How to Grind a Telescope Mirror in 10 Easy Steps

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Telescope Mirror Grinding

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  • Obtain the right glass
  • Choose solid grinding tools
  • Choose the best position to work on mirrors by hand
  • Grind the back flat
  • Try and prevent astigmatism
  • After having the ground and face of the mirror flat through the 220, you are good to begin rough grinding the curve into the mirror
  • Do some fine grinding
  • Proceed and trying getting a sphere
  • After getting your final abrasive, you can now perform polishing

There is a question on whether to build or buy a telescope mirror amongst most astronomers. The whole deal is not about saving a few dollars.

But instead, making your mirror will cost you more than you would spend buying the instrument. You probably have to spend a lot of hours creating it. 

However, most amateur telescope mirror makers decide to build their instruments to understand the whole phenomena behind reflectors. It also offers an advantage in much better surface qualities compared to commercial ones.

The building process of a telescope mirror is the most rewarding when correctly following the guidelines. 

This article gives you adequate information on simple easy steps that will help you build a reflector telescope, starting from obtaining a glass blank to getting a polished parabola surface. 

Grinding  

For more than a decade, astronomers and amateur telescope makers suggested that primary telescope mirrors must meet the ratio of at least 6:1 for thickness and diameter. But in the last few years, that is increasingly eroding as most of the primary telescope mirrors you find are thinner than the standard 6:1 thickness.

However, leaner mirrors are now giving excellent performance and trending at a higher rate. You can no longer find a telescope project with over 12 apertures and has a ratio of 6:1 thickness.

So here is how you do it.

First Phase: Obtaining Glass

First, begin by obtaining a pyrex sheet glass which is often a problem. You will also have to select from either coarse annealed and fine annealed glass.

Additionally, the available pyrex sheet glass comes in a variety of thicknesses and large square sheets.  Therefore, It is essential to specify the ideal diameter and thickness upon ordering the glass. 

The telescope mirror is round in shape, and you will need to cut off the four square corners into a perfectly circular piece of glass. That should get you a glass blank with a ratio of 16:60 slides and a flat surface of at most 1:8. You will then need to contrive a method to grind the empty glass flat and round. 

Note that grinding the glass round is easier when there are enough sides. All you need is grinding off the high points using a hand-size tile and 80 sizes of grit.

You may opt for diamond-generate that will Blanchard the back flat, generate a radius and make the edges round within a few minutes. It is a risky task, and finding the right professional can be cumbersome—instances where the glass cracks make you lose the blank, but it is not so common. 

However, without the professionals, you need to rely on your tools. Keep in mind that the thickness choice is determinable by the diameter of the blank and how you want to mount the reflector in the telescope. 

Second Phase: Solid tools selection

After finding the perfect blank for the mirror, you will have another selection for grinding tools. The piece can either be a plaster tool or a solid tool covered by a layer of hard ceramic tile.

In case you have previous experience or best taste will help you determine what material to use. 

The selection of the tools should ensure that they are not too large as the size of the mirror. The ideal length can be 75% of the mirror’s diameter.

Any tool less than 75% happens to be deficient in designing a perfect spherical curve when grinding. The size of the mirror to work on will highly determine the various types of tools you are to use. 

It is vital to keep a check on the thickness of your tools. Too lean tools are likely to bend while grinding and adding a problem on another.

First is astigmatism in the blank that won’t grind out for a device that happens to conform to the astigmatic contour. Second is the weight you put on the back of the tool may force the center of that tool to grind harder.

That generates a deeper curve than the spherical curve.

However, the problem is not that severe, but you will require extra polishing at the center. The grinding tools are only in two options: Segmented and solid.

The segmented tools consist of plaster covered with ceramic tiles. While the solid comprises a glass material. It can be a disk cut off from a sheet of glass or consist of glass plates binding. 

  •  Segmented Tools

Segmented tools are ideal for an already formed curve. The devices come in a ceramic tile grinder that is flat or curved to fit a generated blank.

However, the materials on the surface can either be plaster, glass, or aluminum then glued to the tile with epoxy. Some may use a pitch instead of epoxy but not made for rough grinding. 

Those who choose plaster support can wrap it with a metal or cardboard dam just within the mirror some inches higher. Then smear soap on the glass surface of the blank to avoid sticking plaster, then spread it on the plaster.

In case the blank has a curve, the method will give a mating curve on the plaster. 

Third Phase: Working Place

Choose the best position to work on mirrors by hand. You can opt for a hard top of a drum filled with pounds of sand for better stability. Or even a studying counter and maybe utilize your knees as the best support for polish.

Having two different working stations is better than one. You can utilize one for grinding purposes while the other one is a polishing station.

