132 replies to this topic

[Polyphemos]

Fascinating information, Iylver.

[Lagrange]

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.

The He-Ne laser is notable for being the first ever continuous wave laser. Prior to it you had the likes of ruby lasers (first ever laser) which could only produce brief pulses of intense light that weren't much use for interferometric testing of optics.

 

He-Ne lasers have additional benefits such as being highly reliable, having long operational lives, long coherence lengths and highly monochromatic compared to other lasers without being hugely expensive. I think most of the other wavelength options for interferometry light sources use diode-pumped solid state lasers which is similar technology to that found in green laser pointers (808nm aluminium gallium arsenide IR laser diode as the energy source for a 1064nm Nd-doped yttrium aluminium garnet / yttrium orthovanadate laser crystal whose output is fed to a potassium titanyl phosphate crystal which halves its wavelength to 532nm).

[Lagrange]

There is an interesting question;  is it possible to make an aspheric objective as highly accurate as a spherical one and if not, does the aspherizing alone allow the large lens to compensate and exceed the overall quality possible with a high-end spherical configuration?  My guess is, yes.  Since AP has been doing it for decades.

It's possible to make aspheric optics to the same precision as spherical ones but it's more challenging to do and they're not as straightforward to test, so production costs are higher.

 

Consumer optics have benefitted hugely from the availability of low-cost aspheric lenses and mirrors that are typically made using moulding techniques. They don't have to be figured and polished to the same standard as a telescope objective though, so I'm not sure how well you could apply the same methods to making telescope optics. As well as the figure and polish, there's also the limitation that precision glass moulding is only practical up to lenses of a certain size which probably rules it out as a method for making something like a 6" telescope.

[Elusivephotons]

gkevinw wrote this in the AP155CS thread:

"I was happy that Roland answered a question for me today at NEAF: are these new scopes optimized for visual or photographic use and are they slightly over corrected (or under?) for best performance under real dynamic cool down conditions typical at night?

The answer (paraphrasing, sorry): yes, corrected for real nighttime cooldown conditions and optimized with visual as well as photographic use in mind! Roland was specific in that some other manufacturers optimized their “apos” for green which is wrong, yellow is best for visual. Also nebula detail for photographic is best in red (and I think he might have said yellow here also) but there is not much detail at all in OIII green. He emphasized that these new CS systems are quite uncompromising and have broader spectral correction.

As a strictly visual observer I was very happy with this answer. I’m tempted by these large aperture systems out of China but they seem to be intended for the now much larger photographic market. Are any advertised as visual?

I did not get a chance to speak with The Takahashi rep about there new ortho-apochromat as his English was not quite there, but the test results they showed for their new 150 looked like broad spectrum corrected as well."


So after years of reading here that telescopes should be nulled in green, I'm now questioning this once again. Maybe the answer is not so black & white afterall?

Edited by Elusivephotons, 08 April 2025 - 11:59 PM.

[davidgmd]

For visual observation:

  

The wavelengths the eye is most sensitive to are important, as are the wavelengths most commonly emitted/reflected. The wavelengths we can’t see don’t really matter.

  
You might have to do some fancy math comparing relative retinal sensitivities to the relative preponderance of observable wavelengths to come up with the ideal wavelength to null for. It would vary depending on the preferred targets. And whether you’re using rods or cones, which are most sensitive at different wavelengths.

  
Too many variables. Complicated to the point of impossibility. I think Paul came up with the best simple answer early in the thread. Green makes sense because it’s between red and blue and if you null for green, neither red nor blue will be that far off.

  
I'd also like to see a masked trial to see how sensitive the typical observer is to over- and under-correction. I have nothing to back this up, but I suspect DPAC results have to be pretty far off before there’s a visible difference to the average astronomer.

[Drothgeb]

gkevinw wrote this in the AP155CS thread:

"I was happy that Roland answered a question for me today at NEAF: are these new scopes optimized for visual or photographic use and are they slightly over corrected (or under?) for best performance under real dynamic cool down conditions typical at night?

