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How much strehl difference is actually detectable?

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#201 LDW47

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Posted 07 May 2021 - 12:18 PM

The post has been deleted, Bob. Not for you, but out of respect to the moderator and TOS!

Your acidic nature and posts mean nothing to me. They only serve to inflame the conversation.

If you had read the post you would understand I was speaking of braggarts who look down on what others have. Nothing to do with those who have expensive equipment!

And yes, I can give specific examples.  Very specific examples. But I’ll not play your juvenile game of inciting an argument.

I would like a copy of your post, I will enlarge it and hang it in my scope room !


 

#202 LDW47

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Posted 07 May 2021 - 12:20 PM

Indeed.  

 

Maybe there is something to this...  Maybe we're anti-social because we're out alone using our high-end, better-then-yours telescopes?  More seriously, I have never seen anyone overtly "brag" about a telescope in a manner which made another member feel inferior.  I have seen - many times - the opposite occur.  Where folks who have modest equipment will bait those that have boutique brand stuff into defending why they laid the cash out for such high-end equipment.  Some of the very same member(s) participating in this thread are guilty of this.  

 

In any event, I posted a comment early on about a direct comparison between my TEC160FL, which is an honest 1/8 wave optic and an AP155EDF that is at least (per A-P) .984 Strehl and 1/10 wave PtV.  I could detect a difference in the the star test, as could a friend.  The A-P was the better scope.  It put slightly less light into the diffraction ring(s) and has less "junk" floating around in the diffraction rings.   This was only noticeable in the star test at very high power.  I will say that to me, the TEC seemed to have slightly better color correction. 

 

Anyway, based on the lack of commentary on what I wrote, perhaps no one is really interested in real world experiences trying to address the OP's question.

You are correct most aren’t I would suspect !


 

#203 Wildetelescope

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Posted 07 May 2021 - 12:21 PM

For example, suppose you have 2 scopes of identical aperture (and preferable same brand and model). One tested .94 while the other tested .98 (again, just an example)

 

What I want to know is:

 

1. How much magnification was needed to see the difference? For example, did it take 400x to even begin to see the difference or was it obvious at just 150x?

2. Any difference in Lunar/planetary or deep-sky contrast?

3. Are the differences detectable by the casual observer or did it take an experienced amateur to see it?

4. Anything else?

The Strehl ratio is the ratio of the aberrated image intensity of a point source compared to the image intensity of a hypothetical perfect optic, with no aberrations.   The math does not care what kind of aberrations are considered, since it is a comparison of measured vs ideal.  SO, in a controlled laboratory setting the assumption is the that the only contribution to deviations from the ideal case are due to aberrations within the optics.  This is why this measurement is PRIMARILY used by opticians when they are figuring the optics.   However, in the field, atmospheric disturbances, tube currents etc.. will all conspire to drop your "observed" Strehl on a given night.  If you have two scopes side by side that are different but close in optical quality, the quality difference will become more evident undergo better seeing conditions.  Strehl is ALSO a function of wavelength.   It is important to understand that Strehl is a SINGLE measurement that is a bit of a black box. It is possible to have a lens that has a >.9 Strehl in the green that drops off precipitously as you go into the red and blue.  Most measurements provided by manufacturers are for 1 wavelength in the green part of the spectrum.

 

Personally, side by side, I can tell the difference between a scope with a measured Strehl in the green of ~0.94 from a mass produced scope of the same design (likely between 0.85 and 0.9 at a guess) on an poor day of seeing(~4-5 on the Pickering scale).  The key difference is just a bit more pop in the contrast between the ice cap, planet and a dust storm on Mars.  The 0.94 scope was also easier to get into focus.  These differences were subtle in these conditions and I probably would not have noticed them if I had not had the scopes side by side.   This of course may have been different on a better night of seeing, or if the difference was between say 0.94 and 0.98.  It is all dependent on the conditions and the observer.  

