132 replies to this topic

[bobhen]

It’s hard to pin down the entire astronomical community with regard to interest. There’s solar, planetary, double stars, carbon stars, nebula, globular and open clusters, Astro astronomy, EAA, and a whole lot more. I would agree that many if not most of us are generalists, and I certainly am. There still remains the fact that Vixen and others null in other than green, however, and Sasa’s bet as to why is as good as any. What is clear is that optical engineers do make choices to null in a variety of wavelengths and likely for a multitude of reasons.

 

I, personally, don’t advocate any particular wavelength to null to so long as correction is reasonably good across the entirety of the visual spectrum, and so far I haven’t encountered an “apo” scope that wasn’t. So I’m fine with letting the designers chose what they feel is best for the scopes that they’ve designing, and I weigh the available information thoroughly before making a decision to buy one that best serves my particular interests, as I imagine most of us do.

You can't have reasonably good correction across the spectrum by nulling in red. You can only have reasonably good correction across the spectrum by nulling in green.

 

Any refractor maker marketing refractors to a public that is expecting a "general use" refractor should null in green. If they do not, because it's easier, cheaper, etc. then they are fooling the public. A refractor maker that makes refractors for specialized purposes should market those refractors as such. Back in the early 80s, Astro-Physics did just that.

 

Bob

[Jon Isaacs]

You can't have reasonably good correction across the spectrum by nulling in red. You can only have reasonably good correction across the spectrum by nulling in green.

 

Any refractor maker marketing refractors to a public that is expecting a "general use" refractor should null in green. If they do not, because it's easier, cheaper, etc. then they are fooling the public. A refractor maker that makes refractors for specialized purposes should market those refractors as such. Back in the early 80s, Astro-Physics did just that.

 

Bob

 

 

Paul's DPAC test of a SVX180T is a good example of what happens when a scope is nulled in red. It's the very good in red, OK in green and not so good in blue.

 

Jon

[Polyphemos]

Bob and Jon, thanks for your thoughts.

 

Here is the spot diagram for the Takahashi FC-100DC, which is presently Takahashi’s best corrected 100mm refractor. It would appear the FC-100DC is nulled in orange-red:

 

 

This is the spot diagram for the Takahashi FOA-60, which Takahashi says is the best corrected scope they’ve ever made. Here it appears Takahashi has split the difference between between green and red.

 

 

Both Vixen and Takahashi, at least in the scopes discussed, have strayed from green more or less in the direction of red. Not red, but decidedly not green either.

 

As with many things perhaps the true answer lies in the middle. I’ve not read any criticism of any of these Takahashi and Vixen scopes, all are well corrected across the visual spectrum, and none are nulled in green.

[bobhen]

I don't read Japanese but you don't need multiple lines to show at what part of the spectrum the lens is best corrected. I don't believe that chart is specific enough. 

 

Notice the Astro-Physics Strehl chart: one line with the wave length and Strehl easily called out. One can see in what part of the spectrum the lens is best corrected. And you really don't need a chart at all, as on the left the wavelength and associated Strehl are numerically indicated.  A spot diagram will not show that.

 

The second chart is from Agema optics. That chart shows "7 different telescopes" including some from Takahashi. ALL are nulled in the green. Again, one line for each scope.

 

Bob

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Edited by bobhen, 23 July 2024 - 11:03 AM.

[Polyphemos]

I don't read Japanese but you don't need multiple lines to show at what part of the spectrum the lens is best corrected. I don't believe that chart is specific enough. 

 

Notice the Astro-Physics Strehl chart: one line with the wave length and Strehl easily called out. One can see in what part of the spectrum the lens is best corrected. And you really don't need a chart at all, as on the left the wavelength and associated Strehl are numerically indicated.  A spot diagram will not show that.

 

The second chart is from Agema optics. That chart shows "7 different telescopes" including some from Takahashi. ALL are nulled in the green. Again, one line for each scope.

 

Bob

Good charts, Bob.

[peleuba]

 

Paul's DPAC test of a SVX180T is a good example of what happens when a scope is nulled in red. It's the very good in red, OK in green and not so good in blue.

 

Hi Jon - Here are the Strehl ratios I gleaned from my testing of the gen 1 SVX180.  There was another thread where this was brought up that Clyde and I were participating in so I had reason to revisit the data.  I don't think I've ever published this.  I am guessing its safe to do so 16 months later...  

 

.891 Green

.984 Red
.604 Blue

 

Scaled appropriately and reduced down, the Strehl in white light would be .866 or so.  Truth-be-told, I was glad to see the tests showed the red to be super-excellent as this corroborates the SV cert which was .994 or something like that in red.    

