Dome thickness...

[Ale Reynoso 0]

Hi:

I´m planning a DIY UW housing for my Nikon DSLR. I´m intending to have an 8 or 9 inch dome blown from an acrylic shop...how thick is a commercial dome?

Thank you!

Alejandro

[oskar 6]

Sorry I cant answer your question, but it sounds excellent if they can do that with optical quality, what will they charge? The optical application requires the surfaces to be very good approximations of a spherical surface.

 

My guess is that for the sake of water pressure, it does not have to be very think, but it should be designed not to crack if hit by anything

 

 

Cheers

[Tom_Kline 137]

Hi:

I´m planning a DIY UW housing for my Nikon DSLR. I´m intending to have an 8 or 9 inch dome blown from an acrylic shop...how thick is a commercial dome?

Thank you!

Alejandro

 

 

The 8" acrylic domes on my Hasselblad EL housing are about 1/4" thick.

[Ale Reynoso 0]

Thank you!

I found this about a 6´Ikelite dome (in a page from "cameras underwater" website regarding ikelite domes)

 

Optical data: Port material: Acrylic, Refractive index: nD=1.492. Inside radius: 3", 76.2mm. Nominal thickness: 0.218", 5.537mm.

 

I will ask for a 6mm thick dome. It´s cheap I think (about U$ 30). I have to test the optical qualities, because they manage general purposes acrylic. That´s why I´m asking for a big dome (may be I will try a 9-10´, as I understand I will have less problems with chromatic aberrations, etc.). Meanwhile I will be looking for domes already made that could meet optical quality.

 

Anyway my housing is in an embrionic state. I have a machinist working in some push buttons, so I will have to wait for a while until I could do some test...

 

Thank you!!

Best regards

Alejandro

[Balrog 5]

As the dome radius increases (with constant port diameter), the form stiffness benefit will decrease up to the point where you have effectively a flat port (infinite raduis) with all its structural implications.

 

I'm not an optical engineer but base principles tell me that aberration will be at a minimum when the light path is perpendicular to the acrylic water/air boundary. Hence flat port for macro, dome port for wide angle.

[davelew 4]

Sorry I cant answer your question, but it sounds excellent if they can do that with optical quality, what will they charge? The optical application requires the surfaces to be very good approximations of a spherical surface.

 

My guess is that for the sake of water pressure, it does not have to be very think, but it should be designed not to crack if hit by anything

 

 

Cheers

 

Besides cracking, there's also the issue of water pressure bending the dome enough to change the optical properties and give you soft corners. Here's an analysis I did of how a 175mm diameter, 3mm thick dome will deflect at 30m depth:

 

 

(note that deflections are scaled up 137 times in that picture so that they're easier to see)

 

You can see that the center of the dome keeps the spherical shape, but as it moves 0.15mm down, the edges of the dome get a funky curvature. I haven't run this deformed shape through zemax, but I would guess that it would give you soft corners when using something with a super wide field of view like the Nikon 10.5mm lens.

 

Edited to add: the peak stress in that model was about 6.0 MPa, or 870 psi, which gives a safety factor of about 10X for an average acrylic, ignoring the stress concentrations from a scratch.

Edited January 7, 2010 by davelew

[Balrog 5]

That's a nice and interesting FEA davelew

[Ale Reynoso 0]

Very interesting...thank you!!!

[segal3 0]

Besides cracking, there's also the issue of water pressure bending the dome enough to change the optical properties and give you soft corners. Here's an analysis I did of how a 175mm diameter, 3mm thick dome will deflect at 30m depth

...

You can see that the center of the dome keeps the spherical shape, but as it moves 0.15mm down, the edges of the dome get a funky curvature. I haven't run this deformed shape through zemax, but I would guess that it would give you soft corners when using something with a super wide field of view like the Nikon 10.5mm lens.

COSMOS? To put this all in perspective, the deflection you list is 0.17% of the dome radius (not 1.7% or 17%, but less than one-fifth of one percent). That's a very, very, very small deflection.

