Jump to content


Photo

Melt variation on relative partial dispersion

  • Please log in to reply
5 replies to this topic

#1 Sasa

Sasa

    Viking 1

  • *****
  • Posts: 862
  • Joined: 03 Nov 2010
  • Loc: Ricany, Czech Republic

Posted 10 December 2012 - 07:36 AM

Hello,

I would like to know what is a typical melt-to-melt variation on partial dispersion (lets say for e,C lines) for a given glass.

I know that dispersion (nF-nC) could be controlled at level of +-0.00002 (and if one asks the company even on 6th digit level), and that melt-to-melt variations are even larger. But I can't find the number for relative partial dispersion. Since it is a ratio of two dispersions, the melt-to-melt variations in refractive indices probably mostly cancel out. Is it safe to assume, that the relative partial dispersion is controlled between various melts at the level of 0.0001 (the glass catalogues usually state relative partial dispersion on 4 digits)?

#2 ed_turco

ed_turco

    Apollo

  • *****
  • Posts: 1389
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 10 December 2012 - 08:55 AM

Others may disagree, but I don't think the partials change that much. Frinst: Take a peek between all the BK glasses and you'll find they all are pretty close partials to BK7. A small difference but not as much as one would hope.

I did my first research on this problem by calculating the partials out of an old Schott catalog, using the one of first digital calculators, a Friden, which weighed 15 pounds and had 4 functions!

WOW!

Ed

#3 Mike I. Jones

Mike I. Jones

    Gemini

  • *****
  • Posts: 3241
  • Joined: 02 Jul 2006
  • Loc: Fort Worth TX

Posted 10 December 2012 - 09:23 AM

That's my experience as well. I've seen melt pedigrees vary as much as 0.0003 from catalog, but the variation is usually constant over the g-C band. I've never seen partials change more than 1 or 2 in the third decimal.

This is not the case in the ultraviolet (like for U-BK7), where partials can and do vary more.

Great question!
Mike

#4 Sasa

Sasa

    Viking 1

  • *****
  • Posts: 862
  • Joined: 03 Nov 2010
  • Loc: Ricany, Czech Republic

Posted 11 December 2012 - 04:05 AM

Thanks, for the interesting information, this is what I was looking for. The 0.0003 variation is at the limit for what I'm trying to do (a design of long f/15 apochromat without any special ED or lanthanum glasses, the color shift is controlled by the uncertainty on relative partial dispersion and would be about f/df=3/err_theta, so with the err_theta=0.0003 I would still get respectable color performance but I would like it to keep it at this level).

Do you know if one can ask the glass companies to measure it with 0.0001 precision (for reasonable price)? I know, one can ask for example OHara company to measure dispersion at +-0.000003 level, but this would still give about +-0.0004 precision on relative partial dispersion for e,C lines. But may be there is some other clever way how to measure the relative partial dispersion.

#5 Mike I. Jones

Mike I. Jones

    Gemini

  • *****
  • Posts: 3241
  • Joined: 02 Jul 2006
  • Loc: Fort Worth TX

Posted 11 December 2012 - 06:48 AM

Give Arnie Bazinski at Schott Glass a call at 714-396-2157, and ask him about it. Tell him I said howdy.

What's not clear to me is your high sensitivity to index and partials. Normally one does a "melt re-comp" on a lens based on real glass data instead of catalog values. After real glass blanks are received along with their melt pedigree, a separate glass catalog is created in the design software that exactly models the measured glass pieces. The design is then re-optimized and re-toleranced using real data, which usually restores full performance. If you're just making one lens, this is no biggie. If you're making a small production run of, say, 100 lens assemblies, you'd better already have all the glass on hand, with melt data, otherwise you'll be tweaking tooling during production, which eats up profit.
Mike

#6 Sasa

Sasa

    Viking 1

  • *****
  • Posts: 862
  • Joined: 03 Nov 2010
  • Loc: Ricany, Czech Republic

Posted 11 December 2012 - 07:55 AM

Thanks Mike for the contact, so far the design is just for fun (an old style long oil apo triplet without special ED and lanthanum glasses) and I'm trying to understand its limitations and find some showstoppers. I could not find any except the large sensitivity to the level of control of relative partial dispersion (about 5-10x more sensitive than standard designs). Of course, one day I would like to build it (if I can find the required glasses at all).

Sorry, if I was not clear enough. My original hope was that I would not need to worry about melt-to-melt differences if the relative partial dispersion stays at about +-0.0002 level from the catalogue listed numbers. Of course, once I know the relative partial dispersions, I can optimize the performance accordingly.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics