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False colour visual tests on 5 apos


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I have owned five apo/semi-apo refractors over the past decade. From the start, I’ve subjected them to the same rudimentary visual tests, one of which is to determine how well they control false colour. I thought my impressions might be worth sharing after I recently took delivery of a little 60ED which performs rather well.
Not much science involved, I must confess. I simply test them with an artificial star, and then observing tree branches against a bright background sky. I have mostly used orthoscopic eyepieces for the tests, together with Plossls and high power Takahashi TOE eyepieces. I also test them with Baader Zeiss T2 prism and Baader T2 BBHS mirror diagonals. And I repeat the tests over time, whenever possible side by side, to ensure comparisons are as meaningful as possible.

The telescopes are as follows:     
SW Equinox 80ED, FPL53, F/6.25 doublet.  
Takahashi FC-100DC, F/7.4, fluorite doublet.  
TeleVue 85,F/7, ED doublet.   
Takahashi TSA-120, F/7.5, FPL53 triplet.   
Tecnosky 60ED, F/6, FPL53 doublet.   
       
Here’s how they perform in ascending order, with CA control rated out of ten:

5th:  6/10 - SW Equinox 80ED
CA was most obvious with the Skywatcher. Violet edges to branches and on the lunar limb, even in focus. Though it always gave sharp and pleasing views, even the planets at 180x, its relatively fast speed was clear on bright objects.

4th: 7/10 - TV85
The TV85 is a wonderful scope. So flexible and beautifully built. But it can exhibit small amounts of CA under demanding conditions. When doing the tree test, marginally out of focus branches show purple fringing. And at very high magnifications (200x and over), colour can be perceived in bright sunlight. Although it’s only fourth in this comparison, the TV85 is probably the scope I’d keep if I could only have one refractor - its value as a solar/travel/spotting/night vision/grab and go instrument means it gets the most use of all my current scopes.

3rd: 8/10 - Tak FC-100DC
Third is the Tak FC-100DC. Such a lovely scope, pretty much colour free when focused. But at high magnifications, just slight amounts of CA were discernible…… I’m being very picky with both the TV85 and FC-100 here. Both would impress the vast majority of visual observers.         

2: 9/10 - Tecnosky 60ED.
Well here’s a surprise. The fastest telescope tested. Chinese. A little over £300 new. Absolutely colour free in daytime tests. Even defocused branches don’t display any false colour. To see if the diagonal made any difference, I swapped the mirror for the Zeiss prism - reputed to not work so well with fast scopes - but the same result. Completely colourless branches, in or out of focus. Doing a bright star test, then the familiar purple and yellow rings are obvious when defocused, but a very impressive performer. 

1: 10/10 - Takahashi TSA-120.
Zero colour on the brightest stars/branches in or out of focus. Superb colour correction on all targets - but at a price!

Although I no longer have the Equinox or FC-100, all these scopes were/are great to my eyes. Unless false colour is noticeably diminishing views, then it’s of marginal importance. Hence why I would be happy to keep the TV85 as my one ‘do everything’ scope. But visual tests like these are still of technical interest, and show that Chinese FPL53 doublets can reach or even surpass some of the more expensive brands in colour correction. 

D1860A8F-F7F2-4BB6-A413-11CA5C45F1A9.jpeg

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Very interesting results Mark :smiley:

I guess with aperture being part of the CA equation, the 60mm F/6 using an FPL-53 element was always going to be in with a good chance.

Likewise at 120mm and F/7.5 it takes a triplet, also involving an FPL-53 element to completely control visible CA.

I'm sure the views are lovely with all of them ! :thumbright:

Do the chinese scopes use Japanese glass or do Ohara produce FPL-53 in China now as well as elsewhere ?

 

Edited by John
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Interesting comparison @Highburymark. A smaller aperture will always show less false colour than a larger one so I guess the 60ED with fpl-53 glass can be expected to perform well. The 80ED being worse at the same focal ratio demonstrates that.

I found the FC100DC to be a reasonably clear step up from the TV85 despite the larger aperture, although I agree the 85 is still a highly desirable scope.

