When in processing a mirror you will reach a refined and smooth surface with abrasives in grain 800 or outside, We must establish for prepare the tool covered with pitch, to proceed in the following two important final processing stages, that are the transformation of the frosted surface but opaque, in a reflective glossy surface; and the subsequent precise parabolization of the spheroid since here obtained.
To understand exactly what many people do summarily, but it's not convenient especially for beginners , (unless you have the same great experience, awareness and professionalism of Gordon Waite), is necessary to see the images of the two following movies ....
Reading then this, we will understand why.
abrasives, also if in fine grain used up to now, They were simply interposed between the two glass panes, tool and mirror, and then acted only by mechanical action of mutual fracture of the abrasive grain and glass. They not have been able to further improve the surface structure at the ultra-microscopic level, like it is instead necessary that occurs for the optics of a telescope.
On the other hand it is understood that an extreme precision is required, because if for example, you think that the resolving power (resolving power that is, the least visible object) a telescope with a lens diameter of 36cm and F5 focal ratio is large 0.38 arcseconds; and an underlying object to those 0.38 arcseconds, presents itself to the focus of the instrument with a diameter of 3.3 microns (see Note below *), it becomes clear that the image will be enlarged with an eyepiece 300 or 400 Sometimes in order to be observed.
But that enlargement will extended at any imperfections still present on the reflective surface.
So the degree of gloss of the reflecting surface must be at least a few orders of magnitude better than the minimum object observable.
Note *:
The resolving power of a telescope = 1.22 *Lambda[mm] / Ø mirror [mm] = (1.22*0.00055) / 360 = 0.000001863 radians.
Then 0.000001863 * 206265 arc-seconds in a radiant = 0:38″[ arc-seconds]
The apparent size of an object at focus of the telescope, is calculated by the astro-photographers with the following simplified formula:
dimensions subject at focus = Focal telescope[mm] * Ø object[arc-seconds] / 206265
= (360*5*0.38) / 206265 = 0.00337 mm (= 3.37 microns)
End note
The basic rule is that the optical surface of one for telescope mirror must not contain defects in shape more than one-eighth of the wavelength of 550 millionths of a millimeter of the green light, where the eye is most sensitive, that is 68.75 millionths of a millimeter.
For surely such a degree of "smooth", We must rely on joint action, on one side: fine abrasive (cerium oxide or zirconium) encrusted in pitch, who “polish bright”, not allowing its rolling on the glass surface; and on the other side: physical action provided by the pitch in contact with the glass. It seems impossible, but it is true that the pitch, during processing, is able to literally SPREADS an infinitesimal amount of glass made locally plastic by friction between pitch-glass, going to "plug" gradually craters and grooves left by the mechanical action of the abrasive used previously rolling the use of pitch. (This happens because the glass is free of crystallization, and in fact is considered to be a "liquid with viscosity infinite at room temperature")
This strange action is visible under a microscope, When you look at the edges of certain craters originally wel chipped, that occur with the progress of polishing, gradually going with rounded edges, up to completely disappear without a trace.
The glass super gloss that way, It is so smooth at the molecular level, which can be pierced by sunlight without emitting any dispersed reflection. As with a ray of sun that enters the water surface.
The pitch is then still an indispensable element for the quality of the optical, but a resin extremely fragile at room temperature (it chip and crumbles easily), ed a 30° (for example during processing due to friction) He begins to soften SLIGHTLY, adapting continuously to the surface to be machined, useful feature optician, but which, however, it makes it sticky, get dirty so often indelible, on any object that is at human "body" temperature.
That said: Any measures to limit its crumbling/Adhesion/proliferation, It is considered good "for peace in the family".
The BLACK PITCH it is the best, but the most famous brand, while it is of production Switzerland, now in our times it is hardly affordable and sporadically in Europe, and it is surely only available from commercial sites overseas. Its lack of traceability means that there can be alternately orient on the most readily available Pece Greca, or Colophone (also called ROSIN).
But for a neophyte is a significant complication in addition as it is having to find experimentally the right working hardness at our specific room temperature, named "temper":
The BLACK PITCH (Gugolz) Is’ the best because’ availables in four grades with SOFTENING TEMPERATURES GUARANTEED in four different shades, the two extremes (soft and hard) are the degree #55 (which melts at about 50 °) and able #91 (which melts at about 70 °).
The Colophone instead it melts at a high temperature around 130 ° (very dangerous for the breakage of glass ... in case you wanted to pour directly above the tool in glass, in that case it would be absolutely preheated).
But the worst is that the temperature of rammollimento (italian = "rammollimento") that MUST reach the pitch by friction during processing, so that it can perform its important function polishing, It is too high for the Rosin, with respect to the sum of the ambient temperature of the place where you work, plus the more one caused by the friction of work.
To reduce the high softening temperature of Rosin is necessary to vary its “"Temper"” adding hot and so very experimental, beeswax and / or turpentine and / or linseed oil, unfortunately without the guidance of a precise recipe; cooling a teaspoon at a time to test more or less his yielding grade with your fingernail or other empirical methods , but always taking care not to make the mixture reach the boiling temperature, that would result in the loss of solvents, making the pitch further undermining the work harder.
A pitch too hard or too soft is not good, because instead of yielding only gradually while maintaining a certain balance between the shape of its surface and yhat of the mirror, if it is too hard and does not give in “"digs"” on the mirror side of the zonal defects, or vice versa, If it's too soft, it not corrects at all, because too much yelding, making ungovernable the delicate process of parabolization.
