The Pits of the Mirror

In the first picture, the pits came from the rough grinding or "hogging out"the mirror, which is the first stage of making a telescope mirror. We hogged out the mirror using a 60 grit silicon carbide.

The second stage is fine grinding.After finishing with the 60 grit, we started using a 120 grit, also a silicon carbide. Fine grinding is taking the pits out from when we did rough grinding. After the first charge of 120 grit half of the pits were gone. At the end of a 120 grit, there were 6 pits left. We marked these pits on the back side of the mirror, to make the back rougher and to cheek on our progress.

We then went to the 25 grit White Aluminium Oxide. We crossed-hatched many times to make sure we still had good contact when the lines rubbed off quickly. We had 1 pit left. After this we went to a 15 grit White Aluminium Oxide. We now have no large pits when the mirror is viewed through a bright light source!! At the end of the 9 White Aluminium Oxide, we had 1 very small scratch which we will polish out. The mirror is very smooth, but has a frosted look. The frosted look comes from the many tiny pits. To get rid of this frosted look, just quirt water on the mirror.

There are no pits!!!! YAH.

Astronomy for the Kindergartener

http://www.nasa.gov/audience/forstudents/k-4/index.html

The fun facts of astronomy are found at this great website, run by NASA. The NASA website has games, easy read books, picture dictionary, and homework topics to cover for your Kindergartener.

The NASA website's games encourage the mind to learn about space while having fun. One of the games is a matching game. Through this game, the child will learn about stars, planets, space shuttles, telescopes, and satellites. The website has a picture as well as a brief summary about each one. The game starts out easy, but with each level it becomes harder and you learn more things about space.

Children are able to see and read about space with NASA's section on easy read books. Robots Storybook is about robots and the useful things that they do for NASA. The robots have names such as Robonaut, Mars Rovers, Remote Manipulator System, and the Stardust. Children will learn about how a robot works and ways that they are different from us.

Children can learn about the abcs of space with this picture dictionary. The dictionary covers things from what a aircraft is to the change in weather. The dictionary also has a summary of each one.

G is for Galaxy. The earth is found in the Milky Way Galaxy. This Galaxy is the home of millions of stars, and are held together by gravity. M is for Matter. Matter is anything that takes up space. It is matter if you can see, hear, taste, feel, or smell it.

Children will learn a variety of information from the homework topic and will have questions answered. The children learn about why Astronauts go on space walks and how they are able to do things when there is no gravity to hold them down.

Together you can learn fascinating information about space with your kindergarten.

Cross-Hatch

We cross-hatched our mirror to see where we had contact between our mirror and the tool. The tool is convex and the mirror needs to be concave, so that it will reflect the light from the stars better.

This is the line test. The mirror has lines on it, so that what when grinding it you can see where you have contact and where you don't. The lines came off after the second charge of 60 grit. We now have good contact!!

The Grinding Tool

Hi, Anna here. I'm the new editor-in-chief. This is the begining of making the grinding tool. We placed plastic around the mirror and then put the tiles on the mirror. We made a ring around the mirror using the last frosted flakes box. We poured plaster of Paris into the ring. After awhile we have a perfect replica of the shape of our mirror.


Daddy hammering the tiles to form a circle to lay on top of the mirror.

New Blog Editor

Kenning Through Astronomy Divine has a new editor. It is our very own Anna. Doesn't she look cute in her "anti-abrasive" get-up? It seems that she is allergic to Silicon Carbide. She gets all itchy, pink and splotchy whenever we grind on the mirror. So rather than torture her, we have decided to make her editor-in-chief.

It should be easy for her to take over. As a teenage girl, she has a natural pre-disposition to "run the show." Peter will be explaining to her what we have done each step along the way and she will post explaining to all of our dear friends what we are doing.

It should be fun.

Hogging Out

Since we got grit, we decided to start "hogging out" the mirror. In the Old Days, we would have used the same plate glass tool throughout the grinding and polishing of the mirror blank, but there have been cost improvements. The very roughest grinding is now its own separate step and is called hogging out.

The purpose of hogging out is to remove a lot of glass quickly with a sub-diameter tool. The tool for hogging out can be almost anything. On advise from another ATM, we thought about using a can filled with mortar.

As an experiment, we used a can without the mortar. I thought that the bottom of the can would be cut away too quickly. You can see it to the left of mirror. However, using the can allowed me to get the feel for what was needed, so I went out to my toolbox and got an 1 1/4 inch socket. This cost $0.25 at a pawn shop, and it was perfect.

Here we see Anna busily grinding. If you have never ground Silicon Carbide (SiC) against glass, prepare yourself for noise. It is loud, but gets quieter as the SiC is ground down.

One of the things that you will quickly learn is when to recharge the grit. There are 2 ways. First, the sound of the grinding changes from very loud to merely loud. Second, you can feel the grit rolling under the tool when you first start. As the charge of grit wears down, you can feel the tool sliding across the mirror.

After you have ground out several chargers of grit, a thick sludge will develop in the hollow of the mirror. This sludge must be removed. We are using a small red tub that we used in the summer to rinse our feet before getting in the pool. Since we don't have a water faucet downstairs, we get our water from emptying the de-humidifier.

