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Equipment Discussions >> Reflectors

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ckwastro
Pooh-Bah


Reged: 11/23/05

Loc: Tempe, AZ
Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5701630 - 02/26/13 12:59 PM

Quote:

Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.




Quote:

Exactly right. Quartz changes about 1/6 as much as Supremax/Pyrex, and Zerodur really doesn't change at all. Of course one pays a lot of money for the properties of these materials.





These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear. I suppose one could always use fans to assist the equilibration, but it seems to be trading one problem for another that is probably more damaging to the final image.

Has anyone with a Quartz or Zerodur mirror ever noticed this to be the case in actual application?


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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5701842 - 02/26/13 03:10 PM

CZ's smoothness criteria is spot on the money. I even like to see it a bit more stringent than this to describe an optic's capability in reaching what might be possible to see with it. Bravo.
M.


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DHurst
super member


Reged: 03/10/06

Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5701848 - 02/26/13 03:13 PM

Sorry to digress, from all the off topic banter, but what happened to
Ed Stevens?
I have a wonderful 14.5" mirror he made and a great 10" re-annealed and refigured by him.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5701918 - 02/26/13 03:53 PM

Per given aperture, the quartz mirror can be made thinner and thus lighter-weight than the Pyrex mirror. This creates a mirror of less mass. Less mass will cool faster. Mike Lockwood is figuring 20" quartz mirror with only 1.25" edge thickness. Typical Pyrex at 20" would be 1.5" edge thickness - a lot heavier than the quartz mirror. I think the 20" quartz mirror would cool faster than the 20" Pyrex mirror given the same f/ratio and sagitta curves.

Quote:

Quote:

Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.




Quote:

Exactly right. Quartz changes about 1/6 as much as Supremax/Pyrex, and Zerodur really doesn't change at all. Of course one pays a lot of money for the properties of these materials.





These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear. I suppose one could always use fans to assist the equilibration, but it seems to be trading one problem for another that is probably more damaging to the final image.

Has anyone with a Quartz or Zerodur mirror ever noticed this to be the case in actual application?




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Sgt
Pooh-Bah
*****

Reged: 12/17/05

Loc: Under the southern horn of the...
Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5702001 - 02/26/13 04:24 PM

Quote:

Quote:

Actually, substrates like zerodur or quartz only fix the issue of the figure changing during cooling. They do nothing about the boundary layer caused by the temperature difference between the air and the mirror, which is the more damaging one in my experience.




Quote:

Exactly right. Quartz changes about 1/6 as much as Supremax/Pyrex, and Zerodur really doesn't change at all. Of course one pays a lot of money for the properties of these materials.





These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear. I suppose one could always use fans to assist the equilibration, but it seems to be trading one problem for another that is probably more damaging to the final image.

Has anyone with a Quartz or Zerodur mirror ever noticed this to be the case in actual application?




Are you sure you heard it right from your friend? A Google search indicates quartz has about 1.3 times the thermal conductivity of Pyrex and slightly lower specific heat capacity. I'd also be interested to hear real life experiences.

edit: that should be 1.3 times the thermal conductivity

Edited by Sgt (02/26/13 06:56 PM)


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Tom Polakis
Carpal Tunnel
*****

Reged: 12/20/04

Loc: Tempe, Arizona
Re: Top Opticians in the US for reflectors new [Re: ckwastro]
      #5702018 - 02/26/13 04:33 PM

Quote:

These materials may even make the boundary layer much more of an issue than it is with Pyrex.

A few years I had a conversation with a friend of mine who is an optics guy. We were discussing the the properties of these two materials and whether or not they are worth the additional cost. He had done some research on the heat transfer coefficient of Quartz and Zerodur and concluded that they retain heat much longer than Pyrex, so while the figure won't change, the effect is that those substances might result in a boundary layer that takes much longer to clear.





Heat transfer coefficient is not a intrinsic material property, rather, it's a function of the surface condition, air velocity, and other factors. The thermal properties of interest for mirror cooling would be density, specific heat, and conductivity. I looked them up, and they are not significantly different between the three materials. As was mentioned in reply to your post, the quartz mirror can be made thinner, reducing the mass, which is always good news thermally.

Anyway, here are the properties, which I will admit are not taken from one source. Note the striking improvements over Pyrex in thermal expansion coefficient.


Density Conductivity Specific Heat Thermal Expansion Coeff.
(g/cm^3) (W/m-K) (cal/g-C) (C ^-1)


Pyrex 2.23 1.06 0.180 3.25E-06


Quartz 2.20 1.30 0.176 5.50E-07


Zerodur 2.53 1.46 0.196 2.00E-08


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ckwastro
Pooh-Bah


Reged: 11/23/05

Loc: Tempe, AZ
Re: Top Opticians in the US for reflectors new [Re: Tom Polakis]
      #5702107 - 02/26/13 05:15 PM

Hi Tom,

It's been too long, good to hear from you. Thanks for the material properties.

Honestly (to the previous posters as well), this conversation was several years ago when we were just starting to see quartz coming into play in the amateur market, and we may have been talking about specific heat or some other property, rather than heat transfer. I do apologize for my fuzzy memory on this (which is unusual for me) , but whatever it was he came away thinking that the quartz would hold heat much longer and cause a completely different problem. I'm not sure exactly what prompted that conclusion at this point, but all I can say is that he's a meticulous researcher, and didn't just jump to that conclusion, even it turned out to be incorrect.

It's certainly possible we had not considered the ability to use thinner blanks at that time, which prompted my question about actual application rather than just theoretical. I was curious whether or not that was a noticeable problem with quartz & Zerodur.

Thanks for clearing up my questions about it.

Edited by ckwastro (02/26/13 05:18 PM)


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