ZWB1, 8mm and lens UV-reach

[ulf]

I did a few series oftest shots to compare my ZWB1, 8mm with my U-340, 4 + 4mm - stack to see if my 8mm filter was flawed.

Both filter sets were OK in that respect.

 

However I was disturbed by the difficulty of WB those shots.

my camera is a full spectrum modified Sony A7III where both dust-shaker and BG-glass has been removed.

Jonathan (JMC) has earlier showed that the sensor's protective window do mot give this camera a very deep UV-reach, making it usable for exploring UV-B.

So far this has not given me any problems as I am not into such more extreme UV-photography. It works quite well for UV-images including the false-yellow colour band.

 

For the filter test above I used my EL-Nikkor 80mm lens that is one of the better accidental UV-capable lenses.

The UV-reach of this lens is normally greater than needed and many lenses with less reach work well too.

As I had problems doing a proper WB I decided to redo the test with a lens that has a really good UV-reach down to 200nm. 

 

I tested with my 70mm fused silica PCX-assembly and found that lens gave images that was easy to WB in a normal way.

My FS PCX lens element is uncoated and has a transmission similar to the graph under the "Graphs"-tab on this page:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=123

 

Here are some images selected from my tests.

they are jut screen shots from FastRawViewer after WB at the roof surface in the foreground.

 

ZWB1, 8mm on EL-Nikkor 80mm old metal type

 

ZWB1, 8mm on PCX FS 70mm assembly

 

ZWB1, 8mm on PCX FS 70mm assembly, 100% detail:

[colinbm]

Thanks Ulf
The white vertical panel of the far building has a mis-match in the paint.

[Stefano]

A little unrelated to this topic, I am trying to design lenses and your fused silica lens looks pretty sharp for being a singlet. Was it stopped down? If you have its specifications (radius of curvature and central thickness) I can simulate it. I'm going to ask more questions, perhaps in a dedicated topic, when I will have more information about lens design in general (I am still learning).

[ulf]

3 hours ago, colinbm said:

Thanks Ulf
The white vertical panel of the far building has a mis-match in the paint.

That is right.

The original 23 years old plaster (white area) was repaired over a big area and replaced with new UV-absorbing plaster later on.

There are two different shades of plaster (VIS) on our buildings. 

None of that is visible in the normal VIS range.

if you look at the 100% detail you will see three darker dots close below the darker boards. They are patches over where scaffolding was anchored during a few times with different repair and installation. All the nine story buildings here has gotten solar panels on their roofs.

[ulf]

2 hours ago, Stefano said:

A little unrelated to this topic, I am trying to design lenses and your fused silica lens looks pretty sharp for being a singlet. Was it stopped down? If you have its specifications (radius of curvature and central thickness) I can simulate it. I'm going to ask more questions, perhaps in a dedicated topic, when I will have more information about lens design in general (I am still learning).

Yes it was stopped down and I think it was specified as a fl of 70mm.

Difficult to tell the aperture value as the iris do not have any click stops, but I think it was something like f/8 - f/11.

The exposure time was 6s.

It is a quality lens like those you get from the big companies like Edmund Optics and Thorlabs.

I bought it at eBay from a small independent distributer that closed down the operation.

 

A lens that is designed for UV (105mm) is quite a bit sharper:

[Stefano]

By simulating some lenses, I have noticed that plano-convex lenses (when the rays come from infinity) often have little spherical aberration, both in the center (perpendicular rays) and off-center (rays entering at an angle), but the main issue is field curvature. The rays that hit the lens off-axis are focused closer, and this is the main reason for the blurriness in the edges of the image. You should be able to focus your lens in a way that makes the edges sharp(er) and the center out-of-focus. Maybe you have already noticed that.

 

Also, is that 105 mm lens corrected for chromatic aberration in UV?

[ulf]

20 minutes ago, Stefano said:

By simulating some lenses, I have noticed that plano-convex lenses (when the rays come from infinity) often have little spherical aberration, both in the center (perpendicular rays) and off-center (rays entering at an angle), but the main issue is field curvature. The rays that hit the lens off-axis are focused closer, and this is the main reason for the blurriness in the edges of the image. You should be able to focus your lens in a way that makes the edges sharp(er) and the center out-of-focus. Maybe you have already noticed that.

Yes I have seen that, but i did not try to focus very carefully as this test was mainly about WB problems even with lenses that normally perform quite well for more normal UV-photography.

