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Fluorite Refractors

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#251 SandyHouTex

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Posted 11 September 2018 - 12:30 PM

You have observed that there is more scatter in your FS-152 than in your TSA-102, so you have one piece of evidence saying the 152 has less scatter than the TSA-102. 

 

If you're claiming the reduced scatter is due to the use of fluorite in the FS-152, I'd like to know how you came to that conclusion, 

 

Clear skies, Alan

You switched it around.  I said in post 223 that there is less scatter in the FS-152 than the TSA-102.  And I already explained how I came to that conclusion in the same post.


Edited by SandyHouTex, 11 September 2018 - 12:31 PM.


#252 Alan French

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Posted 11 September 2018 - 01:04 PM

Yes, I wrote it backwards in the first half of the sentence. The second half had it right. I will correct the first part. 

 

Yes, you said how you concluded there was more scatter in the TSA-102. I am not debating that. 

 

Why do you think that is true? If you believe it's because of the lack of scatter (no microbubbles) in the fluorite, how did you determine that? (As opposed to the difference in having four air to glass surfaces versus six, for instance.) 

 

Clear skies, Alan


Edited by Alan French, 11 September 2018 - 01:12 PM.

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#253 jeremiah2229

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Posted 11 September 2018 - 01:10 PM

How does one detect scatter in the eyepiece?

 

 

Peace...



#254 Element79

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Posted 11 September 2018 - 01:49 PM

How does one detect scatter in the eyepiece?

 

 

Peace...

 

Personally I do the following: place a bright object such as the moon or a really bright star in the field of view and then slew the telescope so that this object is just out of the field of view and now judge how dark and scatter free the image is.  Others probably have different and probably better ways to do this...



#255 SandyHouTex

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Posted 11 September 2018 - 02:12 PM

Yes, I wrote it backwards in the first half of the sentence. The second half had it right. I will correct the first part. 

 

Yes, you said how you concluded there was more scatter in the TSA-102. I am not debating that. 

 

Why do you think that is true? If you believe it's because of the lack of scatter (no microbubbles) in the fluorite, how did you determine that? (As opposed to the difference in having four air to glass surfaces versus six, for instance.) 

 

Clear skies, Alan

A well applied anti-reflection coating shouldn't contribute to scatter nor should the number of elements.  Scatter is typically due, in a refractor, to poor polish or micro-bubbles in the material.  Being Taks, I'm virtually certain the they have a good polish.



#256 Alan French

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Posted 11 September 2018 - 03:03 PM

If there's no scatter, why does a green laser beam show as a bright dot where it enters or leaves a lens surface? Why do we cringe when we shine a bright flashlight at any optical surface? There's a good deal of empirical evidence that optics scatter light.

 

Although they increase light transmission coating do scatter light. Do a search on "scatter by optical coatings."

 

With time, optical surfaces accumulate dirt and grime, which increases scatter.

 

Suiter's "Star Testing" has a brief but interesting discussion of scattering.

 

I'll stick with the simple, obvious solution. Any difference in scattering is due to fewer surfaces to scatter light. High end optics are not going to have significant differences in polish, especially since both scopes are by Tak, and I think your claim that the microbubbles are significant is far fetched. You certainly haven't shown it is more than a belief on your part. 

 

Clear skies, Alan


Edited by Alan French, 11 September 2018 - 06:25 PM.

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#257 jeremiah2229

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Posted 11 September 2018 - 04:33 PM

Personally I do the following: place a bright object such as the moon or a really bright star in the field of view and then slew the telescope so that this object is just out of the field of view and now judge how dark and scatter free the image is.  Others probably have different and probably better ways to do this...

Thank you for sharing.

 

Can an eyepiece do the same thing? I mean cause scatter with this kind of test?

 

Thanks...



#258 Alan French

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Posted 11 September 2018 - 04:37 PM

Thank you for sharing.

 

Can an eyepiece do the same thing? I mean cause scatter with this kind of test?

 

Thanks...

 

Personally I do the following: place a bright object such as the moon or a really bright star in the field of view and then slew the telescope so that this object is just out of the field of view and now judge how dark and scatter free the image is.  Others probably have different and probably better ways to do this...

