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Tag Archives: glass

Trying to Figure Out Problems With a Century-Old Refractor

22 Sunday Sep 2019

Posted by gfbrandenburg in astronomy, History, Optics, Telescope Making, Uncategorized

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Tags

antique, crown, flint, glass, Kiess, refractor, Telescope, testing

I am disassembling the lens cell of the >100 year old 6” f/14 Kiess refractor that produces horrible results on star tests.

There is absolutely no information inscribed anywhere inside the cell, inside the tube or outside it, nor on the edges of the lens elements. I can only guess as to what type of glass they used, and figuring it out won’t be easy. The least destructive method I can think of beginning to do this is by weighing them and calculating out their precise volumes, and from that calculating their densities. A graduate gemologist could probably calculate their indices of refraction, but not me.

Tomorrow I plan to measure the curvatures of the lens elements; perhaps someone familiar with old telescopes will then have clues as to who might have made this particular type of optical prescription.

The shims seem to me to be intact, so I think I can rule out astigmatism from lens elements put in crooked. [OTOH, someone on the Antique Telescopes Facebook group says that the large number of small black spacers in between the lenses may itself be causing the massive astigmatism problem that we found in the star test. I don’t have enough experience to be able to tell whether that’s correct or not.]

The small chips on the edge of the second (meniscus? Flint?) lens element were already there when I got it. I was also surprised to find that the first (biconvex, crown?) lens element has a small bubble very close to the center. It’s probably not significant, but I will check for strain as well.

 

Gently tapping off the lens cell from the tube
Gently tapping off the lens cell from the tube
Note that the retaining ring holding the front of the first lens merely slides into the cell; it’s held in place by four screws. The threading is on the inside of the ring, and the outside is smooth
Note that the retaining ring holding the front of the first lens merely slides into the cell; it’s held in place by four screws. The threading is on the inside of the ring, and the outside is smooth
You can see the black tape and tan cardboard spacers
You can see the black tape and tan cardboard spacers
Me looking puzzled
Me looking puzzled
The cardboard spacers around the edges
The cardboard spacers around the edges
The two lenses together; note the multiple, small black tape spacers between the pieces of glass
The two lenses together; note the multiple, small black tape spacers between the pieces of glass
The original chips on the second lens element
The original chips on the second lens element
The empty lens cell. Note that they didn't make it black
The empty lens cell. Note that they didn’t make it black

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