26 replies to this topic

[gitane71]

Getting ready to work on a blank that was sold to me by a reputable glass company as F2, I noticed  'BK7' stamped into the blank.  It is obviously heavier than the crown.  Measuring the weight and volume of the glass, density figures to be 3.82gr/cc but the 'given' density should apparently be 3.6  .   How much variation is allowed in a density of a particular type of glass ?  Could this really be F2??

[triplemon]

Companies like Schott sell tiered with various limits on all their basic optical specs. Widest one is "step 3", nd +/- 0.0005

Comparing their F2 (nd=1.620, rho = 3.60) to the next denser flint SF2 (nd=1.648, rho=3.86) it sounds like it would be far more than what they would allow for F2 and it could rather be SF2 ?

 

But there is no universal standard what to call any given glass composition or tolerances. You often also read "comparable to company xx type yy". Which means its different in composition but still "close enough" in optical performance. And even what is "close enough" isn't a universal natural constant.

 

If its a somewhat flat blank, measureing the brewster angle with Pfunds method on a thicker blank would yield the actual refractive index accurate enough to tell 1.62 from 1.648.

 

If you mistrust your density number, weigh it twice - first in air and then submerged in water to get a fairly exact density without having to determine exact volume from mechanical measurements.

Edited by triplemon, 07 March 2025 - 04:47 AM.

[davidc135]

Hi Drew,

Looking at old catalogues from Pilkington and Schott they both give the nd of F2 as 1.62004 with s.gs of 3.63 and 3.61 respectively, so I expect that's the scale of the density tolerance.

The 2nd weighing in water gives the volume accurately, but maybe that's what you did.

 

David

Edited by davidc135, 07 March 2025 - 04:04 AM.

[gitane71]

Thanks guys !

  The blank is very flat, cast on one side and ground on the other, flat..

  I didn't know where the most accurate scales would be, but took it to the post office.  They gave me weights that gave pounds, ounces, and the ounces were were to the hundredths of an ounce (but both were '0'), so at least to the tenths of ounces.  

  I haven't weighed it in water and don't have scales to do that, would have to figure out where to get that done.  

  I have not tried a laser through the glass  yet ( assume the ground side would need to be smoothed up significantly) and am not  familiar with, haven't looked up the Pfund method yet.  

   On the blank I see 'BK7' in one place and on the same side, different place, I see 'SCR236-BK7', but the weight tells me this is definitely something else.   

   I bought the glass in the early '90's' as a sort of savings for retirement, hoped to start working some of this stuff.  We had a fire and I lost the papers, but definitely remember seeing (and thinking I was buying) F2 and it was wrapped up and labeled as F2. With the weight indicating SF2 am inclined to go with that. ?  

  Would that be unwise?  I need to do more testing ?  I am wondering about going to a different project.  

[triplemon]

With all that confusing labeling and density and some doubts about the volume (not the weight - you can trust the government for that scale) I would not suggest to proceed under any assumptions. You will want to do more testing and debugging until you clearly break the tie. Its IMO as unlikely a major glass manufacturer would mislabel a blank this badly than you getting the density totally wrong.

 

The Pfund method does not require polishing. If one side is cast, its smooth enough for the laser to enter, you will want the other to be ground.

https://www.lehigh.e...fundsMethod.pdf

Edited by triplemon, 07 March 2025 - 12:55 PM.

[davidc135]

To weigh in water you could make a balance from a regular length of wood. The distance from the glass suspension point to the fulcrum is known as is the distance required to the accurately known weight to balance the scale.

 

The suspension points and fulcrum should be on the same plane and the weight of the wood taken into account. (or just place the fulcrum at halfway point of beam) Should be fairly accurate if the total length of beam is 1.5m say and distances measured to 1mm.

 

David

Edited by davidc135, 07 March 2025 - 02:20 PM.

[gitane71]

Thanks, Triplemom for that link !!  And thanks David for that set up.  

 

I can Certainly understand that they would be highly unlikely to get things labeled wrong.

 

The blank is ...

 

8   9/16" diameter

1 and 1/32" thick

 

and has this stamped in it..

 

 

 

 

??

 

 

Attached Thumbnails

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[gitane71]

Oh !!  

And it weighs , according to the post office,

 

8 pounds. 1.40 ounces

[IslandPink]

That's looks pretty emphatic !

Looking at it, especially from the edge, does it look a different colour to your crown lens ?

 

Density tolerance on glass would be in the range ±0.1% , since this is the weighing accuracy that is required for the constituent oxides, to guarantee the optical properties.

I checked with the old Ohara PBM2 , to see if this was any different but it's also 3.61

 

Mark ( lens designer ) 

[Oregon-raybender]

It looks like the glass type was molded into the blank

and not hand written. So I would trust that it is BK7

It was remolded from a sheet I guess.

 

Starry Nights

[gitane71]

Hi Mark,

   From the edges, their color is very similar, 'clear'  ( or ? 'white') 

  I understand plate should be greenish.

but shouldn't 'normal' crown and flints be clear ?  

