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Re: Roddier Cell Measuring (Was: [ATM] Use of bearings in (static)mirror cells)

Hi,

matt wrote:
> maybe one day when your software will work with masks than one can place
> over the scope aperture this will be doable ;-)
> The method is not new, professional observatories have been doing this for
> about a century now.
> 
> We amateurs are in the stone age of passive mirror support , we discovered
> whiffle tree and load distribution was optimized with FEA (PLOP), we
> rediscovered the wheel and astatic mirror support bringing us into the 19th
> century, now  we just imght get lucky and rediscover Hartman testing and
> active cells.

Yes, we are amateurs, and most of us use smaller, simpler instruments. 
  They simply don't require very advanced technology to function up to 
99% of their theoretical performance in their current form.  Making 
thinner mirrors demands better support, but figuring such mirrors is 
much more difficult than a thicker mirror.

For those who are pushing the envelope in size (24"+), these ideas 
start to become more appealing (and are indeed late in gaining 
popularity).  However, support is not the only problem.  Active 
temperature control is vital too.

> My honest opinion why people are NOT using such methods is they don't want
> to really know their beautiful 1/99 wave scopes are actually 2 waves at the
> eyepiece. Thad would take away the bragging rights and the pleasure of
> pursuing optical perfection.

I think one reason people are not using such methods is because a 
simpler, exceedingly sensitive test is available when the seeing 
permits - the star test.  If my scopes were 2 waves at the eyepiece 
(and they aren't), then it would show up conspicuously in the star 
test.  This is my ultimate test, and it requires no extra equipment.

For the visual observer, the star test is a natural option.  For 
imagers, I think the Roddier method is an interesting option.

	Mike Lockwood

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