This is a telescope in the making. It’s almost ready, the only thing missing
here is the reflective coating. Once finished, it will actually be a complete
Schmidt-Cassegrain telescope, be it a very tiny one. But what is maybe more remarkable than its
modest size, is the fact that it is made of one single piece of glass. Today we are going to meet up with the inventor
and maker of this type of telescope. It’s Rik ter Horst, who is an optical engineer
at NOVA-Astron. Rik is one of the few people who have mastered
the craft of making optical components to near perfection. And in a world where optics is mainly mass
manufactured, he is actually still making these tiny monolithic telescopes completely
by hand. Of course, I’m very curious to see how he
does it. Rik lives in the northern part of the Netherlands,
and it’s quite a drive, especially in today’s Dutch weather. But this gives me the opportunity to give
you a bit of general background information about telescopes before we meet up with him. Telescopes come in different varieties but
in essence they all have one thing in common: In order to achieve a very high magnification,
they need to have a long focal length. This is because the focal length determines
the size of the projected image. A lens with a short focal length, like for
example the lens in your eye, creates a relatively small image in the focal plane, in this case
on your retina. A lens with a longer focal length will create
a larger image in the focal plane. So, if we had bigger eyes and the same type
of retina, we would actually able to discern smaller details in objects. But a sharper image can also be achieved with
more photoreceptors per area in the retina. This is the case for the eagle. Believe it or not, but the eyes of an eagle
are the same size as human eyes, and they up a large part of their sculls. But eagles can see smaller details because
their retinas have a photoreceptor density which is 5-6 times higher than that of humans. The bottom line is that, if we have 2 optical
elements with different focal lengths and we place for example an identical CCD-sensors
in both focal planes, we will be able to discern smaller details in the image of the lens with
the longer focal distance. Because the detail level will very likely
be limited by the pixel resolution of the CCD, and not by the lens. A long focal length is also the reason why
early telescopes had a long linear construction. And an extreme example is found in the Yerkes
telescope in Williams Bay, Wisconsin, which contains a lens with a focal length of more
than 19m. Now, even though you can achieve a very high
magnification in this way, it is obviously not a very practical instrument. For example: we need an elevator to move the
floor up and down in order to look at the sky under different angles. But luckily there is a way around making ever
longer telescopes to achieve a higher magnification and that is to fold up the optical path. By using a specific combination of curved
mirrors, we can make a system that effectively has the same focal length, but is much more
compact. And this principle is found in almost all
the large modern telescopes, because it allows for increasing the telescopes’ aperture
and focal length, while at the same time keeping the size of the telescope itself relatively
limited. One of the most widely known folded telescope
designs is the Cassegrain telescope, named after Laurent Cassegrain, who published this
design already in 1672. His design is actually based on a mathematical
model for light that enters the telescope exactly parallel to the optical axis. Under these conditions all light will be focused
perfectly into a single point. But this design has a few limitations: light
entering under an angle will not have the same perfect projection, because the image
field is actually curved and we observe coma when we move away from the optical axis. On top of that, the optical model that the
Cassegrain is based on requires the reflective surfaces to be aspherical. The primary mirror is a concave parabola and
the secondary mirror is a convex hyperbola. And later in the video we will see that these
types of surfaces are relatively difficult to manufacture. So, since the invention, many improvements
have been made on the initial design of the Cassegrain. And one of these improvements is the addition
of a thin corrective lens called a Schmidt plate. Apart from correcting for the field curvature,
this lens allows for the use of a spherical primary mirror, instead of a parabola, which
greatly simplifies its manufacture. As a bonus, the Schmidt plate can be used
to mechanically hold the secondary mirror in place. And due to this relatively simple construction
and excellent image quality, Schmidt Cassegrain telescopes have gained a high popularity among
both amateur- as well as professional astronomers. So, after all of this background, we have
now come to the essence of the monolithic telescopes. Consider the conventional Schmidt Cassegrain
telescope like depicted here schematically. It consists of the optical elements, the Schmidt
