Now all these years later after a break in telescope making, I decided to make Cassegrain optics and maybe iater try my hand at a Maksutov corrector. For this I would need a spherometer. I referred to articles on the subject in the following books I have.
There are three main designs; the ring type, two leg and three leg using pins or balls. A micrometer or dial indicator can be used. It would appear that the micrometer is the more accurate. On a dial gauge reading to. 01 mm. with a 35mm. dial 1 mm. travel of the plunger is multiplied 110 times by gearing. Thre could be tooth errors causing periodic error. Fillmore says he checked his with Gauge blocks and noted the errors. D. Everett Taylor in ATM Book 2 prefers a dial gauge as there is not the problem of feel when using a micrometer. Horne says that measurements of .00001" are possible with a micrometer. This would be a .0001' direct reading type (no Vernier) with a 2" diameter thimble. I believe that with a magnifier I could read to. 00001". But I doubt I would have much feel on a thimble that size. With the 9/16" dia. thimble on my micrometer I would have some feel and could read to .00005'.
I decided to do some calculations to see how accurate a reading was
necessary for what I had in mind. As I wanted to use the same tooling
for making the optics for the Maksutov as the Cassegrain, I selected
a f/15 design of John Gregory. This was the same that Fillmore made.
In the following calculations I shall use Fillmores dimensions and his
Spherometer. In the grinding specifications for the primary and corrector
he gives a tolerance of plus and minus .005". Can diamond milling be
done to this degree of accuracy? I don't know. How was the radius measured?
With a spherometer? John Gregory puts a tolerance of plus and minus
.500" on the primary and .070" on the secondary. More practical. Let
us now investigate this. Below is a drawing of a mirror blank showing
the three dimensions we are interested in. Also the equations I used
for all my calculations. I used a caiculater giving answers to ten decimal