Making an Optical Flat Mirror

One does not hear about making flat mirror in ATM too often. It seems that even people making their own primary mirrors contend with buying diagonal mirror from commercial sources. Part of it may be that making a good diagonal is a more demanding job that grinding and polishing a lot larger paraboloid mirror which it will be used with - draw your own conclusions from it :-)

Description below is intended for those interested in persuing diagonal building though it is worth pointing out that flat optical surface has other applications as well. Large optical flat mirror can be used as as coelestat in solar telescope. It may constitute back side of corrector plate. As another example, objective prism contains two flat surfaces.

I have build my first diagonal during construction of a 6 inch f4 Dobsonian. Description below lists details and progress of making large diagonal mirror (5 inch minor axis) for 24 inch Dobsonian under construction by Kingston Centre - RASC.

Technique I am using is derived from usually scant descriptions found in a relatively few old ATM books.



Albert G. Ingalls (ed.) Amateur Telescope Making, Scientific Americam Inc. 1926, 1928, 1933 &1949


Allyn J. Thompson, Making Your Own Telescope, Sky Publishing Corp. 1947

I ended up with a combination of those methods based on personal preference and accumulating experience.

One is not enough ...

The paradox of making a flat mirror is that they like company. Actually, the easiest and least frustrating way is to make three of them at the same time. Why?

  1. During grinding, the only way to get a flat surface is to grind three pieces of glass alternatively against each other.
  2. During polishing optical surface needs to be tested against master flat (if you have one you trust to be flat already) or three optical surfaces are tested against each other and curvature/flatness of each can be then calculated.

So what to do with extra flats - afterall you are probably building just one scope. Or aren't you :-) ? I ended up with using extra one in a 20 inch Dobsonian.

Step 1. Grinding

I have started with three pyrex disks - each 7 inch in diameter. I have marked them as A, B and C.

Those blanks were then ground with #400 carborundum in a following order: A on B, B on A, C on B, B on C, C on A and A on C. Then the second cycle started with B on A, A on B, B on C etc. And finally the first cycle was repeated. Carborundum was charged only once for each disk combination (6 times in each cycle) and each charge was ground with quater stroke and some pressure for three circles around the stand. All that took about 2 hours.

Images below show the appearence of one disk prior and after #400 grinding.



original 7 inch Pyrex blank


flat surface after #400 grit


Then A, B and C mirrors were groud in the same alternating order using #600 carborundum (3 cycles) and finally #1200 aluminum oxide (3 cycles).

Step 2. Polishing

Step 3. Cutting elliptical mirror

Back to 20 inch scope

Number of visitors:

Jan Wisniewski