132 replies to this topic

[Polyphemos]

The issue of whether scopes should be nulled in green or red has been raised in another thread.  Correction color choice isn’t the theme of that thread, but I’m sufficiently interested in the question and so I present it here. The title is posed in the form of a question because I don’t think it’s been critically examined sufficiently to put in the form of a statement, and I invite that necessary critical examination so that the question, modified into the form of a statement, can be confidential declared or discarded. Since some discussion has already been made on the topic I’ll repeat those opening remarks to save both time and effort.

 

I wrote the following in the aforementioned thread:

 

“I am beginning to wonder if the conventional wisdom advocating nulling telescopes in green is completely wrong.

Stars radiate across the entirety of the spectrum, but the most commonly used classifications for stars are based upon their peak stellar spectra. Tallying the observable stars, ie., the ones we look at, it’s found that 95.6% of stars emit in the yellow (G, 7.6%), light orange (K, 12%), or orangish-red (O, 76%) in wavelengths longer than green, and that only 0.12% emit in the blue dominated wavelengths shorter than green. The remaining 3.61%, the white and yellowish-white A and F stars, are agnostic.

What’s worth paying closer attention to is that 76% of stars are classified as orangish-red stars with spectrums significantly closer to red than green. There are no green, cyan, indigo, or violet stars. The spectrum for Mars peaks around 640nm - it’s not for no reason it’s called the “Red Planet”. Some planets, like Jupiter, also favor red, though not markedly so, and others, like Saturn, lean moderately green, though like Jupiter only a bit.

So why null in green? I’m unable to postulate any plausible astronomical reason to do so since three quarters of the observable stars should be better resolved in a scope nulled in red. The invariably stated reason to null in green is because that’s the color our eyes are most sensitive to. So what? It would seem that we are nulling for a color conspicuously absent in the night sky at the expense of the colors that are abundant to accommodate our eyes to see what isn’t there. Our eyes resolve best in green because that’s the color of vegetation, not because it’s the color of starlight.

Consider the average star, emitting predominantly in the orangish-red spectrum, and the light from that star passing through a scope nulled to a very high standard in red. The light from that star focused through that scope will reach the observers eye well corrected around it’s dominant orangish-red color, and less so for colors less dominant. The focused light from that same star, but now through a scope nulled in green, will be well corrected around non-dominant green, but less so around its predominant color spectra, orangish-red. So, in essence, the green nulled scope will under perform optically in the predominant orangish-red spectra of our observable celestial neighborhood because it’s nulled in a part of the spectrum where there is far less light emitted. We could call this less-well corrected orangish-red part of the spectra in the green nulled scope “red bloat” because optically speaking it is, and “unfortunate” because stars seem to have strong proclivity for it.

I can appreciate the logic of nulling a terrestrial spotting scope in green, say for orchid hunting, because that’s the dominant spectra in the environment it will be observing in. Applying the same logic, nulling astronomical telescopes scopes towards the red side of the spectrum would seem to answer best because that’s the dominant spectra in the environment it will be observing in.

….”

Edited by Polyphemos, 19 July 2024 - 01:04 PM.

[Polyphemos]

Iylver responded to my post with the following very well documented explanation on the function and limitations of the eye; I provide a link only to Iylver’s excellent post because the important charts and tables Iylver has presented will be omitted in the transfer of the text:

https://www.cloudyni...7#entry13575380

I responded to Iylver’s post with the following:

“Iylver, thank you for the detailed explanation. I did also read your previous post, which was likewise very helpful.

 

The problem as I see it is that you are looking at the issue almost exclusively from the perspective of the eye, the receptor, and not at all from the primary source, orangish-red stars. The same criticism can be made against me in that I’m looking at the issue almost exclusively from the source, and not at all from the receptor. We can therefore be both completely correct and still miss the mark because neither of us are adequately considering the entire issue. To ameliorate that deficiency I propose the following equation:

 

Optical Experience = A(receptor) + B(source)

 

I’ve admittedly simplified the situation, but I think it will help move us towards a more common understanding. I’ve left out the optics themselves because I assume they’re equally perfect for whichever color they’re corrected for. I’m also ignoring Astro photography as camera sensors change with development and so what is best now may not be best later, while in contrast our eyes are relatively static as far as development goes.

