Image of the day

Captured by
Terry Wood

Jupiter (clearer) Nov 5th 2023 w/Mewlon 180c

My Account

New to Astromart?

Register an account...

Need Help?

a severe hyperbola

Started by Phil Barker, 11/29/2003 12:26AM
Posted 11/29/2003 12:26AM Opening Post
A member of the local Society asked me to bring a mirror he's had lying round back to book.

Its a 7 inch f-7 plate mirror 25mm thick. The polish is good except for some gray at the edge.

The problem is its a severe hyperbola like I've never seen. I'm faced with how to attack it to get it spherical again. In the minimal ammount of time as well as needless to say I'll be charging him crumbs to do it.

I can think of 2 approaches

1. accentuated pressure from the edge to .5 tool on top.
This will take a long time given the seriously deep centre.

2. Full tool with graduated facets so more glass goes from the edge and less from the centre. This way it should catch up. I'll then have to recast the tool and go from there.

Any other ideas to fix this up.

Phil b
Posted 11/29/2003 04:43AM #1
Phil, what the literature usually recomends is working the 70% zone, since this is the zone that takes the least degree of deformation during refiguring a sphere into parabola. Hyperbola is only an "overdone" parabola, which means that the center is deeper and the edge zone lower than what they are supposed to be (with the edge being more overdone than the center). To compensate for this, the 70% zone needs to be lowered too, in order to bring the curve closer to parabola. With hyperbola being shallower than parabola, it is the edge that needs to be worked the least, with most of glass being removed from around 70% zone towards mirror center.

To determine best course of action, it is necessary to know more specifically what the surface look like, and to have some practical experience with it. I'm sure Mark Harry would know what is the best way to do it.