 

Whether or not you would detect a difference between a 0.94 and a 0.98 Strehl optic depends on several things including:

1. your experience as an observer

2. the quality of your seeing conditions

3. the target you are looking at

4. the magnification that you are using. 

 

ASSUMING that the Strehl measurement for a single wavelength accurately captures the general optical quality for both optics across all variables, the differences would likely be subtle, best observed on targets like Jupiter and close double stars, and most noticeable in a side by side comparison.  

 

I think that summarizes my own experience and what a number of very knowledgeable folks here have already said. Multiple times.  

 

Sadly, a discussion like this ALWAYS gets convolved with economic considerations.  I am sympathetic to the point of view that newbies spend WAY too much time worrying about getting the last % of optical quality in their purchase instead of purchasing an affordable telescope with the confidence that they will have fun. MOST refractors today using ED glass (and even a lot of achromats) are quite good and will show you much more than your eyes alone.  Part of the issue is that people seeking information get into the middle of conversations between very experienced observers, who do not always provide detailed context for their choices and are debating very fine differences.  That leaves many folks with the impression that they have to spend thousands of dollars on a refractor in order to not have huge flares of color or to be able to get a pleasing image. This is of course is silly. 

 

On the other side, there is a vocal group of people who  imply or directly state that anyone that pays the price for a Tak, AP telescope or a Televue, Nikon, Zeiss eyepiece are either foolish, or pretentious or have more money than sense since there is no discernible difference between one of those and mass production telescope or eyepiece.   This is also clearly not true.  Whether or not the differences are enough to justify a purchase is subjective and varies from individual to individual.  People who are accomplished observers, appreciate the differences that they see, and have the cash to purchase premium brand optics are not fanatics or deluded.   Honest folks can come to completely different conclusions regarding relative value based on their own expectations. The simple fact is that a high end refractor costs less than a motorcycle, a nice Bass boat, snowmobile or some hunting rifles.  ALL hobbies can get expensive, and dollar for dollar, astronomy is actually quite cheap in my experience.  If Astronomy is your main hobby and you want to have the best, go forward and God Bless.    

 

In the end, the best way to determine relative value is to look through the scope and see if you like it.  Barring that, price is probably the best indicator of quality for optics.  That is not a satisfying answer, but it is what we have got to work with.  If you can afford a Tak or an AP, you are curious about how good, good can be, and you will USE the scope consistently, then buy one and you will very likely not be disappointed.  If you cannot, then buy what you can, use it and have fun knowing that your views and/or images exceed what was possible for the professional during most of Gallileo, Newton and Einstein's lives. 

 

Finally, saying that there is a subtle difference between what you would see in optics with 0.94 and 0.98 strehl under good conditions is NOT the same thing as saying there is no meaningful difference and claiming otherwise oversimplifies things and can be misleading.  I can acknowledge and appreciate the quality of views I have been privileged to observe from a modern Astrophysics scope and still be completely satisfied using my 80 mm Astrotech EDT that costs almost 1/5th the price for a comparable aperture.   In the end, the hobby is about what is in the sky, not the gear.  The universe is infinite and life is too short to spend much time worrying about these things.

 

Cheers!

 

JMD


Edited by Wildetelescope, 07 May 2021 - 12:26 PM.

 

#204 RLK1

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Posted 07 May 2021 - 12:35 PM

The Strehl ratio is the ratio of the aberrated image intensity of a point source compared to the image intensity of a hypothetical perfect optic, with no aberrations.   The math does not care what kind of aberrations are considered, since it is a comparison of measured vs ideal.  SO, in a controlled laboratory setting the assumption is the that the only contribution to deviations from the ideal case are due to aberrations within the optics.  This is why this measurement is PRIMARILY used by opticians when they are figuring the optics.   However, in the field, atmospheric disturbances, tube currents etc.. will all conspire to drop your "observed" Strehl on a given night.  If you have two scopes side by side that are different but close in optical quality, the quality difference will become more evident undergo better seeing conditions.  Strehl is ALSO a function of wavelength.   It is important to understand that Strehl is a SINGLE measurement that is a bit of a black box. It is possible to have a lens that has a >.9 Strehl in the green that drops off precipitously as you go into the red and blue.  Most measurements provided by manufacturers are for 1 wavelength in the green part of the spectrum.

