Re: [ATM] Quartz blanks

[Mike Lockwood <melockwo@uiuc.edu>]

Ken,

Ken Hunter wrote:
> Price??? Best guess I've been able to come up with is around
> $5000.00 per disk ready to roll.
> Earlier I posted a price for some 20 inch Fused Quartz blanks that 
> I have available. I forgot to mention that this is very competative
>  with the lowest price I could find from someone that had an 
> honest-to-goodness storefront.

I appreciate the offer, but $5000 per disk is just too high.  We'll 
keep investigating the other sources that have been mentioned. 
Honestly I doubt we will find a disk in the price range we would 
accept, but we wanted to check anyway.

> You can OCCASIONALLY see Fused Quartz optical pieces on eBay for a 
> LOT LESS but rarely at this size and always with a disclaimer that 
> the seller doesn't have the facilities to guarantee what he is 
> selling and you have to take the chance on what you are getting...

A very wise warning.

> I think Mike was wanting Quartz to eliminate a focus shift problem 
> in an assembled instrument under actual operating conditions. I do 
> not think that a Quartz blank alone will give him that. True, the 
> quartz is going to be ultra stable (and a joy to work) but it's 
> miniscule focal length change will be dwarfed by the CTE of the 
> telescope/instrument that he puts it in. Wouldn't be fair to think 
> you are home-free by spending $5K for glass if you are going to use
>  aluminum truss tubes to hold the magnifying secondary out front.

The scope is a 20" F/3 photographic Newtonian with a flat mirror at 
the top and a custom coma corrector.  Yes, we understand the issues 
with the CTE of metals.  The future owner of the scope is 
investigating low-expansion structural materials, too.

The size was chosen to maximize light gathering.  F/3 was chosen (60" 
FL) to fit it in an existing building.  Thickness of 1.25" was chosen 
to reduce weight so that the scope could be placed on an existing 
mount.  These specs are really not negotiable.

I am in charge of making the primary, and F/3 parabola, and providing 
other construction ideas.

> Once you commit to Quartz for the optics, you also have to commit 
> to zero-expansion or compensating materials for the rest of the 
> instrument or you've wasting your investment. While shopping for 
> Quartz, it would be prudent to also shop for INVAR etc.

I think it is likely we will end up using Pyrex, and in that case the 
change in dimension of the telescope structure will make up for some 
of the change in focal length of a Pyrex mirror.

Again, thanks to all those for their on- or off-list posts regarding 
quartz, sources, Pyrex, etc.  They have been quite useful.

	Mike Lockwood

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