Introduction
and
Warning
Back
in the mid 70's, I made (and briefly used)
a 6" aluminum telescope mirror. The aluminum mirror blanks came from a
company called
Innovative Optical Materiel. While they claimed that
their aluminum
alloy was "special", just about any aluminum should work.
This company is long out of business, but I still have a copy of their grinding instructions, shown below. Please note that I did not write the document below. There's some milling and power tools mentioned in their document; please use appropriate care (eye protection, etc.).
The original document is copyright 1975, Innovative Optical Materiel.
If you decide to make an aluminum mirror, I'd like to hear about your experiences. NASA Tech Brief 1831, Process for Polishing Bare Aluminum to High Optical Quality describes using India Ink to polish the aluminum surface. This is critical is you want a high quality telescope mirror.
This company is long out of business, but I still have a copy of their grinding instructions, shown below. Please note that I did not write the document below. There's some milling and power tools mentioned in their document; please use appropriate care (eye protection, etc.).
The original document is copyright 1975, Innovative Optical Materiel.
If you decide to make an aluminum mirror, I'd like to hear about your experiences. NASA Tech Brief 1831, Process for Polishing Bare Aluminum to High Optical Quality describes using India Ink to polish the aluminum surface. This is critical is you want a high quality telescope mirror.
Aluminum
Mirrors
Innovative
Optical
Materiel
Tech Bulletin 753
OPTICAL PROCESSING OF ALLOY ALUMINUM MIRROR BLANKS
(Revised 10-75)GENERAL PROCEDURES
Alloy blanks are ground and polished using the same general conventional mirror making techniques appearing in numerous books on the subject of amateur telescope making. If you have never ground and polished a conventional glass mirror, consult one of these texts before beginning. The following information essentially describes the manner in which conventional methods should be modified in order to obtain best results in finishing our alloy mirror blanks. You will find that use of this alloy results in comparatively fast grinding and polishing. Take advantage of this by assuring yourself that each stage is thoroughly complete before proceeding to the next stage. Be sure that the blanks and your work area are scrupulously cleaned between abrasive stages; this is even more important than when working with glass.
MATERIAL REQUIREMENTS
(1) Two alloy aluminum blanks (one used as mirror, the other as tool). Aluminum tool facet material attached to a rigid substrate may be used as a tool in place of a second mirror blank. (2) Abrasive sequence similar to #120, 220, 320 silicon carbide and 15 micron aluminum oxide. (3) Polishing pitch of medium hardness. We recommend a mixture of #64 and #73 Swiss pitch. Pitch must be free of contaminants which may otherwise scratch the mirror during final polishing. (4)Polishing compound. We recommend our 0.06 micron alumina finishing powder. Optical rouge may be used although sleeking may result. (5) Liquid soap (mild dishwashing detergent). (6) Distilled water to be used during final polishing.
USING ALUMINUM TOOL FACET MATERIAL
(I) Deburr all facets with a file and round off all four 1/4 inch long edges slightly. (2) Position one facet on substrate such tha position of "x" is over center of substrate.
(3)
Place
remaining facets around central facet with l/8"separation to form a
grid. (4) Mark cutting line for all facets which extend beyond required
tool diameter. (5) Cut with a hacksaw and deburr. Use remnants to
complete the tool grid. A perfect grid is not necessary. Straight cuts
for partial facets at the circumference of the tool are sufficient.
Diameter of working tool surface should be no greater than mirror
diameter at any point. (6) Number the facets so that they can be
cemented in proper position. (7) Carefully clean all facets and
substrate. Cement using epoxy cement. (8) After cement has fully
set, tool surface can
be brought to approximate convex radius of curvature using a file or
powder sander. Do not permit Facets to become overheated or epoxy may
fail.
