[time-nuts] Re: Is SC the most stable cut for lowest phase noise?

[Bruce Griffiths]

Its far more likely that the glass blank from which the front element is machined is annealed over six months rather than the element itself as precision optical element fabrication doesn't add strain to the element. Some low dispersion glasses have relatively high TCE and are very sensitive to thermal shock. Elements made from such glasses are rarely used as front elements particularly in larger sizes to avoid element fracture due to thermal shock.

Bruce

> On 11/06/2022 10:38 Lux, Jim via time-nuts <time-nuts at lists.febo.com> wrote:
> 
>  
> On 6/10/22 1:57 PM, Dr. David Kirkby wrote:
> > On Fri, 10 Jun 2022 at 17:39, Lux, Jim via time-nuts 
> > <time-nuts at lists.febo.com> wrote:
> >
> >     On the subject of rapid warm up. I suppose if you had a need, one
> >     could
> >     dump as much power as you need into the heater. Turn on oscillator,
> >     lights in room dim for a few moments.
> >
> >
> > Is that not likely to damage a crystal? Different parts of the crystal 
> > and likely to be at significantly different temperatures at the same 
> > time, putting a lot of stress on the crystal due to a thermal 
> > gradient. It's probably a bit academic, as nobody is going to make an 
> > oven that heats up in fractions of a second, but if one did, I suspect 
> > it might not do the crystal a lot of good. This is only an educated 
> > guess - I don't have anything to back it up.
> Oh, it would be disastrous, although quartz is pretty strong, all the 
> rest of the mounting components might not be.
> >
> > At the other extreme,  would there be any advantage in actually 
> > heating the crystal very slowly, over the course of an hour/day/week, 
> > so the temperature gradient across the crystal is very small? Of 
> > course, if an oven took ages to reach the correct temperature, it 
> > would be inconvenient for most applications, but for some 
> > applications, the advantages might outweigh the disadvantages. Of 
> > course, if one does this, I suspect one would have to cool the crystal 
> > slowly too to prevent a significant thermal gradient across the crystal.
> >
> > I know it's a bit different, but I have a 600 mm f4 Nikon camera lens. 
> > I was told that Nikon cools the front element over a period of 6 
> > months to reduce stresses in the glass.
> 
> Big glass mirrors for telescopes do the same.
> 
> 
> 
> >
> > Dave
> 
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