False colour visual tests on 5 apos

[vlaiv]

Just now, Steve said:

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. 

How so?

 

[Steve]

12 minutes ago, vlaiv said:

How so?

I don’t know. It is something I have always assumed. Curved glass bends light, causing it to act as a prism, separating light into its components. If the lens is larger/thicker then it is more likely to act like a prism as light passes through it. And if the resulting refracted light must travel further then colour separation will be more pronounced when it arrives at the Crown element. 

This is only my amateur understanding. Am happy to hear I am wrong, then learn the real reason 🙂

[Highburymark]

4 hours ago, vlaiv said:

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.

Makes sense?

This does make sense, yes.
Though presumably the lenses would have to be thicker on the larger scope to retain the same refracting power?

[vlaiv]

Just now, Steve said:

If the lens is larger/thicker then it is more likely to act like a prism as light passes through it. And if the refracted light must travel further then colour separation will be more pronounced when it arrives at the Crown element. 

It is about angles and refractive indices of glass and air.

Say that light travels further thru glass because glass is thicker, right, and colors are more "separated" because of this - well it turns out that after longer journey - they hit part of the lens that is less curved or at a shallower angle and that offsets precisely difference in "spread" that happened.

For lens calculation - it is curve of the front and back surface that matters (distance between those matter as well).

Another way to think about it:

Instead of lens let's have "black box" that bends the light - we don't really know how it happens. Top one is 100mm F/6 - so 600mm of FL, while bottom one is 60mm F/6 - so 360mm of FL.

We now observe ray that is 10mm of optical axis going into first black box and second black box - which one bends it harder?

[vlaiv]

Just now, Highburymark said:

This does make sense, yes.
Though presumably the lenses would have to be thicker on the larger scope to retain the same refracting power?

I'm not entirely sure about that, but yes - if you keep front and back radii of curvature the same and have "higher" lens - then in the middle it has to be thicker.

[vlaiv]

3 minutes ago, Highburymark said:

This does make sense, yes.
Though presumably the lenses would have to be thicker on the larger scope to retain the same refracting power?

Actually, having same radii of curvature back and front would create same FL lens - so longer FL lens is actually less curved than short FL one.

Not sure what is dependence on thickness when designing larger lens - if it really needs to be that much thicker as it is larger in diameter (for same F/ratio).

[Adam J]

31 minutes ago, vlaiv said:

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:

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.

Am talking about lateral dispersion not angular dispersion. But the point is that you can't just scale the lens as it needs to become disproportionately thicker for mechanical reasons. Could be wrong I'll look for a reference. 

[vlaiv]

Just now, Adam J said:

Am talking about lateral dispersion not angular dispersion. But the point is that you can't just scale the lens as it needs to become disproportionately thicker for mechanical reasons. Could be wrong I'll look for a reference. 

I think you are right - you can't just scale lens as it will keep focal length the same, but if you apply above "black box" approach - you'll see that larger lens actually bends light less for same distance of optical axis or the same if we measure distance from optical axis in relative units of focal length or 1/focal length (f-ratio). or simply scaled to 0-1.

In any case - there is no more dispersion due to this for same F/ratio.

[Sunshine]

What is this false color I'm reading about? 🤣 kidding aside, I don't believe any scope can be 100 percent devoid of false color, maybe less than what our eyes can perceive, I guess.

Edited January 3, 2022 by Sunshine

[Andrew_B]

7 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.

Just a small correction. My understanding is that if you have two scopes of the same design with different apertures and the same focal ratio then the smaller one will have less false colour. In order for the bigger scope to have the same CA as the smaller one, its focal ratio would have to increased by the same amount as the increase in aperture.

[jetstream]

Hows does this one compare?

 

[vlaiv]

7 hours ago, jetstream said:

Hows does this one compare?

 

That look very good, but it really depends on other things not show in that graph - like what is X scale in this image and what is focal length of the telescope?

I'm guessing it is TSA120?

[John]

8 hours ago, jetstream said:

Hows does this one compare?

 

Or this ? - I've no idea myself

I think we are into colour crossings here, which is well over my head !

 

Edited January 4, 2022 by John

[vlaiv]

2 minutes ago, John said:

Or this ? - I've no idea myself

Actual reading of that graph is easy, but problem is interpreting it.

Each line on such graph is one wavelength of light - usually used are Fraunhofer lines: https://en.wikipedia.org/wiki/Fraunhofer_lines

(in above linked wiki article there are letter designations like e line is 546nm and C line is 656 or H-alpha transition)

Once you pick any line - vertical axis represents distance from center of the lens. In the image you linked, above graph it says Pupil radius of 65mm - so that is for scope of 130mm diameter. Y value represents point on the lens that is Y millimeters away from center of the lens and towards the edge of the lens. Graph assumes that lens is spherically symmetric and just shows what happens along one radius of it.