 

I agree entirely with everything you’ve written from your A(receptor) perspective. There’s no distance between us on that. What I hope you will do is comment on my B(source) perspective and, more specifically, whether it is correct or not. If you decide it’s incorrect that’s perfectly fine and I hope you will explain why, but I also hope that you will concede that optics nulled in red will better correct objects emitting in the orangish-red spectra.

 

If you’re comfortable with the notion that the source matters, and as I have already fully accepted your explanation on the function of the eye, we’ll have agreed on what are arguably the two most salient points of perceived optical experience. More importantly, we’ll both have acknowledged that it’s necessary to consider both the source and the receptor and that neither can be safely ignored. All that will then  remain for us to do is to attribute relative values to the constants A and B.

Once again, thank you, Iylver, for your detailed explanations and the effort you made to present them.”

 

 

This is where things now stand. I look forward to any and all assistance in answering the question, “Should Telescopes be Nulled in Red”, but I ask that everyone keep in mind that the question is not specific to one scope, but to all visual astronomy refractors generally.

[Souldrop]

I'll let others speak towards other facets of the discussion.

 

From a physics perspective (Planck's law) and making the assumption that most stars can be roughly approximated as blackbodies. 

Just because a star's peak emission is more red does not mean that it is not also emitting green energy. Actually the further the peak emission is red shifted the more equally the energy emitted is distributed across the EM spectrum arounds its peak. If you are picking one approximate color to null in (for both visual users and imagers) green does make a fair amount of sense. The further blue shifted an object the "peakier" it's emission spectra is. If you are nulled in red this is going to increase the potential for blue bloat especially for someone imaging. A visual user may or may not find themselves sensitive. Ultimately it's an issue of compromise on behalf of the scope designer/producer/seller.

 

FWIW perhaps a red nulled scope does make sense for some people for some use cases. I use night vision to assist my stargazing. I could likely be coaxed into wanting a scope nulled in red. I just don't expect a red nulled frac to be a preferable option for many users.

[Astrojensen]

 

I can appreciate the logic of nulling a terrestrial spotting scope in green, say for orchid hunting, because that’s the dominant spectra in the environment it will be observing in. Applying the same logic, nulling astronomical telescopes scopes towards the red side of the spectrum would seem to answer best because that’s the dominant spectra in the environment it will be observing in.

 

You very obviously do not understand how optics work or how the eye work. 

 

The reason you do not want to null refractors in red, is because you then make spherical aberration MUCH worse in blue, and thus impact their overall performance dramatically more, than if you balance the spherochromatism around green, which is in the middle of the spectrum. And this will also be visible on red stars, because even if a star is predominantly red, it most certainly also shines in green and blue. 

 

And for planetary observing, the blue-green part of the spectrum is more important than the red, so it's fairly crucial to have the scope nulled in green, if you want it to perform well on planets. 

 

A lot of refractor owners are quite keen on having their telescopes perform well on the planets... 

 

The ONLY case, where it makes sense to null the scope in red, is if it's going to be used exclusively for monochromatic H-alpha observing.  

 

 

Clear skies!

Thomas, Denmark

[Polyphemos]

I'll let others speak towards other facets of the discussion.

 

From a physics perspective (Planck's law) and making the assumption that most stars can be roughly approximated as blackbodies. 

Just because a star's peak emission is more red does not mean that it is not also emitting green energy. Actually the further the peak emission is red shifted the more equally the energy emitted is distributed across the EM spectrum arounds its peak. If you are picking one approximate color to null in (for both visual users and imagers) green does make a fair amount of sense. The further blue shifted an object the "peakier" it's emission spectra is. If you are nulled in red this is going to increase the potential for blue bloat especially for someone imaging. A visual user may or may not find themselves sensitive. Ultimately it's an issue of compromise on behalf of the scope designer/producer/seller.

 

FWIW perhaps a red nulled scope does make sense for some people for some use cases. I use night vision to assist my stargazing. I could likely be coaxed into wanting a scope nulled in red. I just don't expect a red nulled frac to be a preferable option for many users.

I’ve stated that stars emit across the entire visual spectrum, and beyond for that matter, so we agree there. The question is what is the relative importance of the quantity of red, green, or blue light emitted, and should we null towards the more or less quantity. You observe that a scope nulled in red increases the potential for “blue bloat” when it is equally true that a scope nulled in green increases the potential for “red bloat”.