The answer (paraphrasing, sorry): yes, corrected for real nighttime cooldown conditions and optimized with visual as well as photographic use in mind! Roland was specific in that some other manufacturers optimized their “apos” for green which is wrong, yellow is best for visual. Also nebula detail for photographic is best in red (and I think he might have said yellow here also) but there is not much detail at all in OIII green. He emphasized that these new CS systems are quite uncompromising and have broader spectral correction.

As a strictly visual observer I was very happy with this answer. I’m tempted by these large aperture systems out of China but they seem to be intended for the now much larger photographic market. Are any advertised as visual?

I did not get a chance to speak with The Takahashi rep about there new ortho-apochromat as his English was not quite there, but the test results they showed for their new 150 looked like broad spectrum corrected as well."


So after years of reading here that telescopes should be nulled in green, I'm now questioning this once again. Maybe the answer is not so black & white afterall?

 

For visual observation:

  

The wavelengths the eye is most sensitive to are important, as are the wavelengths most commonly emitted/reflected. The wavelengths we can’t see don’t really matter.

  
You might have to do some fancy math comparing relative retinal sensitivities to the relative preponderance of observable wavelengths to come up with the ideal wavelength to null for. It would vary depending on the preferred targets. And whether you’re using rods or cones, which are most sensitive at different wavelengths.

  
Too many variables. Complicated to the point of impossibility. I think Paul came up with the best simple answer early in the thread. Green makes sense because it’s between red and blue and if you null for green, neither red nor blue will be that far off.

  
I'd also like to see a masked trial to see how sensitive the typical observer is to over- and under-correction. I have nothing to back this up, but I suspect DPAC results have to be pretty far off before there’s a visible difference to the average astronomer.

I’ve DPAC tested most of my scopes. Generally I’m seeing straight bars in red and green, with a slight bit of over correction in blue. So for the most part, they do peak do in green/yellow.

[Polyphemos]

https://www.cloudyni...ced/?p=14065357
 

 

“Roland was specific in that some other manufacturers optimized their “apos” for green which is wrong, yellow is best for visual”.

 

Assuming that gkevinw did not misunderstand Roland’s message it’s always nice to hear Roland’s thoughts on optical issues, and not the least because Roland appears to give little weight to conventional wisdom and is often in direct opposition to it. As to nulling in yellow, Iylver, Sasha, and Scott99 have also provided information that tracks with gkevinw’s interpretation of Roland’s approach.

[121601]

“…Roland was specific in that some other manufacturers optimized their “apos” for green which is wrong, yellow is best for visual…”

 

Does Roland use an interferometer with a green laser for a fact? CS

[Polyphemos]

“…Roland was specific in that some other manufacturers optimized their “apos” for green which is wrong, yellow is best for visual…”

 

Does Roland use an interferometer with a green laser for a fact? CS

I have no idea how Roland nulls his telescopes other than what gkevinw conveyed after his conversation with Roland.

[121601]

I have no idea how Roland nulls his telescopes other than what gkevinw conveyed after his conversation with Roland.

 

I have no idea how Roland nulls his telescopes other than what gkevinw conveyed after his conversation with Roland.

It’s conceivable that “yellow” may be the ⭐️ best compromise⭐️ to construct scopes which perform optimally for both visual and astrophotographic use (sort of saying the true Physics is somewhere in the middle.)

Edited by 121601, 19 April 2025 - 07:24 PM.

[Polyphemos]

 

It’s conceivable that “yellow” may be the ⭐️ best compromise⭐️ to construct scopes which perform optimally for both visual and astrophotographic use (sort of saying the true Physics is somewhere in the middle.)

 

Almost everything we ever encounter requires a compromise of one sort or another, so sure, nulling in yellow isn’t an absolute but rather just another compromise.

