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testing convex sphere

Started by Phil Barker, 06/28/2012 04:38PM
Posted 06/28/2012 04:38PM Opening Post
I've reregistered after a few years away from astromart in part to speak to fellow minded person with telescope making addictions.


One of my current projects is making a 8 inch Dall Kirkham and at the moment I've just finished polishing out the convex secondary

the scope will be around f-16 the primary is around f-4.5 and the secondary is a 2.5 inch pyrex blank purchased from willmen bell a few years back.

I decided to stick with a Dall kirk because the correction is still reasonable even with 2 inch eyepieces with these specs. Around the same coma as a f-8 newt in terms of distance in mm from the centre of the field obviously not field size with the longer f ratio.

Also the mirrors are easier to make and I back myself to make an f-4.5 elipse of around -.65 given I've now done around 20 parabolas up to 18 inch f-4.8.

Re the convex sphere I had a largely unseccussful go at a mock up interference tester using the concave as a reference but a friend told me to try fine grinding and polishing the back and testing from behind with a monochromatic light source ronchi and knife edge if possible.

After polishing the back and the convex sphere I tried this and found I had a very poor surface rough and with a hole in the centre and turned up edge.

I've now done some careful work smoothing the surface regularly pressing etc and using a ronchi then a knife edge with a green led lightsource I appear to have a very good sphere. I remade the polishing tool and am using thick medium hardness pitch. I realise its possibly not as sensitive as an interference test although this relies on the concave being accurate also.

Has anyone else tried this to me it really does appear to be a sound way of testing a convex sphere?

Posted 06/28/2012 05:09PM #1
Phil Barker said:

I've reregistered after a few years away from astromart in part to speak to fellow minded person with telescope making addictions.


One of my current projects is making a 8 inch Dall Kirkham and at the moment I've just finished polishing out the convex secondary

the scope will be around f-16 the primary is around f-4.5 and the secondary is a 2.5 inch pyrex blank purchased from willmen bell a few years back.

I decided to stick with a Dall kirk because the correction is still reasonable even with 2 inch eyepieces with these specs. Around the same coma as a f-8 newt in terms of distance in mm from the centre of the field obviously not field size with the longer f ratio.

Also the mirrors are easier to make and I back myself to make an f-4.5 elipse of around -.65 given I've now done around 20 parabolas up to 18 inch f-4.8.

Re the convex sphere I had a largely unseccussful go at a mock up interference tester using the concave as a reference but a friend told me to try fine grinding and polishing the back and testing from behind with a monochromatic light source ronchi and knife edge if possible.

After polishing the back and the convex sphere I tried this and found I had a very poor surface rough and with a hole in the centre and turned up edge.

I've now done some careful work smoothing the surface regularly pressing etc and using a ronchi then a knife edge with a green led lightsource I appear to have a very good sphere. I remade the polishing tool and am using thick medium hardness pitch. I realise its possibly not as sensitive as an interference test although this relies on the concave being accurate also.

Has anyone else tried this to me it really does appear to be a sound way of testing a convex sphere?


If you polish the secondary correctly and carefully you should have something close to a sphere. You could then null test it with the primary and make corrections to it (primary). Never done that but I usually end up with decent spheres after polishing my concave mirrors - maybe the same is true with convex surfaces.

Jim J

"Much of the social history of the Western world, over the past three decades, has been a history of replacing what worked with what sounded good." - Thomas Sowell