How to Make a Parabolic Mirror for a Telescope? [The Only Guide You’ll Need]

by Ali Ahsanul karim

How to Make a Parabolic Mirror for a Telescope? [The Only Guide You’ll Need]

Do you have an interest in the telescope? Telescopes are great tools that can be both fun and fascinating. If you enjoy watching the stars at night and you’re also interested in the constellations, having a telescope will surely make you look a lot more fun.

People use telescopes to profoundly precede objects for many reasons. A home telescope is used to search for comets or some astronomical occurrence that may occur in the sky. For the study of astronomy, large telescopes are used.

They are also used to examine the structure and movement path of the stars and their atmosphere. Telescopes are the main tools to observe the night sky.

But do you know what is the object in the telescope makes it possible to see precisely anything? It is its lenses and/ or mirrors which used to make it easier for the viewer to see distant objects by magnifying them or increasing the effective brightness of a dark object.

The mirrors in a telescope are parabolic, so you can see the object wider to focus the light at one point. The main advantage of the parabolic mirror is it’s the virtue of the parable’s reflective properties which helps it to have a well-defined focus.

Do you know you can make a parabolic mirror for a telescope if you want? You can think it is absurd but no, it is possible. It is possible for a telescope manually. In this writing, I will tell you how to make a parabolic mirror for a telescope? Let’s know how to make it.

Why Is A Parabolic Mirror Used in A Telescope?

how to make a parabolic mirror for a telescope

You may be confused about why the parabolic mirror is that important, right? Before knowing how to make it, we should know about the parabolic mirror and its function.

At first, we should know what is a parabola?

Parabola can be defined as the plane curve created by the point moving. In that situation, its distance from the fixed point and distance from the fixed-line is equal, and also the right circular cone and an element of the cone’s plane-parallel intersect.

So, a parabolic mirror is a concave mirror with a more acute shape than a part of a circle, whereby the center is deeper than an arc. In the case of a mirror-reflecting telescope, it is designed to bring the image to a more central point. It can also be used in other optical devices such as radiation intensification.

Parabolic mirrors are better than spherical mirrors; the main reason is that they have a single focus point. On the other hand, spherical mirrors have many points as the focus.

A single-focus mirror is important for the telescope to capture super-sharp, crystal clear images. That’s why we cannot compromise with the full sharp focus mirror.

See also  What Can You See with a 100mm Telescope?

Steps On How to Make a Parabolic Mirror for a Telescope

Although you can buy a commercial mirror and set them into your telescope. But I think that creating a mirror for a telescope will a fascinating work you have ever done. You can get a nearly ideal parabola but with facade errors 20 toward 60 nanometers of the range of a rough glass blank.

First Grinding (Roughly)

The first step is to make one part of glass to concave. Earlier we know that the parabolic mirror is a curved in mirror, so our main goal is to make the mirror concave.

Your mirror will have a general focal ratio by grinding it roughly. If you are deepening your mirror in the middle, you will create a quick telescope that can be used for deep and dense sky monitoring. If the mirror is shallow, it will be best for the moon as well as planet.

The concept of grinding is if you are using a substance with higher hardness, glass can be removed. For this task, powders silicon carbide (also known as carborundum) is used. It is inexpensive and can vary from different sizes.

It is important to rub the glass with abrasive with a great deal of pressure. For this task, a tool is needed, which size and also weight should be the same like the glass. The tool is built with ceramic tiles that paste with plaster or plate of plywood.

Flattening one side is the first task you prefer to do by rubbing it adjacent to a flat glass piece (an old window). We can begin to curve the front when the backside is smooth. The assumption is if you polish the equal stiffness two circular blanks in between using an abrasive, a convex and a curved in blank will be created.

Our objective is to own the mirror which is concave and tool which is curved. Gravity will support us here when we will give more pressure in the overhanging blank’s middle and the borders of the base.

After one or two hours of grinding with a coarse abrasive, your mirror will become an approximately spherical face. If you are aiming for a potent high focal telescope, you don’t need to have to chafe for too much. But if your aim is for a quick scope it may take a lot of time.

In the next phase, we have to ensure the surface is further spherical once the desired sagitta (is the distance from the arc’s center to its base’s center) has been achieved. Now it’s the point for the next step; fine grinding to attain the final shape.

Check our latest guide about: How Does A Refracting Telescope Work?

