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C8 corrector plate glass

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#1 terraclarke

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Posted 01 November 2018 - 10:17 AM

This question was asked (by another CN’er, not me) over in the ‘Cats’ forum nearly two months ago and dispite much beating around the bush and the tangential pontificating, as you might guess,  it was never satisfactorily answered. However, I am interested in the answer so I thought I would ask our Classic SCT experts here. Just when did Celestron stop using soda lime glass and begin using water white glass for their corrector plates?


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#2 starman876

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Posted 01 November 2018 - 10:31 AM

good question.  wish I had an answer.


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#3 DAVIDG

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Posted 01 November 2018 - 10:35 AM

 Sorry I don't have the exact  timing but do know they switched back and fourth. My tri-color C8's ( 2 of them ) and tri color C5 have water white correctors. Having worked on many  of the more common  orange and black OTAs  they all have had the green plate glass correctors.  My guess is that the switch back to water white correctors happens when the fabrication of the optics switched  to China.

   What is more important to me is what was the quality of the glass used ? True float glass can be made optically fairly flat and uniform. So if one takes the time to test it, areas can be located that are very good. The result is that you don't need to figure the "flat" side of the corrector and the over all figure of the corrector can be very good as well.  

 

                  - Dave 


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#4 clamchip

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Posted 01 November 2018 - 10:59 AM

It was about the same time the Starbright coatings became Starbright XLT coatings, early 2000's

https://www.celestro...ptical-coatings

 

Robert


Edited by clamchip, 01 November 2018 - 11:02 AM.

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#5 terraclarke

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Posted 01 November 2018 - 12:07 PM

 Sorry I don't have the exact  timing but do know they switched back and fourth. My tri-color C8's ( 2 of them ) and tri color C5 have water white correctors. Having worked on many  of the more common  orange and black OTAs  they all have had the green plate glass correctors.  My guess is that the switch back to water white correctors happens when the fabrication of the optics switched  to China.

   What is more important to me is what was the quality of the glass used ? True float glass can be made optically fairly flat and uniform. So if one takes the time to test it, areas can be located that are very good. The result is that you don't need to figure the "flat" side of the corrector and the over all figure of the corrector can be very good as well.  

 

                  - Dave 

Dave, is the optical transmission properties of the two glasses really very substantial? I have ‘heard’ that it is. 

 

This is an unverified quote to that effect, taken from the ‘other’ forum:

 

“From Pilkington's data their ordinary 'green' float absorbs around 1% light per mm whilst their 'white' float absorbs approx.1% per 10mm glass thickness, from memory. So roughly the same improvement in a thin corrector as gained by AR coatings per surface but a small contribution compared to the accumulated benefits of modern coatings on all the surfaces in the optical train.”


Edited by terraclarke, 01 November 2018 - 12:13 PM.


#6 rolo

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Posted 01 November 2018 - 12:36 PM

Terra, My 72 C8 has clear water white glass. As far as I know, The first production C8  to use it was the Ultima 2000. 

 

Celestron States that high transmission Water White glass transmits about 90.5% without anti-reflective coatings which results in 3.5% better transmission than uncoated Soda Lime glass. When Water White glass is used in conjunction with StarBright XLT anti-reflective coatings, the average transmission reaches a high 97.4% – an 8% improvement.

Attached Thumbnails

  • water white.jpg

Edited by rolo, 01 November 2018 - 12:40 PM.

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#7 DAVIDG

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Posted 01 November 2018 - 12:55 PM

Dave, is the optical transmission properties of the two glasses really very substantial? I have ‘heard’ that it is. 

 

This is an unverified quote to that effect, taken from the ‘other’ forum:

 

“From Pilkington's data their ordinary 'green' float absorbs around 1% light per mm whilst their 'white' float absorbs approx.1% per 10mm glass thickness, from memory. So roughly the same improvement in a thin corrector as gained by AR coatings per surface but a small contribution compared to the accumulated benefits of modern coatings on all the surfaces in the optical train.”

 Terra,

   I was just going to comment about the small transmission difference.  I would need to see the exact transmission  spectra because one glass might have slightly better transmission in the near IR then the other but since visual observation aren't done there it won't make any difference. As stated what  is going to make the biggest difference in transmission is  the antireflective coatings that are used. 

  As I said before what would make the biggest difference is surface quality of the glass. Since the "flat" surface of the corrector is the one that is pulled up against the Master Block,  defects in the back are  that transmitted to the front  The front is ground and polished flat and when released it has the Schmidt curve profile. So any of the defects in the back turn into the negative profile on the front that is superimposed onto the Schmidt curve. The problem is they  don't line up with the defects in the back so they  don't  fully cancel. This is the  main issue I see in  Criterion correctors. So if you start off with a  flat surfaces or ones that has very small defects, you can make correctors that will work very well without the need to first grind and  polish the " flat surface" optically smooth. 

