What is the "Wave" component of a scope's spec?

[FZ1]

Another basic question!

I see things like 1/8th wave or 1/10th wave in high end scope specs.  I get the idea that the higher the denominator, the better the optic, but please can anyone explain what it means? (or provide a link that does).

Thanks,

Jon

Edited October 18, 2019 by FZ1

[FZ1]

After a bit more searching,  I found an answer that works for me (and answers my own question), but in case anyone else is interested ............

This thread from 2015 has a simple explanation and also clarification about Lambda and RMS values.  It also has a simple explanation of the  Strehl ratio as a bonus!

cheers,

Jon

 

 

Edited October 18, 2019 by FZ1

[Captain Scarlet]

My shaky understanding is this: it’s the maximum amount of deviation-from-perfect-spherical of the light waves after bouncing off the mirror towards focus, measured as a fraction of the wavelength of the light (not sure exactly which colour of light though). 1/10 is very good, ¼ is less good, and what people tend to start to call “diffraction limited”. I'm sure @vlaiv or @andrew s can explain much better...

Cheers, Magnus

Edited October 18, 2019 by Captain Magenta

[FZ1]

Thanks Magnus, much appreciated.

Good point about "which wavelength".  Given that the wavelength of visible light is (about) 0.4 to 0.7 micrometres, presumably the figures for red and blue light may be significantly different, so I assume there's an agreed standard.

Jon

 

Edited October 18, 2019 by FZ1

[vlaiv]

For wave specification, I think green light about 500nm is used (I've seen figures from 500 to 550nm used). This is because it is peak of human vision sensitivity curve. Sometimes that figure is given along 1/x wave specification.

Often when working with refracting telescopes, red, green and blue correction is specified because they differ.

And yes, explanation is correct - it is deviation from perfect figure in terms of wavefront - to which part in single wavelength does it deviate. Not overly accurate figure because it does not say how it deviates - it can be smooth or "pointy", but in most cases it is in fact smooth deviation.

 

 

[andrew s]

The only additional point I can make is the some use peak to peak values and others an average e.g. rms. Averages ,look better that p to p for the sa e optic.

Regards Andrew 

[FZ1]

Thanks gents, much appreciated.

Jon