The TSA120 is a cracker I’m sure, and as an f7.5 triplet from a top end brand should always be the winner, or you would hope so! I would expect the colour correction to be a smidge better than my LZOS 130mm f6 due to the smaller aperture and longer focal ratio but would be interesting to compare. Should hopefully be close though!

It would be interesting to compare equivalent aperture scopes of similar focal ratio so that the glass and manufacturer are the key differences eg FC100DC vs Technosky 102mm f7, or an FC100DL vs 100ED.

The best I can hope for currently are some clear skies to actually use the kit I’ve got, so for me it will be about observing more with whatever I happen to have put at the time. Fingers crossed! 🤞

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The TSA 120 is a real beauty. I wonder how good the 150mm is? When I get my lotto winnings I'll let you know 😜

With my 102mm Starfield f7 ED fpl-53 I've yet to detect any false colour. The moon is superbly clear and colour free. It would indeed be good to get similar sized scopes together and give them a workout.

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Thanks chaps. 
Of course colour correction is only one part of the equation. The artificial star test exposes other interesting features. The TSA and TV85 are almost textbook - identical in and out of focus (apart from the TV85 showing the expected colour difference - purple one side, yellow/green the other), but otherwise perfect, clearly contrasted rings. The Tecnosky, in comparison, shows clear, black rings one side, and much fuzzier rings on the other. Slightly less foggy with a green filter. Possibly due to spherical aberration? To determine that, I’ll need to do the 33% obstruction test. Must admit, as someone who never did sciences beyond O level, a lot of this stuff is over my head. I’m fascinated, but only as a layman. All these scopes work very well indeed.

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Personally, if the CA control is what I would expect from the aperture / glass type / focal ratio then it's the star test quality (extra, intra and at focus) that really sells a scope to me and what I think will give a scope the edge in performance, particularly at high magnifications. It says more about the optical quality (ie: figure and polish) of the objective than the colour correction does I feel, although we would like that to be good too of course :smiley:

 

 

 

Edited by John
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3 hours ago, Highburymark said:

The Tecnosky, in comparison, shows clear, black rings one side, and much fuzzier rings on the other. Slightly less foggy with a green filter.

I have certainly found this with my Starwave 102ED-R, but good performance in everyday observing. 
 

4 hours ago, Stu said:

A smaller aperture will always show less false colour than a larger one

Can you remind why this is  the case Stu? (assuming it’s possible to explain it in simple terms!) 

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1 hour ago, RobertI said:

Can you remind why this is  the case Stu? (assuming it’s possible to explain it in simple terms!) 

My knowledge is fairly limited to the content of this diagram which has been posted many times by others.

363ADFCA-53F7-4EC3-8366-F6019F8E7715.jpeg

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1 hour ago, Stu said:

My knowledge is fairly limited to the content of this diagram which has been posted many times by others.

363ADFCA-53F7-4EC3-8366-F6019F8E7715.jpeg

I remember this diagram but didn’t remember the relative difference in CA control between big and small apertures. It certainly suggests my experience with the 60ED is not particularly unusual.
But why is a good question. Is it easier to manufacture smaller aperture lenses accurately to iron out false colour, or is the light being bent less? A quick Google search suggests it’s the latter. 

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@Highburymark excellent report.

I personally like triplet APO refractors and my best is the TSA120. Rumour has it the TOA130 edges it out but I'm not sure how this is possible even though those reporting it I do believe. Many of the refractors these days are top notch thats for sure with some knocking the SW series off the stump IMHO.

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On 01/01/2022 at 02:16, Highburymark said:

I remember this diagram but didn’t remember the relative difference in CA control between big and small apertures. It certainly suggests my experience with the 60ED is not particularly unusual.
But why is a good question. Is it easier to manufacture smaller aperture lenses accurately to iron out false colour, or is the light being bent less? A quick Google search suggests it’s the latter. 

From what I understand, all things being equal the focal ratio needs to scale with the aperture in order to maintain the same level of colour correction. That would mean a 100mm doublet scope would need to be an f/10 to have as little false colour as an equivalent 60mm f/6, which sounds similar to your experience.

