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Acrylic and Polycarbonate Achromatic Lens?

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#1 Jason H.

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Posted 23 September 2011 - 01:44 PM

Hi, I'm a relative newbie to optics (with several failed explorations and some on the back burner) and just pondering the following lens combination:

Acrylic - index of refraction (n) = 1.49-1.492 (589nm??)
Polystyrene - n = 1.49 at 589nm
Polycarbonate - n = 1.584 - 1.586

Seems from a novices perspective that one could use an acrylic and Polycarbonate combination (with a similar spread to crown and flint glass) to make an achromatic lens (at least with better color-focus correction than these substances would have alone.)

Has anybody here seen the refractive indices for other frequencies of light for these plastics? (i.e. green, red blue etc.)

Has anybody seen something like an Abbe V-number's run on these plastics?

Could one use the lensmaker's formula in a straight forward calculation on these materials? (or is there some other fudge factor like hazing, polarization, double-refraction or other weird color effects that throw a wrench in the works?)

I'd love to do an experiment on this if it seems that I'm not re-inventing the wheel (which I'd imagine I would be considering how long these materials have been around.)

Any tips would be highly appreciated!!!!

Jason W. Higley

#2 Mike I. Jones

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Posted 23 September 2011 - 06:38 PM

Acrylic and polycarbonate can theoretically form an achromatic, coma-corrected pair. Three things about plastic make them unsuitable to barely suitable for precision optics applications, though:

1. The index homogeneity of plastics is typically poor, about like making a lens out of Pyrex.

2. The change in index of refraction with temperature is much higher than glass. This may or may not be an issue with a long-focus plastic refractor objective.

3. The surface finish is not so hot, on the order of 100 Angstroms RMS, creating scattered light on bright objects. Typical smoothly polished glass roughness is on the order of 5-15 Angstroms RMS, and 1 Angstrom is possible.

Having said that, here's a simple airspaced design for an acrylic/polycarbonate doublet, 60mm aperture, 600mm EFL. If you have OSLO-EDU or other tools, you can key it in and see for yourself.

Entrance pupil diameter: 60.0 mm
Wavelengths: 486, 587, 656 nm
Semi-field: 1.0 degrees

R1 = 350.57 mm
T1 = 7.0 mm
Glass = acrylic
R2 = -222.03 mm

Airspace = 0.5 mm

R3 = -222.03 mm
T3 = 5.0 mm
Glass = polycarbonate
R4 = -852.18 mm
BFL = 593.05 mm

Plastics are better suited for less demanding applications, like TV projector lenses, etc. However, if you want to make this or another lens just to see what it does, have at it! Keep us informed of your progress.

Mike

#3 Ed Jones

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Posted 23 September 2011 - 07:21 PM

One more problem at least with acrylic. It absorbs moisture from the air which diffuses into the plastic changing the refractive index and shape. This is especially troublesome at the edge of a lens. Not sure if polycarbonate acts the same.

#4 Jason H.

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Posted 23 September 2011 - 07:38 PM

Thanks Mike,
I see OSLO-EDU on google, have to get it and play with it. I've read about the various "R" surface curvatures; is the "T" lens-element center thickness? (reading Roger Ceraglioli's Refractor Construction Page I see that I'm already in over my head, but it's still fascinating enough to give it a try on a cloudymonth :)

Are you aware of anyone attempting a larger lens of this type?

Is the the sag from weight worse than glass? (i.e. does the weight reduction of the plastics permit a larger lens with really big scales?)

Does tensioning the edges like a mason jar help? (I don't expect you to know the answer to that one :)

Probably most important, what amount of surface curve error can one get away with as a minimum? Although you had mentioned 100 angstrom "roughness", isn't that like 1/40th of a wavelength of violet light; how do people get that kind of accuracy? (i.e. what's the actual curve error tolerance?)

Thanks again for that information, I guess I'll have to look into whether it's possible to find plastics with known homogenous refractive indices (those eyeglass makers must have polycarbonate down very well, perhaps it's just an acrylic problem?)

Regards, Jason W. Higley

#5 Crayfordjon

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Posted 24 September 2011 - 01:00 AM

I tried to make a doublet in acrylic ( perspex) and polycarbonate back in 1984, the acrylic polished up with a brilliance that cannot be achieved with glass, but the polycarbonate would not take a polish at all,it remained milky in texture, I abandoned the project. I wish you luck.

#6 jbattleson

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Posted 24 September 2011 - 01:10 PM

One more problem at least with acrylic. It absorbs moisture from the air which diffuses into the plastic changing the refractive index and shape. This is especially troublesome at the edge of a lens. Not sure if polycarbonate acts the same.

