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Secondary confusion (con't)

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#1 Bryan Greer

Bryan Greer


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Posted 03 April 2006 - 11:57 AM

(Note: this thread is continued from one that started in the ATM forum.)

> Since you were kind enough to post a reponse, can you
> verify my understanding that your pyrex and quartz flats
> will produce pretty much the same image in the
> eyepiece?

Hi Randy,

I haven't been able to see a difference. I have swapped mirrors in my 6" f/10 (3/4" secondary), and my 8" f/5 (1.83"). I've never been able to see a difference, though it's not a particularly scientific comparison. There is always a gap in time between comparisons while you swap the mirrors. I have not done a careful startest comparison on my artificial star, either.

The reason the ULS parts were introduced is because in the mid-1990s I had a growing portion of the secondary mirror business going to non-astronomy customers for optical bench set-ups, or specialized optical equipment. (I suspect it was because our parts sell for less than the big optical companies.) Many of these customers needed certs with the flats. For a couple of years, I just had Pyrex parts individually certified on an as-needed basis, but that wasn't particularly economical. I also could not get interferograms for all of the Pyrex parts, bake that into the price, and still hope to sell many at the higher price. So, the ULS parts were born principally to satisfy the people who needed certification.

It may not be obvious, but fused silica doesn't have any particular advantage to the telescope user in the field. Both Pyrex and fused silica secondaries work just fine in a telescope as far as thermal stability is concerned. However, it does mean something to the person actually making the parts. With fused silica, what you see on the block during polishing is essentially what you have after deblocking. With Pyrex, they change a bit after they come to room temperature. Thus, you can typically get a little better surface accuracy with fused silica. Since the ULS parts are all certified, and nothing under 1/10th is sold, using fused silica made sense (i.e., it's a more expensive raw material, but we have fewer rejects). Also, most laser applications require fused silica since it polishes more smoothly, and thus scatters the laser beam less.

What I tell people is if you just need a flat that will perform well (i.e., not show astigmatism in the startest), use our Pyrex part. However, if you have invested $1,000+ in a custom made primary (which undoubtedly has certs), it is foolish to save $50 and not get a certified secondary.

Bryan Greer

#2 JoesDad


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Posted 03 April 2006 - 01:14 PM

Thanks for all your input, Bryan. I really appreciate the info!

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