X axis on the graph represents change in focal length for that particular ray.

Say we want to interpret these two points on green (around 520nm) line on above graph:

It really means this:

Ray with 520nm coming in at 6.5mm away from central optical axis will fall a bit short of true focus position (X tells us how much shorter in mm), and ray coming in at 58.5mm (or 65mm - one 1/10 mark and that is 6.5mm) will be a bit further away than true focus point.

If we examine any given line on such graph - how much it deviates from straight line - represents level of spherical aberration on that wavelength (spherochromatism). It's position left or right tells us how much defocus for that wavelength will there be.

Graph you linked shows that most of residual color comes from spherochromatism rather than chromatic aberration as all lines are in fact very close to 0 but are bent.

As a contrast - look at this graph:

This graph shows F/10 4" achromat, and shows two different graphs. Larger graph shows defocus for particular wavelength (right on Y axis there is wavelength scale and certain points are marked with their Fraunhofer line letter designations). That graph assumes 0 spherochromatism - or simply does not show it, but it shows all wavelengths of light in single continuous curve - each point on the curve having its own defocus / focal shift.

Small graph in top left corner is graph that we are talking here about. Most lines are fairly straight and vertical - small change in X if any, so there is very small amount of spherochromatism but they are separated in X axis - clearly showing residual spectrum that comes from defocus.

Hope this explains how to read above graph - curved lines = spherochromatism, lines away from X=0 equals secondary spectrum due to defocus (what we normally think of as residual color).

Problem is that one can't really easily tell level of it from the graph, because it depends on focal length of the scope and in relation to that focal shift. Slower scopes will have larger critical focus zone - so same defocus might not have same implications for slow and fast scope.

What will star look like at particular wavelength also depends on focus position. Graphs are made so that green is "in focus" - but sometimes best focus is in different place and once we change focus - things change because waves align differently and interference pattern is different.

Remember it is about waves rather than rays, in the end, image is formed as waves "traveling" along these lines arrive out of phase with one another and then interfere constructively or destructively depending on phase shift.

 

[Highburymark]

Appreciate all the contributions to an interesting thread. 300 years after the first crown and flint doublet, it’s amazing how something as simple as two discs of polished glass still challenges our curiosity.

[jetstream]

3 hours ago, vlaiv said:

That look very good, but it really depends on other things not show in that graph - like what is X scale in this image and what is focal length of the telescope?

I'm guessing it is TSA120?

Yes its the TSA120 from Tak Japan web site

[jetstream]

Check out the TOA130...

https://www.takahashijapan.com/products.html

[JeremyS]

59 minutes ago, jetstream said:

Check out the TOA130...

https://www.takahashijapan.com/products.html

Have they removed the lenses? 🤔

 

🤣

[John]

1 hour ago, jetstream said:

Check out the TOA130...

https://www.takahashijapan.com/products.html

Two ED elements used in the TOA triplet objective I believe. "Practically perfect in every way" 

[Deadlake]

4 minutes ago, John said:

Two ED elements used in the TOA triplet objective I believe. "Practically perfect in every way" 

After it's acclimated and been hoisted onto the mount. 😁

The Founder Optics scopes also have the same profile, same Cooke design as TOA.

Edited January 4, 2022 by Deadlake

[John]

The good thing about Mark's tests and his narrative are that they attempt to quantify the experience that an observer will actually have with the scopes with regard to CA. For me, no amount of interferometer results, charts, figures etc can actually do that.

 

 

Edited January 4, 2022 by John

[Highburymark]

There’s one other thing about the ‘branches test’ that’s informative. When focusing on one particular branch, the majority of other branches in the fov are inevitably slightly out of focus. So if you see purple quilting all around, but your focused branch is colour free, then that seems to be a perfect demonstration that your telescope’s control of CA is pretty good. The background sky does need to be fairly bright to provide a stern enough test, and deally, conditions will be still, so your chosen target isn’t blowing around. 

[jetstream]

2 hours ago, John said:

Two ED elements used in the TOA triplet objective I believe. "Practically perfect in every way" 

This scope might be the most technically perfect scope out there IMHO. I better quit thinking about it or ...

[Mr Spock]

It doesn't matter how good it is, it's still only 130mm... 

[jetstream]

3 hours ago, JeremyS said:

Have they removed the lenses? 🤔

 

🤣

Its hard to fathom how Tak engineered perfection into this telescope.