 

I agree that it ultimately is an issue of compromise in the balancing of conflicting issues, which is step forward from the position of one color suits all circumstances.

[saemark30]

No telescope can resolve stars into disks.

If you are a photographer using only red sensitive photographic film then you will need optics nulled to red.

[Souldrop]

I’ve stated that stars emit across the entire visual spectrum, and beyond for that matter, so we agree there. The question is what is the relative importance of the quantity of red, green, or blue light emitted, and should we null towards the more or less quantity. You observe that a scope nulled in red increases the potential for “blue bloat” when it is equally true that a scope nulled in green increases the potential for “red bloat”.

 

I agree that it ultimately is an issue of compromise in the balancing of conflicting issues, which is step forward from the position of one color suits all circumstances.

Fair, but green is in the middle of the visual range while red is at the end. To put it simply when nulled in red green is compromised with blue being "doubly" compromised. When nulled in green both blue and red are just "singularly" compromised. The further away an wavelength is from the nulled region the more aberration that will manifest.

[peleuba]

The issue of whether scopes should be nulled in green or red has been raised in another thread.  Correction color choice isn’t the theme of that thread, but I’m sufficiently interested in the question and so I present it here. The title is posed in the form of a question because I don’t think it’s been critically examined sufficiently to put in the form of a statement, and I invite that necessary critical examination so that the question, modified into the form of a statement, can be confidential declared or discarded. Since some discussion has already been made on the topic I’ll repeat those opening remarks to save both time and effort.

 

 

 

Hi Jim,

 

I don't have a lot of input other then this nugget:  If green is well corrected, and red and blue are balanced the overall correction in white is the best that it can possibly be.  I demonstrated this with the see-saw analogy in the other thread.   Another way to say it is - with green corrected and red and blue balanced with equal but opposite correction, white stars will look white.  

 

If a lens is corrected for red, then green will be overcorrected and blue will be grossly overcorrected.  White stars will tend to look yellowish - due to the unfocused blue light and there may even be a blue haze.

Edited by peleuba, 19 July 2024 - 02:51 PM.

[Polyphemos]

You very obviously do not understand how optics work or how the eye work. 

 

The reason you do not want to null refractors in red, is because you then make spherical aberration MUCH worse in blue, and thus impact their overall performance dramatically more, than if you balance the spherochromatism around green, which is in the middle of the spectrum. And this will also be visible on red stars, because even if a star is predominantly red, it most certainly also shines in green and blue. 

 

And for planetary observing, the blue-green part of the spectrum is more important than the red, so it's fairly crucial to have the scope nulled in green, if you want it to perform well on planets. 

 

A lot of refractor owners are quite keen on having their telescopes perform well on the planets... 

 

The ONLY case, where it makes sense to null the scope in red, is if it's going to be used exclusively for monochromatic H-alpha observing.  

 

 

Clear skies!

Thomas, Denmark

Thomas, whether or not I understand optics “at all” isn’t relevant to the discussion. I think it preferable to avoid ad hominem statements because they are, like all logical fallacies, undesirable in that they mislead the simpleton and serve only to dilute the core of your argument. I trust there are no simpletons amongst us here and so there can be no benefit in an appeal to that particular audience.

 

Granting that stars emit across the entirety of the spectrum, but that they emit in some spectra far more strongly than others, if 95.6% of stars dominant emissions are shifted to the red side of the spectrum, and only 0.12% of stars dominant emissions are shifted to the blue side of the spectrum, shouldn’t we also consider the adverse impact of spherical aberration in the red? I understand the value in taking a middle ground compromise position, and that nulling in green serves to accommodate our eyes, but considering the dominance of emissions towards the red end of the spectrum isn’t it logical to consider that an adjustment towards that end of the spectrum isn’t unreasonable.

 

Lets say that neither red nor green are the perfect colors to null to, but something in between. What might that perfect color be? I propose that there is no magic wavelengths to null to, that refractors will always require some compromise in their optical figure to balance conflicting goals, and that those goals can be different for different circumstances. You’ve put an exceedingly narrow constraint on those circumstances but scope makers, including Takahashi, have nulled scopes to varying wavelengths and so it would seem that at least they, and by extension Canon Optron, believe there is some wiggle room.

 

As for planets Mars is decidedly red and Jupiter is somewhat so.

[BigC]

Note that most depiction of the Sun in print are yellow,it might be interesting to try a scope nulled in the transition zone from orange to yellow.

