Glass dome coating. Which side(s)?

I'm making myself a CFWA 11cm glass dome port from an old 70's Oceanic Housing for a SWC Hassy and make it work on my Ike. This glass dome was always too small for the Hassy SWC to properly focus (it really needed a larger 8" dome) and is now perfect to do CFWA with Tok10-17 FE + 1.4x TE on a modified old Ike port.

Before I attach/glue the dome to the port, I was thinking of getting the dome optically coated to reduce reflection and improve image quality.

I have never seen a coated glass dome to know which side or sides are coated. I know that the glass SeaCam and Sea&Sea domes are coated and the owners swear on their optical quality, but I have no clue which side has the coating.

I'm assuming that coating the inside of the dome would make sense when shooting into the sun, but would coating the outside of the dome be worthwhile?

Any insight be much appreciated .

Bo

Before I attach/glue the dome to the port, I was thinking of getting the dome optically coated to reduce reflection and improve image quality.

 

I have never seen a coated glass dome to know which side or sides are coated.

Bo

Hello Bo,

The glass domes I had quoted have the coatings on both the inside and the outside. Do you have a company that will coat your dome? I would suspect that the coating process is expensive since the domes I had quoted were $300 USD more than uncoated domes.

Good luck,

Barry

www.creativeillusionsphoto.com

www.marinekeywords.com

Hello Bo,

 

The glass domes I had quoted have the coatings on both the inside and the outside. Do you have a company that will coat your dome? I would suspect that the coating process is expensive since the domes I had quoted were $300 USD more than uncoated domes.

 

Good luck,

Barry

www.creativeillusionsphoto.com

www.marinekeywords.com

Ahhh.....thank you! Barry.

Your report on your DIY CFWA Nexus port inspired me and made me think to try my project.

In all the decades of having this Hassy housing I never realized it was a glass port till I took it out (duhh). I have a few light surface scratches and deposit fringe I want to polish first with cerium oxide (I will report on the success of polishing the glass, since I'm experienced in polishing acrylic domes and fearless )

If I have not ruined the dome, I have several local resources like Cascade optical that can do the coating. Now that I know I should get both sides coated I can get a better estimate.

Ofcourse I now need to investigate which type of coating to get for each side. Anti reflective on the inside makes sense, but perhaps high reflective on the outside may increase contrast and resolution? Anybody here knowledgable on this?

I will try to have the dome mounted in such a way that I can de-mount it easily and allow me to do some pool testing before and after the coating process, if and when I get it done.

If I get to this coating decision, I may also decide to break the process down to 2 stages (depending on costs) and see how much difference the outside coating makes in pool tests. Ofcourse I will have to get myself one of those snorkel rubber duckies everybody here uses for test standardization... .

Thank you for the best wishes. I will probably need much luck with this project, but I'm fearless and stupid enough to try it. I see it as a learning experience to report on and others to perhaps benefit from. If nothing else, I can make the recommendation "Don't try this at home, kids!"

If I can get a coated glass dome for my CFWA for a few hundred bucks, it is worth trying. No???

Keep you posted on this project. Thank you again for your reply and input.

Bo

Hello Bo,

 

The glass domes I had quoted have the coatings on both the inside and the outside. Do you have a company that will coat your dome? I would suspect that the coating process is expensive since the domes I had quoted were $300 USD more than uncoated domes.

 

Good luck,

Barry

www.creativeillusionsphoto.com

www.marinekeywords.com

Hi Barry,

My local source is asking $400 for one side and $800 for both with MgF2 (Magnesium Flouride) antireflective coating.

Interestingly enough they provided me with the coating specifications and Reflectance (%) vs Wavelength (nm) for each side and both sides of the dome. As it shows, the better side of the dome to get coated is the outside of the dome. The coating MgF2 provides better anti-reflective effect on the outside of the dome between water and glass (R<0.4%) in comparison to inside between glass and air (R<2%). Anti-reflective properties are best when both sides are coated (an amazing R<0.1%). These readings are at 550nm and 0.0 degree angle and a single layer of MgF2 coating.

So, for now I will just try to clean up the dome and use it uncoated until I can find a company that can coat it for $200 or less, I will get the outside of the dome done.

Bo

I am surprised that antireflective coatings do anything on the outside of a dome. I learned something here.

I thought anti-reflective coatings on the outside of a dome would probably only help for over/under type shots.

I am surprised that antireflective coatings do anything on the outside of a dome. I learned something here.