 

Personally, side by side, I can tell the difference between a scope with a measured Strehl in the green of ~0.94 from a mass produced scope of the same design (likely between 0.85 and 0.9 at a guess) on an poor day of seeing(~4-5 on the Pickering scale).  The key difference is just a bit more pop in the contrast between the ice cap, planet and a dust storm on Mars.  The 0.94 scope was also easier to get into focus.  These differences were subtle in these conditions and I probably would not have noticed them if I had not had the scopes side by side.   This of course may have been different on a better night of seeing, or if the difference was between say 0.94 and 0.98.  It is all dependent on the conditions and the observer.  

 

Whether or not you would detect a difference between a 0.94 and a 0.98 Strehl optic depends on several things including:

1. your experience as an observer

2. the quality of your seeing conditions

3. the target you are looking at

4. the magnification that you are using. 

 

ASSUMING that the Strehl measurement for a single wavelength accurately captures the general optical quality for both optics across all variables, the differences would likely be subtle, best observed on targets like Jupiter and close double stars, and most noticeable in a side by side comparison.  

 

I think that summarizes my own experience and what a number of very knowledgeable folks here have already said. Multiple times.  

 

Sadly, a discussion like this ALWAYS gets convolved with economic considerations.  I am sympathetic to the point of few that newbies spend WAY too much time worrying about getting the last % of optical quality in their purchase instead of purchasing an affordable telescope with the confidence that they will have fun. MOST refractors today using ED glass (and even a lot of achromats) are quite good and will show you much more than your eyes alone.  Part of the issue is that people seeking information get into the middle of conversations between very experienced observers, who do not always provide detailed context for their choices and are debating very fine differences.  That leaves many folks with the impression that they have to spend thousands of dollars on a refractor in order to not have huge flares of color or to be able to get a pleasing image. This is of course is silly. 

 

On the other side, there is a vocal group of people who  imply or directly state that anyone that pays the price for a Tak, AP telescope or a Televue, Nikon, Zeiss eyepiece are either foolish, or pretentious or have more money than sense since there is no discernible difference between one of those and mass production telescope or eyepiece.   This is also clearly not true.  Whether or not the differences are enough to justify a purchase is subjective and varies from individual to individual.  People who are accomplished observers, appreciate the differences that they see, and have the cash to purchase premium brand optics are not fanatics or deluded.   Honest folks can come to completely different conclusions regarding relative value based on their own expectations. The simple fact is that a high end refractor costs less than a motorcycle, a nice Bass boat, snowmobile or some hunting rifles.  ALL hobbies can get expensive, and dollar for dollar, astronomy is actually quite cheap in my experience.  If Astronomy is your main hobby and you want to have the best, go forward and God Bless.    

 

In the end, the best way to determine relative value is to look through the scope and see if you like it.  Barring that, price is probably the best indicator of quality for optics.  That is not a satisfying answer, but it is what we have got to work with.  If you can afford a Tak or an AP, you are curious about how good, good can be, and you will USE the scope consistently, then buy one and you will very likely not be disappointed.  If you cannot, then buy what you can, use it and have fun knowing that your views and/or images exceed what was possible for the professional during most of Gallileo, Newton and Einstein's lives. 

 

Finally, saying that there is a subtle difference between what you would see in optics with 0.94 and 0.98 strehl under good conditions is NOT the same thing as saying there is no meaningful difference and claiming otherwise oversimplifies things and can be misleading.  I can acknowledge and appreciate the quality of views I have been privileged to observe from a modern Astrophysics scope and still be completely satisfied using my 80 mm Astrotech EDT that costs almost 1/5th the price for a comparable aperture.   In the end, the hobby is about what is in the sky, not the gear.  The universe is infinite and life is too short to spend much time worrying about these things.