I am convinced that this problem would be smaller if my camera had gotten a replacement of the sensor window, giving it a better UV-reach.

21 minutes ago, Stefano said:

Also, is that 105 mm lens corrected for chromatic aberration in UV?

Yes it is a complex well corrected lens dedicated for research and UV-photography.

[LarsHP]

On 8/31/2022 at 8:14 PM, ulf said:

...

Is the sample image with the UV Nikkor taken with the ZWB1 in 8mm thickness? It looks sharp, so if that's the case, I might try that super-thick filter.

[ulf]

6 hours ago, LarsHP said:

Is the sample image with the UV Nikkor taken with the ZWB1 in 8mm thickness? It looks sharp, so if that's the case, I might try that super-thick filter.

Yes it is.

Please be aware of that I could only do a meaningful WB with the two lenses with a really deep UV reach.

The EL-Nikkor  80mm did nor have enough UV-reach to work in combination with my camera and that filter. Naturally not all of that big UV-reach of the UV-Nikkor is needed, but I have no idea what is enough, as I do not have any lenses with a bit more reach than the El-Nikkor to compare with.

 

The very thick filter glass round is a bit tricky to mount as there are no filter rings that are made for such thick filters.

Also Andy Perrin had bad luck with his sample of that thick glass.

---

I bought five of the filter rounds, verified the flatness, lack of internal bubbles and striations, measured transmission and mounted them in a usable filter cell surrounded by a, individually serial numbered label. I kept one and sold the rest.

(The odd diameter spec is from my request, for what I needed to mount them.)

The filter round itself is rather cheap, but there is much work measuring and mounting, so I had to charge substantially more than the used materials.

Also there might be a dud as the one Andy got when ordering. 

I'm not sure I would like to do that again, as it took me many hours for just those five filters.

---

'

1.) In my collection of usable UV-pass filters this is 8mm one is one I like to use to explore the shorter UV-A without any contribution of the light closer to 400nm.

2.) Then I have what I think of as a Baader U style, ionic filter stack of U-360 2mm + S8612 2mm.

3.) Finally I have a SEU-3 filter that mostly include the spectrum just below 400nm 

 

Those three filter solutions give a wavelength focus on UV-A short-, mid- and long-wave that I find interesting.

 

Then there are other dimensions of this filter type space, with other UV-pass filter glass types together with a suitable IR-blocking BG-glass, but that is a different story.

[LarsHP]

1 hour ago, ulf said:

Yes it is.

Please be aware of that I could only do a meaningful WB with the two lenses with a really deep UV reach.

The EL-Nikkor  80mm did nor have enough UV-reach to work in combination with my camera and that filter. Naturally not all of that big UV-reach of the UV-Nikkor is needed, but I have no idea what is enough, as I do not have any lenses with a bit more reach than the El-Nikkor to compare with.

 

The very thick filter glass round is a bit tricky to mount as there are no filter rings that are made for such thick filters.

Also Andy Perrin had bad luck with his sample of that thick glass.

---

I bought five of the filter rounds, verified the flatness, lack of  internal bubbles and striations, measured transmission and mounted them in a usable filter cell surrounded by a, individually serial numbered label. I kept one and sold the rest.

(The odd diameter spec is from my request, for what I needed to mount them.)

The filter round itself is rather cheap, but there is much work measuring and mounting, so I had to charge substantially more than the used materials.

Also there might be a dud as the one Andy got when ordering. 

I'm not sure I would like to do that again, as it took many hours for just those five filters.

---

'

1.) In my collection of usable UV-pass filters this is 8mm one is one I like to use to explore the shorter UV-A without any contribution of the light closer to 400nm.

2.) Then I have what I think of as a Baader U style, ionic filter stack of U-360 2mm + S8612 2mm.

3.) Finally I have a SEU-3 filter that mostly include the spectrum just below 400nm 

 

Those three filter solutions give a wavelength focus on UV-A short-, mid- and long-wave that I find interesting.

 

Then there are other dimensions of this filter type space, with other UV-pass filter glass types together with a suitable IR-blocking BG-glass, but that is a different story.

Thanks for your detailed response. I may order such a chunk of glass, specifically asking them to check that the sample is flawless. 

 

Regarding the trio of UV filter selections, that is along the lines of what I am thinking. Using this ZWB1 8mm for the shortest UVA, then a Baader U (Venus or Sloan), and finally a TSN575 2mm + perhaps ZWB2 1.5mm for the longer UV to borderline visual.