      "Water white" glass is usually better quality then green plate glass so in theory it should make for a better corrector if the surface(s) are good enough optically. My guess is that Celestron found a supplier of glass that has  the needed specs and with  the optics being fabricated in China, the supplier of the better glass is most likely less expensive. 

   Anytime I see a manufacture change parts in a product, 95% of the time it for a cost savings. 

   

                       - Dave 


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#8 davidc135

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Posted 01 November 2018 - 01:45 PM

PA110498b.jpg An example of glass that Criterion shouldn't have chosen for their DX-8 correctors. Not sure if the stripes are in the glass or on the surface. I could check with interference. The up turned zone near the edge is out by around 11/2 waves.

 

I remember talk of 'crystallex' being available back in the 80s or earlier in the US. These days both sorts are very cheap. I bought the minimum 50cms square sheet of 6mm white float for 24 pounds- enough for 4x 10in correctors. So for thinner glass for 8in plates and buying in quantity- a couple of dollars each?  David


Edited by davidc135, 01 November 2018 - 01:56 PM.


#9 bremms

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Posted 01 November 2018 - 02:47 PM

Dave, is the optical transmission properties of the two glasses really very substantial? I have ‘heard’ that it is. 

 

This is an unverified quote to that effect, taken from the ‘other’ forum:

 

“From Pilkington's data their ordinary 'green' float absorbs around 1% light per mm whilst their 'white' float absorbs approx.1% per 10mm glass thickness, from memory. So roughly the same improvement in a thin corrector as gained by AR coatings per surface but a small contribution compared to the accumulated benefits of modern coatings on all the surfaces in the optical train.”

Thick green plate about 8-10mm has noticeable absorption.


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#10 Gil V

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Posted 01 November 2018 - 05:25 PM

Wanted to chime in - nice photo btw.

I think the combo of poor material/one side polish technique is the root cause of the Dynamax poor reputation.

Doesn’t matter, worked there six years, I’ll always love the scope.

Just wonder if the experts out here might agree...

Edited by Gil V, 01 November 2018 - 05:25 PM.

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#11 DAVIDG

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Posted 02 November 2018 - 10:33 AM

Being a  nerdy scientist I like to make conclusion on  data vs guessing or taking at faith what has been published in ads. My good friend Jim Daley who is the author of the Schupmann Telescope book  and Master optician who  ran MIT's optical lab at Lincoln Labs gave me one of the Schmidt corrector blanks that they used in the lab to fabricate the correctors. Jim told me he made many corrector over his career using these type of blanks.  

  So I brought the corrector blank into my lab  this morning and measured it's transmission. Here are a couple of pictures. One is the blank itself on white paper. You can see the slight green tint since it looks to be made from soda lime float glass.The corrector  in place being measured and the results showing 93.9% transmission over the visible spectra. The corrector is not AR coated, just bare glass.   AR coating would bring the transmission to around 97%. While water white glass would be better in transmission, it is not going to make a huge difference since the corrector are not very thick.  The coatings on the two surfaces of the mirror is were the major loss of light is happening. 

  So while Celestron may say that water white glass is better in transmission which is true, it not a major factor in the total transmission but more of  a selling  point. 

  As I said before what would be more important is the quality of the "flat" surface since this would result is better optically corrected, correctors.

   

 

                          - Dave 

 

schmidtcorrectorblank.jpg

measuringsctcorrectorblank.jpg

correcttransmission results.jpg


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#12 jhayes_tucson

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Posted 02 November 2018 - 07:53 PM

My understanding is that Celestron has used White Water float glass for a long time.  The corrector plates aren't very thick so I don't think that the difference in transmission between White Water glass and something like BK-7 is very significant (BK-7 should be better.)  The biggest difference will be cost.  White Water float glass is dirt cheap compared to BK-7, which as an optical glass, isn't very pricy.  In my view, the one and only reason to use White Water float glass is to get the cost down--and that's what Celestron does well.  The next big difference will be in bubble content and striations.  I believe that Celestron has worked with the manufacturer to minimize striations and pull lines and I've seen no evidence of either in any of the C14 Edge systems that I've interferometrically tested.  That's not necessarily true on some older systems and I understand that this may have been an issue with some older production batches.  Pull lines and striations will cause differential light scatter around bright stars and it will be very noticeable with long exposure imaging.  This is a problem that's been solved for quite a while so it won't occur on current scopes.  On the other hand, the corrector on my scope is loaded with bubbles.  They are fairly small and even though many break the surfaces, the good news is that they don't have a very big effect on imaging performance.  I personally would pay more for a system with a Schott BK-7 corrector; but, Celestron will never produce such a system so that's a pipe dream.  Celestron works hard to maximize value, which requires getting the performance "good enough"at a price point that will appeal to a wide market.  Ultimately, if you want something better, the options include PlaneWave, AG Optical, and many others; but, you'll pay a lot more.

 

John


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