 

I thought anti-reflective coatings on the outside of a dome would probably only help for over/under type shots.

Between Google and Wikipedia searches and email inquiries it has become quite an education and appreciation for those that offer coated domes.

Hoya here in Ca. will only do large quantities and only flat glass discs (no domes). Another US service "only" charges $950.-, but that is for double coating on outside and single inside. All respondents report 3 weeks turn around.

Being a "DIY tinkerer" I also looked into doing the coating myself and find out more what it took to do so. A local supplier can provide me with 99.99% pure Magnesium Flouride Crystals powder for $60/gram. The problem is that the MgF2 has a very high melting/vapor point and needs to be applied in a vacuum (This is a machine that can do this).

So, I don't see much profit in those that make coated domes and ask around 1K$ for special coated optical ports and have us complain how expensive they are.

For now I will be shelving the idea of getting my port AR coated and focus on getting this dome cleaned and polishing out the light scratches. I already have the glass polishing kit and just waiting for the ultrasonic cleaner. I will report on my restoring the glass dome on the forum about "removing scratches from a dome", since most information is on acrylic domes and little on glass domes. Polishing glass is very different from polishing acrylic, of which I have done plenty of.

It is my hope to be able to do a comparison between the traditional 6" acrylic Ike dome versus my 11cm glass dome modification and see if it is even possible to get over/under with a Tok10-17 & Kenko 1.4 TE CFWA set up behind a small glass dome. We'll see. Stay tuned.

Bo

Quick update: I have been getting more quotes on the MgF2 coating. Ranging from as low as $300 for single coat on both sides to as high as $950 for double on the outside and single layer on the inside of the dome. I'm still waiting for more responses to inquiries. Who knows, I may just get a quote within my budget yet.

I'm having a hell of a time with polishing out the scratches and a small pit from the outside of the dome. The glass repair kit I bought is way too slow in abrasiveness and getting to the damage. After hours of polishing with an low rpm electric drill, I still see the imperfections. It is a slow process of 20 seconds of buffing followed with a water cooling spay to prevent the dome from cracking from heat build up. It is also very messy, with buffing slurry flying all over. Glass is a much harder material than acrylic, where it is easy to just sand surface down to the scratch base and then polish back to a shine. Not that easy with glass.

I'm going to attack the domes small visible scratches now with a rougher jewelers buffing compound and a dremel tool, and see if I have better results. It is a task of patience and "keeping it cool"

Stay tuned.

Bo

I'm going to attack the domes small visible scratches now with a rougher jewelers buffing compound and a dremel tool, and see if I have better results. It is a task of patience and "keeping it cool"

Stay tuned.

Bo

Hi Bo,

Better yet, go to a jeweler's or lapidary supply and purchase a round disk impregnated with diamond. You can get the disks with grit as small as 64,000. A 120-320 grit may be good to start with. These disks are to be used with water, so a wet sponge may help. Be careful as the disks cut fast.

I am afraid the Dremel tool will be too fast and cause heating too soon to be effective.

Barry

www.marinekeywords.com

Better yet, go to a jeweler's or lapidary supply and purchase a round disk impregnated with diamond. You can get the disks with grit as small as 64,000. A 120-320 grit may be good to start with. These disks are to be used with water, so a wet sponge may help. Be careful as the disks cut fast.

 

I am afraid the Dremel tool will be too fast and cause heating too soon to be effective.

 

Barry

www.marinekeywords.com

Thanks Barry!

I will look for the diamond disks.

The Dremel tool I was planning to use has variable speeds control and able to keep it under the recommended 2500 rpm's.

Also I'm trying to rig up a small water pump with tubing to provide a constant water flow to the buffing area.

Bo

Bo:

The fastest way to polish domes is to mount the dome in a lathe and have it rotate about 60 - 80 rpm. As it is turning, move your polishing pad across it, creating figure "8" on the dome. This will provide an even polishing pattern and not localized discontinunities such as you would get with a Dremel moor. Kerosene is a good lubricant for polishing plastic.

Allan

Bo:

 

The fastest way to polish domes is to mount the dome in a lathe and have it rotate about 60 - 80 rpm. As it is turning, move your polishing pad across it, creating figure "8" on the dome. This will provide an even polishing pattern and not localized discontinunities such as you would get with a Dremel moor. Kerosene is a good lubricant for polishing plastic.

 

Allan

Hi Allan,

I like your lathe idea, but don't have access to one. But I do have a standing drill press where I can put the buffing wheel into and press the glass dome up against by hand and better control the pressure, motion and distribution of the buffer.

