Plastic lenses for Astronomy

[Tomjo59]

A random musing for a Sunday evening.

The lenses in my spectacles are plastic. They correct my short-sightedness and astigmatism perfectly.

Has anybody or any company tried creating lenses for serious astronomical use using plastic, the equivalent of apochromatics, for instance?

Obviously, I’m not talking about the lenses in cheapo refractors from department stores.

I assume there are good technical reasons why this has not already been done, rather than using exotic and expensive glass; I would just be interested to know the reasons.

 

 

 

[PeterStudz]

Good question.

I’m very short-sighted. In the past I use to be able to get glass lenses for my spectacles. In fact I preferred glass, as compared to plastic it was thinner (I have to pay a lot more for thinner plastic lenses), as they were thinner they were lighter, and they didn’t scratch as easily.

I assume glass became obsolete for safety reasons. 

[Zermelo]

48 minutes ago, Tomjo59 said:

Has anybody or any company tried creating lenses for serious astronomical use using plastic

I don't know if you count this as serious, but I did read somewhere that this series contains a plastic element:

https://www.svbony.com/optional-lens-4mm-10mm-23mm-eyepiece/#F9301AA

I have a pair of the 23mm.

[Vash]

Plastic lenses can be of very high quality. All mobile phone camera lenses are injection moulded plastic lens. 

Plastic lenses usually have lower refractive indexes than glasses and the range of abbe numbers are more limited. Lens designers can make up for those short comings by using extended aspheric lens shapes. Just Google 'mobile phone camera lens design' to see some examples. 

 

When it comes to telescopes, plastic lenses could probably be used but it would probably mean additional elements to produce a well corrected image. 

Injection moulding lenses is also very expensive in small quantities but becomes incredibly cheap if you are making millions of lenses.

I would guess that the volume of sales of telescopes makes plastic lenses not competitive with ground glass lenses. 

[Ags]

I wonder, glass is harder and can take a higher polish I expect. My plastic 'glasses' are fine for everyday use at 1x magnification, but a telescope magnifies substantially. I wonder how phone lenses really work at all, as they are so small their resolution is surely severely impacted?

I have a pair of glasses for long distance that I use only for astronomy. They seem to correct my astigmatism nicely but as far as I know they only correct the major components of astigmatism, so the satisfactory view of stars may be more of an illusion created by zero magnification.

[michael8554]

The zoom lenses that budget DSLRs kits come with usually have plastic lenses, for cost reasons.

"I wonder how phone lenses really work at all, as they are so small their resolution is surely severely impacted?"

Resolution is based on the number of pixels in the sensor.

A 12Mp sensor with 1um pixels is tiny.

All you need then is a lens diameter large enough to fully illuminate the sensor.

So "so small" is not a resolution problem.

Michael

[Ags]

13 hours ago, michael8554 said:

Resolution is based on the number of pixels in the sensor.

I mean the diffraction effects of a small aperture. Stopping down a camera lens to f22 causes a loss of sharpness (at least on bench tests) so a tiny lens that is really not much more than a pinhole must suffer similarly? 

[Louis D]

I did a bit sleuthing online and found these two rather informed sounding posts on the subject of plastic and aspherical lenses:

https://www.photo.net/forums/topic/390353-plastic-elements-in-aspherical-lenses/?do=findComment&comment=3991482

https://www.photo.net/forums/topic/390353-plastic-elements-in-aspherical-lenses/?do=findComment&comment=3992009

[Louis D]

On 05/02/2024 at 16:43, Ags said:

I mean the diffraction effects of a small aperture. Stopping down a camera lens to f22 causes a loss of sharpness (at least on bench tests) so a tiny lens that is really not much more than a pinhole must suffer similarly? 

Remember, it's the f-ratio that determines diffraction effects.  Since most phone cameras have quite fast f-ratios, diffraction effects are pretty negligible.

[Chaz2b]

I would think that there is huge safety issues if any plastic lens was used for solar viewing, especially if a filter wasn't in place first.

Chaz

[michael8554]

"huge safety issues if any plastic lens was used "

Maybe not, the sun isn't spot-focused on any of the elements ?

"especially if a filter wasn't in place first."

After an eclipse, camera hire companies get a lot returned with holes burned into the shutter or sensor ............ :-<

Michael