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False colour visual tests on 5 apos


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Just now, Mr Spock said:

It doesn't matter how good it is, it's still only 130mm... :wink2:

Yes thats the thing- take a true "1/4" wave (including secondary) newt with 8" + of aperture and it will show more, seeing considered.

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3 minutes ago, jetstream said:

Yes thats the thing- take a true "1/4" wave (including secondary) newt with 8" + of aperture and it will show more, seeing considered.

So why is a TOA considered one of the best AG's out there if all you need is aperture?

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16 minutes ago, jetstream said:

Yes thats the thing- take a true "1/4" wave (including secondary) newt with 8" + of aperture and it will show more, seeing considered.

True enough. Gonna sell the TSA then? 😉

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1 hour ago, jetstream said:

Yes thats the thing- take a true "1/4" wave (including secondary) newt with 8" + of aperture and it will show more, seeing considered.

This is why I'd never buy a Takahashi.      🤔

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Earlier in the thread it was asked why a smaller aperture objective (other things being equal) would show less false colour. Here's my take:

A and B show two different sized objectives of the same F ratio. Singlets are shown in the diagram where the longitudinal spectrum is a function of the focal length in each case but the principle is the same for achros etc. However, the Airy disc diameter is a function of the focal ratio and therefore remains exactly the same.

If all rays are sufficiently well contained within the Airy disc for lens A then colour correction for the smaller lens B will be twice as good. Halving lens C's F ratio results in the same colour correction as lens A.

If lens A was an apo and B an ED then at some point the smaller lens would have the same correction as the larger. An F/6 60 ED compared well with an excellent F/7.5 120 apo in the study.

David

PS Apologies if the same point has been made earlier and I've missed it.

 

PB291795.thumb.JPG.c5f5ea487850fc57fb8850c474541778.JPG

Edited by davidc135
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9 minutes ago, davidc135 said:

f all rays are sufficiently well contained within the Airy disc for lens A then colour correction for the smaller lens B will be twice as good.

Can you explain this bit?

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39 minutes ago, vlaiv said:

Can you explain this bit?

For objectives of the same type and F ratio; if, at best focus, the blur of least confusion for all visual colours varies with aperture as in the sketch but the Airy disc diameter remains a constant then what I posted is so.

I should have written ...if all rays are sufficiently well contained within (or outside) the Airy disc for a given standard of colour correction to be achieved...  My sketch showing the two Airy discs of equal size could be clearer.

For achromats, having red and blue spot diagrams no more than 3x the Airy disc is sometimes given as a reasonable standard, for example.

David

Edited by davidc135
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2 minutes ago, davidc135 said:

For objectives of the same type and F ratio; if, at best focus, the blur of least confusion for all visual colours varies with aperture as in the sketch but the Airy disc diameter remains a constant then what I posted is so.

I should have written ...if all rays are sufficiently well contained within the Airy disc for a given standard of colour correction to be achieved...  My sketch showing the two Airy discs of equal size could be clearer.

David

Just to see if we are on the same page here, do the following (draw diagrams):

Say you have 50mm F/6 scope and 100mm F/6 scope. 50mm scope has red color being 1mm short in focus, so it has focal length of 299mm instead of 300mm, with scaled up scope what do you think will happen? If we scale up shift in focal length of red color - to 2 mm instead of one as everything is x2 in size (aperture, focal length) - and is 598mm, then if you draw the diagram red "hits Airy disk" in exactly the same place with 50mm scope as does with 100mm scope

If we assume that focal length is changed by same amount instead of doubling - then on 100mm scope it will have 599 will 'hit Airy disk" closer to center.

It needs to change more than doubling in order for "geometric" explanation to be able to explain things, and I don't see how can that happen.

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34 minutes ago, vlaiv said:

Just to see if we are on the same page here, do the following (draw diagrams):

Say you have 50mm F/6 scope and 100mm F/6 scope. 50mm scope has red color being 1mm short in focus, so it has focal length of 299mm instead of 300mm, with scaled up scope what do you think will happen? If we scale up shift in focal length of red color - to 2 mm instead of one as everything is x2 in size (aperture, focal length) - and is 598mm, then if you draw the diagram red "hits Airy disk" in exactly the same place with 50mm scope as does with 100mm scope

 

If, in your two examples, red rays cross the axis at 299mm and 598mm and if the green Airy disc is the same linear diameter in both cases then, plainly, those from the larger aperture hit the Airy disc twice as far out as they have twice the distance to travel but at the same angle.

David

Edited by davidc135
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3 minutes ago, davidc135 said:

If, in your two examples, red rays cross the axis at 299mm and 598mm and if the green Airy disc is the same linear diameter in both cases then, plainly, those from the larger aperture hit the Airy disc twice as far out as they have twice the distance to travel but at the same angle.

David

Yes, you are actually right.

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5 minutes ago, davidc135 said:

If, in your two examples, red rays cross the axis at 299mm and 598mm and if the green Airy disc is the same linear diameter in both cases then, plainly, those from the larger aperture hit the Airy disc twice as far out as they have twice the distance to travel but at the same angle.

David

What I actually wanted to point out was this:

Angular size of say violet halo will be the same between two scopes. I confused myself with airy disk and the fact that it is smaller with larger aperture (angular size).