The details are way over my head but I get the impression that when designing a lens you have a complex set of compromises and an optician needs to decide how to balance the different aspects of performance. If you were creating a scope aimed at the visual observer then you might choose to maximise resolution in green light to correspond to the peak of the eye's sensitivity and allow the lens to be much less corrected in violet or deep red due to our relative lack of sensitivity to those wavelengths. I suspect that might have been done with the FS-60 because although it's a superbly sharp and contrasty visual scope, it's obviously a lot less well corrected at short wavelengths, especially compared to newer designs from Tak like the FC-series. Correcting the g-line to get rid of blue bloat would likely appeal to imagers but it could slightly lower performance for the visual observer or might impact correction in the yellows and reds which could show up at high magnifications when doing planetary observing.

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1 hour ago, Space Hopper said:

Interesting, but the Tak TSA120 is a triplet and all the others are doublets.

A well designed and figured triplet will always out perform a comparable doublet ?

Provided the optics don't differ in terms of using more advanced designs such as all-spherical vs aspheric or foil spaced vs wide air gaps, a well made triplet will be better than a doublet using the same ED glass. A triplet can also let you get away with using cheaper glass, so an FPL-51 triplet can be as good or better than an FPL-53 doublet of the same focal ratio. The downside is that you've got more work and cost to grind, polish, and coat 6 surfaces rather than 4 and the extra glass and larger lens cell can significantly increase the weight and cool down time.

Edit - aside from controlling simple chromatic aberration, having the additional element of a triplet can apparently be useful in correcting spherochromatism and might allow the designer to reduce other aberrations as well.

Edited by Andrew_B
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1 hour ago, Space Hopper said:

..A well designed and figured triplet will always out perform a comparable doublet ?

Often with regards to CA correction, but not 100% of the time though. I've read reports where a doublet that uses an FPL-53 element shows less CA than a triplet that uses a FPL-51 (or similar) element.

CA correction is one aspect of performance - there are others of course.

 

Edited by John
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10 hours ago, Andrew_B said:

A triplet can also let you get away with using cheaper glass, so an FPL-51 triplet can be as good or better than an FPL-53 doublet of the same focal ratio

This is likely true, but I’ve also looked through a triplet which was not as well colour corrected as a good doublet so it’s all about the design and actual implementation as to what the results are.

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On 31/12/2021 at 16:41, Highburymark said:

I have owned five apo/semi-apo refractors over the past decade. From the start, I’ve subjected them to the same rudimentary visual tests, one of which is to determine how well they control false colour. I thought my impressions might be worth sharing after I recently took delivery of a little 60ED which performs rather well.
Not much science involved, I must confess. I simply test them with an artificial star, and then observing tree branches against a bright background sky. I have mostly used orthoscopic eyepieces for the tests, together with Plossls and high power Takahashi TOE eyepieces. I also test them with Baader Zeiss T2 prism and Baader T2 BBHS mirror diagonals. And I repeat the tests over time, whenever possible side by side, to ensure comparisons are as meaningful as possible.

The telescopes are as follows:     
SW Equinox 80ED, FPL53, F/6.25 doublet.  
Takahashi FC-100DC, F/7.4, fluorite doublet.  
TeleVue 85,F/7, ED doublet.   
Takahashi TSA-120, F/7.5, FPL53 triplet.   
Tecnosky 60ED, F/6, FPL53 doublet.   
       
Here’s how they perform in ascending order, with CA control rated out of ten:

5th:  6/10 - SW Equinox 80ED
CA was most obvious with the Skywatcher. Violet edges to branches and on the lunar limb, even in focus. Though it always gave sharp and pleasing views, even the planets at 180x, its relatively fast speed was clear on bright objects.

4th: 7/10 - TV85
The TV85 is a wonderful scope. So flexible and beautifully built. But it can exhibit small amounts of CA under demanding conditions. When doing the tree test, marginally out of focus branches show purple fringing. And at very high magnifications (200x and over), colour can be perceived in bright sunlight. Although it’s only fourth in this comparison, the TV85 is probably the scope I’d keep if I could only have one refractor - its value as a solar/travel/spotting/night vision/grab and go instrument means it gets the most use of all my current scopes.

