Mirror surface accuracy

[John]

I was reading something the other day that illustrated how accurately mirrors need to be figured for astro scopes. The example was to imagine a mirror 1 mile in diameter - the variations in accuracy that would be acceptable for it to be a decent quality mirror would be about the thickness of a playing card at that scale

I was a bit incredulous when I read this but, thinking it, that could be about right

Or is this example exaggerated ?

[barkis]

I would think your analogy is about correct John.

If I remember rightly, an 8" f7 paraboloid varied from a sphere at the edge zone by 23 millionths of an inch. The zones from centre to edge all have different radii, the edge having the longest of course.

Ron.

[Moonshane]

wow, I love amazing big number facts like these.

amazing that such accuracy can be achieved on top of a workmate in your garden shed.

[johninderby]

I was reading something the other day that illustrated how accurately mirrors need to be figured for astro scopes. The example was to imagine a mirror 1 mile in diameter - the variations in accuracy that would be acceptable for it to be a decent quality mirror would be about the thickness of a playing card at that scale

I was a bit incredulous when I read this but, thinking it, that could be about right

Or is this example exaggerated ?

The degree of precision needed for a good mirror is a bit mind-boggling isn't it.

Mass market machine made mirrors such as those used by Skywather etc are amazingly good optically given the price point, but to reach the levels of precision of the very best mirrors it takes a lot of very highly skilled hand labour to achieve. That's why the costs skyrocket as you move up in optical quality.

John

[Moonshane]

this is why it always makes me smirk when people look at a mirror by eye and then say 'mirror looks good'.

[AndyH]

Wow !

That's incredible.

[FraserClarke]

The one I always like is that the surface quality of the VLT primary mirrors (8 meters) is equivilent to having no waves higher than 2cm on an ocean the size of the Atlantic...

(which translates to about 1/10 wave I think).