I think I will pass on the kerosene idea. Glass polishing is heat generating and I'm afraid any fumes may make my project go "POOF" in my face

Bo

Hi Barry,

 

My local source is asking $400 for one side and $800 for both with MgF2 (Magnesium Flouride) antireflective coating.

 

Interestingly enough they provided me with the coating specifications and Reflectance (%) vs Wavelength (nm) for each side and both sides of the dome. As it shows, the better side of the dome to get coated is the outside of the dome. The coating MgF2 provides better anti-reflective effect on the outside of the dome between water and glass (R<0.4%) in comparison to inside between glass and air (R<2%). Anti-reflective properties are best when both sides are coated (an amazing R<0.1%). These readings are at 550nm and 0.0 degree angle and a single layer of MgF2 coating.

 

So, for now I will just try to clean up the dome and use it uncoated until I can find a company that can coat it for $200 or less, I will get the outside of the dome done.

 

Bo

Bo,

I believe you have not been given full information on how to interpret the specifications. I have worked with coatings, designing and developing them, for 7 years of my career and still is involved with it on a day to day basis after that for another 15 years.

1) You will get the better benefit coating the inside of the dome. Getting R<2% is rather correct with a single layer of MgF2.

2) The "better" anti-reflective effect on the outside of the dome is simply because of the very small change of refractive index between water and glass, not due to the MgF2 layer. Do a simple verification. Ask the local source about the amount of reflection at the front of the dome when immersed in water and without any coating.

3) If on one surface you get R<0.4% and another you get R<2% (I believe R~1.5% will be more accurate if the glass is BK7), it will not add up to being R<0.1% with both surfaces coated. You get a total transmission of (.996 x .98 =) 0.976, which means a transmission loss of about 2.4%, excluding other kind of losses.

4) Coating on the outside will help you with over-under kind of photography as the top half of the outside of the dome will then be in the exact same situation as the back of the dome, that is, both have light passing from air to glass or glass to air, which is no different, and that's when you get the benefit of the MgF2 coating giving you R<2%. It will barely have any benefit for the part of the dome underwater.

5) If someday you still want to coat your dome and you want to save on cost, just do the coating on the inside. If you want to have the benefit also when doing over-under shots, go ahead and also coat the outside, but beware the coating might not stay long on the glass surface due to interaction with the sea water.

6) "... but perhaps high reflective on the outside may increase contrast and resolution? Anybody here knowledgable on this? " No, high reflective on the outside do not increase contrast and resolution. They just look nice on sunglasses, and keep your eyes comfortable by reducing the light intensitiy to a level that your eyes can be comfortable with. It will be like putting sunglasses on your dome port, or equivalently, neutral density filters.

Hope this helps you or anyone else following this.

Regards

Peng

P.S. to Don in Colorado, I think you can stay assured with what you know and not feel that it was incorrect.

Bo,

 

I believe you have not been given full information on how to interpret the specifications. I have worked with coatings, designing and developing them, for 7 years of my career and still is involved with it on a day to day basis after that for another 15 years.........

 

..............Hope this helps you or anyone else following this.

 

Regards

Peng

 

P.S. to Don in Colorado, I think you can stay assured with what you know and not feel that it was incorrect.

Hi Peng,

Thank you so much for sharing your knowledge and extensive experience with this. I have to trust yours more than the sales rep.

For now the coating project has to wait til it is more affordable for me. I will re-inquire for inside coating with the lowest bid from NY and see if it is less than their $300 quote for the outside of the dome.

I don't think that over/under shots are much of a consideration with this small a dome (110mm diameter), but my primary intent is for CFWA use with a Tokina FE 10-17 with a 1.4 TE.

Since you work in the industry, do you know of another type of AR coating that is less expensive? The AR coating on reading glasses must be cheaper, no?

Again, thank you for your input

Cheers,

Bo

Bo,

I am glad to be of what little help I may be.

I believe the cost is in the fact that it will be a one-off custom job. Coatings on reading glasses nowadays tends to be multilayer types and will work very well. However, there is economy of scale involved with the coating of hundreds of them. It is also likely that some special mechanical holders have to be made to hold the dome window and that is why it is not likely to come cheap.

You will have to weigh whether you want to spend the money on getting that extra ~3% improvement in transmission to have the inside coated. After all, you automatically get more than that for the front side already simply by going underwater.

Cheers

Peng