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Fluorite Refractors

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#201 25585

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Posted 06 September 2018 - 11:35 PM

Baader make a flourite field flattener https://www.astrosho...er-ffc-/p,10816 would that be useful in cancelling out CA visually? 


Edited by 25585, 06 September 2018 - 11:36 PM.


#202 junomike

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Posted 07 September 2018 - 07:04 AM

Baader make a flourite field flattener https://www.astrosho...er-ffc-/p,10816 would that be useful in cancelling out CA visually? 

I wouldn't expect to cancel an CA but it most likely won't add any either.



#203 Element79

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Posted 07 September 2018 - 07:30 AM

It's hard to reduce CA at the focuser end of the telescope.  It has been looked at and to a degree achieved by Roland back in the 1980s with what I believed he called a 'tri-color reducer' but it seems that the effect was limited.  To really reduce CA you need to attack it immediately so that the colors don't disperse as they travel down the optical tube and that means right there at the objective lens.


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#204 John Huntley

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Posted 07 September 2018 - 01:37 PM

It's hard to reduce CA at the focuser end of the telescope.  It has been looked at and to a degree achieved by Roland back in the 1980s with what I believed he called a 'tri-color reducer' but it seems that the effect was limited.  To really reduce CA you need to attack it immediately so that the colors don't disperse as they travel down the optical tube and that means right there at the objective lens.

The Chromacor did a pretty good job of correcting CA and SA, if matched and installed correctly.


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#205 Mike Clemens

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Posted 07 September 2018 - 01:52 PM

For whatever its worth, my experience has been that I have seen significantly more scatter with a TEC 140 compared to a TEC 180. Specifically I'm talking about the area immediately around Saturn. Is that due to the fluorite in the 180, the extra aperture, variation in scopes, some other factor?  I have no idea.  And I didnt have them side by side at the same time, and the EPs I used may have been different!  So yes it could have been caused by a lot of factors.  But anyway I thought I'd mention it. 

Sky humidity differences would be my guess.


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#206 Paul G

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Posted 08 September 2018 - 12:19 AM

Sky humidity differences would be my guess.

In my area the scatter caused by humidity usually swamps any small differences between scopes or eyepieces. Comparisons need to be side by side.


Edited by Paul G, 08 September 2018 - 12:20 AM.

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#207 mark8888

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Posted 08 September 2018 - 03:17 AM

In my area the scatter caused by humidity usually swamps any small differences between scopes or eyepieces. Comparisons need to be side by side.

 

Yeah.  But... I can say that with the ED I consistently saw scatter, and with the FL I consistently have not.


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#208 garret

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Posted 08 September 2018 - 09:34 AM

 

Yeah.  But... I can say that with the ED I consistently saw scatter, and with the FL I consistently have not.

Because fluorite is a single grown crystal which contain no air bubbles, ED and any other melted glass may have more or less tiny air bubbles inside depending on the manufacturing quality and also on what quality you order.

Here is what Nikon does to control air bubbles inside their own glass: https://www.imaging-...i-glass-factory

(scroll 25% down)

Tiny invisible air bubbles lower contrast 'considerable'.

 

Garrett van der Veen


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#209 Steve Allison

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Posted 08 September 2018 - 02:19 PM

Interesting link, Garrett. Thank you for posting it.

 

Steve



#210 Swanny

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Posted 08 September 2018 - 05:14 PM

If you are in the AP group, Roland can educate many of the people posting on here from one of his recent posts. Here is a brief capture from his post:

By the way there is no real difference in color correction between any of the high end ED glasses, FPL53, FPL55, FCD100, FK56. They are all capable of extremely high correction if you choose the correct matching glass, and you have at least one airspace as a variable (oil spaced lenses with all-spherical surfaces have almost no options - you basically have to aspherize one surface to get the best color correction).

Edited by Swanny, 08 September 2018 - 05:16 PM.

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#211 mark8888

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Posted 08 September 2018 - 05:46 PM

Because fluorite is a single grown crystal which contain no air bubbles, ED and any other melted glass may have more or less tiny air bubbles inside depending on the manufacturing quality and also on what quality you order.

