Ronchi Screens or Ronchi Gratings have been used for many years as a quick qualitative method to evaluate overall mirror surface smoothness and figure. We stock a series of 7 gratings: 50, 75, 100, 125, 150, 175, and 200 lines per inch with equally spaced opaque lines that occupy 50 percent of the total area. The grating is a highly accurate photographic reproduction on a thin, transparent, and durable film. The thin film base minimizes annoying diffraction effect common to thick glass substrates.

Slower cutting, ideal for softer flint, crown and low-expansion optical glasses.

Aluminum Oxide is slower cutting, ideal for softer flint, crown and low-expansion optical glasses. In its naturally occurring crystalline state, aluminum oxide, or corundum has been used as an abrasive for thousands of years. The purest, natural form is the gem-white sapphire. Blue sapphires and rubies are also composed of aluminum oxide but have been colored by mineral oxides. Emery, another common natural abrasive, is impure corundum containing iron oxides. The naturally occurring abrasives of this family were subject to large variations in quality. Occasionally, a very fine pocket of aluminum oxide would be mined and provide the optician with exceptional finishes, but more commonly, quality was very uneven and the material had to be cleaned and separated by the difficult, time consuming water flotation process. As you read some of the older books on telescope making you will understand how serious the quality control problems were, and the great pains taken to purify abrasives.

Today, modern technology is capable of producing consistently high quality aluminum oxide which has practically replaced the natural forms for precision optical work. Aluminum oxide, like silicon carbide, is made in the electric furnace. Bauxite (aluminum ore), coke and powered iron are combined, heated, cooled, crushed, treated to produce a uniformly shaped particle and finally graded. This results in a nearly pure aluminum oxide crystal with a hardness of 9 on a Mohs scale. It is somewhat slower cutting than silicon carbide and is therefore ideally suited for smoothing operations involving the softer optical glasses used for refractors, corrector plates and optical windows.

Throughout the grinding and final lapping stages, the objective has been to reduce pits and scratches to the smallest possible size. However, no amount of grinding can produce a surface smooth enough or sufficiently transparent to meet the needs of a first-class telescope objective. Different techniques are needed. While there is still considerable discussion on the subject, it appears that polishing is a result of one or more of the following: removal / wear, athermic surface flow, or the formation of a silica-gel surface by hydrolysis. The result, whatever the cause, is an incredibly smooth surface, if proper techniques and optical grade supplies are used.

Until recently there were three principle types of optical polishing compounds available; Barnesite, cerium oxide, and the iron oxides (or more commonly, rouge). Barnesite production was severely restricted when the manufacturer determined that the process could not meet federal anti-pollution controls. Fortunately, cerium oxide and the various rouges are more than sufficient to meet the needs of the precision lens maker.

All of our polishing compounds will give a first-class polish, but each one differs from the other in speed, action, cleanliness and cost. By changing from one to another during the polishing and figuring stages, it is possible to finish a mirror or lens faster and more accurately because the primary difference is speed. Therefore you may want to polish out your surface rapidly with cerium oxide and then switch to one of the slower acting rouges that allow you literally to creep up to the exact figure you are working for without fear of overshooting.

Low cost, time-proven high quality polishing.

Red rouge is produced by combining ferrous sulfate, ammonium hydroxide and ammonia water. The solution is then filtered, burned in the presence of air and finally powered and graded. Its close cousin is iron rust, although optical rouge is many times finer and purer. Generations of skilled craftsmen have used rouge to produce surfaces of the highest quality.

Our red rouge is soft, slow polishing, and leaves no sleeks. It gives the polishing lap a smooth, velvety feel. Care is needed in its use since it does not readily wash out of clothes. Ideal for the final 8 to 10 minutes of figuring when fast changes can cause you to overshoot the desired curve.

Ideal for tempering laps.

Rosin is a solid resinous material that occurs naturally in the oleo resin of the pine tree. Commercially, pine trees are tapped for their sap or gum. The collected gum is thinned with turpentine and a small amount of oxalic acid is added to remove the trace of iron that southern pine trees absorb from the red soils. This solution is then steam heated to between 180 and 200°C., filtered and washed with water to remove the remaining oxalic acid and other soluble acids. Finally, high pressure steam is used to drive off the turpentine, and the liquid is cooled into a solid. The rosin mass does not flow like pitch but rather remains stable over a reasonably wide temperature range. The highest quality rosin is amber colored and is transparent. Rosin is soluble in most organic solvents — turpentine or ethyl alcohol. It is a valuable additive to pitch (where it acts to temper the pitch so that it flows more predictably and evenly) as an ingredient in blocking pitches, or as a lap in very high temperature zones.

The key to outstanding surfaces.

India Astronomy Club IAC pitch, like rosin, begins with the gum from a pine tree. However, unlike rosin, pitch is produced by heating the gum in the absence of air. As the temperature increases the very volatile elements, then light through heavy oils, boil off, until only cyclic organic acids or pine-tar pitch remains. It is cleaned and processed until it is a clear burgundy colored homogeneous mass. Pitch is soluble in organic solvents.

India Astronomy Club IAC offers pure pitch in soft grade and in a special tempered, ready to use formula. The pure pitches can be used alone or as a base to which rosin, beeswax and linseed oil can be added.

For the beginner or advanced worker who does not want to mix his own pitch we offer tempered burgundy pitch, a unique combination of rosin, beeswax and pitch that melts at 174°F, flows accurately for normal focal length mirrors and flats and is consistent from package to package and year after year. This pitch is the favorite of thousands of ATMs and professionals.