6 replies to this topic

[mvdmitri]

Manufacturing the replica of the 7 feet Herschel telescope with 6.2 inch  bronze mirror in Japan:

 

http://www.ne.jp/asa...mmirror-j1.html

 

http://www.ne.jp/asa...mmirror-j2.html

 

http://www.ne.jp/asa...mmirror-j3.html

 

http://www.ne.jp/asa...mmirror-j4.html

 

http://www.ne.jp/asa...mmirror-j5.html

 

http://www.ne.jp/asa...mmirror-j6.html

 

http://www.ne.jp/asa...mmirror-j7.html

 

http://www.ne.jp/asa...mmirror-j8.html

 

 

 

Experimental bronze mirrors with different tin content:

https://okita-tenmon...almirror_x.html

https://youtu.be/j15kgNF1qEo.

Casting bronze telescope mirror for the Herschel telescope replica.

Edited by mvdmitri, 22 March 2021 - 10:53 AM.

[dgreyson]

Why bronze? Seems like a lot of maintenance to keep it optically usable. Or is this a sculpture rather than an instrument??

[DAVIDG]

 Bronze is misnomer since the metals used to cast bronze are  the same to used to cast speculum metal which is actually what the mirror is made from. 

 

                        - Dave 

[RalphMeisterTigerMan]

Correct me if I am wrong, but weren't speculum mirrors made 2-parts tin and 1-part copper? I never heard of speculum being made of bronze.

 

Clear skies and keep looking up!

RalphMeisterTigerMan

[AstroKerr]

Correct me if I am wrong, but weren't speculum mirrors made 2-parts tin and 1-part copper? I never heard of speculum being made of bronze.

 

Clear skies and keep looking up!

RalphMeisterTigerMan

more like 67% Copper, 33% Tin. Modern Bronze about 88% / 12% - varies. I believe older Bronzes had up to 30% Tin. Wiki, Britannica, et al. 

Edited by AstroKerr, 05 April 2021 - 06:50 AM.

[RalphMeisterTigerMan]

AstroKerr, thanks for the correction. I always get those two (copper and tin) mixed up. What I would like to know is how did guys like William Herschel test their mirrors to make sure that they were parabolic let alone how good they were? I don't think anyone was using anything like wave-front error (1/2-wave, 1/4-wave, 1/8-wave) or Ronchi test during the 1700's. Even if they wanted to see how good the mirror was during figuring, how did they manage it? I would love to know.

 

Clear skies and keep looking up!

RalphMeisterTigerMan

[Ken Launie]

The Rev John Edwards published in 1781 the results of testing more than 70 different speculum metal formulations. Most include arsenic in the formula, and his favorite was #47: 64% Copper, 30% Tin, 2% Brass (yeah, I know it's an alloy that includes copper), 2% Silver and 2% Arsenic. He called it "a most excellent metal, being by much the whitest, hardest, and the most reflective I have ever met with." Zinc, Antimony and even Platinum are among other ingredients he lists in some of the formulas.

 

My copy of this is within "Nautical Almanac Additions to 1812", published in London in 1813 by the Commissioners of Longitude. Edwards also gives suggestions on mirror making and testing, recommending an elliptical polishing tool as a way to parabolize a circular mirror.

 

--Ken

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