At focal plane defocus disk from say red color will be twice as large for larger scope, but since focal length is also twice as larger - defocus disk is has same angular size in both scopes.

If you look at unresolved star and notice violet halo - it will be of same size in both scopes at the same magnification.

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35 minutes ago, vlaiv said:

 

At focal plane defocus disk from say red color will be twice as large for larger scope, but since focal length is also twice as larger - defocus disk is has same angular size in both scopes.

Yes, but you must agree that this is a bad thing and explains the ca diagram that Stu posted on page 1

David

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5 minutes ago, davidc135 said:

Yes, but you must agree that this is a bad thing.

David

I would not qualify it as either good or bad.

To me it just shows that we can't explain less CA on smaller aperture with same F/ratio by geometric optics alone. It is in my view inherently wave phenomena.

Your initial argument actually supports this. I did not realize it at first but I do now. I don't know if you were aware of that or not, but the fact is that you compared where rays land with size of airy disk which is purely wave phenomena (nothing in geometric optics with rays explains airy disk / pattern) and as such is related to wavelength and not geometry of telescope. Larger aperture means smaller airy disk because we leave wavelength the same (we can't change wavelength of light by changing the size of scope) - and relation of the two define size of airy disk.

In contrast, if we observe only rays cast in geometric optics - we arrive to conclusion that both scopes give defocus blur of same angular size, and at same magnification when looking at the star - both halos would look the same in both scopes - that is to the contrary of experience and what is really going on - where smaller scope throws up smaller residual color defocus blur.

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13 hours ago, John said:

Or this ? - I've no idea myself :icon_scratch:

Longitudinal_Abberation.jpg.bc5eb0fe1201716f7b79d36f18f367ea.jpg

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

 

I don't understand how to interpret these graphs 🤔😢 

You said I was smart, mom!

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Just now, Sunshine said:

I don't understand how to interpret these graphs 🤔😢

Me neither, despite @vlaiv's valiant attempts :rolleyes2:

The views with the scope are excellent though, so that is what really matters :smiley:

 

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14 minutes ago, Sunshine said:

I don't understand how to interpret these graphs 🤔😢 

You said I was smart, mom!

I also need to read a lot about these diagrams to understand them one day. But i suppose a diagram that shows all coloured lines crossing at a close point on the vertical axis would mean the diagram doesnt belong to the Celestron Astromaster 130

37A30771-708D-43AC-8614-BB020E679439.jpeg

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1 hour ago, vlaiv said:

I would not qualify it as either good or bad.

To me it just shows that we can't explain less CA on smaller aperture with same F/ratio by geometric optics alone. It is in my view inherently wave phenomena.

Your initial argument actually supports this. I did not realize it at first but I do now. I don't know if you were aware of that or not, but the fact is that you compared where rays land with size of airy disk which is purely wave phenomena (nothing in geometric optics with rays explains airy disk / pattern) and as such is related to wavelength and not geometry of telescope. Larger aperture means smaller airy disk because we leave wavelength the same (we can't change wavelength of light by changing the size of scope) - and relation of the two define size of airy disk.

In contrast, if we observe only rays cast in geometric optics - we arrive to conclusion that both scopes give defocus blur of same angular size, and at same magnification when looking at the star - both halos would look the same in both scopes - that is to the contrary of experience and what is really going on - where smaller scope throws up smaller residual color defocus blur.

I say that it's bad because the resolution of larger scopes would remain that of smaller scopes if angular colour blurs stay the same. But I suppose that in practice it's more to do with definition and contrast than resolution.

I'd like to think that geometric optics are a vehicle for the subtle wave stuff and can usefully predict and explain. How could the ideas in your last paragraph be put to the test? Shouldn't be too hard. As you say, geometry predicts that 3'' and 6''  F/8 achros will have the same blue and red blurs at green focus at the same magnification. I'll go with that.

It could be that false colour blurs appear smaller in smaller scopes because the green Airy disc is becoming relatively larger.

David

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33 minutes ago, Robindonne said:

I also need to read a lot about these diagrams to understand them one day. But i suppose a diagram that shows all coloured lines crossing at a close point on the vertical axis would mean the diagram doesnt belong to the Celestron Astromaster 130

37A30771-708D-43AC-8614-BB020E679439.jpeg

Actually the Astromaster 130 is a newtonian (I think) so it should be apochromatic.

 

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10 minutes ago, John said:

Actually the Astromaster 130 is a newtonian (I think) so it should be apochromatic.

 

Oh lol. There you go.  I need to read a lot more to understand these diagrams😄

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21 minutes ago, Stu said:

Thought not! That’s the crazy thing about these lovely fracs!

The purity of view is astounding in these scopes, thats what keeps grabbing my attention. No scatter, superb colour rendition and the ability to take super high mag- cant beat it.The TSA120 is not fussy on diagonals either- even if a prism "de values" the optics a bit, the specs are so high to begin with its not noticed. The TOA 130, from what Ive heard is the technically "perfect" refractor.

What is really remarkable though is that other, more reasonably priced refractors are performing so well, which I believe their graphs etc would indicate.IMHO.

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