CURVE GENERATING
The basic curves may be generated on a flat mirror blank and solid tool by machining or hand scraping. Coarse grinding in a convenflonol manner using #80 a brasive is not recommended because material can be removed at a far faster rate by simple hand scraping. A suitable scraper may be formed by sharpening a piece of tool steel or the end of an old file. Templates of proper concave and convex curvature should be made to compare with the surface of the mirror and tool as you work. If blanks are to be machined, the method used to generate curves on metal laps as described on page 55 of Amateur Telescope Making II may be employed. Unless you have access to metal shop facilities, the labor charge to have this work done is likety to be quite expensive. For smaller sizes, a common wood lathe may be pressed into service by drilling a hole half way through the back of the tool blank at the exact center and tapping it so that it will screw tightly on the lathe spindle. Attach the mirror blank to the face of the tool blank with a strip of masking tape and an adjustable hose clamp around the edge. Rough in the curve on the mirror blank with a machining bit or end of a flat file, comparing the curve to a suitable template as you work. Use lower speeds and always wear eye protection! When complete, the mirror blank may be removed and a convex curve may be generated on the tool in a similar fashion. Above all, avoid gouging the surface of the mirror blank while generating the curve. After curve generation s complete, apply a beveled edge to the blanks using a fine metal file stroking away from the center of the blank. This will prevent formation of a burr at the edge of the blanks.
SMOOTHING
The purpose of the first abrasive stage is to remove tool marks from curve generation and bring the mirror and tool into contact at all points. The choice of initial abrasive size depends upon the smoothness and sphericity obtained in curve generation and the diameter of the mirror. If it is possible get the job done with #220 in one hour, there is no point in starting with #120; it will probably take an hour of work later with #220 to remove the pits left by the #120 stage. On the other hand, if it appears that it will likely require three hours or more to remove the tool marks using #220, it is probably wise to start over with #120. With careful machine work during curve generation or if you are making a small mirror, it is possible that the work can be accomplished with #320. Use center-over-center strokes of total length up to 2/3 the diameter of the mirror. Apply abrasive dry, add water from a spray bottle (put a few drops of liquid soap in the water), and mix and spread with your finger before starting to grind each wet. Add water as required and rinse the mirror and tool and add fresh abrasive whenever the grinding noise has subsided. When smoothing appears complete, the tool blank (if a solid one is being used) should be channeled like a pitch lap in order to minimize scratching in later stages and to assure that good contact is maintained throughout fine grinding. A one to one-and-one-quarter inch square grid can be cut 1/8 inch deep and wide using a mill end or carbide tipped router bit in a drill press. Alternately, a hack saw and edge of flat file and a miter box may be used. For very smll mirrors, make two perpendicular channels which divide the blank into four unequal areas. In large sizes, it may be advantageous to round the corners of the square facets on the tool. Heat generated during fine grinding may cause the sharp corners to expand and scratch. When channeling is complete, scrape off any burrs and grind for one or two additional wets applying minimal pressure to the blanks.
FINE GRINDING
At this point, your mirror is a very dull dark gray. Although no tool marks remain on the mirror, there are pits and probably some scratches. The following discussion assumes that the smoothing work was accomplished using #120 abrasive. It will require at least an hour's work with #220 abrasive to remove pits from the #120 stage. Pits will not be easy to see because the surface cannot be illuminated from behind. Continue until the surface appears uniform. W-strokes are recommended. Remember that pits will be removed fastest at the center of the mirror when the mirror is working over the tool. Pits at the edge of the mirror are removed fastest when the tool is working over the mirror. Both positions should be used, but experience has shown that the mirror is less prone to be scratched when it is on the bottom. Do not worry at this point about small scratches, they will be ground out. If severe scratching persists, the cause is generally one or more of the following: (1) Usual reasons common to glass grinding (e.g. contaminated abrasive charge). (2) Poor contact between mirror and tool. (3) Abrasive-water mixture too thin. (4) Too much pressure being applied. (5) Existence of a large temperature difference between mirror and tool at the beginning of a wet (expansion effects).
When you are satisfied that the surface is uniform and that no pits larger than from the #220 stage are present, clean up and proceed to #320 About half way through the #320 stage, it is a good idea to start each wet by supporting the top blank slightly until the largest grains in the charge are broken down.
