Golden age of Singlet lenses

The 18th century lens from a german mirror has arrived safely to me after a month of waiting.

The diameter 101mm the lens was grinded on one side and polished on both but a little on the flat side to remain the old felt original polish from the 18th century. Tested on auto-collimation foucault showed a good spherical surface after re-polishing, similar to Campani lenses being a Campani lens as good as new in early days.
The Venetian glass being very good and was easy to re-polish.
Some little defects at the edge and something extremely slightly visible in the center as a small bulge that does not influence barely visible in the auto-collimation foucault test. Overall looks like freshly made Giuseppe Campani lense. The edge had to be beveled a lot because of the chips, it was decided to cut it to 101mm, it can be used at 93mm to 85mm diameter.

The lens perfectly resembles that of Giuseppe Campani from the Museo della Specola Bologna having about the same focal length 12m, the same color ( see photos).
When I hold it obliquely in the sky light it is green, in daylight in the hand the edge reflects a yellowish color with a greenish tint, the same has Campani lesnes from Venetian mirror glass.
The objective cell is ready and the lens can be used in observations has soon has the weather clears up.

In the last images a comparison with modern fused silica glass being the 9.2m lens 95mm diameter.
Photos: Venetian glass, vs modern glass down below
Simplet lens made of float glass 42mm focal length 1.8m (Tavi's Optical) left in my hand VS 18th century lens made from Venetian glass in the same image SRPAC 2024 award modern float glass next to the 18th century objective lens with focal length 12m.

Objective lens

Lentila obiectiv de secol 18

Comparison in color with John Marshall objective lens ( left) to my lens ( right)

Comparison in color with Giuseppe Campani objective lens ( right ) to my lens ( left)

Comparison in color with Giuseppe Campani objective lens ( left ) to my lens ( right)

Comparison in color with Giuseppe Campani objective lens ( right ) to my lens (left)

Modern soda lime glass color ( left) and the old lens in the right

Comparison with a modern crown lens in the right to the old glass lens in the left

The 18th century lens in the right in comparison with modern float glass

The 18th century lens in the right in comparison with modern float glass 2

The Optical Examiner, that is me, after today's physical examination session of the 18th century lens glass, I was amazed to discover a lot of air bubbles inside and small pits on the surface.

Examining the glass very carefully with the light of the phone held at the edge of the lens, I was amazed to discover a lot of pits on the surface of the glass on both sides as well as air bubbles all superimposed on each other.

The bubbles can be distinguished from the pits left by grinding by tilting the lens and moving it from side to side.

Obviously these glasses are far from perfect, even if the lens was polished on both sides to make it shinier (the glass on the flat side was worse at first), it did not eliminate all the defects, but it gave it an ok surface.

I'm surprised my colleagues didn't notice this (because these inclusions are too small, you just had to hold the light at an angle to see them).

Anyway, I didn't notice them either when I just looked at it from the front with a flashlight. It didn't occur to me to hold the flashlight at an angle and only in the dark. I didn't see them until the evening when I examined it more closely with the LED glued to the edge of the lens.

This is how these small bubbles or dots are magnified, as most of them are large dots under a millimeter that reflect the light, especially the pits in the glass.

I noticed about two large bubbles, one at the edge on the convex side and one in the center of the lens. They are smaller than a mm (small medium large dots) in the vast majority, the two bubbles are 1mm.

A number of small pits on the convex side.

Small scratches on the flat and convex sides and a large scratch on the flat side.It has to be examined carefully to distinguish the pit from the small point air bubbles in the glass.I blew off the remaining dust with an air pump and a fine brush.Unfortunately the weather is bad, so I couldn't check it optically in the sky. In the pictures there are also small bubbles and dimples on the surface, all overlapping. Most of the bubbles inside are very small, like white dots that reflect light, but there are a few larger ones. I am very happy that I found dimples and bubbles in the 18th century lens, this proves its uniqueness and originality.

To wash the lens without rubbing the already fragile surface, I bathe it. I put pure water, dishwashing liquid, and rubbing alcohol in a stainless steel bowl and place the lens on a pad for 10-15 minutes on one side, then 10 minutes on the other.

Finally, I rinse it under the tap with pure water (I have a water filter). And that's it, then I let it drain and dry its surface.There are many marks left, I take a fine napkin and with Baader Planetarium Solution I walk lightly on the surface and they disappear, then all the marks remain clean.It dries instantly with Baader solution and leaves a clean surface. This is my method, which I discovered after many experiments. Especially with this old glass you need a method that does not scratch it.

18th century objective lens in the support cell ready for observations only the weather is not with us.Several apertures have been made starting with 94mm aperture mounted from the inside to start then I will reduce the diameter to the optimal aperture for the focal length of 12m.I have 4 cardboard aperture diaphragms 94mm, 88mm, 86m and 81mm diameter.