Re: [ATM] Flexed mirrors

["Marinus van der Lugt" <lugt@xs4all.nl>]

I wonder about astigmatism.
For zenit observations I see no problems, but what about near-horizon
observations?

Regards,
Rien.

> 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
>> _______________________________________________
>> ATM mailing list http://www.atmlist.net/
>>
>
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