A new UV Bandpass filter

[rfcurry (1950-2024)]

I have been working on several new UV-bandpass filters. Today's testing yielded very positive results for the CopperU (working name) filter. The CopperU weighs 93 grams, has a standard 52mm thread mount, and accepts a 52mm hood. The length is 33.17mm and the maximum diameter is 56.9mm.

 

I will be adjusting the CuSO4 concentration over the next few days, looking for the optimum blend of transmission and NIR cancelling. Below are some photos comparing the CopperU to an early PrecisionU (I can't keep the latest model on hand). The camera is a Lumix GF1 modified to full spectrum, the lens is a Meyer-Optics Gorlitz Primagon 35/4.5. All shots taken with the lens wide open, indoors in natural sunlight. In-camera WB done for each filter. No post processing except for reduction to 1200px width.

 

A one second shot with the CopperU

http://uvroptics.com/images/CopperU1200px761.jpg

 

a one second shot with the Old PrecisionU

http://uvroptics.com/images/OldPrecisionU1200px764.jpg

 

 

A thirty second shot with an IR 72 near-IR long pass filter over the Old PrecisionU

http://uvroptics.com/images/OldPrecisionU30sNIR1200px757.jpg

 

A thirty second shot with the IR 72 over the CopperU

http://uvroptics.com/images/CopperU30sNIR1200px756.jpg

 

 

I found the comparison interesting.

[enricosavazzi]

Just like a thick glass filter, a thick liquid cell introduces a number of optical aberrations, mostly spherical and related to spherical. This is the same effect, for instance, caused by using cover glass of wrong thickness with high-NA microscope objectives. It gets much worse with fast lenses.

 

To reduce these aberrations, I would recommend using a cell as thin as possible. It may be possible to increase the copper sulphate concentration to compensate for a lower thickness.

[nfoto]

I hope the filter cell is physically very strong as it's easy to envision the hardships such a filter might face in actual field use ...

 

Agree with Enrico that the thinner the filter the better.

[colinbm]

Fantastic work Reed

Col

 

PS, One problem I had with the copper sulfate solution was getting it pure enough, so that no sludge formed on the bottom or glass of the cell, it looks like a sulphur sludge.

Col

[baffe]

That was the hint I was waiting for!

 

Beautiful work Reed!

 

I usually filter light but not the cam. So ca's don't mind me at all.

 

I find it a great chance to filter with a liquid because i can handle the enormous power dissipiation (and it's distribution) much easier in a liquid. Hopefully.

 

I was watching your "bassin filtering" also. I do not know how I manage that all but I have a sticker on my desk "buy CuSo4" now.

[colinbm]

I have two qualities of copper sulfate, & I have mixed them in both tap water & distilled water, they both mix & cut well.

Both after standing over night get a sludge that looks like sulphur, settling on the bottom of the container. Ever if I have filtered a number of times & left the sludge behind each time, still some sludge has formed. My latest efforts at eliminating the sludge, is to use a double ended bottle. I am leaving the sludge to settle on the bottom cap, which when I see no more sludge (a few days) I will open the bottom carefully with the top still closed & remove the sludge. Then I can filter the clean copper sulfate solution & hopefully it will remain clean.

Col

[baffe]

Is the concentration critical?

[colinbm]

That I haven't tried yet, Stefan.

There is not a huge difference to the colour of saturated copper sulfate & 12.5% copper sulfate, but there could be in the transmittance.

It would be interesting to see what the different thicknesses of the cells, make to the transmittance too.

I am sure that this was well documented 70 years ago, but what can be found today, might be quicker to experiment.

I have just been involved with getting a pure of a mix as is possible, for me to get, without any more of the sludge forming & settling to the bottom of the cell.

I am still preferring the perspex for the windows, even though it has been attracting the sludge, hence the need to remove or prevent the sludge first.

