I'm really close to grinding down my polished back just to nix the reflections, but if there IS an alternative, I'm all ears.



#28 BGRE

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Posted 27 July 2018 - 05:48 PM

index matching paint.

#29 jwestervelt

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Posted 27 July 2018 - 06:14 PM

ahh...  next question, where does one find the index on existing paints, or are there specialty paints made for this?



#30 BGRE

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Posted 27 July 2018 - 06:37 PM

Paints etc used for edge blackening of lenses are supposedly index matched.
NB the Fresnel reflection formulae also include the imaginary components of the refractive indices.
The imaginary component is due to the absorption in the material. If this is too high such as in metals the reflectivity is dominated by this component.

Acrylic lacquers may work well.

Try some on a sample and use a laser with a thick glass sample so that the various beams can be easily identified when the beam strikes the surface at an angle that isn't normal to the surface.

#31 Pinbout

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Posted 27 July 2018 - 06:39 PM

 

My thought is that the mirror has a hint of a central hill still, and a bit of turned edge.  The diffraction ring is on both left and right sides, with a bit of a gap at top and bottom.  I'm having a bit of difficulty interpreting the reflection, as my mirror is an old Pyrex with the 1/4" wide raised lip on the underside.  It almost looks like this is messing with the clear reading of the edge at times.  Your thoughts?

You need to test the edge with a ke not ronchi

 

#5 shows a little hill, #6 doesnt

 

looks like everything is going in the right direction.



#32 BGRE

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Posted 27 July 2018 - 06:56 PM

Paints etc used for edge blackening of lenses are supposedly index matched.
NB the Fresnel reflection formulae also include the imaginary components of the refractive indices.
The imaginary component is due to the absorption in the material. If this is too high such as in metals the reflectivity is dominated by this component.

Acrylic lacquers may work well.

Try some on a sample and use a laser with a thick glass sample so that the various beams can be easily identified when the beam strikes the surface at an angle that isn't normal to the surface.

http://www.physics.l...fractometer.pdf

indicates why white paint isnt useful.
Its also a simple way to measure the refractive index of a piece of glass.
Black acrylic paint should help.

#33 Pinbout

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Posted 27 July 2018 - 07:03 PM

Painting the back, is a no no no, just say no, real loud.

 

i draw sharpie circles but that’s it, ok maybe a dragon.

 

if you have to you grind it flat. 



#34 BGRE

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Posted 27 July 2018 - 07:10 PM

Removing paint from a ground surface could be "interesting".
One could try oiling a black glass plate to the surface or maybe even oil/glycerin on black paper.

#35 Sam Danigelis

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Posted 27 July 2018 - 07:11 PM

Okay, some clarification.  Sorry, my ronchi pix were posted after working all night.  A mistake.  #2 and #6 are inside the R.O.C.  Everything else is "outside."  Now, a couple pictures of the knife edge 1" inside R.O.C. to test for turned edge, Per Sam Brown.  So, what does it look like to you?  BTW, I noticed that when I view these pics on my phone, they turn vertical, as they should appear.  On the computer, however, they remain horizontal.  I'm clearly a noob at this forum.

Attached Thumbnails

  • edge test 2.jpg
  • edge test 1.jpg


#36 Pinbout

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Posted 27 July 2018 - 07:18 PM

636A7F06-6E1B-4895-A208-9027FD785EDE.jpeg Put the ke just outside RoC and see if you get a bright edge on the knife side hard to take pic



#37 Sam Danigelis

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Posted 27 July 2018 - 07:28 PM

Thanks Danny.  I've taken hundreds with the cellphone.  Here are 3.

Attached Thumbnails

  • diffraction ring 3.jpg
  • diffraction ring 2.jpg
  • diffraction ring 1.jpg


#38 Sam Danigelis

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Posted 27 July 2018 - 07:38 PM

I am curious about the suggestion of painting the back with "index matching" (flat black, I assume) paint.  I suppose this might help kill a bit of the rear reflection off of the raised ring around the back of mirror.  If I did, it would only be with water-based acrylics.  I've read that any leftover oil-based material in the cracks and crevices of a mirror can mess with the aluminizing later.  Not sure.  If I can verify that the edge is okay, I may just skip this step.  I take it that's why you're discouraging this, Mr. Pinbout?



#39 BGRE

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Posted 27 July 2018 - 07:59 PM

Outgassing from residual paint can be an issue in vacuum systems, however there are paints with low outgassing.
Outgassing of coatings/paints etc can be problematic for instruments intended for operation in orbit and beyond.
Deposition of volatiles from paint etc on windows etc can be a serious issue.

Actually gloss black should also be fine and may scatter less.

#40 Pinbout

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Posted 27 July 2018 - 08:25 PM

Just stay away from painting the back.

 

your edge looks good.




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