If one were making a glass mirror, the above procedure would continue with finer and finer abrasive until the pits remaining on the surface were small enough to be polished out with rouge on a pitch lap. Aluminum is too soft to be worked in this manner. Regardless of any preventive measures you may take, it will be nearly impossible to grind with the "hard" tool with abrasive grains smaller than 15&emdash;30 micron size without the risk of scratches (Small mirrors are more forgiving than large ones). We avoid the problem entirely by stopping with #320 . This is not to suggest that the surface is ready for final polishing . We won’t skip the intermediate abrasive stages, but instead of working on the hard tool, a pitch lap is used.
PITCH LAP AND POLISHING WITH ABRASIVES
Make up a pitch lap of medium hardness; hard pitch may scratch the mirror surface. The lap may be formed by casting (on wax paper) strips of the melted pitch 1/8 inch thick and slightly narrower than the width of the metal facets on the tool (see Jean Texereau’s, How to Make a Telescope, page 46, for additional details). Cut into squares, melt the waxed side slightly by holding near a flame and apply to the heated tool. Before further work can proceed, the surface of the lap must be made to conform to the mirror by hot pressing. Put some liquid soap on the lap after heating it under hot tap water, and work the mirror over the surface until good contact is established. When not actually working, a piece of polyethylene "kitchen wrap" should be placed between the mirror and lap. The slight decrease in curvature radius of lap will be removed as work begins. The surface of the lap should be trimmed to about 90% of the diameter of the mirror in order to counter an otherwise strong tendency for the edge of the mirror to become "turned down". After good contact is established, continue "grinding" with #320 abrasive. On pitch, this grade makes far shallower pits and will remove any scratches left from the "hard" grind. Use short, straight strokes so that the mirror does not lose its sphericity. Avoid elliptical strokes as they will almost surely turn down the edge of the mirror. The polishing lap will become clogged with aluminum mud which should be removed (more than by simple rinsing) every 20 to 30 minutes. This is accomplished by applying several drops of liquid detergent to the rinsed lap and performing the usual grinding strokes, adding water as needed. After two or three minutes, the lap will begin to approach its original color and grinding may again proceed with a fresh charge. Depending on the quantity of abrasive and the action of particles embedded in the lap, a very bright polish may appear on the mirror from time to time. It is too early for this to be of any beneficial importance other than it will permit close examination of the condition of the surface. Scratches and deep pits, if they exist, will be extremely apparent It is hoped that after about 1 1/2 hours of work, no deep pits or scratches will be visible. If pits are deep and cover the entire surface, only a return to the "hard" tool will remove them-earlier fine grinding was not sufficiently thorough at one or more stages. If the surface is in satisfactory condition, continue work for another 30 minutes without adding fresh abrasive. When complete, unclog the polishing lap once again
POLISHING THEORY
You should have observed two distinct processes while working with #320 abrasive on pitch. At the beginning of each wet the action was one of grinding. As the free abrasive either became embedded in the lap or was pushed over the edge of the tool, polishing occurred (notice that the mirror became brighter). Both processes are necessary: Grinding removes material at a fairly fast rate but leaves small pits in the surface. Polishing is a shaving action which leaves the surface bright but removes material (and pits) very slowly. Both grinding and polishing will be occurring throughout the finishing process.
Before further work can proceed, the lap should be sealed. First hold the lap near a heat source to melt the surface slightly. Then paint a coat of melted pitch over the surface. Press again for perfect contact.
POLISHING
Polishing is done in two stages. The first stage uses a fine finishing abrasive of about 15 micron size to remove the last pits and leave the surface of mirror sleeked but brilliant. Use short strokes, adding diluted soap solution as required. After about 15 minutes, begin adding solution more frequently so that eventually you are rinsing all loose particles off the lap. If not already so, the surface will become quite brilliant. Foucault testing of the surface should begin at this point. All gross irregularities should be removed from the surface before the second stage of polishing begins. You may use a fine slit or pin hole for testing since reflectivity at this point is already 10 to 20 times as great as glass. The very high thermal conductivity of aluminum practically eliminates localized temperature effects on the mirror. However, high thermal coefficient of expansion make an even working temperature desirable during polishing and figuring. A wooden disc taped to the back of the mirror while working will prevent transfer of heat from your hands. As additional abrasive is added to the lap, brilliance of the surface may decrease temporarily, but it can always be restored at the end of the wet by rinsing loose particles off the lap and continuing. Actual polishing action is at its greatest when the mirror working over the lap "squeaks". Wnen the figure is uniform and the surface is free from all but the very finest pits, seal the surface as before, press for contact, and begin final polishing with 0.06 micron alumina. Polishing compound should be applied to the lap in a distilled water suspension. Figuring to final shape should begin when the surface is evenly brilliant and no pits are visible to the eye. Keep in mind that successful specular polishing of aluminum requires that the abrasive particles be well-bound to the polishing lap. The very last work done on the mirror should be accomplished using distilled water to continuously rinse loose particles off the lap. Practically anything loose on the surface of the lap will cause sleeks.