plate, the primary and secondary mirrors. And apart from the optics, we need a tube
to hold everything together. We need the mounts for the mirrors and we
need a baffle for blocking stray light. Now the sheer genius behind the concept of
the solid telescope is that we can get rid of … everything you see here. What we can do is just replace the space between
the optical surfaces with glass. And by giving each of these surfaces the right
shape and reflectivity, this single piece of glass is equivalent to the complete optical-
and mechanical system of a Schmidt-Cassegrain. Even things like baffles can be incorporated
directly in the glass. And we have a bonus optical surface here,
which is not present in the standard Schmidt Cassegrain. And this surface can for example be used for
additional field corrections. We are almost there now, and I’m pretty
excited about this visit. This is actually the first time that I do
an interview in a video and for Rik this is also a first. So in the beginning it will probably be a
bit uncomfortable. But please stick with us for a while because
it is going to be really interesting. [Jeroen] Okay Rik, thanks for having me here,
for the invitation You’re welcome. Nice to see you again
[Jeroen] nice to see you. Can you tell us a bit about the background
of the project? Why did you start making these tiny telescopes? I was experimenting a lot in the past, just
like I do today. I was making a lot of molds for the ophthalmic
industry. So these are small glass cylinders with a
small concave surface and they used it for pressing lenses for cataracts so for artificial
lenses that replace the natural lens. So I made hundreds, maybe thousands of these. And at some point I thought, well you could
look at it as being a secondary mirror of a Cassegrain telescope, just for fun. So I thought, when I make this surface convex
I have a really small telescope. That is how it started. So I made one for fun and then I thought well
maybe it could actually work. I started looking for ways to design a configuration
that actually works, and so over time I succeeded in the end to make a small telescope that
actually worked So, from what you have told me, these are
almost exclusively made by hand right? [Rik] Yep [Jeroen] How long does it typically
take you to finish one? That depends on the design and the requirements
of course. If I really have time and I have everything
prepared, tooling, grinding glass, requirement glass, test glass, I could do it in a week. But the current version that I’m working
on takes way more time. It’s designed for a space project in the
US and the requirements are quite strong and it takes way more time to make this aspherical
surface to the right accuracy. SO that takes more like months than weeks. [Jeroen] Okay. Is it possible to tell us a something more
about this space project? What exactly are you looking at? Yeah, this is a student space project from
the University of Portland. So, students are developing a CubeSat. And they were looking for a camera with sufficient
resolution so they could see the campus from space. And the normal existing cameras, so the low
budget cameras could not get this done. Then they found an article about this telescope
that I made somewhere in (19)93 and 94. And then they thought, well that is exactly
what we are looking for: it is compact it still has a long focal length. And they could use it for their projects and
that is their application in the end. [Jeroen] So what is the focal length of these
telescopes, let’s say this one or the other The one I’m making for the US has a focal
length of 125 mm but this one has a focal length of almost 300. I has a smaller obstruction, longer focal
length and it is more relaxed in terms of accuracy because it is a bit longer. Now given the high-tech and innovative character
of these telescopes you’d expect that making them requires some pretty advanced equipment. But the opposite is actually true: the whole
process is done on just 1m2 in a small corner of Rik’s house. [Jeroen] So Rik, this is where the magic happens
right? Exactly. Or Magic, this is all I use for making these
telescopes. So I have a measuring device for measuring
parallelism of the glass blank, this is how it started in fact. I have a tool for measuring thickness, I have
test glasses that I use for checking the curvature of the surfaces. And it’s a feet drive spindle, and this
is in fact all you need. [Jeroen] Apart from the fact that you of course
need to drill these cores. Yeah that is the only thing that is done externally. So at my work in Dwingelo I have drilled tis
baffle in fact. I could show you how it is polished. At the moment I’m polishing this convex
surface and it’s done by hand on pitch, very traditional but it is in my opinion still
the best way to polish. [Jeroen] SO basically you just do this for
a few hours or something like that for each surface? Yeah it takes quite some time, it depends. This is a spherical surface, this is quite
