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What does XLT stand for?

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#26 jjack's

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Posted 21 December 2018 - 03:20 AM

I bought a C8 XLT in december 2009. The coating on the corrector was deep blue and very forgiving. Sometime it disapear from the view ! they seller tell me it was hafnium coated. It feel like this picture.

 

blue xlt.JPG

 

I broke accidentaly the corrector (i am a little ggofy smile.gif ). The scope is now repaired with a new corrector. it seems to work well but the coating is now green and apparently not so dark than before.

 

IMG_5241.JPG

 

Do you know if hafnium coating is now always applied ? this is a rare earth material and cost reduction can drive Celestron compagny to change their strategy...


Edited by jjack's, 21 December 2018 - 03:29 AM.


#27 Recretos

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Posted 28 December 2018 - 05:41 AM

Hafnium is still applied to Starbright XLT OTA's, and the corrector plate actually has a very good transmission of ~97%.

The thing we must not forget is that Celestron provided cumulative system reflectivity/transmission numbers. Meaning how much of the light that reaches the scope, actually ends up in your eyepiece (focal plane).

To do this we have to calculate the light loss at the corrector plate, primary mirror and secondary mirror.

For Celestron XLT, the corrector plate has 97% average transmission, primarry mirror is average at 94% rsflectivity and secondary the same at 94%. I say average because the reflectivity varies over the wavelength spectrum.

0.97×0.94×0.94=0.857

This means that roughly 86% of the light that gets into the aperture of the scope, pass the secondary obstruction, ends up in the eyepiece of your fancy Celestron SCT with XLT coatings. :)

If we take the most widespread newts, lets say synta, they have average reflectivity around 91-92%. If we say 92% for primary and secondary:

0.92×0.92=0.846

That means that your average asian newt has roughly 85% system reflectivity which is on average 1% less than the SCT with XLT. But 1% is pretty much nothing, so we can say that the same amount of passing light per aperture ends up in your eyepiece. The actual amount/volume of light is modified by central obstructions.

This is light % at the focal plane. From here to your eyes, it has to go through an eyepiece. And it is the quality of that eyepiece that usually has a bigger impact on the image, rather than a few % difference in reflectivity.

Edited by Recretos, 28 December 2018 - 05:56 AM.


#28 Recretos

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Posted 28 December 2018 - 07:06 AM

I give up. How does an uncoated corrector drop the throughput to 61%? A polished, uncoated optical surfaces only reflects 5%. Glass absorbs like 2% per inch of thickness, so for a 3/8 inch corrector that equals about 2/3%, and then the last surface of the corrector another 5%. So it should allow 89 1/3% through.

That was for total system transition.

Starbright XLT corrector plate has average 97,4% total (with coatings) average transmission across the spectrum.

The water white glass that is used for the new corrector plates, has 90.5% transmission without any coatings.
The old soda lime glass had 87% transmission uncoated. So 3.5% improvement was done by using better materials. The XLT corrector plate anti-reflective coatings then give further 7-8% improvement, to 97-98% total transmission across the spectrum.

Still, you wont be able to notice these improvements in new coatings when looking into the (same) eyepiece.

Edited by Recretos, 28 December 2018 - 07:08 AM.


#29 jjack's

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Posted 28 December 2018 - 06:33 PM

Ok. I am just curious why the same hafnium coating can give so differents  reflectives colors.



#30 Recretos

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Posted 29 December 2018 - 05:06 AM

The green/blue colour is likely not from hafnium alone, but from different overcoats.

#31 jjack's

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Posted 30 December 2018 - 05:08 AM

thanks Recretos...you are living into a wild country !




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