 

In some eyepieces I see a distinct glow in part of the field when the bright lunar limb is placed just past the field stop. I take that to be stray light and inadequate baffling somewhere - not scatter, which tends to be more diffuse and indistinct. 

 

Clear skies, Alan



#259 jeremiah2229

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Posted 11 September 2018 - 04:42 PM

In some eyepieces I see a distinct glow in part of the field when the bright lunar limb is placed just past the field stop. I take that to be stray light and inadequate baffling somewhere - not scatter, which tends to be more diffuse and indistinct. 

 

Clear skies, Alan

Thank you.

 

 

Peace...



#260 daquad

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Posted 11 September 2018 - 06:46 PM

In some eyepieces I see a distinct glow in part of the field when the bright lunar limb is placed just past the field stop. I take that to be stray light and inadequate baffling somewhere - not scatter, which tends to be more diffuse and indistinct. 

 

Clear skies, Alan

Perhaps, but the atmosphere also scatters light from the moon.  Could it be that some of the "stray light" we see is that caused by atmospheric scattering?  Just wondering.

 

Dom Q.



#261 SandyHouTex

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Posted 11 September 2018 - 06:53 PM

If there's no scatter, why does a green laser beam show as a bright dot where it enters or leaves a lens surface? Why do we cringe when we shine a bright flashlight at any optical surface? There's a good deal of empirical evidence that optics scatter light.

 

Although they increase light transmission coating do scatter light. Do a search on "scatter by optical coatings."

 

With time, optical surfaces accumulate dirt and grime, which increases scatter.

 

Suiter's "Star Testing" has a brief but interesting discussion of scattering.

 

I'll stick with the simple, obvious solution. Any difference in scattering is due to fewer surfaces to scatter light. High end optics are not going to have significant differences in polish, especially since both scopes are by Tak, and I think your claim that the microbubbles are significant is far fetched. You certainly haven't shown it is more than a belief on your part. 

 

Clear skies, Alan

The bright spot you see where the laser enters the fluorite lens is the reflection from the front polished surface.  All polished glass surfaces (I'm assuming fluorite is similar to glass in this respect) reflect about 4% of the incident light uncoated and 1% coated.  Plus lasers are really bright.



#262 Kent10

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Posted 11 September 2018 - 06:57 PM

I am not sure this is the correct thread for a long answer to scatter, and with no mention of Fluorite, but there are questions of scatter in eyepieces and I am very interested in this topic.  I test scatter a lot especially while viewing brighter double stars and sometimes the planets.  I have a lot of eyepieces and enjoy comparing them for differences.

 

I evaluate the area size of the scatter but also how bright the scatter is.

 

I believe it was Thomas Back who recommends using averted vision to judge the amount of scatter.  You can see more scatter by doing that, of course.

 

I see scatter differences among my eyepieces but I haven't really evaluated how that effects the view of a planet, for example.  How much smearing is done with the scatter?  Supposedly more scatter will smear details but I haven't tested this yet.  I imagine you would need very good seeing and conditions change constantly.

 

I am certainly not an expert on this and am still learning a lot so I don't want to appear that my results are "correct."  These are my impressions with my Tec 180FL refractor.  There are so many variables as well, of course, as in the humidity in the sky, and sometimes I wonder if I have slightly fogged the eyepiece so I have a Rocket Blower handy at times.  Even when it isn't cold I might be fogging the eyepiece because my warm eye is so close to the lens.  Sometimes I wear a hood and lately a mosquito net so that may cause fogging.

 

I have some favorite eyepieces that in my impression (again I don't want anyone to take this as correct) the scatter is obviously lower than other eyepieces.  I do have some floaters and that can impact the view as well so I consider that.

 

For double stars, just last night, I took my entire line of Pentax .965 orthos and viewed some doubles with each of them from lower power to higher power.  Really beautiful.  I see some scatter but not a lot.  Not last night, but I have compared to my Delos and Pentax XW and for me there is a noticeable increase in the scatter and I don't like the view of doubles nearly as much through these eyepieces.  I wonder at times if the different field of view is contributing to seeing differences.

