Foucault tester-dial indictor

[dark star]

I am in the process of making a Foucault tester and need to get a dial indicator/ gauge. Does this need to have 1 inch (25.4mm) of travel? I am making the tester based on the one on the American Stellafane ATM website, and it appears that it needs to have one inch of travel, but it is not clear. Or have I misunderstood? Most dial gauges I have seen advertised have 10mm of travel., or less.

David

[Astrobits]

Chronos have some metric and imperial dial gauges that might suit:

http://www.chronos.ltd.uk/acatalog/Engineering_Menu_Dial_Gauges__Dial_Indicators____Magnetic_Bases_228.html

Nigel

[dark star]

I ended up getting a dial indicator from Machine Mart, as there is a branch not too far from where I live. It is has 10mm of travel. If this turns out to be not enough for the Foucault Tester I will get another one. I could then possibly use this one to try to make a Spherometer (if I got the word right) to measure the curvature of the mirror at different points. I do not know much about this, as I only just discovered such a thing exists from another American ATM webiste!

David

[fwm891]

Length of travel will depend on the focal length/f-ratio your working on. Do your cals for knife edge movement, decide whether the knife edge and source move together (halve the distance - less accurate) add 25% more travel for the early stages of testing where zones may be longer than anticipated and your about there.

Or you could use a linear bar style, that will give you about 100mm to .01mm approx: this is Axminster but there are others: http://www.axminster.co.uk/axminster-digital-read-out-scales-prod21966/

[dark star]

At the moment I am grinding a 150mm f/5 mirror, focal ratio x diameter of the main mirror = focal length = 750mm, assuming I haven't mixed anything up! But I hope to grind a 350mm f/5 mirror if this one is successful = 1750 focal length. I will do the other calculations at the weekend, as I will have to do a fair bit of reading to figure out how to do them!

The linear bar type indicator is interesting, although I would have to understand how to fit it on to the Foucault tester, or modify the design. I will post some pictures of the tester when I have made some more progress in constructing it.

David