spaceview telescope_making

Mirror Making Process :

1) Rough and fine grinding

1) Rough and fine grinding

This process is amazingly very simple.

Don't be afraid of seemingly complex details.
In practice it is extremely easy.
Once you actually see or initiated in the process,
you will be astonished how simple it is.

I'm going to describe a lot; and about a number of different things concerning grinding and polishing of a main mirror of telescope so you read through this entire set of pages and get familiar with it.

I also suggest that you find other web pages describing this process and compare them with what's here. While comparing all or various methods you will find that essentially the same process has been expressed in different ways. People narrate things in different ways and think that certain parts of the process of grinding and polishing a mirror are more important than others.

(I) Imagine you have two round stone disks. Keep one on top of other. Pour some water in between these disks. Slide top disk with some pressure over bottom disc. This is the grinding process. You will see that both discs are grinding or losing some material. Their shape is also changing.

The same process is involved in grinding telescope Mirror. Instead of two stone disks, we take two glass disks, pour mixture of different numbers of grit/abrasive and water in between them and grind these in specific angles.

Lo and behold the Mirror for Telescope starts taking shape according to your design

Yes, it is so easy.

Added advantage is that the Telescope Mirrors made by hand are better than the one made purely by machines. Hence, often times you see in the adds - "Mirror made by hand grinding'.

Take two round glass discs of 4.5". If glass disks are not champhered (edges/corners not

grinded), then take a 120 number floor grinding stone (available at hardware shops)and grind both discks at the edges (to cut their sides). Always pour plenty of water on the stone/glass i.e. keep the stone wet. Or you can take a small Kadahi, pour some mixture of 120 number grit and water in it and grind the sides of the both the glasses. The champhering (edge grinding) may be around 1.0mm.

For making telescope mirror, we give these two glass discs a special way of grinding. We will call the upper disc Mirror (M) and place it 1/3rd to 1/2nd out from any side of the below one, called Tool (T). We will put 'T' on any heavy stool or table. Secure T by three wooden square cleats of 20x20mm and 5-6mm thick by screwing these around T Disk, 120 degree apart.

Put some thick paper beneath 'T'. Pour some quantity around half a small tea-spoon of abrasive grit no.80 and water mixture as shown in top left picture.

As shown above, stand at 12'o clock position. Slide M downwards so that it comes to 2nd position over T, then slide upwards at 3rd position, then 4th. In this manner, as also shown in last diagram (also see left moving picture) give sliding/grinding movement to M over T. Do these strokes around 30 in number. Always keep M 1/3 to ½ out from one side of T, as shown in above diagrams. These are called side-only orchordal strokes.

The speed of forward and back motion should neither be too fast, jerky nor too slow. For 4.5" disk speed of around 60-80 motions in a minute is alright.After giving 30 strokes in 12'0 clock position now come at 4'0 clock position. Also bring M at your side. Keep M at this position also 1/3rd to 1/2nd out from T disc as shown in left picture. Give same 30 grinding side-only strokes as described above.

Again after 30 strokes , come at 8'0 clock position as shown in picture. Keep M 1/3rd to 1/2nd out from T as already described and give same 30 numbers of side-only forward and back motion to the top disk M over T.

(II) After every three top disk rotation (i.e. first 12'0, 2nd 4'0 and third 8'0 clock), and giving 30 numbers each of grinding chordal strokes, lift the top disk M, give one rotation to the bottom disk T in 45 degree angle. For example put any mark over the edge of T. Suppose this mark was at 1'30 clock position now bring this mark at 3'0 clock position so that the bottom disc T moves 45 degrees, as shown in 1st picture above. Also rotate M around 90 degrees.

At the start of grinding, you will hear a peculiar cutting/screeching noise. It denotes that by grinding process abrasive and water mixture is cutting/grinding both glasses. This sound will decrease progressively and after around 90 grinding strokes of Mirror over Tool (MoT) you will observe that the screeching noise is almost gone and the grit-water mixture has converted into a gray paste. This would be the time to pour fresh grit and water mixture between the glasses.

Now repeat the same process as described in (I). but with a fresh mixture of grit and water which should sustain atleast one round. Keep the top disc M over T in the same fashion 1/3rd to ½ out from botton-one T as described in above (I) paras. Stand in 12'0 clock position. Pour fresh #80 grit and water mixture between M and T give same side-only grinding strokes to M over T in 30 numbers. Then come at 4'o clock position (i.e.120 degrees apart from previous position) and give 30 side-only strokes. Again come at 8'0 clock position (120 degrees apart from previous position) and repeat same grinding process.

Now, you have again given M three rotations and each time 30 number of side-only strokes, so there is time now to rotate the bottom disc T by another 45 degrees and M another 90 degrees. Previously T came at 3'0 clock from 1'30 position now move it from 3'0 clock to 6'0 clock position. Again the same process as described in (I) and ( II) of grinding three times MoT (Mirror on Top) will follow.

So, after every three rotation (120 degrees) of M one rotation of T (45 degrees) and M (90 degrees) will be there. If you do not want yourself to rotate in 12, 4 or 8'0 clock position, standing at one place you can give rotation of 120 degrees to T and after three such rotations, rotate both T by 45 degrees and M by 90 degrees.

You have to compulsorily give 3 rotations to T and after that compulsorily rotate the position of T and M so that grinding is not at the same place but at every place.. You have not to perfectly gauge the changing angles. These should be roughly around that.

You have to pour grit-water mixure in such quantity which should last minimum one round i.e. 3 rotations or 6 or 9. This would be better though you can change the mixture even after every 30 strokes.

By this grinding process you will see that Mirror is grinding down all over from centre and Tool is grinding down all over from the sides. Thus M will convert into a concave surface and T into convex surface, as shown in left diagrams.

This process of grinding will last till such time until the saggitta (S) of M reaches according to our design which in our case is .03" or 0.75mm

While grinding you have to gauge this saggitta (depth) S of the Mirror's concave surface from time to time until

you achieve this. It is better to reach uptil 0.8mm (slightly over from 0.75mm) as afterwards it tends to decrease.

You can test the saggitta using steel rule and small rod, bar, coin, piece of drill bit of feeler gauge of right thickness or spherometer.

As and when the proper saggitta has reached in the Mirror, you have to stop giving side-only grinding strokes. Now onwards another type of strokes will start and remain till the end.

Please click for continued Rough and fine grinding