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Fluorite vs ED

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#76 JJK

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Posted 11 August 2015 - 11:02 AM


We have three pages and no one has mentioned coatings. Roland seems to think them important enough to use what he considers the best. The ones he specifies are expensive and are claimed to have a very high transmissibility.


Roland does not think of his big refractors as telescopes. He thinks of them as astrographs, and based on comments to me once, I think he might believe that using one of his scopes visually is a waste. For planetary performance, he pointed me to a 10" f/6 reflector as being a better choice than a 155 EDF.

The purpose of the coatings is I believe to allow greater transmission into the near infra red and ultraviolet. This is what rings his bell. A quote:

"The acid test was, of course, imaging and for that I added the dedicated field flattener and my STL1100 CCD camera. The results were everything I had hoped for. All the colors came to the same exact focus - no focus tweek required for any of them. Stars showed similar size and full width half max resolution with all 8 filter positions (RGB, L, H-alpha, OII and SII)..."

For visual use, you rea​lly don't need to go very wide. This is why the red Airy Disk is often allowed to be twice the diameter of the green Airy Disk in many designs. When it is kept to this size, the human eye simply can't see it. But the camera misses nothing. And for many photos, having high transmission at these more exotic wavelengths increases the ability to image large H-alpha features.

For visual use, coatings simply don't need to be so wide. For an astrograph though, it starts to become more important.

The AP180 f/9 says "hi".

#77 Sky Muse

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Posted 11 August 2015 - 11:44 AM

In that event, would that the upcoming James Webb telescope might be a refractor instead.



#78 Daniel Mounsey

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Posted 11 August 2015 - 12:26 PM

Been looking at TAK FSQ's and trying to decide if I want a Fluorite version or the new ED version. I have read advantages of both but I found an article on Takahashi's site and this is what it said.:

 

If you seek optical and mechanical perfection in a truly modern telescope, look no further. Takahashi is the answer. Other telescope manufacturers may claim that ED (Extra-low Dispersion) glass is the equivalent of fluorite or that their older designs will work as well. Unfortunately, they are not being honest. While ED and fluoro-crown lenses can achieve Abbe-coefficients approaching fluorite, they tend to absorb more light in the visible spectrum. This means that fluorite yields a brighter, higher contrast image. Leica, Zeiss, and Kowa have all gone to fluorite in their spotting scopes and telescopes to achieve the maximum performance levels their customers demand. Most of them previously used ED glass. Obviously, they know the difference between fluorite and ED. You will too. Takahashi pioneered the use of fluorite in astronomical telescopes and they are still the leader. Accept no substitute. Get the fluorite advantage. Get Takahashi!

 

Now I have heard that part of the reason they stopped using Fluorite is due to environmental reasons, but that can't be true, being TAK still produces Fluorite scopes. I am just curious if they actually found a better way with ED glass to where it truly is a better color correct glass. Not trying to bash the new Q's I am just wondering why if Fluorite was such a great glass and to read from TAK itself, why they would switch. Seems they are now not being "Honest". 

 

Never really found a good discussion about this online.

 

So let the debate begin. :grin:

 

 

Both of these will perform better than most imagers even know how to process.



#79 Edd Weninger

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Posted 11 August 2015 - 02:17 PM

Questar offers a "Broadband" coating vs a "Standard" (less expensive) coating. 

 

Making the assumption the "Broadband" means a wider spectral transmission, and although I've never had an opportunity to compare them, I've read the difference is clearly noticeable visually.  I think this requires about a 10% difference to be perceived visually?  This might be analogous to A-P wider spectral acceptance coatings. 

 

As far as I know, very little data is available about coatings.  So, how do we factor that into triplet/doublet comparisons?  What does Takahashi use for fluorite?  The other Chinese manufacturers?

 

If we quibble 0.5% delta glass transmission, coatings are worth considering. 

 

 

 

 

We have three pages and no one has mentioned coatings.  Roland seems to think them important enough to use what he considers the best.  The ones he specifies are expensive and are claimed to have a very high transmissibility.  

 

Roland does not think of his big refractors as telescopes.  He thinks of them as astrographs, and based on comments to me once, I think he might believe that using one of his scopes visually is a waste.   For planetary performance, he pointed me to a 10" f/6 reflector as being a better choice than a 155 EDF.

 

The purpose of the coatings is I believe to allow greater transmission into the near infra red and ultraviolet.  This is what rings his bell.  A quote:

 

"The acid test was, of course, imaging and for that I added the dedicated field flattener and my STL1100 CCD camera. The results were everything I had hoped for. All the colors came to the same exact focus - no focus tweek required for any of them. Stars showed similar size and full width half max resolution with all 8 filter positions (RGB, L, H-alpha, OII and SII)..."