 

What’s interesting is that conventional wisdom advocated green as the best spectrum to null in because that’s what our eyes are best accommodated for. That seemed to make logical sense but apparently it’s more nuanced than that, and Vixen, Takahashi, and others believe so as well, choosing to null in spectrum other than green.

Edited by Polyphemos, 20 April 2025 - 05:23 AM.

[Marino]

Almost everything we ever encounter requires a compromise of one sort or another, so sure, nulling in yellow is likely just another compromise.

 

What’s different is that convention wisdom advocated green as the best spectrum to null in because that’s what our eyes are best accommodate for. Turns out the universe and physics cares little for us or our eyes, which can be problematic for those who insisted it’s green or nothing.  Turns our they’re likely wrong and Vixen, Takahashi, and others felt otherwise as well, choosing to null in spectrum other than green.

So, just perhaps:

A(receptor) + B(source) = Yellow

Edited by Marino, 19 April 2025 - 09:36 PM.

[Yuri]

 

It’s conceivable that “yellow” may be the ⭐️ best compromise⭐️ to construct scopes which perform optimally for both visual ...

 

Since the human eye changes the spectral sensitivity with age due to physiological changes in the eye lens, retina, etc. the ⭐best compromise⭐️ will shift.
When we are young the peak sensitivity at night (scotopic) is at green-blue. With age transmission of the blue end of spectrum is diminishing due to eye lens yellowing...
So, when you are ordering TEC scopes, please specify your age and I will null the lens accordingly
Going "blue" is more difficult, going "red" is easier for obvious reasons.

[Polyphemos]

Since the human eye changes the spectral sensitivity with age due to physiological changes in the eye lens, retina, etc. the ⭐best compromise⭐️ will shift.
When we are young the peak sensitivity at night (scotopic) is at green-blue. With age transmission of the blue end of spectrum is diminishing due to eye lens yellowing...
So, when you are ordering TEC scopes, please specify your age and I will null the lens accordingly
Going "blue" is more difficult, going "red" is easier for obvious reasons.

Wow! I don’t know whether you’re serious or not, but either way I’m highly amused by your proposition and honored that you’ve contributed to this thread in which I’ve learned far more than I could have hoped for.  Thank you, Yuri.

[Dean J.]

Since the human eye changes the spectral sensitivity with age due to physiological changes in the eye lens, retina, etc. the ⭐best compromise⭐️ will shift.
When we are young the peak sensitivity at night (scotopic) is at green-blue. With age transmission of the blue end of spectrum is diminishing due to eye lens yellowing...
So, when you are ordering TEC scopes, please specify your age and I will null the lens accordingly
Going "blue" is more difficult, going "red" is easier for obvious reasons.

Interesting. 

 

So Yuri, if one does not specify {I didn't when ordering my APO140FL - serial number 123} then where is the default for nulling your 140FL lenses?

[Yuri]

Interesting. 

 

So Yuri, if one does not specify {I didn't when ordering my APO140FL - serial number 123} then where is the default for nulling your 140FL lenses?

Dean, all triplets and doublets are nulled in Green. I also have blue and red lasers, the latter one could be used for nulling the optics for solar imaging (by request).

Yuri

[Dean J.]

Dean, all triplets and doublets are nulled in Green. I also have blue and red lasers, the latter one could be used for nulling the optics for solar imaging (by request).

Yuri

Thanks Yuri.  I really enjoy using my 140FL.  

Edited by Dean J., 20 April 2025 - 04:56 PM.

[ABQJeff]

Green is the best color to null in unless scope will be used solely for solar H-a.  For daytime terrestrial the eye sensitivity peaks in yellow green, at night peak sensitivity drifts more to blue green (so even less sensitive in red).  ~530-550 nm “Green” is a good middle range to match both eye sensitivity and so that the Red and Blue ends of the spectrum aren’t terrible.

Edited by ABQJeff, 20 April 2025 - 05:35 PM.

[ABQJeff]

And yes most stars are red, as in red dwarfs, but they are so dim you can’t see the vast majority of them.