Edited by peleuba, 23 July 2024 - 11:08 AM.

[Scott99]

You can't have reasonably good correction across the spectrum by nulling in red. You can only have reasonably good correction across the spectrum by nulling in green.

Not so sure about this - if the Strehl stays above 95% across the entire spectrum, does it matter where it's "nulled"?   Here is the Strehl curve RC posted for the AP 160EDF.......more grist for the mill....this lens was the best I've looked through out of a couple dozen apo refractors......
 

 

 

[Souldrop]

Not so sure about this - if the Strehl stays above 95% across the entire spectrum, does it matter where it's "nulled"? Here is the Strehl curve RC posted for the AP 160EDF.......more grist for the mill....this lens was the best I've looked through out of a couple dozen apo refractors......

160strehl:wvlngth.jpg


160colorcurve3.jpg

That looks nulled in greenish as well. Notice the curve towards the blue end. That would be exacerbated if it had been nulled in red.

Something to be aware of that I don’t believe has been mentioned is the refractive index of a material can have a wavelength dependence that adds to the challenge of lens design. (Typically it seems refractive index increases the more blue a wavelength) …yet another knob that requires tuning

[Jon Isaacs]

For what it's worth:

 

The FC100DZ is a doublet..

 

.jon

[peleuba]

this lens was the best I've looked through out of a couple dozen apo refractors......

 

Me too - the best scope I've ever personally viewed through.  The TOA130 is a close second.  But I'll take the AP any day.  

 

I looked through Philip Blanda's AP160 at the 2005 Black Forest Star Party.  Such a beautiful piece of equipment made by a true master - air spaced and, likely, aspherized for best correction.

 

Photo from the 2005 BFSP.

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Edited by peleuba, 23 July 2024 - 12:43 PM.

[Scott99]

Me too - the best scope I've ever personally viewed through.  The TOA130 is a close second.  But I'll take the AP any day.  

 

I looked through Philip Blanda's AP160 at the 2005 Black Forest Star Party.  Such a beautiful piece of equipment made by a true master - air spaced and, likely, aspherized for best correction.

 

Photo from the 2005 BFSP.

I have not been able to look through any TOA's yet....too bad.  The first night out with the 160, seeing was horrible.  But I noticed something amazing, even the waves of turblence washing over Jupiter were defined with incredible sharpness, something I"d never seen before.   Over the years the scope displayed extraordinary sharpness.

 

I think all the AP scopes since that time, 2004-5, are pretty much the same way.  I think there is a visual payoff from these air-spaced triplets with uber-high color correction for imaging.   My current 6-inch shows no false color at all, yet has a very slight softness to the view compared to the 160EDF, I assume that's from a little more spherochromatism.
 

Edited by Scott99, 23 July 2024 - 01:02 PM.

[Polyphemos]

Me too - the best scope I've ever personally viewed through.  The TOA130 is a close second.  But I'll take the AP any day.  

 

I looked through Philip Blanda's AP160 at the 2005 Black Forest Star Party.  Such a beautiful piece of equipment made by a true master - air spaced and, likely, aspherized for best correction.

 

Photo from the 2005 BFSP.

That really is a beautiful telescope

[Polyphemos]

For what it's worth:

 

The FC100DZ is a doublet..

 

.jon

Jon, can you expand on why a doublet might benefit from being nulled towards the reddish end of the spectrum while, presumably, a triplet might favor greener pastures?

[bobhen]

Not so sure about this - if the Strehl stays above 95% across the entire spectrum, does it matter where it's "nulled"?   Here is the Strehl curve RC posted for the AP 160EDF.......more grist for the mill....this lens was the best I've looked through out of a couple dozen apo refractors......
 

160strehl:wvlngth.jpg

 

 

160colorcurve3.jpg

That scope is nulled in green. Look at the wavelength numbers and the corresponding shift. The least shift is right is the middle of the green at .5254. The highest Strehl is .991 which corresponds almost exactly at .5285 green.

 

The Strehl only stays that high across the spectrum "if the the scope is nulled in green". Otherwise, the green will drop and one of the ends will drop even farther.

 

Bob

Edited by bobhen, 23 July 2024 - 01:50 PM.

[Scott99]

That scope is nulled in green.

Of course he knows what he's doing!     I never said it wasn't nulled in green - just suggested that, if the correction is high enough, e.g. in the TOA's, it won't matter where the "null" is, the whole thing will be pretty good.

