GRINDING AN ASTRONOMICAL TELESCOPE MIRROR

May 10, 2002 - Larry Gilstrap

Disclaimer

 Ahem! - This page is part instructional document, and part personal notes gleaned from my own experience. No claim is made that this is the right way, or even a right way to produce a telescope mirror. At the time of this writing the mirror is still in the polishing stage, though all seems to be going well enough. Follow these directions at your own risk, and using your best judgment as to how these experiences and suggestions may apply to your own situation.

EQUIPMENT & SUPPLIES

1. Mirror Kit - 8" f/8 Generated Curves on both mirror and tool Purchased from Newport Glassworks of Stanton, CA.
2. Work Bench - must be sturdy, and not wobble or rock
3. Wash Basin - plastic, ! 14" across, 6" deep
4. Stiff Bristle Scrub Brush - not wire bristle
5. Containers for Abrasive Canning/Jelly jars Cheap Ziploc bins Ziploc Sandwich bags Masking Tape
6. Magnifier Glass
7. Newspapers - lots of old, clean newspapers
8. Miscellaneous
   a. Plastic spoons - to dish out grit / abrasives
   b. Sharpie indelible marker - for marking mirror & tool
   c. Spray bottle
   d. Water bucket
   e. Paper towels
    
9. 9. Books, Web sites, Information on Grinding. (Polishing, figuring and testing will be covered in a separate document.)

SET UP & PREPARATION

1. Divvy up abrasives and polishing compounds into working supply (in jars and bins) and reserve supply (in original plastic pouches placed inside ziploc baggies).
2. Clean a work area at work bench Remove sources of grit contamination, dust, etc.
3. Clean Mirror and Tool Blanks Some grit/abrasive material stuck from curve generating process.

BEGINNING

 Since the curves of the mirror and tool are pre-generated, they should closely match each other. However, the practicality of the matter is that the curves must become much more close for effective grinding and polishing to continue. Thus, the purpose of using the #220 Silicon Carbide abrasive is to work both the mirror and the tool into closely matching curves.

 It should be noted that the tool I received had a series of concentric channels molded into the blank. The channels retain water, and catch and perhaps recycle some of the spent abrasive and debris from the grinding process. Rather than use cleats (called for in some ATM books), I chose the alternative method of holding the tool in place. Wet the tool and place it on a folded, thin section of newspaper. The resulting water-vacuum was quite sufficient for the mild forces I was exerting.

 To "charge" the tool, simply wet the working surface, then sprinkle on a small amount (1/2 teaspoon for #220 grit) of abrasive using the plastic (disposable) spoon. Gently, lay the mirror on top of the wetted, abrasive- charged tool. Using a series of 1/3 diameter strokes (mirror center passing directly over tool center), push and pull the mirror across the tool, in the same direction, perhaps five or six times.

 Now begin the rotation sequence. Take one small step to the left (clockwise around the tool). Next turn the mirror about ten degrees counterclockwise. Push the mirror across the tool again, for another set of five or six strokes. After each set of strokes, step and rotate. After a while, the grating sound is greatly reduced, or the abrasive mixture starts looking more like a pasty film, than an abrasive slurry. Then it is time to "recharge" the abrasives on the tool.

CHECKING FOR CONFORMITY AND EVENNESS OF WEAR

 With the Sharpie indelible ink marker, draw straight lines crossing the diameters of both the mirror and the tool. During the next "wet" of grinding, note how these lines wear and fade. Having a uniform fade, is a sign of good contact between the mirror blank and the tool.

 Normally with mirror on top (MOT), the center will wear away first, while the outer portions of the tool will also wear away. To even out the wear for the outer portions of the mirror, perform a few wets of grinding with the Tool on Top (TOT). Do not advance to the next stage until the lines wear more or less uniformly.

Before	After

ADVANCING THROUGH THE "GRADES"

 Once the tool and mirror come into close conformity of curve, the next objective is to grind away any pits caused by larger abrasives, using an abrasive grade that is half the size (twice as fine) as the previous. The main problem here is determining when to change to the next finer grade in the series.

 After four or five wets, you may wish to inspect the mirror for evenness of wear. Rinse both the tool and mirror blank thoroughly, and scrub them to make sure that very little remaining grit or debris clings to the surface. Rinse the mirror surface with some rubbing alcohol and some soft lint-free cloths or cotton balls. Do this very lightly; the point is to remove any residue, while avoiding further scratches or pitting of the optical surface.