[Polyphemos]

Hi Jim,

 

I don't have a lot of input other then this nugget:  If green is well corrected, and red and blue are balanced the overall white correction is the best that it can possibly be.  I demonstrated this with the see-saw analogy in the other thread.   Another way to say it is - with green corrected and red and blue balanced, with equal but opposite correction, white stars will look white.  

 

If a lens is corrected for red, then green will be overcorrected and blue will be grossly overcorrected.  White stars will tend to look yellowish - due to the unfocused blue light and there may even be a blue haze.

Hi Paul, and thanks for stopping by.

 

Paul, what happens if there’s a red rhinoceros on one side of the allegorical see-saw, and blue mouse on the other? Archimedes might suggest moving the lever.

 

All kidding aside, it’s great to have you here. Can I in some way induce you to play the role of Devil’s Advocate and take my side for a while? Devil’s Advocate is an important and honorable position that was until recently held in the Vatican. That was, of course, until the DA made the error of taking the job seriously, which soon led to the unforgivable error of probing too close to the truth. Naturally he had to be dismissed. I make no claims with regard to the truth, but you might find the exercise entertaining and I don’t believe there will be any repercussions in the afterlife beyond being limited to observing through a scope nulled in red.

[Polyphemos]

Note that most depiction of the Sun in print are yellow,it might be interesting to try a scope nulled in the transition zone from orange to yellow.

Interestingly, the depiction of scopes varies from culture to culture. In Japan, for example, the sun is commonly depicted in red.

[hyia]

...

So why null in green? I’m unable to postulate any plausible astronomical reason to do so since three quarters of the observable stars should be better resolved in a scope nulled in red. The invariably stated reason to null in green is because that’s the color our eyes are most sensitive to. So what? It would seem that we are nulling for a color conspicuously absent in the night sky at the expense of the colors that are abundant to accommodate our eyes to see what isn’t there. Our eyes resolve best in green because that’s the color of vegetation, not because it’s the color of starlight.

...

 

The goal is not to null in green.  The goal is to maximize the quality of the lens over the visual spectrum (i.e. polystrehl).  I'm sure you could construct a use case where you would care most about red.  You'd then have a problem if you wanted to look at other wavelengths though.  With a scope that is well balanced over the visual spectrum, however, if I'm interested in just one wavelength, I can use a filter.

 

If the scope nulled in red had strehls above 0.9 in all wavelengths and simply peaked in red, then you could argue that this was similar to the way AP supposedly had scopes tuned slightly different for photo and for visual.  If you just null in red and maximize in red to the SEVERE detriment of other wavelengths, then you have an application specific scope.  That might have limited appeal, but, again, if I could  have scope with a high strehl across all the spectrum and use a filter, I'd rather have that because I want to be able to look at things which aren't red and still get a good image.

[db2005]

When designing optics, one always optimizes the design for the intended purpose.

 

If you think of a refractor as an originally visual instrument, it makes sense to optimize its design for human vision for which the green wavelengths are the most important.

 

Even if using the telescope photographically, it's worth keeping in mind that RGB image sensors are designed to mimic human vision as closely as possible, so the idea of optimizing for green still makes good sense unless you have special applications in mind.

 

In fact, most image sensors are filtered to specifically block out wavelenghts that the human eye aren't sensitive to.

 

It is true, however, that unfiltered CMOS and CCD image sensors are sensitive to wavelengths far beyond the limits of human vision. So the answer to your question might just be: it depends on the application.

 

As a case in point: In the era of photographic emulsions some aprochromatic refractors did not visually produce better views than achromats, but they were designed to compensate for the photographic emulsions being sensitive to wavelengths outside the visual spectrum.

 

If you have deep pockets I'm sure you can ask an optics manufacturer to design and produce a refracting telescope to specific needs, optimized to red or even NIR (near-infrared) wavelengths. But as always with optics, there's no free lunch. The more abberations you want to keep under control, the more optical elements you need, and other optical problems begin to creep in.

 

It would seem to me that for amateur astronomy purposes, composing esoteric designs to correct for specific wavelengths is likely more trouble than it's worth, especially since there's already solution available: slow reflecting telescopes.

Edited by db2005, 20 July 2024 - 01:28 AM.

[lylver]

It has existed and still exist scopes that minimize SA.