CARE OF MIRROR SURFACE
When off the polishing lap, the mirror should be handled as carefully as an aIuminized surface. Prepare the mirror for final testing by rinsing under tap water,swab lightly using a clean cotton ball and mild detergent solution. Rinse under tap again followed by alcohol rinse. Dry the surface completely by applying circular motion with several fresh cotton balls. Do not touch the surface with any other object or with portion of cotton ball that previously contacted your fingers. In time, the mirror will form a hard clear oxide coat which will provide some protection. The above technique is also used to clean a mirror which has become dirty in use, although a lens brush may be lightly used to remove loose dust particles.
NOTES
(1) Do any required machine or cutting work on the mirror blank before grinding begins. Perforations for Cassegrain telescopes can be made with a drill press after curve generation. The pitch lap for a Cass should have a central "hole" slightly larger than the diameter of the mirror perforation. (2) #320 abrasive will erode the pitch lap somewhat. Make sure that the pitch does not become so thin that metal touches metal. Remake the lap if necessary. (3) Keep in mind that #320 abrasive on pitch will remove material at a far faster rate than rouge. It is therefore essential that perfect contact be maintained between the mirror and the lap. (4) If at any point while working on the pitch lap you find particles of the lap adhering to the mirror surface, just add a few drops of liquid soap to the lap and continue polishing. (5) If you suspend polishing work for more than a day or two once the 15 micron stage has begun, it is a good idea to coat the mirror with mineral or vegetable oil to retard oxide formation. A thick oxide coat could interfere with even polishing. Use alcohol to remove all traces of the oil before polishing recommences as it may dissolve the pitch. (6) Occasionally a blank may reveal a few pits while finishing which were not a result of inadequate fine grinding. Impurities present in the blank may "pull out", leaving a small hole in the surface. While certainly disappointing, a few pits (like an exposed bubble in a glass mirror) will not visibly affect performance. In the event that this does occur, it would be possible to go back and grind the pits out, but there is always the possibility that more will be uncovered.
OPTICAL PROCESSING OF ALLOY ALUMINUM MIRROR BLANKS
(Revised 10-75)GENERAL PROCEDURES
Alloy blanks are ground and polished using the same general conventional mirror making techniques appearing in numerous books on the subject of amateur telescope making. If you have never ground and polished a conventional glass mirror, consult one of these texts before beginning. The following information essentially describes the manner in which conventional methods should be modified in order to obtain best results in finishing our alloy mirror blanks. You will find that use of this alloy results in comparatively fast grinding and polishing. Take advantage of this by assuring yourself that each stage is thoroughly complete before proceeding to the next stage. Be sure that the blanks and your work area are scrupulously cleaned between abrasive stages; this is even more important than when working with glass.
MATERIAL REQUIREMENTS
(1) Two alloy aluminum blanks (one used as mirror, the other as tool). Aluminum tool facet material attached to a rigid substrate may be used as a tool in place of a second mirror blank. (2) Abrasive sequence similar to #120, 220, 320 silicon carbide and 15 micron aluminum oxide. (3) Polishing pitch of medium hardness. We recommend a mixture of #64 and #73 Swiss pitch. Pitch must be free of contaminants which may otherwise scratch the mirror during final polishing. (4)Polishing compound. We recommend our 0.06 micron alumina finishing powder. Optical rouge may be used although sleeking may result. (5) Liquid soap (mild dishwashing detergent). (6) Distilled water to be used during final polishing.