Is’ important to know that the vapours of pitch solvents are highly flammable, like as are those of turpentine, which is a pitch natural component.
This is to say that if, unfortunately the substance to catch fire, the flames that would result from the frying pan would be persistent and dangerously high more than a meter.
A basic rule of security therefore, it is to NEVER use an open flame to heat and melt the pitch. But use a heating pad with electrical resistance armored inaccessible, much better if thermostatized (since the melting time to “the lowest temperature” is never short) to prevent boiling and forming “blowholes” with many air bubbles, but, above all, avoiding the loss by evaporation of the precious solvents responsible for its good cold softness.
The technique of preparation of the tool contemplate not so much to cover it with a pitch "patina" coating (Whose term gives the idea of a very thin thickness in antithesis with practical needs); but cover it with a more lasting LAYER, of pitch with a thickness of about 10 mm,.
It must be said that a thickness of pitch layer 10 mm at start polishing, is usually optimal because during processing you taper off until you reach an equally optimal residual thickness at the end of parabolization of only about 2 or 3 mm.
Slim residual thickness that is desirable because it is less elastic than the initial 10mm, and therefore it favors the best guarantee of quality of the mirror surface favoring the operator's work, which it has the task of reducing the error peak / valley of the mirror surface, gradually until it becomes equal to or less of the small 68.75 famous nanometer Lambda / 4 Tolerance.
The initial coat of pitch is then divided into contact surfaces delimited by channels wide about ten mm, be sufficient to permit an expansion of the individual contact areas (expansion due to softening/flattening of the pitch, to follow the shape of the mirror).
Without these grooves, the contact between adjacent squares would correspond a local lifting or raising of the pitch, which soon would produce zonal errors over the mirror under construction.
To avoid some of the worst zonal mistakes , It is also required that the surfaces of individual squares in which are divided the whole tool surface coated of pitch, are absolutely asymmetric with respect to the tool to be covered. For this reason, the tool with the coating layer of pitch, It provides two alternative embodiments techniques:
- Pouring hot pich on aluminum foil covering the pre-heated mirror and making its bordering with cardboard secured by masking tape (Gordon don'have bordered his mirror in the movie, because his tool was in less diameter with the mirros), pour a layer of pitch over the entire surface and positioning of the tool above pitch (see video).
In this case, after solidification of the pitch, You may “saw” or depening with warm edge of a electric soldering iron , a lattice of asymmetrical grooves (in the sense that the tool center must not fall neither in center of a square of pitch, nor in a groove ) so that grooves should be wide enough, so as not to be quickly filled from working, determining need for continuous groove remake. This operation involves the ease of disseminating pitch splinters everywhere ... Or: - Construction of a mold casting made of silicone, melting in it many "candy" of pitch which after will be installed on the tool to form a cover of many asymmetric squares (Even here the word asymmetric means that the tool center must not fall either in a groove or either in the center of a square pitch), and glued them in place simply warming the back facet with the flame of a candle and resting them in place with light finger pressure on the tool.
- The second hypothesis greatly simplifies the entire construction of the tool work, it becomes easy task, quiet and above all absolutely “clean”. But it envisages spending of about thirty euro for buying a jar 1 kg of silicone resin casting Prochima GLS-50 or equivalent. by the use of which will gain the "quiet" domestic cohabitation (because your wife prefers everything that avoids shedding splinters of pitch anywhere).
- The surface of the "candy pitch" will already automatically grooved by copying the mold wood ribs imprinted faithfully in the silicone casting, so it does not need more dusty grooves scoring to perform by hand.
- The size of “squares "Candy"”, for my works are 20x20mm x10mm thickness, They are not critical, and may be even more, increasing the side only in proportion to the mirror diameter to achieve.
- The increase should not affect the thickness, that it is better not exceed 10mm in order to avoid possible downturn.
- I have personally experienced that a thickness of 10mm INITIAL, decrease as You progress in the work, up the end polishing and the following parabolization, with a residual thickness reduced to about 2 or 3 mm, wich is known wery desirable in finish work, for the best quality obtainable.
- After gluing the "candies" on the tool, (simply obtained by heating the back of each picture to the flame of a candle, and pressing it into place on the mirror), follow the usual indispensable tool pressure against the mirror with an interposed cellophane sheet ("maybe a "Saran Wrap" in USA), in order to obtain an indispensable and perfect mutual adaptation of the two surfaces.
- The cellophane sheet, in addition to not allow the pitch to adhere to the mirror, , will display transparency darker the square uniformly adapted to contact with the mirror, and those with lighter zones require additional pressure.
- As the tool is working, its black color gives way to the brown from incorporated white cerium or zirconium oxide .
- the Originally wide channels 10 mm, it will thin up to tend to disappear filled from the softening pitch, and must always keept open..
I use redo channels blasting pitch with a knife or a blade of planer, When the tool is immersed in a basin of warm (tepid) water. Immersion in water avoids the inevitable splinters everywhere; while the temperature of the water reduces the fragility of the pitch and counteracts the formation of large splinters which would ruin the tool contact surfaces, in the light of the technical fact that a square of pitch ruined and no longer in contact, It NOT must still be replaced if not redoing the complete tool).
Of course, the pitch derived from the crumbs of the channels of remaking operation, are retained to again be merged and poured in future work, even if they are encrusted with polishing agent.
I enclose a few photos to show in sequence as an object of the present text:
Clicking on the images will appear magnified