In the 19th century Alvan Clark scopes, including the big Lick and Yerkes refractors do that.

 

They use the d'Alembert-Gauss property to null two wavelengths using the spacing between the crown and flint of an achromat.

So between the two wavelengths the cool fact is that SA is minimized.

 

Takahashi and Vixen did this too (TOA and AX103SD)

 

One big goal for those lenses was spectroscopy and you need to be accurate on a large range of wavelength to show rays and their position.

 

This is useful for human vision too, we have the abilty to focus our attention on details, this not only mental, but effective for eye focus position.

 

I agree with the fact that if you want accurate SA on many wavelength, it is difficult with short f/D scope.

 

This is one reason why I appreciate long achromat even if the range of color focused is narrow : we use filters.

This is an old way to search for specific details when you observe.

This is old fashion now, most of people watch celestial objects for mainly pleasure, so they prefer the all in one capability in a refractors. This cost much and many times not so perfect as in the past.

 

Danjon & Couder wrote about a tautochronism criteria, it meant to have SA small as 1/12 lambda PtV from green (turquoise) 520nm to cherry red 633nm. This is a criteria to catch enough contrast / resolution on the targetted wavelength in this range when you use filters. The eye then focuses in the narrow range of color exposed and can take advantage of this precise analysis.

Coupled to spectroscopy, this is the way astrophysician made hypothesis on the composition of Jupiter, Saturn etc.

 

-----------

 

Another possibility is to use also aspherisation as AGEMA does with their fantastic products.

But sometimes the tools (spacing, aspherisation) are not perfects or have drawbacks.

Edited by lylver, 19 July 2024 - 04:36 PM.

[Polyphemos]

The goal is not to null in green.  The goal is to maximize the quality of the lens over the visual spectrum (i.e. polystrehl).  I'm sure you could construct a use case where you would care most about red.  You'd then have a problem if you wanted to look at other wavelengths though.  With a scope that is well balanced over the visual spectrum, however, if I'm interested in just one wavelength, I can use a filter.

 

If the scope nulled in red had strehls above 0.9 in all wavelengths and simply peaked in red, then you could argue that this was similar to the way AP supposedly had scopes tuned slightly different for photo and for visual.  If you just null in red and maximize in red to the SEVERE detriment of other wavelengths, then you have an application specific scope.  That might have limited appeal, but, again, if I could  have scope with a high strehl across all the spectrum and use a filter, I'd rather have that because I want to be able to look at things which aren't red and still get a good image.

Excellent points. Your comment that AP supposedly figured scopes differently for photo and visual, if true, serves to illustrate the principle that circumstances should dictate the scope and not the other way around.

 

The circumstances and the use case under discussion speaks to stars, the mast majority of which emit most dominantly in red. No optical design for general observation should focus solely on one wavelength to the severe detriment of others, and the best should strive for excellent correction in all colors of interest. The best scopes do.

[Polyphemos]

When designing optics, one always optimizes the design for the intended purpose.

….

 

It would seem to me that for amateur astronomy purposes, composing esoteric designs to correct for specific wavelengths is likely more trouble than it's worth, especially since there's already solution available: slow reflecting telescopes.

Lol, I was wondering when one of our reflector brethren would make an appearance. Welcome?

[Souldrop]

Polyp, something to keep firmly in mind is even a star whose peak is in the red range will have more total energy spread across the green and blue parts of the visible spectrum as opposed to energy available in the visible red spectrum.

Edited by Souldrop, 19 July 2024 - 03:44 PM.

[db2005]

Lol, I was wondering when one of our reflector brethren would make an appearance. Welcome?

I'm not actually a big reflector fan despite having owned several over the years. They don't suit my climate and observing habits well, so I do most of my observing in refractors.

 

But I think we'll have to give credit where credit is due: reflectors don't need to deal with color correction problems.

Edited by db2005, 19 July 2024 - 03:50 PM.

[RichA]

The issue of whether scopes should be nulled in green or red has been raised in another thread.  Correction color choice isn’t the theme of that thread, but I’m sufficiently interested in the question and so I present it here. The title is posed in the form of a question because I don’t think it’s been critically examined sufficiently to put in the form of a statement, and I invite that necessary critical examination so that the question, modified into the form of a statement, can be confidential declared or discarded. Since some discussion has already been made on the topic I’ll repeat those opening remarks to save both time and effort.

