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Re: [ATM] Flexed mirrors
Hi Bill,
As a glass caster this has me intrigued, how can I work out the "ideal"
profile for the back of a (for example) 10" F6 mirror?
I figure if I cast it with the curve already in the back then all I need to
do is grind and polish the F6 sphere into the front, coat and the rest as
they say would be history.
BTW once I have a mold made up I can churn these out at the rate of around
1/week, the blanks anyway, so if there is any interest I might make a more
permanent mold instead of the usual sacrificial mold....
Cheers, Thomas.
PS this strictly a non-commercial idea, I don't see this as a market of
commercial size, but materials and energy costs money so any blanks made
wouldn't be free.......
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Bill Kelley
Sent: Monday, 14 January 2008 2:38 PM
To: David Weinshenker; 'atm_list'
Subject: Re: [ATM] Flexed mirrors
David,
Thanks for your interesting reply. Flexed mirrors may be brought to full
correction when the temperature is changing by slight retuning in the field.
When using the scope for terrestrial work the mirror may be retuned so that
it can even be used as a near range microscope.
Yes, the Dall-Kirkham primary can be refigured by changing the tension so
that it could be used with a flat as a newt.
The correct back shape for a perfect paraboloid is conical ala Royce but
concave from the edge to a slightly raised stub for puller attachment. The
correct concave back shape may be calculated. This milling of the glass back
can be avoided by a puller that distributes the puller tension over the back
of the mirror. Not quite as perfect result as a perfect "shaped back" but
way better than needed even on nights of very low turbulence. I presume our
readers know that turbulence sets limits such that under perfect terrestrial
observing conditions anything larger than a "perfect" 8 inch optic will not
net any gain in resolution.
The flex concept becomes very useful with the trend toward large, fast
mirrors. On these conventional parabolizing by polishing to a smooth,
zone-free figure is a bear of a problem for many atm workers.
It is easier to make good spheres on fast mirrors than slow.
Thanks to all for the friendly thoughtful responses I am getting by re-
introducing flexing as an excellent way to produce superior mirrors with
many "flexible" :<) uses.
Bill Kelley
Imprisoned under dark Arizona skies.
----- Original Message -----
From: "David Weinshenker" <daze39@earthlink.net>
To: "'atm_list'" <atm@atmlist.net>
Sent: Sunday, January 13, 2008 1:06 PM
Subject: Re: [ATM] Flexed mirrors
> Bill Kelley wrote:
>
>> Why flexing for the ATM ? :
>> 1. A spherical surface is the easiest to make.
>> 2. A spherical surface is the easiest to test.
>> 3. Extreme smoothness can be achieved because the tool and mirror fit
>> in
>> all positions of the polishing stroke. The larger
>> and faster the mirror the worse the fit, and consequent small scale
>> surface defects. The resultant light scatter reduces
>> resolution and explains why many refractors provide better
>> resolution
>> than reflectors.
>
> I wonder if this explains why the small mass-produced Maksutov/Cassegrain
> scopes work as well as they seem to?
>
>> 4. Among the many other advantages your flex mirror can be detuned to
>> an
>> appropriate ellipsoid for near range imaging.
>
> Very interesting... or similarly to suit e.g. Dall-Kirkham systems with
> interchangeable secondaries of different curvatures for different final
> magnifications (and readjust the primary to "fully parabolized" for use
> with a flat secondary in the Newtonian manner).
>
> Certainly providing a direct "field adjustment" for spherical "correction"
> might be as useful as having the means to fine-tune the collimation...
> some observers, at least, might value the ability to tune out a visible
> aberration under actual use conditions... grab the big knob sticking out
> of the back of the mirror cell and adjust it until all of the light goes
> through focus at the same time.
>
> I guess the real question is how exactly a bent, figured-spherical
> plate can be made to match the exact desired curve. There might be
> something in a design resembling the "conical" style mirrors that
> are advertised by R.F.Royce, which are supported on a single peg
> at the center and have a thinned, unsupported edge. If something
> like this were figured spherical on the unstressed glass, and then
> flexed by using the center post to pull the edge back against a
> support ring, then the exact thickness profile from center to edge
> might be used as a degree of freedom to establish the exact pattern
> pf bending.
>
> -dave w
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