Cheers

Col

[rfcurry (1950-2024)]

The issue with path length as noted by Enrico is not straightforward. The shorter the path length, the stronger the solution (and/or thicker ionic glass); the stronger the solution, the greater likelihood of crystalization in cool conditions. Crystals in the viewing field are to me a matter of concern. As in most things, compromise is necessary. I will attempt the minimum path length consistent with excellent and prolonged functionality. Another point, previous makers of a CuSO4- based filter used a relatively thick (4mm+) piece of ionic glass, e.g., UG11, to allow for a shorter CuSO4 path - the thick UG11 reduced the NIR leak, allowing a weaker solution in a shorter path length. However, the thick UG11 drives the UV peak down and narrows the FWHM. I believe that my customers want maximum UV throughput, so the UV ionic glass I use is quite thin. The CopperU filter in its present incarnation should provide a peak between 85% and 92% and a FWHM between 264nm and 382nm.

 

BTW, I incorporated expansion buffers in the filter in order to allow for warming and cooling of the liquid. This is not intended to be an astronomical filter kept in a steady state, but a filter for everyday use.

[colinbm]

Sounds very exciting Reed, yes max UV & min IR, the 'holy grail' for us all, here ;)

Col

[baffe]

First I need some of the salt and then I have to make thoughts and then I can begin "to drill and screw"

 

The "sludge" sounds a little strange to me. I used CuSO4 for coppering purposes a long time ago. But I can only remember CuSo4 at the bottom of my containers. At least as long as there was no contamination with other metals.

[colinbm]

Hi Reed

How is the CopperU coming along please ?

Col

[rfcurry (1950-2024)]

Col,

 

The CopperU is just in the final stages. I had hoped to get one out for testing two weeks ago, but the fused quartz I used for the front glass was not of the thickness that I wanted. I should have some new fused quartz today. I am using fused quartz because it doesn't degrade the 90%+ transmission in the 300-1100nm range. Fused silica would get me a lower UV range (<200nm) but no camera sensors will record those wavelengths, so I am avoiding the additional expense of fused silica.

 

Thus far, I am seeing a one to three stop faster image capture and a lot more in the blue, when compared to a Baader U or PrecisionU. I have greatly reduced the path length and the required higher CuSO4 concentration has not been an issue. The size and weight is now not much greater than a thick glass 52mm filter.

 

In short, it is going well and I am looking forward to its practical testing soon.

[colinbm]

Thanks Reed for the update, it all looks promising :)

Col

[Alaun]

Reed, isn't quartz glass and fused silica the same? Second thought: What about sapphir glass, transmits down to 150nm and can take more heat than SiO2-based glass? (recycling of i-phones ...)

 

Werner

[nfoto]

Aha, we can at last have a scratch-free filter.

[rfcurry (1950-2024)]

No, fused quartz and fused silica are different. See http://abrisatechnologies.com/2012/10/fused-silica-vs-quartz/

I am using the GE 124 Fused Quartz.

http://abrisatechnologies.com/wp-content/uploads/2012/10/GE-124.jpg

I use the quartz both as a front glass and as a protective cover to the UV transmitting ionic glass, so that the CuSO4 doesn't affect it.

[Alaun]

Ok, so basicly they are same, only different levels of trace elements, fused quartz glass -made from mineral SiO2 (quartz)- with trace element levels in the ppm range and fused silica glass -made from synthetic SiO2- with trace element levels in the ppb range (from googling on some german glass manufactures sites). So a few ppm make all the difference (by the way, not uncommon within the materials world).

 

Werner

[rfcurry (1950-2024)]

Werner,

 

That sounds right.

What i find interesting as I study the results of the filter, is how much my lenses differ in depth of wavelength transmission. It makes me want a decent quartz lens, to capture the wavelengths below 340nm.

 

Regards,

Reed

[nfoto]

I'm awaiting a sample of the Copper-U for testing - and I do possess the required deep-UV lenses for it (UV-Nikkor, Coastal 60).