I Can Meet with Dead Scientists

Two days later.

The courtyard where Old Su's study was located remained unchanged.

At this very moment.

All the decorations in the courtyard had been removed completely.

In the entire yard, there was only a piece of open ground, a stone table, and a few trees left.

Xu Yun stood in the very center of the courtyard, accompanied by Old Su, Old Jia, Xiaoli, and seven or eight others.

Surrounding them were numerous exposed or sealed items, and...

A donkey.

Once everyone had gathered.

Old Su's eyes gleamed with anticipation, as he gently patted a box containing goods and said to Xu Yun:

"Xiaowang, everything you asked for is here."

Upon hearing, Xu Yun opened a box, took out a piece of coarse glass from inside.

He inspected the glass in front of his eyes and nodded with satisfaction, saying:

"No problem, the quality of the raw materials is excellent."

As a seasoned veteran in the DIY community of later generations, Xu Yun had crafted over three hundred lenses in two lifetimes.

Judging the quality of glass by touch had become a skill ingrained in his soul.

Then Old Su cautiously took a box from Mr. Xie's hands and handed it carefully to Xu Yun, advising:

"Xiaowang, this item is somewhat dangerous, so hold it well."

Xu Yun carefully received the box and placed it stably on the stone table.

Seeing this, Xiaoli couldn't help but ask curiously:

"Wang Lin, what's inside the box?"

Xu Yun glanced at her, and without beating around the bush, said directly:

"Mercury."

Xiaoli blinked her eyes, instinctively stepping back:

"Mercury, that's poisonous. What do you want to do with it?"

Xu Yun chuckled lightly at this, this renowned toxic substance in the Song Dynasty was an indispensable key item for this lens-making stage.

He raised his head, glanced at the sky, and took a deep breath:

"It's certainly for making..... a parabolic surface."

Mentioned earlier.

In the DIY field of later generations, Newton reflector telescopes are generally among the most common types.

Because compared to refractor telescopes, the optical system of the Newtonian reflector is simpler and easier to handle.

Furthermore, the Newtonian reflector has no chromatic aberration, offering greater convenience than refracting telescopes.

By convenience, it means that only one reflective concave surface of the Newtonian reflector is actually involved in imaging.

However, even the most convenient thing, when it comes to optics, often becomes complex.

For instance, in later generations.

The choice of this single concave surface has always been quite debatable.

To be exact.

It's the debate over whether to choose a spherical or parabolic surface.

Fermat's principle in later generations can prove that a parabolic surface perfectly meets the equal optical path conditions for parallel light incidence, hence provides perfect imaging for parallel light.

But on the other hand.

A parabolic surface has only one symmetrical axis and does not fulfill the Abbe sine condition.

So a parabolic surface has obvious coma.

It cannot perfectly image parallel rays that do not enter along the symmetrical axis.

Even if the angle of incidence is very small, the image quality rapidly declines.

Though spherical surfaces have spherical aberration, comparatively, they have an unmatched advantage—they have infinite symmetrical axes.

For a single sphere, there is no concept of light off the axis.

In the telescope scenario.

Although a spherical surface cannot perfectly image star points, the imaging quality across the entire field of view can ensure uniformity.

Considering cost issues, many can't afford the price of parabolic surfaces.

Hence, in later generations, quite a number of people chose spherical surfaces for the single concave.

But don't forget.

The premise for this choice is...in later generations.

The timeline Xu Yun is in now, however, is ancient Song Dynasty.

Therefore, crafting a spherical lens by hand involves an unavoidable challenge:

Xu Yun could not make a Knife Edge Instrument or an Interferometer.

Actually, the Knife Edge Instrument is more manageable; really wanting to craft it by hand still allows for two or three dozen simple prototypes.

But the Interferometer is quite difficult because it requires lasers....

And these two instruments are pivotal, perhaps even the core aspect, in crafting spherical mirrors by hand:

Why can skilled DIY spherical mirrors in later generations completely surpass machine-made ones?

The reason is that these two machines provide extremely precise inspection.

Whenever a deviation is detected, just scrub with a soft asphalt coated in polishing powder.

For example, Canon lenses—especially the high-end L lenses—still have many hand-polished lenses nowadays.

As for the remaining Rotating Instrument Clock and such, they're more straightforward:

Old Su's Water Transporting Celestial Observation Platform is actually the earliest Rotating Instrument Clock, directly connecting to the ancestors' head...

Considering this is a documentary... uh-uh, rigorous novel.

Therefore, from the start, Xu Yun had prepared a plan:

Utilizing rotating mercury to achieve a parabolic surface, and hand grinding a spherical mirror that doesn't require excessive precision as a critical focal point auxiliary, resembling an RC structure. (It was hinted when discussing mercury thermometers, but no one noticed, disappointed.....)

Actually.

The idea of applying liquids in optical devices can always be traced back to Little Niu... or rather Old Niu's era.

But due to numerous engineering and technical difficulties, not until the 19th century did later generations begin attempts at developing reflective liquid components.

So-called reflective liquid paraboloid refers to using highly reflective mercury as the mirror material to create a liquid mirror surface.

By placing it in a container rotating steadily at a constant speed of 8.5 rpm, a parabolic surface can be formed.