Setting up and using a Bath Interferometer (to test telescope mirrors)
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- Опубліковано 29 жов 2024
- This video is many months in the making!
Once you have your Bath interferometer, here is how to use it. How to get everything aligned, common mistakes, and how to get good interferograms.
+Xtilt means right side of mirror is closer
+Ytilt means top side of mirror is closer
+Defocus means outside of focus
Helpful information at gr5.org/bath/
This is fantastic work. Definitely not for the masses but hugely use full to the group of people involved in this type of work. Specifically we want to make our own mirrors for telescopes and finding simple and useful explanations of this industry has been hard. Your doing fantastic work and hope you keep going.
Thanks for the kind words!
Thank you for this clear and concise explanation. The crossing of reverence beam and testing beam is complet new fakt for me, now i know my issue : )
I am trying to put together a bath for testing my mirror. I have a normal keychain laser pointer for cats/presentations/annoying your friends which seems to have a fairly collimated circular beam coming out of it. I can't really see inside the tube so i don't know if they're masking the beam or using a collimating lens or something else entirely.
Rather than opting to use something like this as it has a push button momentary switch on the side to activate it, I opted to order a red laser module off amazon that claimed it was a "dot" (I understand these types are generally not dots and come out a bit rectangular). The module is focusable however that means the beam isn't all that collimated. Is this going to be a problem? My thinking says this is not ideal as the beam will clip the diverging lens up close when i have the beam focused at the distance the mirror is at.
I'm thinking I should find a better laser module. Any advice would be appreciated.
cheap laser pointers all have a collimating lens - an aspheric lens similar to a simple biconvex lens but better. They work great except many have dust on the lens and many are plastic lenses that are difficult to clean (or are already scratched) so I recommend the more expensive $2 lasers that have glass collimating lenses in them. or you can get a laser from me (see end of video).
@@gr575 Appreciate the response. Wanted to circle back on this for others that may come here with the same question. I think just about any laser module you buy that uses a laser diode in it will be rectangular in nature despite its description of being a "dot". This should not be an issue in getting a bath to work. I built my bath using the original laser in question mentioned above and had no issues in getting igrams.
Helpful summary of the common path.
Out of curiosity I'd try this with the beam splitter out of an older optical drive but the size causes issues you mention. 2 small bino prisms oiled together. Can the polarising filter give good enough contrast in that case?
I would be interested to find out how horrible my sphere is. Focault shows all kinds of vague will o wisp shadows but they have no coc which can be measured.
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.
I saw a brand new bath interferometer, which does not have the plane mirror, but the beam splitter is placed at an angle of 45°. I'm wondering if it would be appropriate to use a 1" beam splitter for this style of interferometer, since depending on the light path diagram the light at the edges may be in jeopardy.❤
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.🤔🤔🤔
You think it's too difficult to print Dale's design? He did a good job making it pretty easy to print. For example, none of the parts he designed require support on your typical cheap FFF printer which is not a trivial requirement. Anyway you can buy the parts from me if you think it's too hard. They aren't particularly difficult to print. I do print 3 of the parts in nylon because they work better if they are slightly flexible although dale printed them all in the same hard plastic for his. Nylon is definitely trickier to print. The XYZ stage has no Nylon so it is easier but you still have to drill out the holes after the printing is done and file down some spots where the metal tubes and cylinders fit.
which diverge lense do i need for a f5-f6 mirror ?
Do you have STLs of your mirror holder?
All STL files are published on thingiverse. Look for bath interferometer. Dale's design is there and my modifications are also easy to find.
3 axis stage ,Structural design 3D printing is too difficult
Very difficult to follow. Everything is ok until you talk about getting the return beam above the cube. Then subsequently putting in the paper. I cannot see both breams on the paper since I adjusted it to be above the cube. Also, where does the final image come? Through the cube? Or above the cube. Should the diverger lens be in place during the placement? You talk about the reference beam and the test beam. Which is which?
Finally if you want to do this with this tester, you will want to print it yourself. The one you buy has horrendous print quality.
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.
thinks