 

But what is really puzzling for me, is that my line of TMB Supermonocentrics which are only 3 lenses appear to scatter more than some of my other low lens eyepieces.  I was very surprised by this initially and thought there would be less with only 3 well-coated lenses.  These are some of my favorite eyepieces for smaller groups of stars and I feel I see more stars with them.  I might not be seeing higher transmission because differences are small compared to other quality eyepieces but for me stars appear more pinpoint, very sharp or maybe the background is darker and so they stand out more or it could be the very small FOV so I concentrate on that area more.  But I just love them on smaller clusters and globulars that don't have any bright stars in them.

 

I was curious about the higher scatter of the SMCs and so I searched CN and found that others have noticed this as well so perhaps it is not my imagination.  I wondered and others wondered it there was more scatter because of the higher transmission which would show brighter stars and therefore more scatter.  But any higher transmission is probably not enough to demonstrate this so I am not sure why I see more.  It is not a lot of scatter, but more than some of my other favorites on brighter double stars.

 

I have some Zeiss monocentrics that could have older coatings, maybe single coated, I am not sure, but I also really love the double stars in these.  There is scatter but it is a dull scatter, in others words the scatter is not bright so stars are really beautiful with a dark sky background.  Very sharp too.

 

The Doctor 12.5mm is a wide angle eyepiece but it was really beautiful last night on many objects and double stars.  Scatter seemed low but I didn't do too much comparing.  I believe I did compare with my 12mm Pentax Ortho but there is a small difference in focal length. 

 

Another wide angle eyepiece that seems better than I might expect is the Nikon HW 17mm.  That is lower power though so that might contribute to lower scatter.  I really like it though.

 

Lots of fun!!


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#263 Alan French

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Posted 11 September 2018 - 09:48 PM

The bright spot you see where the laser enters the fluorite lens is the reflection from the front polished surface.  All polished glass surfaces (I'm assuming fluorite is similar to glass in this respect) reflect about 4% of the incident light uncoated and 1% coated.  Plus lasers are really bright.

Yes, lasers are bright and some of their light is reflected off the glass. It's bright enough that you can pick up the reflection, appearing as a spot on your hand or a piece of paper. But you can also see the light, appearing quite bright, entering the glass by looking from anywhere off axis, far away from the reflected beam. That's not a reflection, that's scatter. 

 

In spite of the laser's brightness, the beam's path through the glass is faint, far fainter than the scatter from the lens surfaces. 

 

Clear skies, Alan



#264 nicknacknock

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Posted 11 September 2018 - 11:08 PM

Garret,

 

They do take pre-orders if you email the Baader family. But next run will be available in about a year.



#265 Far Star

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Posted 12 September 2018 - 01:56 AM

Fluorite triplets 105/1000, 135/1080 and 175/1200 - similar to Zeiss APQ - coming:

 

https://astro-theke....aktur/produkte/   (Please scroll down.)



#266 X3782

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Posted 12 September 2018 - 06:46 AM

If there's no scatter, why does a green laser beam show as a bright dot where it enters or leaves a lens surface? Why do we cringe when we shine a bright flashlight at any optical surface? There's a good deal of empirical evidence that optics scatter light.

 

Although they increase light transmission coating do scatter light. Do a search on "scatter by optical coatings."

 

With time, optical surfaces accumulate dirt and grime, which increases scatter.

 

Suiter's "Star Testing" has a brief but interesting discussion of scattering.

 

I'll stick with the simple, obvious solution. Any difference in scattering is due to fewer surfaces to scatter light. High end optics are not going to have significant differences in polish, especially since both scopes are by Tak, and I think your claim that the microbubbles are significant is far fetched. You certainly haven't shown it is more than a belief on your part. 

 

Clear skies, Alan

 

Scatter on bulk inclusions result in the greatest amount of scattered light propagating in the forward direction at small scattering angles, these are the most harmful because they will run almost parallel following the same path as the intended light rays, and cause loss of contrast (the black sky will appear washed out) and a halo-like structure to form around the image.

 

These green-laser photos posted above I guess instead show "backscattered" light, those that left the lens at large scattering angles without doing harm. Obviously at this camera angle, the backscattering on the lens surfaces will look most intense compared to those in the bulk.

 

It depends on the wavelength, but surface scatter become insignificant when the surfaces are polished below 2-3 Angstrom rms roughness. No consumer optic on some amateur telescope costing only a few thousand dollars is likely to have that. The coating has to be high-grade too, but again nothing for amateur telescopes. Generally speaking if the "flouride glass" is of thickness 15-20 mm, then the scattering off the consumer-grade coated lenses surfaces and the scatter/absorption in the bulk will be of similar magnitude. To put it another way, when that threshold is past, glass/coating manufacturers say, "most amateur people will not notice this, so we let it pass".