Also, read the related guide: What Power Telescope to See Saturn Rings?

See also  eVscope 2 vs. eQuinox Telescope

Second Grinding (Finely)

The only purpose of this step is to remove the remaining holes and scratches due to the coarse grinding when preserving the surface shape.

We’re going to use abrasive which can be coarse or finer both to do that. We can move from one-grain dimension to next when all holes of the prior grain have removed. Using a sharpie to spot the larger holes and sand the surface till all them are disappeared. Do exactly for each size of the grain.

By using aluminum oxide, the better grains can be achieved. Now when seen from the side, the surface will begin to shine, if the abrasive motion becomes too low to be seen with the unaided eye, or using a magnifier. Let’s move to the next step, refining.


The target is to eliminate the remaining rough surface of the previous grinding and to gain a surface which is an even reflective surface.

Throughout this step, we must advance to chemical grinding (cerium oxide) from motorized grinding (aluminum oxide and silicon carbide). The change happens only on a microscopic level but the rub stays the same. For eliminating peaks on the surface cerium oxide can act as shears instead of causing cracks in the glass. For this reason, pressure not any more affects the rate of polishing.

We have to use a softer substance instead of the ceramic tool (tile) which can keep the cerium on its facade. Pitch used as most popular soft material.

To design a pitch lap on the similar side of the tile tool of ceramic, we require creating a plywood disk or plaster, bending the parts with tape as well as on it running liquid. It may be molded into a shape of convex once the lap begins to solidify. We will stop it from pushing to the mirror by using fewer ceriums on the pitch when we eventually push the glass in opposition to the pitch chest.

To increase the flow of cerium when the pitch seems solid enough, we have to incise groove on the facade. The Exacto knife will perform the cutting just fine under the flowing water. It is significant to note that the pitch of almost all surfaces is very difficult to remove.

Now the mirror can be polished. After a few hours, it will begin to obtain its deep surface. You will require some more hours to ensure that all the uneven surfaces are gone from the grinding.


This is the lengthiest step. What we are trying to achieve is to transform our superbly sphere-shaped surface to a paraboloid, which is our desired shape.

This involves a bit of calculation: when parallel light rays are reflected on a sphere-shaped surface, they accomplish focus on different points. That’s the reason we are making the parabolic mirror. A paraboloid generally has a wider core and edges that are flatter excepting a spheroid and focus on a single point.

See also  What Can I See with a 130mm Telescope?

There is a very small dissimilarity between the paraboloid and the spheroid and it cannot be determined with the unaided eye. Depending on experience, focal ratio, and luck, hours to weeks could be taking to change from a spheroid shape to be a paraboloid.

Through the test of knife-edge, we can see how the paraboloid looks. You can see that the center is deeper, edges are flatter apart from a sphere and surface is lit from the right.

You finish up with a hyperboloid in that situation and you’ll have to back off to the orb before attempting to parabolize once more. You will require using tools to monitor the figure of the mirror. The Foucault, as well known as the test of knife-edge, is a widely used tool for this.

A mirror can be evaluated as finished when variations are fewer than of green light’s 1/8 of the wavelength between the ideal paraboloid and its surface. This means the distance of a peak to the valley should be below 64 nm. This test is a fine indicator to test the quality of the mirror, but there have other tests too.

Coating of surface

In this final step where your mirror obtains its surface is reflective.  For this purpose, aluminum has been coated on the surface of the mirror. A mirror goes from 5 percent of reflectivity to over 90 – 95 percent of reflectivity relating to the form of treatment applied after aluminizing.

The aluminum coating is rendered in a high vacuum tank by aluminum coating. Aluminum metal evaporates from a wire at the tank’s bottom and covers the mirror spinning at the top of the tank. To save it from oxidation, a defensive SiO2 film is often added to the aluminum.

While it is achievable to build a vacuum chamber of your own and mirrors can be coated. But it is normally not profitable if not you have large number of mirrors for aluminizing. It will be better if you take help from a laboratory to aluminizing for the low cost.

Bottom Line

Now it is clear that we can make a parabolic mirror for a telescope. Making a mirror sure takes some time and tests your patience. But the whole process is really exciting and makes your telescope viewing object for enjoyable because you, yourself make the mirror.

Ali Ahsanul karim

I'm Ali. is my little place on the web to express what I've learned first-hand, specially about the telescope part. I am writing these articles to share my love for astronomy with you.