We just douse the mirror in the water and gently rub its surface to help the grit fall off.

Here is Anna still grinding away. I decided to cover the top of our grinding stand with plastic. Each time we change grit size, we must thoroughly clean up everything to make sure that none of the larger grit is around to make big scratches in our mirror. With the plastic, we can just carefully fold it up and throw it away.

Here is the mirror blank just as it came from Edmund's Scientific all of those years ago. It has some surface irregularities from the molding and annealing process.

Here is the mirror following about 15 minutes of hogging out. All of the surface irregularities have been ground out, and the face is uniformily pitted. The pitting has made the glass very opaque.

Later, when we are polishing the mirror, we will use a cheap laser pointer to look for pits. The pits will cause the laser light to bounce around and prevent it from going through the glass. As we polish the glass, we will see less and less scatter from the pits.

A very well polished mirror is almost invisible, if both sides are well polished.

Tommorow, more detail on the methods involved on "Hogging Out."

Have Fun.

Got Grit

After a brief delay in getting our grinding abrasives, we have resumed our telescope project. In the picture, you can see the mirror blank in the middle. Above the blank is optical pitch. It is literally tree sap and will be used in polishing and figuring the mirror.

To the left of the mirror are the abrasives used in rough grinding. This abrasive is made of Silicon Carbide and are graded by size. We will be using 60, 120, and 220. If you imagine sand paper, it is graded the same way, I believe. So the 60 grit abrasive feels the same to the fingers as 60 grade sand paper.

To the right of the blank are the abrasives that we will be using in the fine grinding and polishing. Three are made of White Aluminium Oxide. We will be using 25, 15, and 9 in the fine grinding. We will be using Cesium Oxide in the polishing.

The Amateur Telescope Makers website (http://www.atmsite.org/) has a number of links to suppliers. I considered 2, Willmann-Bell (http://www.willbell.com/Default.htm) and GotGrit.com (http://gotgrit.com/). Ultimately, I choose GotGrit. The owner, Tom Moulton, is an ATM (amateur telescope maker) himself and his prices were as good or better than the others. Just be aware that he usually ships orders out on Saturday.

The Stand

Here is the completed Grinding stand. I know that it looks like it is leaning but it really is plump and level. The Stand is made from an old paint bucket that I was using as a doose bucket on my outdoor brick oven, concrete, 4" PVC pipe, a 6" 4x4 timber, and an 18" table top blank purchased at Lowe's.

One of the wonders of building such a precise astronomical insturment is how roughly some of the tools and equipment can be built. Our stand is largely built from scronged items that I already had on hand. Before the kids and I went to Lowe's I took stock of what I already had, so that I could get by as cheaply as possible.

When we got to the store, we went straight to the plumbing department. I really wanted to get a galvanized pipe and flange for the pedastal, but I didn't want to spend as much as that was going to require. My next choice was oak, but again price intervened. Ultimately, I decided on the PVC which cost less than $7.

The first step was to cut a 6" 4x4 timber (it had been a part of a fence that I took down). We then planed the corners off so that it would fit inside the PVC pipe. This timber is what the top is secured to. We then mixed a bag and a half of mortar mix left over from my oven project. It had set around for nearly a year and become quite compacted.

Once the mortar was mixed, we filled the bucket with the mortar and then set the pipe in, which we also filled leaving room for the 4x4 block. We used a level to make sure everything was properly plumbed and leveled. All of the weight makes the stand very stable, so that it will not be wobbly when we begin grinding.

I then attached the top using 4 counter-sunk screws, which can be seen through the mirror blank. The cleats, which hold the mirror, is from a flower stake from my garden. The cleat screws are also counter-sunk.

The only thing left to do with the stand is to seal the top. When using the abrasive charges for grinding, water is used as a lubricant, and I am sure that the glue used in the top is not waterproof.

We will using Kilz, left over from painting the bathroom for the sealer. Once that is done, our stand will be completed.

Also, the mortar does not adhere to the PVC, so there is no need to be excessively careful when filling the PVC pipe.

When doing a project like this, do not be afraid to use what you have on hand. Remember the goal is not a beautiful stand, but one that is serviceable for the intended use.

Next week, we will begin "hogging out" the mirror blank and making the grinding tool. Stay tuned.

The Project Begins

The Project began at the school table this morning. We are going to grind a 6" f/8 mirror and put it in a Dobsonian mount. I began by telling them that we will be able to see the a couple of miles of the Moon at a time, see the divisions in the rings of Saturn, and the phases of Venus to build their excitement.

We then went over the basics of grinding the mirror. I emphasized the precision of the mirror, which made Anna a bit nervous since she is fearful of making a mistake. I relieved her anxiety that the errors between 3 careful grinders will average out and should actually produce a mirror with greater precision.

We then went over the need for a very stable stand to grind the mirror on. We followed this up with a trip to Lowe's to get the materials to build our stand.

Tomorrow I will post pictures of the stand and its building.

As Jack Horkheimer says, "Keep looking up."