grinding paste

[perrycas]

pls forgive this very basic question, but my son and I have been discussing lens grinding and have so far been unable to discover
-what this is
and
-where you get it.
We have a lense that we would like to play, with and have some jewellers rouge.

I am sort of assuming that this is not really appropriate??

Thanks
Perry

[raymo]

You can get various grades of grinding materials such as Cerium Oxide
from a lapidary [gemstones] supplier.
raymo

[Blue Skies (Jacquie)]

You're looking for "grits" from lapidary suppliers, as raymo mentioned.

If you go further down in the forum, in the ATM and DYI Projects section, you will find several threads on this topic and that's where the mirror grinders hang out. The first "sticky" at the top of the section is all about mirror making.

Getting the grits is the easy part, however, I found finding the glass much harder to do, but if you really want to try it, it can be done.

My only caveat is do it because you want the experience, not because you want a scope. If you want a scope, go buy it! But if you're simply curious about how its done then by all means go for it. I'm just making this comment as I've found some people don't realise how cheap telescopes are these days and are still under the misconception that its cheaper to make one - no, its not! In fact you'll probably find it cost more to make it yourself than it would to buy it off the shelf.

[Wavytone]

Hi Perry,

As one who made a few mirrors and an eyepiece with 3 elements, there are a few things you will need. Don't be afraid to try, as I did many years ago. Lenses are easy to grind and polish. What is much harder is learning to achieve a specific curvature within a strict tolerance, achieving the correct thickness, and controlling the wedge error. And if you grind off too much ... tough you must start over again, you cannot stick the glass back on !

1. You need a typical ATM mirror grinding kit which includes a range of abrasives starting with silicon carbide grade 80, 120, 220 then aluminium oxide in several grades from 320 to 3200, followed by jewellers rouge on a pitch lap (cerium oxide polishes faster but rouge while slower produces a better end result). While rouge is messy stuff to work with (your wife will hate you), but IMHO is well worth using especially on lenses.

You really need to buy these from ATM sources (usually for mirrors, but lenses are much the same) in order to make sure each grade isn't contaminated with stuff that will leave the odd scratch or sleek, especially the finer grades of abrasive.

Grade 80 is fine for big pyrex mirrors, but way too coarse for grinding small lenses. Optical glass also is much softer than pyrex mirrors so you'll probably find 320 will grind lens blanks to shape plenty fast enough.

2. Glass blanks. You really do need to use optical glass blanks when you are ready to make a "real" lens, from Schott or Hoya. But first make some practice lenses from anything - bits of lead crystal (the bottoms from chipped whiskey glass do OK), window glass and other bits & pieces but optically these will be terrible.

3. Strongly recommend you find and buy the 3 volume set "Amateur Telescope Making" edited by A.G. Ingalls, read and study this thoroughly, these contain all you need to know for making lenses at home.

4. You will need to make a motor-driven spindle to spin the lens while you grind it. A 12mm steel shaft, small ball bearings, a motor scavenged from a drill or even a washing machine will suffice.

5. You will need a micrometer that reads to 0.005mm over 25mm and a spherometer than reads down to 0.001mm (a micron). These are not cheap. The spherometer must have a base slightly smaller than the diameter of the lenses you intend to make, and you need to know how to calculate the radius of the surface from the spherometer reading (it measures the sagitta of the lens surface).

You can buy run-of-the-mill ones that are cheap but the precision is an order of magnitude worse.

6. You will need a metal-working lathe - it is essential for centring lenses as the final step before mounting them, as well as turning the metal rings needed to mount them accurately.

The snag is that during centring, the abrasives you will use to grind the edge down will probably ruin the lathe for other purposes, so do not consider asking someone else to use theirs unless you fully accept the consequences.

7. Lastly you will need to learn a little about lens design (ahem*...) and glass types in order to make a satisfactory lens, assuming this might be a doublet objective or eyepiece of some sort.

* This is far from trivial unless one of you is very mathematically inclined. You'll probably spend more time thinking, doing the maths and designing a lens than it takes to make one even by hand.

You will need a lot of patience though, which is something lacking in many people these days.

If you can make a small achromatic doublet this is a good start. In the process I learned a very healthy respect for what is involved with making a Maksutov corrector and I have no problem paying for one.