Edited by X3782, 12 September 2018 - 07:01 AM.


#267 X3782

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Posted 12 September 2018 - 06:55 AM

I am not sure this is the correct thread for a long answer to scatter, and with no mention of Fluorite, but there are questions of scatter in eyepieces and I am very interested in this topic.  I test scatter a lot especially while viewing brighter double stars and sometimes the planets.  I have a lot of eyepieces and enjoy comparing them for differences.

 

I evaluate the area size of the scatter but also how bright the scatter is.

 

I believe it was Thomas Back who recommends using averted vision to judge the amount of scatter.  You can see more scatter by doing that, of course.

 

I see scatter differences among my eyepieces but I haven't really evaluated how that effects the view of a planet, for example.  How much smearing is done with the scatter?  Supposedly more scatter will smear details but I haven't tested this yet.  I imagine you would need very good seeing and conditions change constantly.

 

I am certainly not an expert on this and am still learning a lot so I don't want to appear that my results are "correct."  These are my impressions with my Tec 180FL refractor.  There are so many variables as well, of course, as in the humidity in the sky, and sometimes I wonder if I have slightly fogged the eyepiece so I have a Rocket Blower handy at times.  Even when it isn't cold I might be fogging the eyepiece because my warm eye is so close to the lens.  Sometimes I wear a hood and lately a mosquito net so that may cause fogging.

 

I have some favorite eyepieces that in my impression (again I don't want anyone to take this as correct) the scatter is obviously lower than other eyepieces.  I do have some floaters and that can impact the view as well so I consider that.

 

For double stars, just last night, I took my entire line of Pentax .965 orthos and viewed some doubles with each of them from lower power to higher power.  Really beautiful.  I see some scatter but not a lot.  Not last night, but I have compared to my Delos and Pentax XW and for me there is a noticeable increase in the scatter and I don't like the view of doubles nearly as much through these eyepieces.  I wonder at times if the different field of view is contributing to seeing differences.

 

But what is really puzzling for me, is that my line of TMB Supermonocentrics which are only 3 lenses appear to scatter more than some of my other low lens eyepieces.  I was very surprised by this initially and thought there would be less with only 3 well-coated lenses.  These are some of my favorite eyepieces for smaller groups of stars and I feel I see more stars with them.  I might not be seeing higher transmission because differences are small compared to other quality eyepieces but for me stars appear more pinpoint, very sharp or maybe the background is darker and so they stand out more or it could be the very small FOV so I concentrate on that area more.  But I just love them on smaller clusters and globulars that don't have any bright stars in them.

 

I was curious about the higher scatter of the SMCs and so I searched CN and found that others have noticed this as well so perhaps it is not my imagination.  I wondered and others wondered it there was more scatter because of the higher transmission which would show brighter stars and therefore more scatter.  But any higher transmission is probably not enough to demonstrate this so I am not sure why I see more.  It is not a lot of scatter, but more than some of my other favorites on brighter double stars.

 

I have some Zeiss monocentrics that could have older coatings, maybe single coated, I am not sure, but I also really love the double stars in these.  There is scatter but it is a dull scatter, in others words the scatter is not bright so stars are really beautiful with a dark sky background.  Very sharp too.

 

The Doctor 12.5mm is a wide angle eyepiece but it was really beautiful last night on many objects and double stars.  Scatter seemed low but I didn't do too much comparing.  I believe I did compare with my 12mm Pentax Ortho but there is a small difference in focal length. 

 

Another wide angle eyepiece that seems better than I might expect is the Nikon HW 17mm.  That is lower power though so that might contribute to lower scatter.  I really like it though.

 

Lots of fun!!

 

Scattering on AR coatings primarily occur because of microroughness of the coating. This often follows the contours of the glass surface, i.e., the rougher the substrate, the rougher the coated surface. The coating "exaggerates" or "magnifies" the roughness of the original bare glass surface, this is why people spend so much effort trying to get the glass surface smooth prior to coating.