 

Cheers!

 

JMD

If you believe there's an observable difference between optics having a strehl ratio of 0.94 vs 0.98 and all other factors accounted for and assuming an experienced observer with excellent conditions, would you, in a hypothetical case, do the following: Return the scope with the lessor strehl ratio(0.94) in favor of one having the higher ratio(0.98), given a 30 day satisfaction guaranteed return policy? 


 

#205 LDW47

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Posted 07 May 2021 - 12:43 PM

The Strehl ratio is the ratio of the aberrated image intensity of a point source compared to the image intensity of a hypothetical perfect optic, with no aberrations.   The math does not care what kind of aberrations are considered, since it is a comparison of measured vs ideal.  SO, in a controlled laboratory setting the assumption is the that the only contribution to deviations from the ideal case are due to aberrations within the optics.  This is why this measurement is PRIMARILY used by opticians when they are figuring the optics.   However, in the field, atmospheric disturbances, tube currents etc.. will all conspire to drop your "observed" Strehl on a given night.  If you have two scopes side by side that are different but close in optical quality, the quality difference will become more evident undergo better seeing conditions.  Strehl is ALSO a function of wavelength.   It is important to understand that Strehl is a SINGLE measurement that is a bit of a black box. It is possible to have a lens that has a >.9 Strehl in the green that drops off precipitously as you go into the red and blue.  Most measurements provided by manufacturers are for 1 wavelength in the green part of the spectrum.

 

Personally, side by side, I can tell the difference between a scope with a measured Strehl in the green of ~0.94 from a mass produced scope of the same design (likely between 0.85 and 0.9 at a guess) on an poor day of seeing(~4-5 on the Pickering scale).  The key difference is just a bit more pop in the contrast between the ice cap, planet and a dust storm on Mars.  The 0.94 scope was also easier to get into focus.  These differences were subtle in these conditions and I probably would not have noticed them if I had not had the scopes side by side.   This of course may have been different on a better night of seeing, or if the difference was between say 0.94 and 0.98.  It is all dependent on the conditions and the observer.  

 

Whether or not you would detect a difference between a 0.94 and a 0.98 Strehl optic depends on several things including:

1. your experience as an observer

2. the quality of your seeing conditions

3. the target you are looking at

4. the magnification that you are using. 

 

ASSUMING that the Strehl measurement for a single wavelength accurately captures the general optical quality for both optics across all variables, the differences would likely be subtle, best observed on targets like Jupiter and close double stars, and most noticeable in a side by side comparison.  

 

I think that summarizes my own experience and what a number of very knowledgeable folks here have already said. Multiple times.  

 

Sadly, a discussion like this ALWAYS gets convolved with economic considerations.  I am sympathetic to the point of view that newbies spend WAY too much time worrying about getting the last % of optical quality in their purchase instead of purchasing an affordable telescope with the confidence that they will have fun. MOST refractors today using ED glass (and even a lot of achromats) are quite good and will show you much more than your eyes alone.  Part of the issue is that people seeking information get into the middle of conversations between very experienced observers, who do not always provide detailed context for their choices and are debating very fine differences.  That leaves many folks with the impression that they have to spend thousands of dollars on a refractor in order to not have huge flares of color or to be able to get a pleasing image. This is of course is silly. 

 

On the other side, there is a vocal group of people who  imply or directly state that anyone that pays the price for a Tak, AP telescope or a Televue, Nikon, Zeiss eyepiece are either foolish, or pretentious or have more money than sense since there is no discernible difference between one of those and mass production telescope or eyepiece.   This is also clearly not true.  Whether or not the differences are enough to justify a purchase is subjective and varies from individual to individual.  People who are accomplished observers, appreciate the differences that they see, and have the cash to purchase premium brand optics are not fanatics or deluded.   Honest folks can come to completely different conclusions regarding relative value based on their own expectations. The simple fact is that a high end refractor costs less than a motorcycle, a nice Bass boat, snowmobile or some hunting rifles.  ALL hobbies can get expensive, and dollar for dollar, astronomy is actually quite cheap in my experience.  If Astronomy is your main hobby and you want to have the best, go forward and God Bless.    

