Re: [ATM] Spherometer calibration, precision, etc.

[Mike Lockwood <melockwo@uiuc.edu>]

Dominic-Luc,

Dominic-Luc Webb wrote:
> Admittedly I do not know much about making Cass secondaries. Can't
> this also be measured by testing the ROC of a glass tool with my
> pinpoint light source and piece of paper? I recall once a long time
> back going through the math and deciding this test was more precise
> than a typical ATM spherometer. I can certainly get the ROC of my
> more conventional concave mirror surfaces accurate to a couple mm,
> and assumed a Cass secondary ROC was also flexible to within a couple
> mm, presumably the position in the optical system could be adjusted
> to accommodate the exact ROC obtained. As I understood it, the goal
> is a particular type of curve (depending on type of Cass), not a very
> exact ROC. The ROC actually obtained simply dictates the
> amplification, which I thought is somewhat flexible. If your design
> is for a hyperbola that is ROC 480 mm, will it actually matter if
> the ROC comes out 479, 480 or 481 mm?

Sure, you can change the design a bit if your secondary curve is off, 
but I like to avoid it if I can.  It affects the secondary's 
prescription (recalculate it after you know the real ROC of the 
secondary), and of course the focal length of the telescope. 
Depending on the final design focal ratio you're trying to hit, the 
tolerance grow tighter as the focal ratio of the primary drops, and 
also as the system focal ratio gets larger.

For most amateur systems small errors won't matter much at all, and 
can be compensated for.  For larger and faster Cassegrains, it becomes 
more worthwhile to hit the radii spot on.

I wrote:
> Well, I agree in concept, for most ATM mirrors, but not in the case of
> a Cassegrain secondary.  Extra precision here, before polish is begun,
> is advantageous.

I was talking about controlling the curves before polish.

I've never had great luck measuring ROCs for unpolished mirrors (by 
wetting them, oiling them, etc.), and I like to watch the ROC closely, 
which implies frequent measurements.  I would finish grinding and 
check the ROC after some polishing and there would be a discrepancy 
that would irritate me.

So, rather than set up the pinhole, I like to just set the spherometer 
on the piece and have my reading in a few seconds, rather than a few 
minutes.  Just my preference.

I will digress here, but it's a similar related topic.....

On larger pieces (primary mirrors), carefully sliding the spherometer 
around can immediately show if there is significant asphericity.  If 
the reading changes, the piece is not spherical, and some areas are 
not getting ground uniformly.

	Mike Lockwood


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