USING ALUMINUM TOOL FACET MATERIAL
(I) Deburr all facets with a file and round off all four 1/4 inch long edges slightly. (2) Position one facet on substrate such tha position of "x" is over center of substrate.
(3)
Place
remaining facets around central facet with l/8"separation to form a
grid. (4) Mark cutting line for all facets which extend beyond required
tool diameter. (5) Cut with a hacksaw and deburr. Use remnants to
complete the tool grid. A perfect grid is not necessary. Straight cuts
for partial facets at the circumference of the tool are sufficient.
Diameter of working tool surface should be no greater than mirror
diameter at any point. (6) Number the facets so that they can be
cemented in proper position. (7) Carefully clean all facets and
substrate. Cement using epoxy cement. (8) After cement has fully
set, tool surface can
be brought to approximate convex radius of curvature using a file or
powder sander. Do not permit Facets to become overheated or epoxy may
fail.CURVE GENERATING
The basic curves may be generated on a flat mirror blank and solid tool by machining or hand scraping. Coarse grinding in a convenflonol manner using #80 a brasive is not recommended because material can be removed at a far faster rate by simple hand scraping. A suitable scraper may be formed by sharpening a piece of tool steel or the end of an old file. Templates of proper concave and convex curvature should be made to compare with the surface of the mirror and tool as you work. If blanks are to be machined, the method used to generate curves on metal laps as described on page 55 of Amateur Telescope Making II may be employed. Unless you have access to metal shop facilities, the labor charge to have this work done is likety to be quite expensive. For smaller sizes, a common wood lathe may be pressed into service by drilling a hole half way through the back of the tool blank at the exact center and tapping it so that it will screw tightly on the lathe spindle. Attach the mirror blank to the face of the tool blank with a strip of masking tape and an adjustable hose clamp around the edge. Rough in the curve on the mirror blank with a machining bit or end of a flat file, comparing the curve to a suitable template as you work. Use lower speeds and always wear eye protection! When complete, the mirror blank may be removed and a convex curve may be generated on the tool in a similar fashion. Above all, avoid gouging the surface of the mirror blank while generating the curve. After curve generation s complete, apply a beveled edge to the blanks using a fine metal file stroking away from the center of the blank. This will prevent formation of a burr at the edge of the blanks.
SMOOTHING
The purpose of the first abrasive stage is to remove tool marks from curve generation and bring the mirror and tool into contact at all points. The choice of initial abrasive size depends upon the smoothness and sphericity obtained in curve generation and the diameter of the mirror. If it is possible get the job done with #220 in one hour, there is no point in starting with #120; it will probably take an hour of work later with #220 to remove the pits left by the #120 stage. On the other hand, if it appears that it will likely require three hours or more to remove the tool marks using #220, it is probably wise to start over with #120. With careful machine work during curve generation or if you are making a small mirror, it is possible that the work can be accomplished with #320. Use center-over-center strokes of total length up to 2/3 the diameter of the mirror. Apply abrasive dry, add water from a spray bottle (put a few drops of liquid soap in the water), and mix and spread with your finger before starting to grind each wet. Add water as required and rinse the mirror and tool and add fresh abrasive whenever the grinding noise has subsided. When smoothing appears complete, the tool blank (if a solid one is being used) should be channeled like a pitch lap in order to minimize scratching in later stages and to assure that good contact is maintained throughout fine grinding. A one to one-and-one-quarter inch square grid can be cut 1/8 inch deep and wide using a mill end or carbide tipped router bit in a drill press. Alternately, a hack saw and edge of flat file and a miter box may be used. For very smll mirrors, make two perpendicular channels which divide the blank into four unequal areas. In large sizes, it may be advantageous to round the corners of the square facets on the tool. Heat generated during fine grinding may cause the sharp corners to expand and scratch. When channeling is complete, scrape off any burrs and grind for one or two additional wets applying minimal pressure to the blanks.