Here is what Nikon does to control air bubbles inside their own glass: https://www.imaging-...i-glass-factory

(scroll 25% down)

Tiny invisible air bubbles lower contrast 'considerable'.

 

Garrett van der Veen

 

Thats right and thanks for the link.  It's interesting though how controversial the idea that FL noticeably lowers scatter is.



#212 Alan French

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Posted 08 September 2018 - 10:10 PM

The small microbubbles are only visible in the bright coherent light of a green laser, and the beam's brightness within the glass is faint compared to where it encounters surfaces. It's hard to imagine there is enough scatter within the glass to be noticeable, let along differentiate between a doublet or triplet with a single fluorite element from one with all glass elements. A lens using fluorite still has one or two glass elements. There are also far too many variables involved to point to a single one as causing a difference. 

 

The bubbles the Nikon article discusses appear to be larger bubbles, visible by eye, and certainly something consumers might object to. I've seen antique lenses with serious bubble problems. The article makes no mention of contrast. 

 

Clear skies, Alan



#213 SandyHouTex

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Posted 09 September 2018 - 12:21 PM

The small microbubbles are only visible in the bright coherent light of a green laser, and the beam's brightness within the glass is faint compared to where it encounters surfaces. It's hard to imagine there is enough scatter within the glass to be noticeable, let along differentiate between a doublet or triplet with a single fluorite element from one with all glass elements. A lens using fluorite still has one or two glass elements. There are also far too many variables involved to point to a single one as causing a difference. 

 

The bubbles the Nikon article discusses appear to be larger bubbles, visible by eye, and certainly something consumers might object to. I've seen antique lenses with serious bubble problems. The article makes no mention of contrast. 

 

Clear skies, Alan

Actually all glasses have micro-bubbles.  CaF2 being one solid crystal does not.  The differnce in scatter is noticable between my FS-152 and TSA-102.  Not a bunch, but easily visible.


Edited by SandyHouTex, 09 September 2018 - 12:22 PM.


#214 Alan French

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Posted 09 September 2018 - 01:27 PM

Actually all glasses have micro-bubbles.  CaF2 being one solid crystal does not.  The differnce in scatter is noticable between my FS-152 and TSA-102.  Not a bunch, but easily visible.

I know. I can see the lack of microbubbles in the center fluorite element of my triplet. 

 

I'm curious. How do you detect the difference in scatter, and how do you know it is because the FS-152 uses a fluorite element?

 

Clear skies, Alan


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#215 Alan French

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Posted 09 September 2018 - 03:44 PM

Actually all glasses have micro-bubbles.  CaF2 being one solid crystal does not.  The differnce in scatter is noticable between my FS-152 and TSA-102.  Not a bunch, but easily visible.

 

A bit from Roland, shared with his permission...

 

"I've made plenty of fluorite lenses and the truth is that internal scatter of any optical lens is only a tiny percentage compared to the surface scatter of the material. Fluorite has low internal scatter but high surface scatter due to the nature of the crystal planes. When a curved surface is polished into the fluorite crystal, the crystal planes come to the surface in concentric rings and this cannot be polished out. This results in surface scatter that can be easily measured.

 

Even Yuri at TEC admits that fluorite has more surface scatter and surface defects than any normal glass including ED glass when both are polished to a high level of smoothness. He gets around that by oil-spacing the fluorite between two cover glasses. A fluorite doublet will have two exposed fluorite surfaces and will have higher scatter than two normal glass surfaces. Even in normal glass the surface scatter is larger than the internal scatter. Just simply coating the glass increases the scatter yet again.

 

I have found also that many lenses that are used on the night sky develop a layer of stuff from the acid dew in the air which produces a thin film or haze that can only be removed with careful cleaning. Attached are images of a customer lens that I cleaned for him, and this was not the worst i have seen. First pix shows the lens before, second one shows one half cleaned compared to the other half not clean. I can just see someone trying to compare two lenses for scatter or contrast, and of course the newest (cleanest) one will always win.

 

How well a lens is corrected for spherical aberration will always be the #1 criteria for contrast.

 

Roland"

 

Lens before cleaning. Next post for lens after half cleaned. 