 

Given the same coating technology, the more coating layers there are, the more the roughness is magnified. A single-layer coating will generally be more smooth than a coating with 50 layers. So there is a design trade-off: either you add a lot of layers and increase transmission but also increase scatter, or you opt for a simpler coating structure and increase reflections but decrease scatter.

 

Or, you go for a more expensive, new coating technology that creates dense "hard" coatings of uniform thickness. Latest Pentax and other companies' expensive lens products have this on some of their surfaces. This is under the constraint that, the lens and coating has to make a profit at an end user price of a few thousand dollars in a low-volume market with potentially lots of excess inventory. Therefore the figure per lens surface cannot be more than a few hundred dollars for objectives, and a few tens dollars for eyepiece lenses.


Edited by X3782, 12 September 2018 - 07:26 AM.


#268 Fomalhaut

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Posted 12 September 2018 - 07:15 AM

The bright spot you see where the laser enters the fluorite lens is the reflection from the front polished surface.  All polished glass surfaces (I'm assuming fluorite is similar to glass in this respect) reflect about 4% of the incident light uncoated and 1% coated.  Plus lasers are really bright.

According to "Telescope Optics" (Rutten/vanVenrooji) reflectivity of uncoated Fluorite is a function of the refractive index and for green light is only 3.19%  per surface (yellow => 3.18%). 

Optical glasses have higher reflectivities (e.g. BaK4 has 4.94% per surface).


Edited by Fomalhaut, 12 September 2018 - 07:15 AM.


#269 Element79

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Posted 12 September 2018 - 07:18 AM

Fluorite triplets 105/1000, 135/1080 and 175/1200 - similar to Zeiss APQ - coming:

 

https://astro-theke....aktur/produkte/   (Please scroll down.)

 

Those are indeed interesting offerings!  I have never heard of the manufacturer and they haven't come up with a price yet but that 105/1000 really looks interesting to me.  A fluorite oiled triplet at f/9.5 should perform as well as optical theory allows!



#270 X3782

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Posted 12 September 2018 - 07:20 AM

For normal incidence it is only a variant of Fresnel equations,

R=(n1-n2/n1+n2)^2,

wherein n1 is the refractive index of e.g. air or oil, n2 of glass or CaF2.

 

So the smaller the difference in refractive index between the two materials,  the smaller the reflection.


Edited by X3782, 12 September 2018 - 07:21 AM.


#271 STE411

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Posted 12 September 2018 - 07:57 AM

Fluorite magic.

In fluorite the speed of light approaches infinity, and that’s why you don’t see the laser beam.

It’s going too fast.

steve



#272 Element79

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Posted 12 September 2018 - 08:08 AM

Fluorite magic.

In fluorite the speed of light approaches infinity, and that’s why you don’t see the laser beam.

It’s going too fast.

steve

 

And here I thought that the fluorite was affecting the flux of the tachyon field...


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#273 SandyHouTex

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Posted 12 September 2018 - 10:33 AM

Yes, lasers are bright and some of their light is reflected off the glass. It's bright enough that you can pick up the reflection, appearing as a spot on your hand or a piece of paper. But you can also see the light, appearing quite bright, entering the glass by looking from anywhere off axis, far away from the reflected beam. That's not a reflection, that's scatter. 

 

In spite of the laser's brightness, the beam's path through the glass is faint, far fainter than the scatter from the lens surfaces. 

 

Clear skies, Alan

On my FS-152 there is no path through the element with a green laser.



#274 SandyHouTex

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Posted 12 September 2018 - 10:34 AM

According to "Telescope Optics" (Rutten/vanVenrooji) reflectivity of uncoated Fluorite is a function of the refractive index and for green light is only 3.19%  per surface (yellow => 3.18%). 

Optical glasses have higher reflectivities (e.g. BaK4 has 4.94% per surface).

That's why I said, "...about 4 percent...".



#275 Far Star

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Posted 12 September 2018 - 11:19 AM

Those are indeed interesting offerings!  I have never heard of the manufacturer and they haven't come up with a price yet but that 105/1000 really looks interesting to me.  A fluorite oiled triplet at f/9.5 should perform as well as optical theory allows!

The optical design is from Dr. Juergen Pudenz (designer of the Zeiss APQ lens). The lenses are manufactured by former Zeiss master optician Peter Grosse, who also had made Zeiss APQ lenses.


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