 

I'd be curious to know if port manufacturers are even able to guarantee tolerances that low between production runs...

[fotoscubo714 0]

Besides cracking, there's also the issue of water pressure bending the dome enough to change the optical properties and give you soft corners. Here's an analysis I did of how a 175mm diameter, 3mm thick dome will deflect at 30m depth:

 

At this depth you should worry about other things than corner softness or dome failure. Head softness and failure maybe? :)Just kidding!

 

Dome integrity will remain better and be stronger in smaller domes. Ikelite 6 inch domes have done fine for years until people needed to do over and under shots. So use large domes in shallow surface water and smaller domes in deep water. You can go for a thinner dome if you keep it small in diameter and deeper in curvature.

 

Long time ago I also had custom plastic domes made and found that the process produced uneven thickness in the dome (thin in the center and thicker near the edges). Although promised to be optical quality, they were awfull in CA and unusable to photograph through. They were great for lenses in DIY movie lights though.

 

Bo

Edited January 8, 2010 by fotoscubo714

[davelew 4]

COSMOS? To put this all in perspective, the deflection you list is 0.17% of the dome radius (not 1.7% or 17%, but less than one-fifth of one percent). That's a very, very, very small deflection.

 

I'd be curious to know if port manufacturers are even able to guarantee tolerances that low between production runs...

 

Basically it was COSMOS. The exact plug-in I used is called SimulationXpress, but I believe that's just the name for the 2009 version COSMOS bundled with my CAD software. It has a very similar feel to the COSMOSExpress that I used with the 2008 version.

 

I agree, 0.15mm is a really small deflection, but I think that it could cause issues in an optical system where small deflections can be significant. I've seen sphericity tolerances of 5 microns on a 10mm diameter lens element of good quality. That means the manufacturing tolerance on sphericity is .10% of the radius. So the 0.17% deflections would add to that, causing the performance of the lens to shift from the 0.10% of a good quality element to the 0.27% of an economy-quality lens element. Or you could RSS the two tolerances together to get .20% sphericity in the deflected shape, but that's still a significant increase in tolerance.

[Ale Reynoso 0]

Long time ago I also had custom plastic domes made and found that the process produced uneven thickness in the dome (thin in the center and thicker near the edges). Although promised to be optical quality, they were awfull in CA and unusable to photograph through.

 

Hi: That´s typical of a blown dome I think, not molded...I´m curious about the deffects you got with those domes, as I probably can get this kind of dome (a little bit thicker in the edges...).

Thank you!!...

[fotoscubo714 0]

Hi: That´s typical of a blown dome I think, not molded...I´m curious about the deffects you got with those domes, as I probably can get this kind of dome (a little bit thicker in the edges...).

Thank you!!...

 

Correct. The domes were made via a vacuum process I think. Too long ago for me to remember, other than that the corners were very soft and even with film had abaration issues. Tried close up filters, but no improvements.

 

Have you looked into domes used for security video cameras?

 

This link may be of interest to you perhaps: http://www.criteriondomes.com

 

 

Bo

[Ale Reynoso 0]

Thanks a lot!

I will check out domes availability in my city.

Thanks again and best regards

Alejandro

[Ale Reynoso 0]

I got an answer from Myra of criterion domes

 

http://www.criteriondomes.com/

 

very, very kind people and willing to help with projects:

 

"We may be able to thermoform a custom product for your application. Do you have a drawing you can send me? If not, please send me the dimensions you need. I will need the dome diameter, dome height (from the inside), and flange diameter. Please let me know if you need any type of mounting holes in the flange.

 

We look forward to working with you on this project."

 

Thank you,

 

Myra

 

 

 

Criterion Technology, Inc.

 

101 McIntosh Parkway

 

Thomaston GA 30286

 

 

 

706-646-7025 (direct line)

 

706-647-5082 (main line)

 

706-647-5052 (fax)