 

In the end, the best way to determine relative value is to look through the scope and see if you like it.  Barring that, price is probably the best indicator of quality for optics.  That is not a satisfying answer, but it is what we have got to work with.  If you can afford a Tak or an AP, you are curious about how good, good can be, and you will USE the scope consistently, then buy one and you will very likely not be disappointed.  If you cannot, then buy what you can, use it and have fun knowing that your views and/or images exceed what was possible for the professional during most of Gallileo, Newton and Einstein's lives. 

 

Finally, saying that there is a subtle difference between what you would see in optics with 0.94 and 0.98 strehl under good conditions is NOT the same thing as saying there is no meaningful difference and claiming otherwise oversimplifies things and can be misleading.  I can acknowledge and appreciate the quality of views I have been privileged to observe from a modern Astrophysics scope and still be completely satisfied using my 80 mm Astrotech EDT that costs almost 1/5th the price for a comparable aperture.   In the end, the hobby is about what is in the sky, not the gear.  The universe is infinite and life is too short to spend much time worrying about these things.

 

Cheers!

 

JMD

So if this is how simple it is, the rational of it all how did this thread go on to over 200 posts ?? There are other factors to this but I am sure not getting into it, it all sounds great ! Unfortunately in the real world, this world it is not always as easy, there is no ‘Pie in the Sky’ ! You figure the rest out !


Edited by LDW47, 07 May 2021 - 01:01 PM.

 

#206 Asbytec

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Posted 07 May 2021 - 01:12 PM

Sadly, a discussion like this ALWAYS gets convolved with economic considerations. 

 

In the end, the best way to determine relative value is to look through the scope and see if you like it...and you will USE the scope consistently, then buy one and you will very likely not be disappointed.  

 

In the end, the hobby is about what is in the sky, not the gear. 

 

Cheers!

 

JMD

I believe it is as simple as this. Well, one minor detail. Remember, the human is the observer, the telescope observes nothing. Observing is a very human endeavor making the best of the scope we chose. Hopefully it's not a dud, chances are it's a good one, and you're fortunate if it is as good as they can be. Whatever the case, learn to emphasize the human element and rely less on the scope to deliver the image in all it's glory on a silver platter. To say it another way, the scope is really only as good as what we can make of the image it offers us. You will not be disappointed, either, with what you can learn to see in a good scope under favorable conditions. So pray for favorable observing conditions. Scope preparation, observer skill, and observing conditions matter more than a few tenths of the Strehl ratio. The best scope is one that get's used, and it gets used because it pleases us to use it. That's two or three minor details, I guess. :) 


 

#207 vahe

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Posted 07 May 2021 - 01:23 PM

"Personally, side by side, I can tell the difference between a scope with a measured Strehl in the green of ~0.94 from a mass produced scope of the same design (likely between 0.85 and 0.9 at a guess) on an poor day of seeing(~4-5 on the Pickering scale).  The key difference is just a bit more pop in the contrast between"

 

Typically in a mass produced scope the Strehl comes from the design, I happen to believe that in mass produced scopes with low price tag it is highly unlikely for the manufacturer to spend that extra time and money to arrive at a measured Strehl.

.

Vahe


Edited by vahe, 07 May 2021 - 01:24 PM.

 

#208 GSBass

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Posted 07 May 2021 - 01:23 PM

Economically speaking… if money was not a concern I would buy the best regardless of whether I could tell the difference but that’s just how I roll… I like the best of everything, it’s why my cars are older, have to wait for them to depreciate to my budget level, unfortunately astronomy market does not work that way


 

#209 peleuba

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Posted 07 May 2021 - 01:46 PM

Typically in a mass produced scope the Strehl comes from the design, I happen to believe that in mass produced scopes with low price tag it is highly unlikely for the manufacturer to spend that extra time and money to arrive at a measured Strehl.