3rd: 8/10 - Tak FC-100DC
Third is the Tak FC-100DC. Such a lovely scope, pretty much colour free when focused. But at high magnifications, just slight amounts of CA were discernible…… I’m being very picky with both the TV85 and FC-100 here. Both would impress the vast majority of visual observers.         

2: 9/10 - Tecnosky 60ED.
Well here’s a surprise. The fastest telescope tested. Chinese. A little over £300 new. Absolutely colour free in daytime tests. Even defocused branches don’t display any false colour. To see if the diagonal made any difference, I swapped the mirror for the Zeiss prism - reputed to not work so well with fast scopes - but the same result. Completely colourless branches, in or out of focus. Doing a bright star test, then the familiar purple and yellow rings are obvious when defocused, but a very impressive performer. 

1: 10/10 - Takahashi TSA-120.
Zero colour on the brightest stars/branches in or out of focus. Superb colour correction on all targets - but at a price!

Although I no longer have the Equinox or FC-100, all these scopes were/are great to my eyes. Unless false colour is noticeably diminishing views, then it’s of marginal importance. Hence why I would be happy to keep the TV85 as my one ‘do everything’ scope. But visual tests like these are still of technical interest, and show that Chinese FPL53 doublets can reach or even surpass some of the more expensive brands in colour correction. 

D1860A8F-F7F2-4BB6-A413-11CA5C45F1A9.jpeg

So the interesting thing is that to my knowledge you can buy better colour correction in exchange for introducing polychromatic Spherical aberration. Also the smaller aperture of the technosky makes thing easier, but I would not be shocked if you told me that it was not the sharpest. 

None of that invalidated the test, just that you can't judge a scopes performance by assessing it against only CA.

Adam

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The question here is why should the average 60mm F/6 scope have better colour correction than a 100mm F/6 scope, assuming they are doublets and use the same glass? I have now read conflicting reports on this. One says the light bends less with the smaller scope - hence is less liable to false colour. Another claims that scopes with the same focal ratio should be equal in CA, no matter the aperture - as Andrew states in his first post above. 
If there is a difference in CA between the two scope sizes, I don’t think it’s because light is bending more. But perhaps it’s because overall, more light is bending? The larger glass is refracting a greater amount/surface area of light away from the central axis of the objective lens. 

1 hour ago, Adam J said:

So the interesting thing is that to my knowledge you can buy better colour correction in exchange for introducing polychromatic Spherical aberration. Also the smaller aperture of the technosky makes thing easier, but I would not be shocked if you told me that it was not the sharpest. 

None of that invalidated the test, just that you can't judge a scopes performance by assessing it against only CA.

Adam

Absolutely. More tests to do.

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18 minutes ago, Highburymark said:

The question here is why should the average 60mm F/6 scope have better colour correction than a 100mm F/6 scope, assuming they are doublets and use the same glass? I have now read conflicting reports on this. One says the light bends less with the smaller scope - hence is less liable to false colour. Another claims that scopes with the same focal ratio should be equal in CA, no matter the aperture - as Andrew states in his first post above. 
If there is a difference in CA between the two scope sizes, I don’t think it’s because light is bending more. But perhaps it’s because overall, more light is bending? The larger glass is refracting a greater amount/surface area of light away from the central axis of the objective lens. 

I think easiest way to explain that is like this:

Both 60mm and 100mm F/6 scopes have same geometry - same light bending, same everything - they are just "scaled" versions - like if you took 60mm and then scaled it by 10/6 factor. Lens diameter scales, focal length scales - but angles don't scale (think triangle - you can scale sides but angle remains the same).

Why would then up-scaled version behave differently than "base" model? Because of the way light works. Almost all phenomena that we observe it telescopes (aberrations, diffraction effects, ...) come from wave nature of the light.