It is important to note that polishing stations are better indoors for cleanliness and moderate temperatures. Grinding is less distracted by climate, but more than thirty microns may be suitable when done indoors when grinding with abrasives.

Fourth Phase: Grinding the back flat

Grinding on both the front and back should help you generate an optical surface. You might find out that the back of the mirror is only a few thousandths of an inch for it to be concave or convex.

However, you should pay attention to any form of astigmatism commonly known as “cylinder” by opticians. At times you may find out that the back has a Blanchard ground, and all you need is to grind it smooth with a 220.

You can also use a tile or glass grinder. 

One of the best methods in grinding is to grind two blanks against each other. That will now give you an end product of two flattened blanks for the work of only one of the two.

Grinding the front, which is at the side, and the curve becomes the ground on a telescope. They all need to be of a lesser degree.

But in the grossly high and low spots near the edges, you will probably miss contact near the edges before grinding the curve. That will cost you more time and more work in flattening the high and low spots into regular blanks.

To grind the low areas near the edges, you need to place the blank face up on a smooth surface to prevent the reflector from rocking. Then make it wet with the appropriate amount of #60 abrasive and the grinding tool(optional), then begin grinding.

After at least four wets, wash off and confirm whether grounding the high points took place excellently. Observe your tools to keep track of the convexity and concavity of the surfaces with a straight edge.

If you realize that your mirror is beginning to go convex, concentrate your grinding to the center accompanied by short strokes. But if the convexity is too much, consider grinding the mirror at its top.

However, it is easy to tolerate isolated small spots because they are small. It isn’t easy to let a cylindrical curve on the back or even an angle on the axis while there is a flat on the other side. 

That will bring concerns on bending mirrors during grinding and polishing. You might spend more time polishing until you realize astigmatism present in the mirror.

Once you finish making proper flatness of the back, you will need to fine-grind it through 120 and 220. Additionally, fine abrasives work as an advantage in exposing low areas or coaster pits and are easily identifiable.

When the back has Blanchard ground, it will not take more time to grind the mirror flat with a 220. 

Fifth Phase: Preventing Astigmatism

The first step in preventing astigmatism is by grinding the back flat. You also need to ensure proper mirror support and do fine grinding and polishing the mirror face up.

One of the main techniques to success is by frequently rotating the mirror when working on it. That prevents non-uniformity into the mirror, which then appears as astigmatism.

It is essential to ensure that the turntable you are using is rigid. You can then choose one among several ways of supporting your mirror on the grind. 

Sixth Phase: Rough Grinding

After having the ground and face of the mirror flat through the 220, you are good to begin rough grinding the curve into the mirror. You can use #60 carborundum tools for grinding.

Note that you should always maintain an angle on the edge of the mirror throughout grinding. In case your mirror is too light to sand enough, as is done for small size mirrors. 

Begin grinding concentrating by focusing the center of the mirror to the edge of the tool. However, when having a mirror ground face up, consider using a smaller instrument that is less than or equal to 75% of the mirror diameter.

Seventh Phase: Fine Grinding

Fine grinding is well known for its long process of grinding. But if you do it correctly, it might just take you a fraction of time.

In all phases listed, fine grinding is the most manageable task in large mirror making. You can roughly work for about 2 hours on each grading.

One of the critical things that makes it easy is the division into two stages: Before #220 and after #220. 

  • Before #220, you aim to get the radius and a smooth curve. 
  • After#220, your purpose is to prevent astigmatism 

Eight Phase: Fine Abrasives

The critical tool in this stage is aluminum oxide, for it has less tendency is causing deep pits in the ground surface. Some processes may differ as fine abrasives may depend on several sequences.

There may also be some difference in a row depending on the quality of abrasive particles. 

Ninth Phase: Getting a sphere

It is the most critical phase that needs attention. Even with the most careful attention after finishing the task with a 60 grit, the curve may remain somehow hyperbolic.

Therefore, the main interest should focus on removing the pits and maintaining a spherical mirror. 

At that point, apart from the support, you need a good piece of carpet that allows easy sliding of the mirror. It would be best if you began by sprinkling the abrasive on the mirror face to make it wet. Avoid being too abrasive.

Then place your tool down with the edge first and start grinding. Ensure you are rotating the device by walking around the barrel.

Note that solid tools trap abrasives to the center of the mirror, contributing to spherical grinding resistance. You can remedy that by using short W strokes and stirring the abrasive regularly until the bubble is gone. 

Tenth Phase: Polishing

After getting your final abrasive, you can now perform polishing. 

You have now completed all the phases of grinding a telescope mirror. If you happen to touch the mirror, make sure to clean the mirror with alcohol.

Final Word

Grinding down a telescope mirror can be a lot of work but if you follow these steps you will be able to get it exactly sa you like.


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