 

 

Agree.  The Strehl coming from design software is ALWAYS optimistic because it assumes a best case scenario where the glass company had a perfect melt, with no striae or inclusions and is annealed well.  Additionally, it assumes that the optician figured the lens perfectly.  Rarely does design = reality.  Even in some of the high-end brands.  It just does not happen.  

 

And what drives me nuts are the folks who see design Strehl listed on the various manufacturer or reseller websites and confuse that with actual reality. 

 

As an aside, all this talk about Strehl ratios that we amateurs participate in is amusing to the professional optics crowd.  They almost exclusively look to RMS as the metric of choice when writing proposal and requirements documents.


Edited by peleuba, 07 May 2021 - 01:53 PM.

 

#210 t.r.

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Posted 07 May 2021 - 01:57 PM

Indeed.  
 
Maybe there is something to this...  Maybe we're anti-social because we're out alone using our high-end, better-then-yours telescopes?  More seriously, I have never seen anyone overtly "brag" about a telescope in a manner which made another member feel inferior.  I have seen - many times - the opposite occur.  Where folks who have modest equipment will bait those that have boutique brand stuff into defending why they laid the cash out for such high-end equipment.  Some of the very same member(s) participating in this thread are guilty of this.  
 
In any event, I posted a comment early on about a direct comparison between my TEC160FL, which is an honest 1/8 wave optic and an AP155EDF that is at least (per A-P) .984 Strehl and 1/10 wave PtV.  I could detect a difference in the the star test, as could a friend.  The A-P was the better scope.  It put slightly less light into the diffraction ring(s) and has less "junk" floating around in the diffraction rings.   This was only noticeable in the star test at very high power.  I will say that to me, the TEC seemed to have slightly better color correction. 
 
Anyway, based on the lack of commentary on what I wrote, perhaps no one is really interested in real world experiences trying to address the OP's question.


Before this thread gets locked down, I'd like to add my comparison between a TEC 140 and an AP 140 as a data point to support Paul's observation. In a side-by-side myself and LLEEGE noted no difference in planetary images between these two refractors however, when we went into star testing and in particular resolving Porimma, we noticed less light in the diffraction ring and the split was cleaner/better in the AP as well over the TEC, so both of us would say, YES, you can visually detect a slight difference in strehl/rms if you know where and how to go look for it! Hint...it isn't in full disk planetary viewing!!!
 

#211 Wildetelescope

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Posted 07 May 2021 - 02:13 PM

If you believe there's an observable difference between optics having a strehl ratio of 0.94 vs 0.98 and all other factors accounted for and assuming an experienced observer with excellent conditions, would you, in a hypothetical case, do the following: Return the scope with the lessor strehl ratio(0.94) in favor of one having the higher ratio(0.98), given a 30 day satisfaction guaranteed return policy? 

I will answer that hypothetical this way.   If I had the two such scopes in my hands, I would spend 3 or 4 nights looking through them and make my decision based on what I see, as well as how it performs mechanically ease of setup etc... If we are talking about two otherwise identical scopes costing the same amount, of course I would take the higher Strehl. Who wouldn't?  that said, if the only option was the 0.94 strehl scope, I would not hesitate to buy it.  

 

The actual example I cited in my previous approach was between two six inch maks.  I had owned and enjoyed the mass produced synta mak for years.  I had the option to get a Used Intes Micro for essentially the same price as I paid for the Synta mak.  I saw a difference, and kept the scope that gave the very slightly better view.  Of course.  That said, I personally would not pay full price for an Intes compared to the Synta mak given the very small differences I saw.   I have a very nice 80 mm EDT that I image with.  No idea what its Strehl rating is, but I have no desire at the moment to run out and look for an AP traveler, or for that matter the very nice AT 92 that my buddy has and is truly excellent.    I know what I want to do, and my expectations are met with the 80 mm EDT.   That does not mean I do not recognize the quality of the other two ~90 mm scopes.  At some point when my imaging skills improve, and finances allow, I might feel the urge to upgrade.    

