A spherometer is actually quite simple to build ,they can be made with 2 or 3 contact points and a measuring device of some type. The spherometer I have built has 3 contacts (refered to as a 3 ball spherometer since the contact points are spherical ) which enables the device to sit flat at all times, but a spherometer with only 2 contacts is in actuality more useful provided it is kept perpendicular to the work piece. I used a cheap micrometer for measurement but one could just as easily use a 1/4 20 tpi ( threads per inch ) bolt. Given that with a 1/4 20 tpi bolt you will get 50 thou travel with one revolution you now have a very economical way to measure the sagitta or depth or depth of curvature of your primary mirror.

Ok here we go! Start by acquiring a 5" long piece of flat bar it can be any type of metal providing it is capable of being tapped. I recommend at least 1/4 " x 1" but 1/2" x 1/2" would be preferable. Now drill 3 holes at 13/64ths (I just use a 3/16" drill bit , it's close enough) be sure to have them spaced out at 2" between them, remember the more accurate your layout is the more accurate your spherometer will be. Now tap them with a 1/4 20tpi tap ,always keep the tap perpendicular to your work piece!!!!! Screw in 2 1/4 20 cap screws 1 1/4" in length and lock them in with a lock nut (an ordinary nut) . Screw on your stainless acorn nuts , stainless steel will be the only metal to actually contact the mirror, because it won't corrode. Get the biggest washer you can find and at equal spacing around the edge place 50 marks with a scratch awl or centre punch,each mark represents 1 thousandth of an inch, if you can't get 50 on it then put 25 which will give you 2 thou per mark or 10 marks will give you 5 thou and so on. Slide your big washer in a 2" bolt and lock it in position with a lock nut screw it in and put on your remaining acorn nut and THERE YOU GO!!


The beautiful thing about it is that you never have to mess around with a straight edge and feeler gauges again ,and this is a lot cheaper to build than to buy the other . Even if your spherometer doesn't turn out as well as you hoped, it's still far more accurate.

You can make your spherometer as large as you like , you wouldn't want to use a 4" spherometer on a 20" mirror it wouldn't be accurate enough so simply scale the spherometer up to something like 18" , or even build one with a longer bar and have as many pre drilled and tapped holes as you want. The thing to remember is when using this spherometer for example on a 10" f6 mirror (which has a 60" focal length) you are not measuring the sagitta for a 10" f6 but for a 4" f15 ,the sagitta you are trying to achieve is for 60" . A 20" f3 , 10" f5 , 6" f10 and a 4" f15 will all have different sagitta when measured from the highest point on the outermost edge of the mirror to the lowest point in the centre, but as far as the spherometer is concerned it's measurements are relative to the focal length not the f-ratio .


First find your desired focal length. Lets say we want to grind a 10" f6 .Our focal length will be 10 x 6 = 60". We know our spherometer is 4" between contacts so using the formula depth = (rxr)/4F , r being the 1/2 of the distance between contacts which is 2" and F being our desired focal length which is 60" we arrive with (2x2)/(4x60) and that reduces down to 4/240 which gives us the final measurement of .01666666666667 when rounded off we have 17 thousandths of an inch or 17/1000 inches.

Here are a few pictures of my 3 contact sperometer .I used a cheap import micrometer I bought at Princess Auto for $9.95.

All I did was cut off the lower fork and turn it down on my lathe to 7/16 inches and threaded it.

I cut out a peice of 1/4 plate aluminium .

Tapped 3 holes at 1/4 20tpi and one in the centre at 7/16 20 tpi and screwed all the pieces together.

And this is what I ended up with.

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