When we scale telescope we also scale its "errors" - but wavelength of light remains the same. We don't scale wavelength of light with telescope as well. If 60mm telescope had one wave of defocus for particular wavelength and we enlarge everything but that particular wavelength - suddenly we have 1.6667 waves of defocus for that wavelength - larger defocus in waves.

Since chromatic error is defocus of all wavelengths but two (and some spherochromatism as well, but let's not complicate things) - larger geometrical error translates in larger wave error since "size of a wave" remains the same.

If light behaved like geometry rays - then it would make no difference - larger focal length - larger image at focal plane but we use longer FL eyepiece to get same magnification and image is the same if magnified the same.

However - light does not fully behave like that - geometric rays are only approximation and it actually behaves by interference of those waves - and we can't scale those.

Makes sense?

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I always thought that scopes of identical design, F ratio and glass types yet different apertures, would give the same level of CA, but that the one with larger aperture would show more obvious CA because of the brighter image. 

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2 hours ago, mikeDnight said:

I always thought that scopes of identical design, F ratio and glass types yet different apertures, would give the same level of CA, but that the one with larger aperture would show more obvious CA because of the brighter image. 

I've certainly found over the years that a smaller doublet ED using FPL53 appears to show less CA than a larger aperture version.

But, it could be that they are the same if tested in a lab , but on the night sky what we see is also affected by the seeing, ice/water in the atmosphere, the altitude of the target, the target itself, our own eyesight, the eyepieces and diagonals we use, and indeed the size of the objective as you say Mike.

 

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4 hours ago, Highburymark said:

The question here is why should the average 60mm F/6 scope have better colour correction than a 100mm F/6 scope, assuming they are doublets and use the same glass? I have now read conflicting reports on this. One says the light bends less with the smaller scope - hence is less liable to false colour. Another claims that scopes with the same focal ratio should be equal in CA, no matter the aperture - as Andrew states in his first post above. 
If there is a difference in CA between the two scope sizes, I don’t think it’s because light is bending more. But perhaps it’s because overall, more light is bending? The larger glass is refracting a greater amount/surface area of light away from the central axis of the objective lens. 

Absolutely. More tests to do.

I think that one factor is that they have to make the glass thicker as aperture increases also increasing dispersion. Also the size of the airy disc gets smaller and the larger aperture supports more magnification hence the tolerance to CA is lowered. 

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6 minutes ago, Adam J said:

I think that one factor is that they have to make the glass thicker as aperture increases also increasing dispersion. Also the size of the airy disc gets smaller and the larger aperture supports more magnification hence the tolerance to CA is lowered. 

https://en.wikipedia.org/wiki/Snell's_law

No mention of lens thickness - angles depend only on indices of refraction between glass and air.

In order to get same F/number, larger lens actually bends light less than smaller lens at same distance from lens center. You can see that if you stop say 100mm F/6 scope to 60mm - then it will turn into 60mm F/10 scope, so edge ray bends only at F/10 not as fast as F/6 like with 60mm f/6 scope.

It is all down to wave nature of light.

It has nothing to do with brightness of the image, or size of airy disk.

Look at this diagram:

image.png.73cc6c1d9a0cb5e5ea960efa2be0aa50.png

It characterizes both spherochromatism and residual spectrum. If you just scale up the scope - increase aperture but leave F/ratio the same, then said graph will "grow" as well. Shape of it won't change - it will be the same - but X and Y axis will grow proportionally.

For any of those lines - difference in X axis between Y0 and Ymax represents "lag" of light wave. That out of phase wave will either constructively or destructively interfere with itself - to produce pattern at said wavelength / frequency of light. If you change X axis - then obviously number of waves between any two points on any line change.

With perfect straight lines - no spherochromatism - scaling up won't introduce spherochromatism, but if there is some spherochromatism in design - making scope larger will worsen it. With perfect straight lines scaling up will only make defocus in waves larger - larger purple halo.

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When a 60 f6 optical design is increased in size to 80 f6 then the larger telescope’s lenses are thicker and spaced further apart. I imagine both factors would increase optical dispersion. It would explain why smaller aperture scopes are less prone to CA. At least I think it does… 🙂

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