 

Cheers

JMD


 

#212 Wildetelescope

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Posted 07 May 2021 - 02:17 PM

"Personally, side by side, I can tell the difference between a scope with a measured Strehl in the green of ~0.94 from a mass produced scope of the same design (likely between 0.85 and 0.9 at a guess) on an poor day of seeing(~4-5 on the Pickering scale).  The key difference is just a bit more pop in the contrast between"

 

Typically in a mass produced scope the Strehl comes from the design, I happen to believe that in mass produced scopes with low price tag it is highly unlikely for the manufacturer to spend that extra time and money to arrive at a measured Strehl.

.

Vahe

My guess was based on how close it performed relative to the Intes.  I believe that it was at least diffraction limited based the star tests I had done over the years with it.   The measured scope was an Intes Micro 603.   

 

JMD


 

#213 peleuba

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Posted 07 May 2021 - 02:32 PM

Before this thread gets locked down, I'd like to add my comparison between a TEC 140 and an AP 140 as a data point to support Paul's observation. In a side-by-side myself and LLEEGE noted no difference in planetary images between these two refractors however, when we went into star testing and in particular resolving Porimma, we noticed less light in the diffraction ring and the split was cleaner/better in the AP as well over the TEC, so both of us would say, YES, you can visually detect a slight difference in strehl/rms if you know where and how to go look for it! Hint...it isn't in full disk planetary viewing!!!

 

Them A-P's are somthin' else, Huh Tim? 

 

lol.gif


 

#214 bobhen

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Posted 07 May 2021 - 02:51 PM

"Personally, side by side, I can tell the difference between a scope with a measured Strehl in the green of ~0.94 from a mass produced scope of the same design (likely between 0.85 and 0.9 at a guess) on an poor day of seeing(~4-5 on the Pickering scale).  The key difference is just a bit more pop in the contrast between"

 

Typically in a mass produced scope the Strehl comes from the design, I happen to believe that in mass produced scopes with low price tag it is highly unlikely for the manufacturer to spend that extra time and money to arrive at a measured Strehl.

.

Vahe

Agree.

Synta/Clestron’s guarantee is "diffraction-limited" only because they know they can deliver that level of quality quickly, consistently and at a competitive price point that “maximizes” profitability. More time spent on the optics is just not profitable.

 

Bob


 

#215 Deadlake

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Posted 07 May 2021 - 02:59 PM

Before this thread gets locked down, I'd like to add my comparison between a TEC 140 and an AP 140 as a data point to support Paul's observation. In a side-by-side myself and LLEEGE noted no difference in planetary images between these two refractors however, when we went into star testing and in particular resolving Porimma, we noticed less light in the diffraction ring and the split was cleaner/better in the AP as well over the TEC, so both of us would say, YES, you can visually detect a slight difference in strehl/rms if you know where and how to go look for it! Hint...it isn't in full disk planetary viewing!!!


I know someone who had a TEC140 and a AP130GTX, on side by side testing the AP edged out the TEC and so the TEC got sold. If this all down to Strehl, then they would be so close as not to be noticeable.

My 2 cents, now I’m getting the pop corn while I read this thread.
 

#216 bobhen

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Posted 07 May 2021 - 03:04 PM

I know someone who had a TEC140 and a AP130GTX, on side by side testing the AP edged out the TEC and so the TEC got sold. If this all down to Strehl, then they would be so close as not to be noticeable.

My 2 cents, now I’m getting the pop corn while I read this thread.

Even between high-end telescopes, differences can be seen. In Ed Ting’s comparison of a Takahashi 152mm and an Astro-Physics 155mm refractor and in another between the Astro-Physics 155 and a Takahashi 150mm FCT triplet, all 5 or 6 observers voted (in secret) for the AP 155. They were very close but the voting was "unanimous" for the AP 

 

HERE is a link

 

HERE is the second link  

 

Bob


 

#217 vahe

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Posted 07 May 2021 - 03:19 PM

Them A-P's are somthin' else, Huh Tim? 