 Place the clean, dry mirror on a dark background material (cloth, cardboard, construction paper, etc. ) and shine a desk lamp at a low angle to the surface. Using a decent quality magnifier, slowly inspect the surface, from center to edge, for variations in grinding-pit size. You may also notice major and minor scratches, edge chips, grit, lint-fibers and such. With a Sharpie marker (or equivalent), mark an 'X' over each offending defect (large-pit or scratch) and resume grinding.

 Once all the X's have worn away, the defects may have disappeared also. Upon an inspection in which the mirror surface shows no major defects, then it is time to advance to the next finer grade of abrasive or grit.

 Reclean everything that will be used in the next grade. Replace old newspaper. For the washbasin, to prevent grit from becoming temporarily embedded in the soft plastic, and later contaminating the finer grade grinding sessions, I lined the basin with a plastic trash can liner (13 gallon). However, the mirror and tool can still have sufficiently sharp edges to cut through the liner, so I first lined the wash basin with a clean old newspaper section, before covering with a liner.

 The grades of abrasive which came with the 8" kit are:

• #220 Silicon Carbide
• 30 Micron Aluminum Oxide (AlOx)
• 25 Micron AlOx
• 20 Micron AlOx
• 15 Micron AlOx
• 9 Micron AlOx

 The time spent at each grade:

Grit	Time Grinding	Duration of Wet	Abrasive in "Charge"	Grinding Force (Pressure on mirror)
#220	~2 hours	10 minute wets	1 tablespoon charge	10 pounds
30 Micron	~ 1 hour	15 minute wets	1/2 tablespoon charge	~5 pounds
25 Micron	~ 1 hour	15 minute wets	1/3 tablespoon charge	~5 pounds
20 Micron	~ 1 hour	15 minute wets	1/3 tablespoon charge	~5 pounds
15 Micron	~ 1 hour	15 minute wets	1/3 tablespoon charge	~5 pounds
9 Micron	~ 1 hour	15 minute wets,	1/3 tablespoon charge	~2 pounds

 After the 9 Micron grit, doubly inspect the surface for any remaining pits or scratches. It will be better to grind them out now, rather than attempt to polish them out later. If you are ready to move on to polishing, reclean the mirror, tool and work area again, removing as many potential contaminants as possible.

MISCELLANEOUS NOTES ON GRINDING

 1. Mirror grinding makes a unique series of sounds. Very coarse grit will produce a sort of medium pitch "raspy-growl", like two stones engaged in mud-wrestling. As the grit becomes finer, the pitch of the growl moderates to more of a "swishing". If you hear a screech, stop immediately, it may be an abnormally large particle plowing a scratch into the surface.

 2. In olden days, the mirror and tool would sometimes seize up during grinding, becoming immovably stuck. This should not be a problem with the channeled tool provided in some kits. However, should you encounter such a situation, try first soaking the pair in a deep bucket of clean, cool water. Should that fail, try applying a cap-full of rubbing alcohol at the seam where the two have stuck. Finally, try aiming a stream of water, from a garden hose at the seam.

 3. If you are grinding with tool on top, and notice that the abrasive slurry is beginning to dry, or form a paste in the wake of the tool, sprinkle a few drops of water from the wash basin, if it has been cleaned and filled since the previous grade (of abrasive).

 4. The mirror and tool should have some degree of a bevel at the circumference. They were originally that way in the curve generated pair in my kit. However, the bevel can wear away during grinding. To create or restore a bevel, use a sharpening stone (fine-grit) and water. With the tool in a half-full wash basin, dip the stone in water, and rub it along the circumference, at a 45-degree angle. Be careful not to chip the edge, nor drag the stone across the working face.

 5. As fine grinding progresses, the mirror will gain the ability to reflect light from a distant light bulb at a relatively low angle. As the surface become more finely ground, the angle at which reflection occurs gets higher.

 6. AVOID GRIT CONTAMINATION - Once you've progressed to the smaller grit sizes, the last thing you want is a boulder from a previous (coarse) grade, carving jagged trenches in your hard-earned optical surface. Be scrupulous between grit changes! Keep grit separate, each grade from the other.

• I used separate Mason Jars with screw top lids to hold each grade of abrasive.
• For the wash basin, to clean the mirror between grades, newspaper lined the bottom, and a relatively waterproof plastic trash bag provided a fair barrier against earlier contaminants. Once the grade was done, the newspaper went in the bag, and the bag in a distant trash receptacle.
• Scrub the table surface at and around the grinding station.
• Put fresh newspaper under the tool.
• Clean your nails, take a shower, wash your hair and hands, put on clean clothes, and a fresh (clean) shop apron if you use one.

This site was last updated on Saturday, January 21, 2006.

Copyright 2006, Larry Gilstrap
 

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