Re: [ATM] Sphereometer accuracy

[Dominic-Luc Webb <dlwebb@canit.se>]

> Dominic-Luc Webb wrote:
> > Exactly my point for years now. If someone REALLY needs such 
> > precision, then maybe measuring the actual ROC with reflected 
> > light source using a micrometer attached to a piece of paper at 
> > ROC would be more appropriate. There are very few of us that 
> > really need the ROC to be this exact using a spherometer.
> 
> That's fine if the mirror is concave, but what if the mirror is 
> convex?  For example, how to you measure a Cassegrain secondary of 
> unknown origin?  In this case a spherometer is the perfect tool.


Agreed, as per my point, most people are not doing this.


> There are also inherent inaccuracies when measuring with a piece 
of 
> paper and a light source. 

Yup, +/- a mm or so... I have thus far survived. I have not 
yet worked a scope that had specs that could not tolerate 
an actual ROC that differed from the specs by this amount. 
Have a lot of (any) ATMs encountered such extreme requirements?

Regarding convexes, I simply measure the tool, which 
should be a very close replica.

> It also takes more time, and unless you set 
> up some equipment it can be tricky to get an accurate 
> measurement when 


Maybe depends on what you have handy. I started doing 
this because it was really fast and seemed at least as 
good as a good spherometer used properly at the optical 
surface.


> > More to the point from my observations, some newbies 
> > get depth easily within useful specs (goal is F/5.0, and 
> > they get F/5.0 +/- negligible error), but are not getting 
> > spheres right from the early stages of grinding.
> 
> the newbie both the ROC AND if it is spherical - just move 
> it around and see if the reading changes.

Excellent point. I rarely see people actually doing this. To 
that end, there is a similar device I have seen used by 
optometrists to make glasses in which they run old lenses 
across a surface which has, essentially, a spherometer that 
records the depth as the optical surface glides under it from 
one end to the other. The curve is displayed on a small screen. 
The only one I have seen seemed to have similar precision as 
the spherometers we are more familiar with. I would appreciate 
if someone here can tell me what this device might be called 
and what precision is typical. 

Dominic-Luc Webb
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