17 replies to this topic

[WallyZ]

Just a general question

 

With all the advancements made in optical engineering and manufacturing what is the general consensus regarding the use of plastic versus glass in the use for making mirrors?

 

Can decent mirrors be made from plastic or is glass still the preferred medium?

 

Does anyone make mirrors from plastic?  Is that a thing?

 

What is the difference in end viewing quality for these two mediums?

[MitchAlsup]

In general, plastic is only useable for lenses for which one does not need an "optical" surface--such as reading glasses.

 

There are certain ceramics that are essentially equivalent to glass. These ceramics have their grain-structure smaller than the wavelength of visible light (cervit, zerodur,...) These are more glass-like than plastic-like.

[Scott E]

The closest thing to plastic I know of is the use of composites for thin aerospace applications but it's been a while since I've heard anything on this. I don't know if they've got the kinks worked out yet or how good they actually are.

[WallyZ]

interesting.

 

The wavelength of the medium was not something I had considered. 

 

Since mirrors are coated with another medium which forms the actual reflective surface how does the underlying material (glass, ceramic or plastic) affect this coating?

[MeridianStarGazer]

Every plastic I know of has a much higher coefficient of thermal expansion.
Their stiffness to weight ratio is much worse.
Their conductivity much lower.
There softness may present challenges with grinding and polishing.
Good luck polishing something that changes size and shape readily under slight heat and pressure.

[MeridianStarGazer]

The only chance I see is a super well calibrated nano smooth mold that accounts for shape change during cooling and leaves a ribbed mirror ultra thin. That would be an expensive mold and expensive trade secret. No way to touch up the figure. Anealing plastic is likely just as important as annealing glass.


Cheap binoculars show softness even at a 5mm exit pupil. Good lenses cost a little money. So do eyepieces. If binos are sold for $15, you know some serious corners were cut.

[ccaissie]

The $5 SvBony eyepieces have molded acrylic lenses.  Not great oculars, but they do work.

[RalphMeisterTigerMan]

Is it not easier to polish glass than to polish plastic? After all, we are all taught that the smoother the optical surface, the better the view thru the eyepiece will be.

 

Clear skies and keep looking up!

RalphMeisterTigerMan

[Pinbout]

 

Is it not easier to polish glass than to polish plastic?

they sell polishing pads for glass as well as plastics

 

you can polish well enough, just can't get a 1/20~ tolerance on the figure, probably more like 2~, so if you don't mind observing with a 35mm eyepiece only, knock yourself out.

 

there's plenty of commericial glass mirrors that are worse than 2~ 

 

  

[Steve Dodds]

The $5 SvBony eyepieces have molded acrylic lenses.  Not great oculars, but they do work.

I have the Svbony set (6-9-15 and 20) and they are all glass and work great at f/10

[Steve Dodds]

Is it not easier to polish glass than to polish plastic? After all, we are all taught that the smoother the optical surface, the better the view thru the eyepiece will be.

 

Clear skies and keep looking up!

RalphMeisterTigerMan

It's A LOT faster to polish plastic, it takes about 1-2 minuets to polish an eyeglass lens.

[MeridianStarGazer]

The $5 SvBony eyepieces have molded acrylic lenses. Not great oculars, but they do work.

The eye lens won't through the light too far off course, being closer to the eye. Even getting eye lash grease on it is somewhat accessible.

[Oregon-raybender]

I worked in the plastic optics industry for 5 years, injection molding and some casting. Plastic optics for lenses works

fine if small. If larger than 25mm the process is complex plus the limited materials is an issue. PolyCab is the best all around.

It does not take in water like acrylic does. Make many aspheric lenses for imaging systems and make the first optical mouse

optics for Apple, MicroSoft and LogicTech. They used the same design. I don't see much improvement in the coming years

were it will become equal to telescope grade. As far as mirrors, no. We tried to make 1/4 wave plano optics, 10mm, but no luck

Most the mirrors make for bar code readers are 2-5 fringes, were most of the plano mirrors are manufactured for.

 

So to answer your question, no  not there yet and not much reason to. Flat optics is make a large enough scale now using

glass. Polishing plastic is not worth the effort.

 

Starry Nights

 

https://sydor.com/ca...ided-polishing/

[RichA]

Just a general question

 

With all the advancements made in optical engineering and manufacturing what is the general consensus regarding the use of plastic versus glass in the use for making mirrors?

 

Can decent mirrors be made from plastic or is glass still the preferred medium?

 

Does anyone make mirrors from plastic?  Is that a thing?

 

On in camera lenses where plastic aspheric lenses are now common, but not in lenses where real precision is required (i.e. the use of high power, long focal lengths for viewing/photography).  They even have fresnel lenses in camera camera telephoto lenses but not in telescope lenses.

[kur3tking]

The closest thing to plastic I know of is the use of composites for thin aerospace applications but it's been a while since I've heard anything on this. I don't know if they've got the kinks worked out yet or how good they actually are.

Yes, the acrylic mirrors were much more difficult to manufacture than glass, mainly due to the very high coeficient of expansion. We tried to keep the temperature within 2 degrees during fabrication but the heat from the large grind/polish motors alone made that unworkable . We made 9 sets or 36- 2.5 meter acrylic mirrors for the CH-47 Chinook helicopter TFPS sims before we talked the government into a shatterproof glass.  Below is a Ronchi shot of a typical acrylic mirror. Disregard the lines and imperfections as they are on a ginned up Ronchi screen.  For the application of viewing from the pilots eyepoint at the mirror FL, the acrylics are magnitudes better than mylar systems but far below the quality of our glass mirrors.

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[jpcannavo]

interesting.

 

The wavelength of the medium was not something I had considered. 

 

Since mirrors are coated with another medium which forms the actual reflective surface how does the underlying material (glass, ceramic or plastic) affect this coating?

Always wonder why there are not more amateur offerings in Zerodur

Joe

[PrestonE]

Always wonder why there are not more amateur offerings in Zerodur

Joe

The Expense something like 2.5 times the cost for the blanks if I remember correctly...

 

Best Regards,

 

Preston

[Steve Dodds]

Always wonder why there are not more amateur offerings in Zerodur

Joe

Zerodur is about $60-$80 per pound  Pyrex is about $15