Both acrylic and polycarbonate absorb moisture. When I was molding them both needed about the same amount of drying time so would assume they absorb moisture the same.

#7 Jason H.

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Posted 24 September 2011 - 05:42 PM

One more problem at least with acrylic. It absorbs moisture from the air which diffuses into the plastic changing the refractive index and shape. This is especially troublesome at the edge of a lens. Not sure if polycarbonate acts the same.


Thanks Ed, that sure seems like a wrench in the works, especially on a dew-laden night. Hmmm, maybe one should just go with an acrylic singlet with filters in front of the lens and monochrome camera?

I believe I saw a mention here awhile back of somebody being amazed at how sharp a singlet can be with a pre-filter. I wish I could find 10" or bigger filters as practical and translucent as the ones for camera :) Anybody here done giant singlets with a filter in front of R1? (to mitigate the other spectrum before it becomes a problem down near the focus.)

Today picked up some old-school 72mm diameter red, green and blue camera filters from Japan from an old camera shop in town, the guy said those are probably the last ones they'll stock as Photoshop basically killed the demand for them. He said "What are you going to do something scientific with them?" :) They also phased out their astronomy gear over the years where it took several display cases and floor space, now it's down to a couple of Celestron items in one case and I'm guessing they won't have anything to do with astronomy shortly, sad as there is no other astronomy hobby gear store SFAIK within 100 miles of Orlando (the other camera shops that at least had a couple of scopes in them all closed within the last few years. Oh well I've digressed.

Thanks for the tip on the detrimental moisture effects! Due to yours and other valuable responses, at the moment I'm thinking to just fiddle with singlets now.

Jason W. Higley

#8 Jason H.

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Posted 24 September 2011 - 05:50 PM

I tried to make a doublet in acrylic ( perspex) and polycarbonate back in 1984, the acrylic polished up with a brilliance that cannot be achieved with glass, but the polycarbonate would not take a polish at all,it remained milky in texture, I abandoned the project. I wish you luck.


Thanks CrayfordJon and John B.

One has to wonder how eyeglasses (like the ones I'm wearing) and Lexan look so good? Whoa, I just looked up Lexan, made with phosgene (WWI chemical weapon.) And I was thinking of cooking sheets to shape them in the oven :)

Jason W. Higley

#9 Sky Captain

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Posted 03 October 2011 - 12:09 AM

Whoa, I just looked up Lexan, made with phosgene (WWI chemical weapon.) And I was thinking of cooking sheets to shape them in the oven


:shocked:Better take that oven out to the back yard... :grin:

On second thought, just make a batch of cookies.

#10 Crayfordjon

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Posted 03 October 2011 - 12:59 AM

You could use Thorium glass eyepieces with the Phosgene lenses and go for gold, who said heroic physics was dead!.

#11 FlorinAndrei

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Posted 03 October 2011 - 02:50 AM

I tried to make a doublet in acrylic ( perspex) and polycarbonate back in 1984, the acrylic polished up with a brilliance that cannot be achieved with glass, but the polycarbonate would not take a polish at all,it remained milky in texture, I abandoned the project. I wish you luck.


Acrylic is said to be more scratch-resistant than lexan. Could this be why lexan doesn't take a polish - too soft to polish well?

#12 John Carruthers

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Posted 03 October 2011 - 06:23 AM

could be, Lexan comes as normal and 'MR' (mar resistant), the edges polish well enough for showcases and cabinetry but not ideal for optics. I see Edmund Sci stock various acrylic lenses.

#13 Crayfordjon

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Posted 03 October 2011 - 10:06 AM

I ground my acrylic lenses with carbo starting with 120, 80 will embed into the surface, finishing with 600. Polish was with rouge on a plasticene polisher, the finish is very transparent, for more than can be achieved with glass, although the optical finish under KE test shows roughness. I am using a singlet and reduction lens combo of my own design for monochromatic imaging, using a medium broad band yellow filter by Endmunds, great on the sun and other bright objects. The optical combination plus the filter give very sharp images up to X400. The system is great for black and white mono photograpy. Some examples of the images, a test image and the moon.

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  • 4841100-moon Helium filter enhanced.jpg


#14 Crayfordjon

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Posted 03 October 2011 - 10:08 AM

And a terrestrial test image

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  • 4841103-Test images in He using Sony camcorder. 004.jpg


#15 Crayfordjon

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Posted 03 October 2011 - 10:16 AM

I you greyscale the images you get.

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  • 4841130-X.jpg


#16 Crayfordjon

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Posted 03 October 2011 - 10:17 AM

And.

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