FINE GRINDING
At this point, your mirror is a very dull dark gray. Although no tool marks remain on the mirror, there are pits and probably some scratches. The following discussion assumes that the smoothing work was accomplished using #120 abrasive. It will require at least an hour's work with #220 abrasive to remove pits from the #120 stage. Pits will not be easy to see because the surface cannot be illuminated from behind. Continue until the surface appears uniform. W-strokes are recommended. Remember that pits will be removed fastest at the center of the mirror when the mirror is working over the tool. Pits at the edge of the mirror are removed fastest when the tool is working over the mirror. Both positions should be used, but experience has shown that the mirror is less prone to be scratched when it is on the bottom. Do not worry at this point about small scratches, they will be ground out. If severe scratching persists, the cause is generally one or more of the following: (1) Usual reasons common to glass grinding (e.g. contaminated abrasive charge). (2) Poor contact between mirror and tool. (3) Abrasive-water mixture too thin. (4) Too much pressure being applied. (5) Existence of a large temperature difference between mirror and tool at the beginning of a wet (expansion effects).
When you are satisfied that the surface is uniform and that no pits larger than from the #220 stage are present, clean up and proceed to #320 About half way through the #320 stage, it is a good idea to start each wet by supporting the top blank slightly until the largest grains in the charge are broken down.
If one were making a glass mirror, the above procedure would continue with finer and finer abrasive until the pits remaining on the surface were small enough to be polished out with rouge on a pitch lap. Aluminum is too soft to be worked in this manner. Regardless of any preventive measures you may take, it will be nearly impossible to grind with the "hard" tool with abrasive grains smaller than 15&emdash;30 micron size without the risk of scratches (Small mirrors are more forgiving than large ones). We avoid the problem entirely by stopping with #320 . This is not to suggest that the surface is ready for final polishing . We won’t skip the intermediate abrasive stages, but instead of working on the hard tool, a pitch lap is used.
PITCH LAP AND POLISHING WITH ABRASIVES
Make up a pitch lap of medium hardness; hard pitch may scratch the mirror surface. The lap may be formed by casting (on wax paper) strips of the melted pitch 1/8 inch thick and slightly narrower than the width of the metal facets on the tool (see Jean Texereau’s, How to Make a Telescope, page 46, for additional details). Cut into squares, melt the waxed side slightly by holding near a flame and apply to the heated tool. Before further work can proceed, the surface of the lap must be made to conform to the mirror by hot pressing. Put some liquid soap on the lap after heating it under hot tap water, and work the mirror over the surface until good contact is established. When not actually working, a piece of polyethylene "kitchen wrap" should be placed between the mirror and lap. The slight decrease in curvature radius of lap will be removed as work begins. The surface of the lap should be trimmed to about 90% of the diameter of the mirror in order to counter an otherwise strong tendency for the edge of the mirror to become "turned down". After good contact is established, continue "grinding" with #320 abrasive. On pitch, this grade makes far shallower pits and will remove any scratches left from the "hard" grind. Use short, straight strokes so that the mirror does not lose its sphericity. Avoid elliptical strokes as they will almost surely turn down the edge of the mirror. The polishing lap will become clogged with aluminum mud which should be removed (more than by simple rinsing) every 20 to 30 minutes. This is accomplished by applying several drops of liquid detergent to the rinsed lap and performing the usual grinding strokes, adding water as needed. After two or three minutes, the lap will begin to approach its original color and grinding may again proceed with a fresh charge. Depending on the quantity of abrasive and the action of particles embedded in the lap, a very bright polish may appear on the mirror from time to time. It is too early for this to be of any beneficial importance other than it will permit close examination of the condition of the surface. Scratches and deep pits, if they exist, will be extremely apparent It is hoped that after about 1 1/2 hours of work, no deep pits or scratches will be visible. If pits are deep and cover the entire surface, only a return to the "hard" tool will remove them-earlier fine grinding was not sufficiently thorough at one or more stages. If the surface is in satisfactory condition, continue work for another 30 minutes without adding fresh abrasive. When complete, unclog the polishing lap once again
POLISHING THEORY
You should have observed two distinct processes while working with #320 abrasive on pitch. At the beginning of each wet the action was one of grinding. As the free abrasive either became embedded in the lap or was pushed over the edge of the tool, polishing occurred (notice that the mirror became brighter). Both processes are necessary: Grinding removes material at a fairly fast rate but leaves small pits in the surface. Polishing is a shaving action which leaves the surface bright but removes material (and pits) very slowly. Both grinding and polishing will be occurring throughout the finishing process.
