Removing coating from a lens

[Grif]

Amazon shows used copies of texereau's book "How to Make a Telescope" for under $10

 

[Grif]

The difference between a mirror in a telescope and a lens is (squint hard when you read this ;-) is actually simple. A mirror is a one sided lens that has a reflective coating, not a transparent one.

 

[Sirius Glass]

... also a mirror reflects all wavelengths while a lens may not transmit all wavelengths completely and some not at all depending on, among other things, lens materials and coatings.

 

[AgX]

There are quite some mirrors that reflect selectively. But I admit the least are employed for imaging but for lighting.

 

[zanxion72]

It is a cheap lens. Sell it for as much as you can get and buy another one.

 

[Dali]

The difference between a mirror in a telescope and a lens is (squint hard when you read this ;-) is actually simple. A mirror is a one sided lens that has a reflective coating, not a transparent one.

Not as simple as you think. A lens needs concentric shapes, the mirror does not. To give a similar optical result, the mirror needs to be better surfaced than a lens. You can shape a mirror from (almost) any type of glass (and even from ceramic) which is not true for a lens. Texereau describes the method to yield an aspheric shape to the mirror (those used in most of the Newtonian telescopes). Aspherical lenses are less common...

 

[AgX]

Meanwhile there are already asymmetrical lenses designed.

 

[Dan Fromm]

Meanwhile there are already asymmetrical lenses designed.

Do you mean asymmetrical like the Tessar or with elements whose surfaces aren't sections of spheres? AFAIK the first lens with an aspherical element to be produced in quantity was Elgeet's 12(?)/1.2. In C-mount, for 16mm cine cameras.

 

[AgX]

I mean the surface of a lens element. And assymetrical, not aspheric.
A rather new technology.

 

[Nodda Duma]

Aspheric optics are now very common compared to, say, 10 or 20 years ago. They've even made significant in-roads into the commercial sector (the slice of the optical design world of most interest here). Every optical shop I know of has an aspheric polisher or diamond-turning machine which are pretty much in continuous operation.

AgX: The term is Free-form optics. The ability to design with them and fabricate today is roughly at the same level of development as aspheres were 20 years ago. However, they will never really see as much use as radially-symmetric aspheres except where folded optical paths are desirable (like in aerospace).

 

[AgX]

I know, I read the papers. We shall see what comes out of that.
Thank you for putting things into historic relation. Likely though technology has evolved to give it a faster pace.