Chromatic aberration, refractive index and the Schmidt corrector plate

[jambouk]

I recall being told some time ago that SCTs don't exhibit chromatic aberration as the last optical surface the light interacts with is a mirror before reaching the observer, which is why there is no chromatic aberration in SCTs. Thinking about this as I prepare a talk for my local astronomical society, this is rubbish, well bits of it anyway. Mirrors which are silvered on the light path side of glass have minimal (no) action in reflecting different wavelegths in different ways; I suspect traditional mirrors which are silvered on the other side of the glass and where light has to travel through the glass, get reflected and then have to travel through the glass again, may be different.

Anyway, this made me wonder [again] why there is no chromatic aberration in SCTs as the corrector plate is a single [simple] lens. 

Is this because the refractive index of the corrector plate is so low, that the amount of chromatic aberration is just so small it is not noticeable? Or is there something else?

Thanks for any replies.

James

 

[johninderby]

An interesting discussion on the subject on CN.

https://www.cloudynights.com/topic/631055-ca-in-a-sct/

[michael.h.f.wilkinson]

The dioptric strength of the corrector plate is essentially zero, it is only designed to correct for spherical aberration. Of course this correction is only optimal for one wavelength, and the correction of spherical aberration is different for different wavelengths. This effect is called spherochromatic aberration, which is distinct from chromatic aberration in that there is no overall difference in magnification or focal length as a function of wavelength, as is the case in lateral and axial chromatic aberrations respectively.

[Peter Drew]

The main potential cause of chromatic effects in Maksutovs and Schmidt-Cassegrain telescopes is mechanical "wedge" across the optical diameter of the corrector elements.  If the wedge is severe, the component operates as an objective prism and introduces false colour. 

[Alkaid]

6 hours ago, Peter Drew said:

The main potential cause of chromatic effects in Maksutovs and Schmidt-Cassegrain telescopes is mechanical "wedge" across the optical diameter of the corrector elements.  If the wedge is severe, the component operates as an objective prism and introduces false colour. 

I'm interested Peter, what is 'wedge'?  Thanks.

[andrew s]

Wedge is when the surfaces of the corrector plate are not parallel and  resemble a weak prism.  As they are thin with near zero power, as said above, they cause little chromatic aberration.  In larger sizes it can be an issue and the UK Schmidt camera has an achromatic  doublet corrector.

Regards Andrew 

Edited April 1, 2021 by andrew s

[Alkaid]

2 hours ago, andrew s said:

Wedge is when the surfaces of the corrector plate are not parallel and  resemble a weak prism.  As they are thin with near zero power, as said above, they cause little chromatic aberration.  In larger sizes it can be an issue and the UK Schmidt camera has an achromatic  doublet corrector.

Regards Andrew 

Thanks, did not know that.

[Peter Drew]

4 hours ago, andrew s said:

Wedge is when the surfaces of the corrector plate are not parallel and  resemble a weak prism.  As they are thin with near zero power, as said above, they cause little chromatic aberration.  In larger sizes it can be an issue and the UK Schmidt camera has an achromatic  doublet corrector.

Regards Andrew 

I actually met and shook hands with the gentleman who made the UK Schmidt  camera corrector.  Sadly nothing rubbed off!      😆

[beka]

Hi All,

In principle as the corrector plate is correcting spherical aberration, then some refraction is taking place at the surfaces and therefore there will be chromatic aberration (it effectively is a very weak over corrected lens). It must be that it is too little to significantly affect the final image.

Best

Edited April 2, 2021 by beka
grammar...

[Peter Drew]

I think that the corrector, being a correcting rather than image forming component works in its favour as it doesn't have a focus.       🙂