Video

I think you are describing a "straight bath". See the black and white diagram on wikipedia "bath interferometer" article. The advantage of this style is the beams are much closer together (beam separation causes astigmatism but you can remove that later so it's not very important). The disadvantage is that you can't test mirrors with lower f/#'s as the conical beams clip the cube (regardless of how big the cube is - bigger cubes will have more beam separation and more distance for the test beam to grow in size). What is the cutoff? I'm not sure but I think around F/5 possibly. Testing F/10 mirrors is definitely not a problem with the straight bath. The cutoff would also depend on the IOR of the splitter glass which changes the geometry.

@@bath_interferometers In fact, I have completed building such an interferometer last weekend😋, but when I only adjust the fringe density or shape, the interference pattern obtained each time is completely different from the wavefront pattern analyzed in DFT. 🧐🧐🧐 I want to know this Does it happen in BATH interferometers with common structures.🤔🤔🤔

Thanks for the kind words!

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.

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

I don't know much about making splitters but to get anywhere close to 50% split (beams equally bright after the split) I think you need a pretty complex coating before "gluing" the two triangles together. The splitter is indeed the most expensive part but I sell them for $35, I think surplus shed for about $30 and you can get them from china for about $15 (I have a link for where to get the good ones for cheap on my web page). 15mm is a good size for a splitter because your diverging lenses are going to be 4-10mm in diameter and things scale badly at some point. I do sell everything you need in a pretty cheap kit - at the end of the video. I think the polalizer trick isn't mandatory as I've gotten good enough without - the polarizing trick only works (I think) if you have a polarizing splitter. Those are the cheapest splitters and have a coating that lets one polarization through and not the other and how it gets an even 50% split of light. non-polarizing splitters tend to have multiple coatings in that diagonal I believe and expect only a certain wavelength of light maybe? I'm not a splitter expert.

When you put in the paper - it's okay if you can't see the returning beams - just raise up the tripod until they hit the paper. Let me start over: use your hand to know where the returning 2 beams are. Move the bath into that 3D location - actually move half way there as when the bath moves up, the returning beams move down the same amount. It might help to just keep watching a bit and then come back to the step you find tricky. You can see where the final image will appear later in the video. You can also see how it works with one of the many diagrams. Plus if you just look at the URL of the diagram you can type in that web page and try out the "diagrammer" web page and maybe that will help you understand where the 2 returning beams are positioned exaclty. Regarding which beam is reference and which is test beam, I explain this maybe 30 times so just keep watching the video. The beams cross over. At least watch until I explain that the beams cross over. It's important to pause the video when I present too much, too fast (mostly the diagrams). quality: I've printed and assembled 10 XYZ stages and about 100 interferometers. I've tested about 10 different interferometers myself over and over on the same reference mirror. Testing different optical parts to make sure they are worthy of selling. I'm not sure what you mean by "bad print quality". They may be ugly but they should all work very nicely. There can indeed be tiny misalignments in the XYZ stage which can be annoying. Every kit I sell there is about 30 minutes spent filing the parts for perfect parallelism in the X,Z stages (lower portion). I'm not sure what you are referring to. The XYZ kits I sell in winter are better because they are PLA. But in summer, to be safe, I print using PETG as I'm worried they will melt/slump in a mail truck on a hot day if temperature exceeds 130F/52C. PETG quality is definitely not as good. Interferometer itself should be quite excellent.

@@bath_interferometers I think your first video is more clear. It shows the actual bath more clearly. However, in the first video you talk about having the two beams superimposed while the later one one say they should be about 15mm apart. Which is it? Or is the goal to align these later? The kit I bought wasn't from you. My kit looked like it was printed on a $10 printer. Things did not fit well, holes were too large and the rods were bent. Overall very flimsy.

@@tomb18ca Can we talk on phone if you are in USA or by skype if not? Email me at thegr5store@gmail.com. It's great that I have competition but sad that the quality sucks. It's so obvious to me that it's hard to remember what it was like when I was new at this. There's 2 beams, separated by 1 or 2 cm going out and then 2 beams return from the mirror. So that's 4 beams? But I think of it as the test beam going out and returning and the ref beam going out and returning. If you watch for another 5 or 10 minutes I think I trace the beams out with paper several times. Over and over again. To make it as clear as possible. The older video from a few years ago didn't explain these things nearly as well. In the video and also in the diagram, when everything is in place the test beam goes through the diverger then spreads out to cover the mirror and the mirror returns the test beam back to the flat, into the splitter and out the back of the splitter. The ref beam starts off split at the splitter, goes off the flat, off the mirror hits the diverging lens the opposite direction from the test beam, and starts expanding away from the mirror. The two cones of light interfere and you can put paper behind the splitter to see the igram. But this is all shown in this newer video we are commenting on.

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

@@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?

@@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.

Lol - yeah. some people build a rail system - kind of like raised railroad tracks at "counter top height". I usually test against a wall now so there is less prone to someone tripping on a tripod. Anyway the tripod method works quite well.

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.

You could also use a pinhole camera

@@AppleSpar How exactly? Do you put the hole in the focus point of the lens and the mirror or use a screen?

@@a_pakhomov I'd put some translucent paper at the focal plane to focus and then replace it with the film or photo image detector. It would be like old time photography

So it turns out you absolutely need a lens. Without the lens you will get bad results. It will *look* like a reasonable igram but it won't. I don't understand why. There is discussion of this on the groups.io/g/Interferometry forum. I think it will have diffraction and/or distortion effects - I forget.

I know im asking the wrong place but does anyone know a method to log back into an Instagram account..? I was stupid forgot the login password. I appreciate any assistance you can offer me

@Carter Mason instablaster :)

@Patrick Adonis thanks so much for your reply. I found the site thru google and I'm trying it out now. Looks like it's gonna take quite some time so I will reply here later when my account password hopefully is recovered.

@Patrick Adonis it did the trick and I actually got access to my account again. I am so happy:D Thank you so much you saved my account !

@Carter Mason glad I could help xD

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

I dont know if anyone gives a shit but the top 2 comments on here are spam