 

Dirty Lens.JPG


Edited by Alan French, 09 September 2018 - 03:47 PM.

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#216 Alan French

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Posted 09 September 2018 - 03:46 PM

Lens half cleaned.

 

Half Cleaned Lens.JPG


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#217 HARRISON SCOPES

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Posted 09 September 2018 - 04:41 PM

Excellent post

#218 daveCollins

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Posted 09 September 2018 - 05:36 PM

This points out the fact that humans are not capable of objective reporting. We are all biased and those biases come into most of what we say. We hear that Fluorite scopes have less scatter and those claims are supported by reports of "subjective" observations. When we hear many times that Fluorite has superior scatter characteristics compared to glass, and that gets repeated over and over, then when we get a Fluorite scope, we "see" less scatter.

 

I understand what Roland is saying, but it leaves me wondering how significant the scatter effect is. It may be measurable, but is it perceptible at the eyepiece? I think what Roland said would lead one to conclude that you will see more scatter at the eyepiece, but he did not say that. So it may be that the scatter is actually insignificant and one may not even see it.

 

I am awaiting a new Fluorite doublet myself, so there you have it, personal bias. I am excited about the scope and the comments above have no effect on my enthusiasm for this scope. But I will surely see significantly less scatter than with any of my other scopes and I'll be sure to report on that characteristic.


Edited by daveCollins, 09 September 2018 - 05:36 PM.

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#219 mark8888

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Posted 09 September 2018 - 06:21 PM

There are varying opinions on the issue...

https://groups.yahoo...s/messages/9802



#220 Steve Allison

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Posted 09 September 2018 - 06:32 PM

For me, a quality fluorite doublet is about as good as it gets for visual observing. This is based on my own experience, and not the often tedious technical bickering that goes on this forum. 

 

Steve


Edited by Steve Allison, 09 September 2018 - 06:46 PM.

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#221 peleuba

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Posted 09 September 2018 - 06:42 PM

 

A fluorite doublet will have two exposed fluorite surfaces and will have higher scatter than two normal glass surfaces. 

 

The sound you just heard is the TAK FS crowd pounding away at the keyboard Googling for something/anything that would dispute this.


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#222 Alan French

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Posted 09 September 2018 - 07:38 PM

Over the years I've enjoyed incredible planetary views in Newtonians, apochromats, Mak-Casses, and Mak-Newts. The biggest three factors, by far, have been aperture, high quality optics, and steady skies.

 

Clear skies, Alan


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#223 SandyHouTex

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Posted 09 September 2018 - 08:34 PM

I know. I can see the lack of microbubbles in the center fluorite element of my triplet. 

 

I'm curious. How do you detect the difference in scatter, and how do you know it is because the FS-152 uses a fluorite element?

 

Clear skies, Alan

So I took the same diagonal, the same eyepiece, and aimed them at the same target, on the same night using my Twilight II mount.  The scatter was less in the FS-152 than the TSA -102.


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#224 SandyHouTex

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Posted 09 September 2018 - 08:37 PM

This points out the fact that humans are not capable of objective reporting. We are all biased and those biases come into most of what we say. We hear that Fluorite scopes have less scatter and those claims are supported by reports of "subjective" observations. When we hear many times that Fluorite has superior scatter characteristics compared to glass, and that gets repeated over and over, then when we get a Fluorite scope, we "see" less scatter.

 

I understand what Roland is saying, but it leaves me wondering how significant the scatter effect is. It may be measurable, but is it perceptible at the eyepiece? I think what Roland said would lead one to conclude that you will see more scatter at the eyepiece, but he did not say that. So it may be that the scatter is actually insignificant and one may not even see it.

 

I am awaiting a new Fluorite doublet myself, so there you have it, personal bias. I am excited about the scope and the comments above have no effect on my enthusiasm for this scope. But I will surely see significantly less scatter than with any of my other scopes and I'll be sure to report on that characteristic.

See my post above.



#225 SandyHouTex

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Posted 09 September 2018 - 08:38 PM

The sound you just heard is the TAK FS crowd pounding away at the keyboard Googling for something/anything that would dispute this.

Nope.  I just would like to know how Alan determined that.




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