[gitane71]

Oregon Raybender,

  The other blank, which I am confident is BK7, is only slightly smaller and weighs 5 pounds, 5.60 ounces.

[triplemon]

Just to confirm your density math, I get 3.744 g/cm3 as the density.
To make this BK7 or any sort of crown glass, there would nerd to be some unnoticed bulge increasing volume by 40%. Thats reasonably unlikely.

Plausible measurement errors would be 1/32 in thickness or 3% in density, bringing sf2 glass nominal density within the margin of error.

Edited by triplemon, 07 March 2025 - 10:48 PM.

[IslandPink]

Hi Mark,

 

but shouldn't 'normal' crown and flints be clear ?  

Let me check with the engineers at work to double check.

You may be right that F2 would look clear, but I think SF2 would have a very slight yellow tint.

I know that SF6 and most lightweight flints definitely look yellow looking into the ground edge. 

[gitane71]

Ok, I must be doing something wrong.

  I got a laser and aimed the light in at 60 degrees (30 degrees from vertical)

  The bottom is ground so the return wasn't 'sharp', but I thought 

   I was seeing a reflected return at 24 degrees from 'normal'. 

   the reflection at the bottom of the blank was where the 'normal' was 

  (so, a total deflection of 54 degrees.)

   from this, I get a refractive index of 1.23 which doesn't sound reasonable at all.

  What am I doing wrong. 

  I tried shining the light through the blank, measuring the angle at exit, but the 

  exit beam of light was too diffuse. 

  ?  

[triplemon]

Not sure how you expect any kind of specular reflection or even refraction from a ground surface.
Pfunds method is about testing where total reflection stops, you measure distances, not angles.

 

See the link I posted.

Edited by triplemon, 16 March 2025 - 11:58 PM.

[luxo II]

 

  I got a laser and aimed the light in at 60 degrees (30 degrees from vertical)

  

So, just how did you measure these angles ? 

Do you understand the maths around the errors in measurements ?

 

This isnt a matter of just eye-balling it...

Edited by luxo II, 16 March 2025 - 08:42 PM.

[gitane71]

Hi Luxo II

  No , probably not.  I've never tested for R.I. before.  Snell's law was what I was trying to use.

  I layed out some towels on the floor, set a box on them.  I cut a cardboard 'base' that allowed the laser to rest on it and I could shine the light into the blank at 60 degrees.  I had more than 1/2 the blank on the box so gravity kept it in place there. 

  On the part that overhung the box, I tried to get a beam that was above or below, wasn't sure where I could measure it, but had a 180 degree plastic protractor to try and measure what I could access, but there wasn't anything clear enough.  

  I guess the bottom of the blank will have to be ground much smoother than it is.  

[gitane71]

Hi Triplemom

   I have trouble with computers.  I have a schedule that includes grandkids and parents.  I am 'retired' but have a lot of other stuff going on.  I thank you for that link.  When I did have time for it, I didn't see it.  I am away from home sometimes and often use the phone, and I don't see things as clearly on that, a Lot smaller a lot of the time.  I am trying to get better at that.  

   Not having done this before, I am not sure what works and what doesn't.  I don't know what the 'limits' are on this stuff.  Yes, I will probably have to work on that surface to get it much better in preparation for trying again.  I am not sure how soon I will get time to work on that surface.  

  Fine ground apparently isn't enough.  It needs to be polished?  

[triplemon]

  It needs to be polished?  

Please, read the document at the link I posted. It at length describes how it works - which is nothing like your idle speculation and using snells law. Pfunds method is very specifically made to test only coarse ground glass blanks.

 

The thing is, there are four hundred years of wisdom in those things. Reading the books, follow and read the links is really the only way to benefit from that. Otherwise you're going to operate like Lippershey and Galileo, having to find out everything the hard way. 

Edited by triplemon, 17 March 2025 - 12:33 PM.

[gitane71]

Triplemom
Thank you very much for the link. That sounds like what I need and is helpful. I knew I had seen it but had been trying a variety of sites and forgotten exactly where I had seen it. I have a variety of things going on and don't often get a chance to concentrate on this. When I checked on Cloudynights, on the phone, I need to have the 'full version' visible I think or the link isn't as noticeable. I will spend some time on it.
Thank you very much !!

[gitane71]

I have used the Pfund method and my numbers indicate I have glass with a refractive index of 1.70
I assume this is not F2 and can't continue with this project

[davidc135]

Can the glass be returned or has too much time passed?  David

[gitane71]

One of my 'strategies' was to save in glass, have glass to work in retirement. This was purchased probably in the early 90's. I am sure it is too long ago.

[davidc135]

That's disappointing. There's someone on 'Astronomy buy sell' here advertising a pair of 175mmx25mm BK7/F2 blanks for £350.00. From Schott with melt data. Advert # 224174. According to him, they'd cost near 4 figures new. Still a bit of money, though.

 

David