 

For visual use, you rea​lly don't need to go very wide.  This is why the red Airy Disk is often allowed to be twice the diameter of the green Airy Disk in many designs.   When it is kept to this size, the human eye simply can't see it.   But the camera misses nothing.    And for many photos, having high transmission at these more exotic wavelengths increases the ability to image large H-alpha features.

 

For visual use, coatings simply don't need to be so wide.   For an astrograph though, it starts to become more important.

 


Edited by Edd Weninger, 11 August 2015 - 02:18 PM.


#80 GJJim

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Posted 11 August 2015 - 08:00 PM

 

Questar offers a "Broadband" coating vs a "Standard" (less expensive) coating. 

 

Making the assumption the "Broadband" means a wider spectral transmission, and although I've never had an opportunity to compare them, I've read the difference is clearly noticeable visually.  I think this requires about a 10% difference to be perceived visually?  This might be analogous to A-P wider spectral acceptance coatings. 

 

As far as I know, very little data is available about coatings.  So, how do we factor that into triplet/doublet comparisons?  What does Takahashi use for fluorite?  The other Chinese manufacturers?

 

If we quibble 0.5% delta glass transmission, coatings are worth considering. 

 

The glass properties, not the AR coatings, sets the passband for optical transmission. Coatings reduce the losses (~ 4% for uncoated) at each air/glass interface. Single-layer coatings are effective at only one wavelength and at normal incidence. Broadband coatings are made of multiple layers and are effective over a wider range of wavelengths.



#81 edif300

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Posted 12 August 2015 - 03:21 AM

Parfocal filters are made basically over same substratum, has same refractive index (if I remember well RI of 1.46 and 1.53). Broadband or narrowband and the bandpass are determined basically by the coatings.

 

A coating can determine the bandpass. In a good design it's a must do. Who needs a scope with 0,95 transmission in 400 nm if the optical system can't focus it at correct point?  using a good broadband blue filter with high blue QE ccd will get a bloating stars.


Edited by edif300, 12 August 2015 - 03:22 AM.


#82 GJJim

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Posted 12 August 2015 - 07:57 AM

Parfocal filters are made basically over same substratum, has same refractive index (if I remember well RI of 1.46 and 1.53). Broadband or narrowband and the bandpass are determined basically by the coatings.

 

A coating can determine the bandpass. In a good design it's a must do. Who needs a scope with 0,95 transmission in 400 nm if the optical system can't focus it at correct point?  using a good broadband blue filter with high blue QE ccd will get a bloating stars.

The implication in an earlier post was that superior coatings allowed the lens to operate over a broader range of wavelengths. Coatings can limit the passband (as in filters) but they cannot increase it beyond what the glasses in the lens will allow. AR coatings, no matter how exotic, will never broaden the passband of a lens.



#83 edif300

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Posted 12 August 2015 - 10:15 AM

It's a clever design to take in mind a broadband coating as a filter in order to adecuate it to the sistem's correction.



#84 Edd Weninger

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Posted 12 August 2015 - 11:40 AM

I am not implying that coatings can add to the spectral transmission of the glasses.  What I am suggesting is that, in this discussion about fluorite and ED glass, the difference in the light transmission of both glasses is very small.  These numbers are available from the glass manufacturers. 

 

There are some suggestions that a scope made with one type is better than the other type because of this difference.

 

I suggest that the type of coatings used in the finished lens have a much greater effect on through-put than the glass itself.  Hence, the type of glass used cannot predict the light transmission of a lens if the coating is not considered.

 

There is very little data from scope manufacturers about coatings used.

 

(not to mention the fineness of the final polish)

 

Cheers,

 

 

 

 

Questar offers a "Broadband" coating vs a "Standard" (less expensive) coating. 

 

Making the assumption the "Broadband" means a wider spectral transmission, and although I've never had an opportunity to compare them, I've read the difference is clearly noticeable visually.  I think this requires about a 10% difference to be perceived visually?  This might be analogous to A-P wider spectral acceptance coatings. 

 

As far as I know, very little data is available about coatings.  So, how do we factor that into triplet/doublet comparisons?  What does Takahashi use for fluorite?  The other Chinese manufacturers?

 

If we quibble 0.5% delta glass transmission, coatings are worth considering. 

 

The glass properties, not the AR coatings, sets the passband for optical transmission. Coatings reduce the losses (~ 4% for uncoated) at each air/glass interface. Single-layer coatings are effective at only one wavelength and at normal incidence. Broadband coatings are made of multiple layers and are effective over a wider range of wavelengths.

 




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