 

I'm more interested in the difference between these design diagrams and the finished lens.....do they always match the design curves?  Maybe not.  Here is an interesting diagram TEC posted some years ago when the 140FL came out.  The 140FL is up top.   It looks like the design strehl for the older 140ED is the bottom graph.  The middle one is "selected melts" w/ 140ED, which I believe refers to tweaking the design for a certain batch of glass ....here is the graph and Yuri's explanation.   It's quite an improvement, and one could imagine the final product might come out worse than the design specs in some cases (not saying TEC - but maybe others?)

 

Are companies like Tak and Agema and others bothering to update the design graph with what actually came out for a given batch of lenses?  I don't think so?

 

 

https://groups.io/g/...329759#msg28743

Edited by Scott99, 23 July 2024 - 02:41 PM.

[Scott99]

a couple more from the archived AP web page FYI.......

 

[Scott99]

one more....so they were using 546nm back in the late 90's-early 2000's timeframe:

 

Edited by Scott99, 23 July 2024 - 02:51 PM.

[bobhen]

Of course he knows what he's doing!     I never said it wasn't nulled in green - just suggested that, if the correction is high enough, e.g. in the TOA's, it won't matter where the "null" is, the whole thing will be pretty good.

 

I'm more interested in the difference between these design diagrams and the finished lens.....do they always match the design curves?  Maybe not.  Here is an interesting diagram TEC posted some years ago when the 140FL came out.  The 140FL is up top.   It looks like the design strehl for the older 140ED is the bottom graph.  The middle one is "selected melts" w/ 140ED, which I believe refers to tweaking the design for a certain batch of glass ....here is the graph and Yuri's explanation.   It's quite an improvement, and one could imagine the final product might come out worse than the design specs in some cases (not saying TEC - but maybe others?)

 

Are companies like Tak and Agema and others bothering to update the design graph with what actually came out for a given batch of lenses?  I don't think so?

 

TEC140WingsStrehl.jpeg

 

https://groups.io/g/...329759#msg28743

Optics are tested on an interferometer throughout the figuring process. If the final optic gets within the "minimum spec" set by the designer and the head of the company, it will go out the door. 

 

But the above is different than designing a lens to be nulled in green and then having the final lens come out nulled in red and still sending it out the door. That would not happen.

 

For example; one AP 160 might be .98 Strehl in green and one might be .99 in green. But if AP's minimum spec is .98 then both will meet AP's advertised minimum spec even if there is a slight difference between the two scopes. But a scope would never go out the door nulled in red.

 

Bob

[Polyphemos]

Of course he knows what he's doing!     I never said it wasn't nulled in green - just suggested that, if the correction is high enough, e.g. in the TOA's, it won't matter where the "null" is, the whole thing will be pretty good.

 

I'm more interested in the difference between these design diagrams and the finished lens.....do they always match the design curves?  Maybe not.  Here is an interesting diagram TEC posted some years ago when the 140FL came out.  The 140FL is up top.   It looks like the design strehl for the older 140ED is the bottom graph.  The middle one is "selected melts" w/ 140ED, which I believe refers to tweaking the design for a certain batch of glass ....here is the graph and Yuri's explanation.   It's quite an improvement, and one could imagine the final product might come out worse than the design specs in some cases (not saying TEC - but maybe others?)

 

Are companies like Tak and Agema and others bothering to update the design graph with what actually came out for a given batch of lenses?  I don't think so?

 

TEC140WingsStrehl.jpeg

 

https://groups.io/g/...329759#msg28743

Eagle, Raven, Raven. I’ll have to look closer to absorb the rest of the information presented. Thanks, Scott.

[Polyphemos]

Here’s an interesting link that converts wavelength in nanometers to color. Pay particular attention to the numbers at the bottom of the page that breaks down the color into its red, green, and blue constituents (RGB Value):

 

https://405nm.com/wavelength-to-color/

 

What we call commonly call red, green, or blue are most often a mixture of the three.

[Souldrop]

Here’s an interesting link that converts wavelength in nanometers to color. Pay particular attention to the numbers at the bottom of the page that breaks down the color into its red, green, and blue constituents (RGB Value):

https://405nm.com/wavelength-to-color/

What we call commonly call red, green, or blue are most often a mixture of the three.

A photon has a discreet wavelength. A laser producing ~520nm light will not have red or blue photons mixed in with the beam even though a picture of said laser would have to combine rgb to give the appearance.

RGB values are more an “emulation” of color and more for technological convenience.


ETA: I had to look this up but you are describing “hue” not color with RGB https://en.m.wikipedia.org/wiki/Hue

Edited by Souldrop, 23 July 2024 - 06:56 PM.

[Polyphemos]

A photon has a discreet wavelength. A laser producing ~520nm light will not have red or blue photons mixed in with the beam even though a picture of said laser would have to combine rgb to give the appearance.