 

I wrote the following in the aforementioned thread:

 
….”

Being nulled in the red is what the SCT makers used to do to post overly-optimistic wavefront figures.

Edited by RichA, 19 July 2024 - 03:50 PM.

[hendric]

I think non linearity can play a part. Since the effect is a circular spot, a deviation of 2, 0, and -2 would give an area factor of 2x2 + -2x-2 = 8. But if all the deviation is pushed to one side so that it becomes 4, 2, and 0 that would give an area factor of 4x4 + 2x2 = 20

 

Since the curves aren't linear in reality, shifting the null could make the far color much worse than this.

[lylver]

Being nulled in the red is what the SCT makers used to do to post overly-optimistic wavefront figures.

This is always a good idea, because you can ajust slightly the null SA with changing the spacing between secondary and primary.

 

They tunes and measure in red 633nm HeNe to get the best and they change the spacing (~mm) to readjust.

When you have a flexible formula like the SCT, this is not a problem. HeNe laser were the only stable and affordable solution for interferometry 30 years ago. Green Nd:YAGs are recents.

 

Maybe they changed to laser green 532nm now at Torrance (because of the Edge HD : I don't know)

Edited by lylver, 19 July 2024 - 04:03 PM.

[Polyphemos]

Polyp, something to keep firmly in mind is even a star whose peak is in the red range will have more total energy spread across the green and blue parts of the visible spectrum as opposed to energy available in the visible red spectrum.

Interesting point, though I confess I don’t know how this might play out in terms of observing. More energy doesn’t mean brighter, and much of the most energetic spectra is invisible to our eyes. 

[Jeff B]

It has existed and still exist scopes that minimize SA.

In the 19th century Alvan Clark scopes, including the big Lick and Yerkes refractors do that.

 

They use the d'Alembert-Gauss property to null two wavelengths using the spacing between the crown and flint of an achromat.

So between the two wavelengths the cool fact is that SA is minimized.

 

Takahashi and Vixen did this too (TOA and AX103SD)

AX103SD.JPG

 

One big goal for those lenses was spectroscopy and you need to be accurate on a large range of wavelength to show rays and their position.

 

This is useful for human vision too, we have the abilty to focus our attention on details, this not only mental, but effective for eye focus position.

 

I agree with the fact that if you want accurate SA on many wavelength, it is difficult with short f/D scope.

 

This is one reason why I appreciate long achromat even if the range of color focused is narrow : we use filters.

This is an old way to search for specific details when you observe.

This is old fashion now, most of people watch celestial objects for mainly pleasure, so they prefer the all in one capability in a refractors. This cost much and many times not so perfect as in the past.

 

Danjon & Couder wrote about a tautochronism criteria, it meant to have SA small as 1/12 lambda PtV from green (turquoise) 520nm to cherry red 633nm. This is a criteria to catch enough contrast / resolution on the targetted wavelength in this range when you use filters. The eye the in the narrow range of color exposed focuses on it and can take advantage of this precise analysis.

Coupled to spectroscopy, this is the way astrophysician made hypothesis on the composition of Jupiter, Saturn etc.

 

-----------

 

Another possibility is to use also aspherisation as AGEMA does with their fantastic products.

But sometimes the tools (spacing, aspherisation) are not perfects or have drawbacks.

You touch on something that has taken me some decades to figure out.

 

Why am I not that bothered by the large amounts of secondary spectrum at red and blue that there can be in achromats?   

 

Or more precisely, for a given aperture, why do I prefer a slow achromat with a red to blue polychromatic Strehl of say, .5, compared to a fast ED doublet/triplet with the same polychromatic Strehl?  

 

Jeff

[RichA]

This is always a good idea, because you can ajust slightly the null SA with changing the spacing between secondary and primary.

 

They tunes and measure in red 633nm HeNe to get the best and they change the spacing (~mm) to readjust.

When you have a flexible formula like the SCT, this is not a problem. HeNe laser were the only stable and affordable solution for interferometry 30 years ago. Green Nd:YAGs are recents.

 

Maybe they changed to laser green 532nm now at Torrance (because of the Edge HD : I don't know)

No, it was a con game as they were very happy to say, "1/25th wave RMS!!!"  In red light.  Celestron even issued certificates.  Then work out that it was really about 1/4 wave P-V in Green, at best, and more often (at the time) 1/2 wave.