Optical experts here? Question about biconvex

Anyone here care to speculate the possible reasons why a very large Zerodur lens I found on ebay has a very pronounced curve on one side while a very shallow one on the other, one side so shallow I had to look close to make sure it isn't plano on that side? Maybe this is an all too basic question, but being new to optics I'm curious about the possibilities.

http://www.ebay.com/itm/390820422881



I opened the cell on this monster to measure the diameter of the lens at a full twelve inches. Although I cannot see any color in the glass while looking through the center, after opening the cell I can see that the edge of the lens has somewhat of a yellow tint to it, so I believe it is made from Zerodur, likely made for infrared use. I would probably be astounded at the original cost of this thing, I imagine many thousands of dollars.

Laser Project said:

Anyone here care to speculate the possible reasons why a very large Zerodur lens I found on ebay has a very pronounced curve on one side while a very shallow one on the other, one side so shallow I had to look close to make sure it isn't plano on that side? Maybe this is an all too basic question, but being new to optics I'm curious about the possibilities.

Large Refracting Lens in Cell 11 75" for Telescope | eBay

I opened the cell on this monster to measure the diameter of the lens at a full twelve inches. Although I cannot see any color in the glass while looking through the center, after opening the cell I can see that the edge of the lens has somewhat of a yellow tint to it, so I believe it is made from Zerodur, likely made for infrared use. I would probably be astounded at the original cost of this thing, I imagine many thousands of dollars.

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Sometimes that type of lens is used in an aspherical set up.

I've been googling lens ray patterns and I was wondering if it wasn't a type of aspherical. When I get home tonight I will take a closer look at its side profile and see. Attaching a photo I just found, the side with the larger curve on my lens isn't as pronounced as the lens shown on the right side of these two photo's, but the back side of my lens has a curve about the same as what this aspherical lens has, maybe it is one. The focal length of my lens is about 32 inches, when placing a piece of paper that far behind it the image from outside my living room window comes into sharp focus on the paper at that distance from the lens.

Laser Project said:

I've been googling lens ray patterns and I was wondering if it wasn't a type of aspherical.

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There can be different combinations of lenses of course, but aspheric uses are pretty common for this type at least.

I have experimented with many lenses over the years and
aspherical lenses have a bonding agent in them which appears
to burn under high power. I still recommend BK-7 with proper
AR coating if dealing with real power.
HMike

OOOPPPSSS -- I was thinking Achromatic lenses.
I'm way off on this. Sorry

Thanks HMike, didn't know that.

I took a picture of the lens, looking at the profile it doesn't appear to be aspherical to me, looks like a smooth curve.

I removed the 12" dia. lens from the cell and took a photo of each side, see a split image showing the two sides below:



Not sure how the difference in radii from side to side might affect the properties of this lens for either a mono-near IR telescope or a beam expander, anyone? Begging for advice I asked someone at Edmund Optics if I could still use this as a beam expander lens and they said yes, but I still want to know what difference it makes if one side has a larger curve than the other. Why the focal point of an image shows up at 32 inches regardless of which side faces the source is another mystery I'd like to find the answer to, I thought there would be a large difference in FL to bring the image into sharpness, depending on which side was facing the object but there isn't.

When I had this lens out of its cell overnight, this morning I found a very small white spot inside the lens, it appears this lens is made from more than one piece (but it sure didn't look like it at the edge) and there was a small 1/32" diameter spot where the glue had apparently separated between layers inside. I put it back in the cell and tightened things down, hoping it will go away

Laser Project said:

Anyone here care to speculate the possible reasons why a very large Zerodur lens I found on ebay has a very pronounced curve on one side while a very shallow one on the other, one side so shallow I had to look close to make sure it isn't plano on that side? Maybe this is an all too basic question, but being new to optics I'm curious about the possibilities.

Large Refracting Lens in Cell 11 75" for Telescope | eBay

I opened the cell on this monster to measure the diameter of the lens at a full twelve inches. Although I cannot see any color in the glass while looking through the center, after opening the cell I can see that the edge of the lens has somewhat of a yellow tint to it, so I believe it is made from Zerodur, likely made for infrared use. I would probably be astounded at the original cost of this thing, I imagine many thousands of dollars.

Click to expand...

I can't speculate the why, but I can tell you you don't have a biconvex lens. A biconvex lens looks like this () What you have is a positive menicus lens. From wikipedia Lens (optics) - Wikipedia, the free encyclopedia

I thought positive meniscus lenses had one side which was slightly concave, both sides of this lens are convex, although one much less than the other.

Laser Project said:

I thought positive meniscus lenses had one side which was slightly concave, both sides of this lens are convex, although one much less than the other.

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Looks like () to me, with one ( being more pronounced than the other )



Which is biconvex.

Its not a symmetrical biconvex though.

This could be part of a lens system that involves converging rays for example, and the focal point is shifted by the difference in the radius of curvature between the two sides.



I did a quickie googling, and found this:

http://www.tecnotticaconsonni.it/en...Biconvex_and_asymmetric_lenses_BCX_-_ABL.html

and this:

http://alumnus.caltech.edu/~leif/infratag/lens_choice.html


Hope it helps!

Thanks, that does help

I really would like to use it as a wide field near IR telescope with a CCD camera but not sure how well suited it is for such with these two curves. I just joined a telescope forum for more answers, this thread is getting into that territory now that I've decided I want to use it for such at IR.

Teej said:

Looks like () to me, with one ( being more pronounced than the other )



Which is biconvex.

Its not a symmetrical biconvex though.

This could be part of a lens system that involves converging rays for example, and the focal point is shifted by the difference in the radius of curvature between the two sides.



I did a quickie googling, and found this:

Biconvex and asymmetric lenses BCX - ABL - Spherical lenses - Production - Tecnottica Consonni - Lavorazioni ottiche, componenti e sistemi ottici dal 1957

and this:

Lens Choice


Hope it helps!

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The op said a shallow curve. I took it to mean inwards
This lens looks like a best form lens. That's the name I've seen Edmund Optics use.

steve001 said:

The op said a shallow curve. I took it to mean inwards
This lens looks like a best form lens. That's the name I've seen Edmund Optics use.

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Yeah a curve could mean convex or concave. ...His pics made it a lot clearer and if you'd seen them beforehand I'm sure it would have cleared it up for you.

We use best formed to get a custom grind...but we don't know what this lens looked like when ordered. ..so, the generic term would just be an asymmetrical biconvex lens...as its a biconvex that has different radius curves on each side.

That's a convergence type lens so it's just another way to get a focus point some where it's needed etc.

Sorry, I couldn't resist.

It's not spherical! It's biconvex an asymmetric biconvex which now appears to be achromatic too since I took it out of its cell and it began separating, arrrrrg....

Laser Project said:

it doesn't appear to be aspherical

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This must be a new type of lens made from an impossible shape.