RGB values are more an “emulation” of color and more for technological convenience.


ETA: I had to look this up but you are describing “hue” not color with RGB https://en.m.wikipedia.org/wiki/Hue

Thanks for the additional information, Souldrop; I had clearly oversimplified the situation. We can still keep in mind that “green”, “red”, and “blue”, are artificial constructs and variable, and the linked site is fun to play with.

[PPPPPP42]

Not sure if its been mentioned in the pages of science homework here, but Stellarvue already did their homework on this.

 

From their website:

In the summer of 2023 based on customer requests we implemented a slight change in our process to move the final optical correction toward the middle of the visual spectrum. During the spring of 2023 we tested this process extensively to ensure that photographic performance would not be compromised due to this change. Both red and green figured, high-Strehl SVX objectives delivered the same excellent color correction free of the dreaded "blue bloat." Since most of our customers are imagers, it was important that we took the time to test the results before we implemented this change. Based on our findings, we modified our final spherical correction slightly to ensure that our objectives are now most accurate in green light.

 

I'm unclear if that means they test all of them in both red and green light, or if they switched to just using green after verifying they weren't losing red accuracy by changing their processes to prioritize green.

Either way it would solve the debate in this thread as it seems that if correctly done in green, it costs you nothing worth noting in red.

 

EDIT: I also recall someone mentioning last time this came up that most places use red lasers simply because they are much cheaper than green in the quality level and power needed for the process.

Edited by PPPPPP42, 23 July 2024 - 09:39 PM.

[Polyphemos]

Not sure if its been mentioned in the pages of science homework here, but Stellarvue already did their homework on this.

 

From their website:

In the summer of 2023 based on customer requests we implemented a slight change in our process to move the final optical correction toward the middle of the visual spectrum. During the spring of 2023 we tested this process extensively to ensure that photographic performance would not be compromised due to this change. Both red and green figured, high-Strehl SVX objectives delivered the same excellent color correction free of the dreaded "blue bloat." Since most of our customers are imagers, it was important that we took the time to test the results before we implemented this change. Based on our findings, we modified our final spherical correction slightly to ensure that our objectives are now most accurate in green light.

 

I'm unclear if that means they test all of them in both red and green light, or if they switched to just using green after verifying they weren't losing red accuracy by changing their processes to prioritize green.

Either way it would solve the debate in this thread as it seems that if correctly done in green, it costs you nothing worth noting in red.

 

EDIT: I also recall someone mentioning last time this came up that most places use red lasers simply because they are much cheaper than green in the quality level and power needed for the process.

This isn’t homework, nor a debate, and it isn’t about a particular scope. It’s more of a broad sharing of information, ideas, and experiences about any and all refractors, celestial objects of interest, and design goals and achievements. What StellarVue does and why is of no more importance in this discussion than what Vixen, Takahashi, Astro Physics, TEC, AstroTech, any other telescope maker does, and arguably much less so because that’s already been discussed ad nauseum. For me at least, and hopefully others, the quality and breadth of the information presented by contributors to this thread has been fascinating, and not least of all because it doesn’t center on StellarVue or anyone else.

 

But to answer your question and far as I understand, StellarVue does the majority of their figuring in red and completes the process in green. I don’t think there’s anything cheap about their or anyone else’s Zygo Interferometer. Presumably they’re comfortable with the results across the visual spectrum and there is plenty of reason to expect that it makes for a satisfactory visual experience. Other scopes we’ve discussed that are not nulled in green likewise provide very satisfactory visual images.

[lylver]

I'm unclear if that means they test all of them in both red and green light, or if they switched to just using green after verifying they weren't losing red accuracy by changing their processes to prioritize green.

Either way it would solve the debate in this thread as it seems that if correctly done in green, it costs you nothing worth noting in red.

 

EDIT: I also recall someone mentioning last time this came up that most places use red lasers simply because they are much cheaper than green in the quality level and power needed for the process.

They needed to verify the move between red 633 to green (532 ? 546 ?)

especially for 4" and more refractors : the change is bigger than 0.99 to 0.98 when you focus to take a picture or even do visual.

 

At 633nm you are close to 656nm that has a criteria of diffraction limited strehl (~ 0.8 or ~0.82 in fact ), so they have to be cautious if they tune the f/D to be the shortest technically possible.

 

---

 

Nd:Yag laser at 532nm are often cheaper than the HeNe, it depends on level of quality you need for the coherence length.

In case of HeNe, the coherence length varies from 20 cm (for multiple longitudinal modes) to more than 100 m.

Edited by lylver, 24 July 2024 - 04:12 AM.