Telescope Reviews: 6" F/15 Refractor Project

First I will show some of my spherometers. The first spherometer I made consisted of a 4" pulley, 3 ball bearings epoxied to the bottom, and a dial indicator clamped in the center. This wasn't the greatest spherometer due to my failure of epoxying the bearings on a common radius about the center. The dial indicator wasn't exactly centered either.

So I made a better one out of aluminum.

This is a 3" spherometer shown on a 3.1" opthalmic glass blank. The ball feet are synthetic ruby balls. The hardness of synthetic ruby greatly extends the life of the feet while testing during rough/fine grinding. I epoxied the balls into the head of a machine screw. I made sure the feet were perpendicular to the base plate before tightening the nuts. The mitutoyo micrometer head was purchased from Ebay for just $25. The micrometer spindle spins as the dial is turned. I used this spherometer to make a 3" test plate and test the sphericity of my 6" lenses.

My next spherometer was a 5.5" with an electronic dial indicator.

I had to use huge ball bearings and I believe this was a mistake. Smaller bearing might have been better. The indicator is held inside its base by a small set screw. Care must be taken not to overtighten. This digital indicator reads down to .0005" which was just not good enough for my needs. So I would call this a failed spherometer.

So I proceeded to make another spherometer.

This is another 5.5" spherometer. This micrometer head is a Starrett with a spindle that does not rotate. I purchased this one from Ebay as well for $5.00. I had to take it apart though in order to mount it onto the aluminum plate. I epoxied the bottom of the micrometer to lock it in place. These ball feet are steel ball bearings. They are also epoxied onto machine screws.

When I first begin making these spherometers, I first draw a circle on the aluminum with a nice compass. I then find the three points where the feet will be using the compass again. After the points are found, I punch a small hole using the head of a finishing nail. The hole helps to keep the drill bit in place when I begin to drill. I start with a small drill bit and then progress to the size I need. The center hole is also done this same way. I cut the spherometer out of aluminum with a jig saw. So no special tools are needed.

Here is the 3" spherometer design on the aluminum before cutting:

Larger Version: http://farm3.static.flickr.com/2117/2397153317_03b3a945e9_b.jpg

You will need an optical flat or some other flat surface to zero your spherometer. I used a granite flat from Grizzly. It was only $12 and has an accuracy of plus or minus .0001".

Another good thing to have is an optical surface where the radius of curvature is known very accurately. This can be a mirror or another lens. You can use this surface to find the radius of your spherometer accurately. The more surfaces you have to check your spherometer against the better. The worst feeling is having doubt in your spherometer when you are grinding. I took apart a couple meade 4.25" mirrors at the university and used them to find the radius of my spherometer. I used my foucault tester in finding the radius of the mirrors. I also used an 8" uncoated mirror and a 10" coated mirror.

Another method is to use the concave R3 surface of the flint lens. You could maybe do 2-3 wets per abrasive grade and make a pitch lap to put a flash polish on the surface. Use a foucault tester to find the radius of curvature and use R3 as your calibration piece.

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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
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How To Make a 6" F/15 Refractor