[Mike W]

And yes most stars are red, as in red dwarfs, but they are so dim you can’t see the vast majority of them.

Except red super giants, Antares, Betelgeuse, etc.

Edited by Mike W, 20 April 2025 - 06:03 PM.

[Elusivephotons]

Green is the best color to null in unless scope will be used solely for solar H-a.  For daytime terrestrial the eye sensitivity peaks in yellow green, at night peak sensitivity drifts more to blue green (so even less sensitive in red).  ~530-550 nm “Green” is a good middle range to match both eye sensitivity and so that the Red and Blue ends of the spectrum aren’t terrible.

While that has been consensus on CN for quite some time and obviously Yuri agrees.  I do find it interesting that Roland is noted above saying that Yellow makes more sense than Green.  Personally, I don't think it really matters so long as the optics are made to a high standard, as there are scopes nulled in Green, Red, and Yellow, that pass the visual acuity tests with flying colors.

Edited by Elusivephotons, 20 April 2025 - 06:07 PM.

[peleuba]

Dean, all triplets and doublets are nulled in Green. I also have blue and red lasers, the latter one could be used for nulling the optics for solar imaging (by request).

Yuri

 

Hi Yuri - I assume you mean red and blue lasers for the interferometer.  Wouldn't you also need the reference sphere (within the interferometer) nulled in whatever wavelength you nulled the optics?  In other words, if normal production scopes are nulled in green you test with a green laser using reference spheres that were nulled in green.   If this is the case, it becomes quite expensive just to make a few one-off scopes having to purchase (or make) new reference spheres.  

 

I also think the best wavelength overall for both visual and imaging is green, and perhaps you could convince that green-yellow might be best.  This allows for (nearly) equal an opposite correction of the blue and red ends.

[peleuba]

While that has been consensus on CN for quite some time and obviously Yuri agrees.  I do find it interesting that Roland is noted above saying that Yellow makes more sense than Green.  Personally, I don't think it really matters so long as the optics are made to a high standard, as there are scopes nulled in Green, Red, and Yellow, that pass the visual acuity tests with flying colors.

 

A few years ago, Roland and I exchanged email about this subject during some sessions where he was providing some optical design advice to me using ATMOS.   While he did mention yellow, I came away with the feeling that for overall best experiences in visual and imaging, green with a trend toward green-yellow is what he thought was best.   In my testing, the Stowaway, GTX110 and GTX130 are all nulled in green.

[Yuri]

And yes most stars are red, as in red dwarfs, but they are so dim you can’t see the vast majority of them.

It is not about the stars, it is all about star #1 that created us, it took a billion years. The spectral sensitivity of our eyes coincides with maximum spectral intensity of the Sun. We are all "children" of the Sun. It developed our vision accordingly, and the vision of most animals (see  Bessy's eyes attached)¹.
If the nearest star was of different spectral class with intensity shifted to red, for example, then our vision would be optimized for red-infra-red (like a vision of some snakes), it also would be of lower resolution.
If the Earth was orbiting around a blue star like Vega, then we all would be "vegans"   but do not expect a better vision - short wavelength (UV) would be partially blocked by the cornea and eye lens, and could damage the eye (cataracts, etc.).

 

I might be mistaken somehow, count the above as Easter day fantasy and correct me, well, "nobody's perfect”

 

¹ Cat's vision, BTW, is of two colors blue-green, no red cones. They for sure would avoid telescopes nulled in RED.

 

[Polyphemos]

A few years ago, Roland and I exchanged email about this subject during some sessions where he was providing some optical design advice to me using ATMOS.   While he did mention yellow, I came away with the feeling that for overall best experiences in visual and imaging, green with a trend toward green-yellow is what he thought was best.   In my testing, the Stowaway, GTX110 and GTX130 are all nulled in green.

Assuming gkevinw heard what he though he heard perhaps Roland’s thinking on the subject has evolved. It would be interesting to hear Roland’s first person thoughts on this.