easy, this is done within an hour. But the aspherical surface which is really
a strong asphere with up to 60 microns asphericity that is different [Jeroen] Sixty or Sixteen? [Rik] sixty [Jeroen] Sixty, okay that is a
strong asphere. [Rik] So this is how we do it. And then it is ready for being tested and
that is what I do with a spherical test glass. [Jeroen] Okay, so that’s this one, right? [Rik/Jeroen] The big one. So I make sure that there is no dust or water. Then I get this one, clean it a bit. And by looking at the fringes whether it’s
spherical or convex or concave. It is very traditional. I can see very well how the fringes are up
to the edge. The only goal is to have a really smooth and
spherical surface with a sharp edge. And that gives me a very good reference for
testing the front surface in the end. Personally, I think watching these polishing
movements is very relaxing. So, before I get too Zen it is probably good
idea to take a step back and explain exactly why it is so much easier to make spherical
surfaces than aspherical surfaces. The reason is pretty simple: The sphere is
the only shape in which the surfaces still fit exactly, even when they are displaced
along the degrees of freedom of the surface, basically tilt and rotation. Now, with aspheres, that is not the case. Take this parabola: the surfaces might fit
very well in one position, but if you displace one of the two surfaces, you’ll find that
the shapes only touch very locally, indicated in red here. And when you are performing an abrasive action
like grinding or polishing, you will remove material only in these areas. So what happens is that if you continue this
for a longer time, the asphere gradually iterates back to the spherical shape. Because that is the most stable shape, where
there is contact over the full surface area. What this also means is that if you want to
make an aspherical surface, like for example the hyperbolic secondary mirror, you cannot
do this easily with a large-size rigid tool. In this case, we actually want the removal
of material to be non-uniform over the surface. So in general, the strategy with aspherics
is to start out with a spherical surface and to use a grinding or polishing tool that is
much smaller than the optical surface. And if we want to get from spherical to hyperbolic,
we can for example vary the contact time or pressure over the surface area in such a way
that we remove more material in the center than at the edges. But we want the end result to be a very smooth
and accurately curved of course, and exactly this requires a lot of “experimenting”
as Rik called it. Next he demonstrates what making an asphere
looks like in practice. But then I have to first finish the front
surface (Schmidt plate), if it is finished I have to make the secondary mirror aspherical. And that is done with a very small polishing
tool. And it looks as if I’m done in just a few
seconds but it takes hours and hours and hours to get the asphere in the right proportion. So making the asphere is nothing else than
polishing a kind of low valley in the middle of the secondary mirror
So making a good asphere is actually pretty difficult, even for an optical expert. But it might be worth the effort if your product
has strategic advantages in a specific application such as a space telescope. [Jeroen] SO the fact that these are solid,
or made from one piece is also and advantage, right I guess in space it will be more thermally
stable, rigid, temperature stable. Correct, the two that I make for the university,
they are made from fused silica so very stable and that is of course an advantage because
you don’t have to make very complicated mechanical mountings for the secondary mirror
or the primary mirror or the baffle, It is all in one solid piece of glass
[Jeroen] never any collimation No collimation, so this means also that you
have to grind them, all surfaces are really well centered in the middle and that is something
you have to du during grinding and polishing. So, after a few inspiring hours, I thanked
Rik for his time and went on my way back. Luckily, the weather had cleared up a bit. Back home I had a moment of contemplation. And I couldn’t stop dreaming about making
one of these for myself…. [Dr Liam] After many months of extensive research
and development, we at Huygens Optics have developed the new generation of so called
“ter Horst” telescope. By increasing the diameter by only a factor
of two, we have managed to create an aperature that is 4 times larger in area. At the same time, we have increased the mass
to aperture ratio by a factor of 2. [interviewer] So what you are saying is that
it’s bigger and heavier, right? Well you know I cannot really comment on that. I mean that is proprietary information, right? Anyway, look at it: it’s still tiny
[interviewer] do you have a name for it? Well in fact we have, yes. Basically we are planning to connect 4 of
these together using interferometers, thereby creating one very large telescope. And that is exactly the name we came up with:
the VLTT, the Very Large Tiny telescope.
