Re: [ATM] Sphereometer accuracy

[Mike Lockwood <melockwo@uiuc.edu>]

Dominic-Luc,

Dominic-Luc Webb wrote:
>> but what if the mirror is convex?
> 
> Agreed, as per my point, most people are not doing this.

True.  But I wish more were!

> 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?

Probably not.  It is not required for making a Newtonian.

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

If you didn't make the mirror, you don't have a tool for it.  :)

> 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.

If it works for you, by all means keep doing it, and please mention 
your procedure!  I got tired of the inaccuracy in my method.  Sounds 
like you have developed yours more.  (Based on Stan's later message, 
maybe I didn't develop mine enough.)

I don't want to discourage anyone from developing or using their own 
test methods.  That is a core skill in telescope making, and it should 
not be stifled!

I have stuck with a spherometer because it is convenient for me.  I 
have lots of projects going, so I really value things that are time 
efficient for me.  I can easily, quickly and accurately measure all of 
the following with my spherometer:

1) ROC of concave mirrors (also can be done by wetting the mirror)
2) ROC of convex mirrors (done by measuring the tool if you have it)
3) Curvature of an unpolished flat (no other simple way to measure it)
4) Gross asphericity of a concave or convex, unpolished surface

>> 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. 

Stan's description of the profilometer sound like the perfect tool for 
this.

	Mike Lockwood

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