 

lol.gif

Astro-Physics and TEC do not offer test results for their scopes, Roland claims a minimum of 1/10 wave PV and TEC 1/8 wave PV. Personally I prefer the PV over Strehl if a scope comes from a reputable manufacturer, the reason I prefer PV is because it is a simple number that tells the story. For Strehl to tell the story it needs to be polychromatic and not just based on single color, with single color even an achromat can offer a Strehl of 0.99 (D&G) but you do not want to know its polychromatic Strehl.

.

Vahe


 

#218 dan_1984

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Posted 07 May 2021 - 04:02 PM

I see this long discution and I keep wondering, do you guys really stress so much over this Strehl number? As I said a few posts back, I got a nice frac from CFF a while back and I don’t know the RMS or Strehl. When I got the scope those numbers were written somewhere ...
When the weather allows, the scope puts up the best views. 
I wonder if the reflector folks worry much about this? 
I have an odd feeling that refractors are so good that the only thing to argue is Strehllol.gif

Or ... when you stress about the Strehl, think of what the reflector folk have to put up with: collimation, thermal management, dew everywhere, dirty mirrors, mirror flop-shift, tube currents, mirror coatings , secondary obstruction, SCTs etc.smile.gif

 


Edited by dan_1984, 07 May 2021 - 04:08 PM.

 

#219 Bowlerhat

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Posted 07 May 2021 - 06:08 PM

I see this long discution and I keep wondering, do you guys really stress so much over this Strehl number? As I said a few posts back, I got a nice frac from CFF a while back and I don’t know the RMS or Strehl. When I got the scope those numbers were written somewhere ...
When the weather allows, the scope puts up the best views. 
I wonder if the reflector folks worry much about this? 
I have an odd feeling that refractors are so good that the only thing to argue is Strehllol.gif

Or ... when you stress about the Strehl, think of what the reflector folk have to put up with: collimation, thermal management, dew everywhere, dirty mirrors, mirror flop-shift, tube currents, mirror coatings , secondary obstruction, SCTs etc.smile.gif

Some people stress over the numbers and data instead of practical observation of what's happening in the field.


 

#220 CHASLX200

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Posted 07 May 2021 - 06:14 PM

Some people stress over the numbers and data instead of practical observation of what's happening in the field.

I don't go by numbers. Just give me Jupiter and it's moons and i will know if got a winner or a so so scope.


 

#221 Suavi

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Posted 07 May 2021 - 07:05 PM

Astro-Physics and TEC do not offer test results for their scopes, Roland claims a minimum of 1/10 wave PV and TEC 1/8 wave PV. Personally I prefer the PV over Strehl if a scope comes from a reputable manufacturer, the reason I prefer PV is because it is a simple number that tells the story. For Strehl to tell the story it needs to be polychromatic and not just based on single color, with single color even an achromat can offer a Strehl of 0.99 (D&G) but you do not want to know its polychromatic Strehl.

.

Vahe

1/8 PV expressed in red wavelength equates to about to 1/5 PV in blue wavelength. Just saying wink.gif


 

#222 LDW47

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Posted 07 May 2021 - 08:00 PM

Some people stress over the numbers and data instead of practical observation of what's happening in the field.

You are so right, lol !


Edited by LDW47, 07 May 2021 - 08:00 PM.

 

#223 LDW47

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Posted 07 May 2021 - 08:02 PM

I think Svbony are starting to give their Strehl numbers, I think ?


 

#224 barbie

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Posted 07 May 2021 - 08:06 PM

The final image at the eyepiece on a night of good seeing tells me all I need to know!!


Edited by barbie, 07 May 2021 - 08:15 PM.

 

#225 teashea

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Posted 07 May 2021 - 09:04 PM

Shots fired lol.gif lol.gif

I better duck...................

 

 

177674383_10222081006908584_6464730682490335532_n.jpg


 


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