Zernike Terms Explained for Telescope Makers

Zernike terms explained especially as they relate to lenses and mirrors and interferometry and DFTFringe software.
Special thanks to Paul Valleli who helped write the text for this video. All mistakes are mine, not his.
Chapters
00:00 Intro and advertisement
00:34 Overall explanation (zernike description, wavefront versus surface error, orthogonality, wavelength, similarity to Fourier Transform)
07:39 Spherical Aberration - what is it, parabolic mirror versus spherical mirror
10:42 Null feature in DFTFringe - what it does, how it works
13:10 Astigmatism - x astig, oblique astig, how to remove it
16:10 Wavefront Inversion - when to invert the wavefront
17:54 Defocus
18:13 Coma

[@Fluffy7048]

Really nice video. It may not be of interest for many people, but its great for everyone it is.

[@PatrickWilson47]

Thanks for all the info. I have been using your interferometer and fumbling with DFTFringe for a while now on a 406mm F6 mirror I am grinding.

[@the_dirac_field6519]

Hi, There is a way to donwload a 2D matrix representing the surface distortions of the mirror once we have set the Zernike terms?

[@nikunjbheda9946]

What software used for this wavefront simulation?

[@pritamshtty]

Thank you so much for a valuable information, btw does this software work for lens aberations too?

[@bath_interferometers]

Yes, DFTFringe works on any kind of circular interferogram of optical lenses, mirrors, or flats. The "hard part" is getting an interferogram of a lens.

[@jlove_you]

Thanks for the video.Please for the C8 eigth zernike term for spherical aberration that you used,Is there a corresponding formula for a lens?

[@bath_interferometers]

Yes but it's more complicated. Equation 8 in the link below does that where q is zero if it's a biconvex lens with equal radii and p is usually -1 so the formula isn't as bad as it looks but it assumes a thin lens so really you should use ray tracing software to measure distortion knowing actual lens thickness (I use oslu edu which is free). Also I assume there is a perfect mirror behind the lens (if you are using a bath interferometer) so you need to expect double the S.A. (I think?). But if this is from a telescope objective then you would have a doublet or triplet and it actually gets simpler as there should be no S.A. Also errors in the main objective are sometimes corrected in one of the other elements. www.telescope-optics.net/lower_order_spherical.htm#:~:text=General%20expression%20for%20the%20aberration,%2Dto%2Dlens%20separation).
Note that you can fudge the null by giving DFTF the correct diameter but lying about the focal length or F/# as focal length and F/# are only used to calculate null and nothing else. Or you can fudge the conic constant. This lets you get any null value you want.

[@jlove_you]

@@bath_interferometers Thank you. I will check it out.

[@niaei]

Wow. Nothing to say. Thank you.

[@user-qp5re8cm6u]

what's the software in the video?

[@bath_interferometers]

DFTFringe. Free and open source on github. I have links to it from my bath page gr5.org/bath/

[@siwatpatamakam897]

What is the name of the software you were using in this video?

[@bath_interferometers]

DFTFringe. It's free and open source created by Dale Eason and you can download it here: github.com/githubdoe/DFTFringe/releases

[@siwatpatamakam897]

@@bath_interferometers OMG thank you so much. Just one things, I’ve just started learning Optical system and facing many problems about the fundamental terms. Could you please recommend me about the easiest optical basic source, I could find on the internet?

[@bath_interferometers]

@@siwatpatamakam897 I don't know which terms you mean but I just use google. For example I didn't understand the word "conjugate" which is very basic in optics but has many other meanings in non-optics (such as math). So I googled "conjugate optics" and that takes you to a great explanation on wikipedia. The article is helpful but I find myself going back to the idea many many times as it's hard for me to get my head around some of these terms. This is just one example. "caustic" is another. And sometimes there are 4 terms in a single sentence and I have to google them all and re-read the sentence many times and it takes me 20 minutes to understand a single sentence. With Zernike terms I understood "orthogonality" and about breaking up a surface into many components from my experience as an electrical engineer as we do something very very similar. But in optics there are many terms I've only learned in the last few years.

[@harriehausenman8623]

Thats one loude mouse button!

[@harryzernike5953]

Frits, not Fritz. Cheers.