Before further work can proceed, the lap should be sealed. First hold the lap near a heat source to melt the surface slightly. Then paint a coat of melted pitch over the surface. Press again for perfect contact.
POLISHING
Polishing is done in two stages. The first stage uses a fine finishing abrasive of about 15 micron size to remove the last pits and leave the surface of mirror sleeked but brilliant. Use short strokes, adding diluted soap solution as required. After about 15 minutes, begin adding solution more frequently so that eventually you are rinsing all loose particles off the lap. If not already so, the surface will become quite brilliant. Foucault testing of the surface should begin at this point. All gross irregularities should be removed from the surface before the second stage of polishing begins. You may use a fine slit or pin hole for testing since reflectivity at this point is already 10 to 20 times as great as glass. The very high thermal conductivity of aluminum practically eliminates localized temperature effects on the mirror. However, high thermal coefficient of expansion make an even working temperature desirable during polishing and figuring. A wooden disc taped to the back of the mirror while working will prevent transfer of heat from your hands. As additional abrasive is added to the lap, brilliance of the surface may decrease temporarily, but it can always be restored at the end of the wet by rinsing loose particles off the lap and continuing. Actual polishing action is at its greatest when the mirror working over the lap "squeaks". Wnen the figure is uniform and the surface is free from all but the very finest pits, seal the surface as before, press for contact, and begin final polishing with 0.06 micron alumina. Polishing compound should be applied to the lap in a distilled water suspension. Figuring to final shape should begin when the surface is evenly brilliant and no pits are visible to the eye. Keep in mind that successful specular polishing of aluminum requires that the abrasive particles be well-bound to the polishing lap. The very last work done on the mirror should be accomplished using distilled water to continuously rinse loose particles off the lap. Practically anything loose on the surface of the lap will cause sleeks.
CARE OF MIRROR SURFACE
When off the polishing lap, the mirror should be handled as carefully as an aIuminized surface. Prepare the mirror for final testing by rinsing under tap water,swab lightly using a clean cotton ball and mild detergent solution. Rinse under tap again followed by alcohol rinse. Dry the surface completely by applying circular motion with several fresh cotton balls. Do not touch the surface with any other object or with portion of cotton ball that previously contacted your fingers. In time, the mirror will form a hard clear oxide coat which will provide some protection. The above technique is also used to clean a mirror which has become dirty in use, although a lens brush may be lightly used to remove loose dust particles.
NOTES
(1) Do any required machine or cutting work on the mirror blank before grinding begins. Perforations for Cassegrain telescopes can be made with a drill press after curve generation. The pitch lap for a Cass should have a central "hole" slightly larger than the diameter of the mirror perforation. (2) #320 abrasive will erode the pitch lap somewhat. Make sure that the pitch does not become so thin that metal touches metal. Remake the lap if necessary. (3) Keep in mind that #320 abrasive on pitch will remove material at a far faster rate than rouge. It is therefore essential that perfect contact be maintained between the mirror and the lap. (4) If at any point while working on the pitch lap you find particles of the lap adhering to the mirror surface, just add a few drops of liquid soap to the lap and continue polishing. (5) If you suspend polishing work for more than a day or two once the 15 micron stage has begun, it is a good idea to coat the mirror with mineral or vegetable oil to retard oxide formation. A thick oxide coat could interfere with even polishing. Use alcohol to remove all traces of the oil before polishing recommences as it may dissolve the pitch. (6) Occasionally a blank may reveal a few pits while finishing which were not a result of inadequate fine grinding. Impurities present in the blank may "pull out", leaving a small hole in the surface. While certainly disappointing, a few pits (like an exposed bubble in a glass mirror) will not visibly affect performance. In the event that this does occur, it would be possible to go back and grind the pits out, but there is always the possibility that more will be uncovered.