DIY Telescope: Polishing the Mirror
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- เผยแพร่เมื่อ 28 ส.ค. 2018
- Up until now, we’ve used abrasive grits to remove the glass, but now we’ll be using a polishing compound called Cerium oxide to put the finishing touches to the mirror allowing it to begin reflecting it’s first light
- วิทยาศาสตร์และเทคโนโลยี
This is so interesting! Really enjoying the edits and the process how this is made! :D
Thanks Geoffrey. The things I have to do to keep myself occupied when I'm injured from running
You can also heat the pitch up with a torch just enough to make it soft enough to manipulate,mate it to your surface you want to polish, and with soap/water combo, lap the two together so the soft heated pitch matches exact to the curve/plane you have on the optic you are making. Then cool the tool off with water. You can also do this by heating up a large container of water and soaking the tool in it if you want a more evenly heated tool. Save you the headache of trying to lap a hard pitch tool to match your work piece
That's a great idea! Thanks for sharing.
Your polishing work looks very very cautious. In the final stages (figuring) it is ok but at first you can really use force to get mirror polished. You can also use different strokes like elliptical or epicycloids, but keeping stroke length in control. Also working tool on top can be useful (watch out for TDE!). The most important is to be sure contact is all the time near perfect. Mastering pitch lap is one of the main skills in mirror making.
Thanks. I think you're right. I was watching some videos of guys making 14 inch mirrors and I was very surprised to see how much for they use when polishing. I have alternated with TOT and MOT. Because when I started testing I had a bit of a doughnut shape. Luckily I have a slightly TUE. Lockdown and everything has sadly stopped our get togethers, and I need to repour my lap, so hoping I can finally finish this mirror off soon.
man australia and new zealand are so beautiful
I thought the thumbnail showed a mirror and a waffle beside to it😂. I'm getting hungry tho
Haha. Putting a waffle in there would be click bait. Wait! Now I'm hungry too
A bit picky here but it is generally recommended to take off any rings, bracelets, watches when doing that. They could bring a few grains of a previous abrasive, or worse make a big scratch on the mirror when handling it... That being said, nice video!
Yeah, you're right. Also, to clean your finger nails before starting out. It only takes one grain of dust to mess up a whole mirror. Thanks for the advice!
I've been looking at doing this, have VERY minimal experience sanding furniture and car repair so wondered what are costs involved? and is it worth it vs buying commercial equivalent?
sorely tempted to emulate Dobson himself after seeing his video and it might make a cool winter project. I have ability to whack together most of the rest of it without major problem
Hey there! Making a telescope is actually surprisingly simple. It does, however take a very long time. There's a guy who made a 4 inch scope over the course of a week. But, he had many years experience and all the equipment. Most of the hours will be grinding and figuring. For less than an 8 inch, probably 40 or so hours. Then making the scope is only as complicated as you want it to be. If you're good with your hands. A weekend construction and another day to pretty it up is all it should take.
Cost wise. The cheapest would probably be buying a second hand telescope. Making one can be very cheap, or expensive. Depends on the design and how frugal you are. You can salvage parts for a scope from many places to keep the costs down. So the only costs would be in the mirror (buying the glass, the grits and polishing compounds, and coating it when you're done)
If there are any clubs nearby, go through and chat to the guys. It's a great hobby and making a scope is incredibly rewarding.
Hope this helps!
Ahhh a fellow South African ATMer! I have just finished my 220 mm f6 primary, sent off to Joburg to get aluminised. Ended with a slight turned down edge but not too bad considering I'm all alone here in Pietermaritzburg. Do you know if we can get polishing pads here in SA?
Hey Gareth! Well done on finishing off the mirror. Is Ronnie coating it for you? It's difficult to get it all done alone. I haven't seen anyone sell the polishing pads locally, but will ask around for you
@@BornAdventures Thanks man I would really appreciate that. I think Rodney coated it for me? I might try using the felt polishing pads you can get from dremel, glued down onto the tool. If it ruins the figure then I guess I can always go back and regrind with 1200. This is for my second and third mirror which I am hoping to complete in a much quicker time.
no way i'm from pmb and kicking off on my first build greets from cape town!
Great video man, hope you're doing well. Just a question: I've seen that most people online use something like barrels or turntables as stands for the mirrors, while you just rotate mirror and tool. Is your method the same thing as having a pivoted plywood disc that you can rotate, but you just rotate the tool instead of the base, or do you use different strokes? (I'm a complete beginner, sorry if this is a dumb question)
Hi There, I'm good thanks. I hope you're well too. For a smaller mirror like an 8 inch or smaller, it's easier to just turn the tool every so often. The main aim is to be quite random in your approach and to make sure you're always either always clockwise or always turning anti clockwise on the tool as well as the mirror. Rotating tables are great, but its easier to introduce flexing in the surface which could cause turned edges or odd shapes.
@@BornAdventures thanks very much
Where did you get so thick glass?
Hi there, sorry for the late reply. I lost the password for this account. It came from our local Astronomy Club ASSA (Astronomy Society South Africa). It's recycled glass from Ngwenya Glass factory. There are a few bubbles, but none that'll make much of an impact on the final image
Mirror name
Sir if I use 12 inch diameter Plano concave lens for reflection telescope with diameter and length secondary mirror will I used?
Hi Dasari. What was the lens used for before? Do you know it's focal length? I suspect if you want to use that piece of glass for a Newtonian, you will have to regrind and refigure it into a parabolic shape before using it. It may be easier to get a fresh piece of glass to if that is the case. I'm a bit stuck with fixing some errors I have with the glass from this video, so will do the calculations for the secondary mirror when I get to that point. But, in the meantime, please visit stellafane.org. They have a wealth of information on how to calculate diagonals and are much more thorough in their instructions than my video
@@BornAdventures Thank you sir
@@BornAdventures Focal length is 2000mm
@@dasarinarasimhulu5974 You could probably use a 2.6inch diagonal. But, just double check all your measurements before on the online calculators
Why not using orbital polishing tool?
Hi There. Orbital polishers would dramatically distort the shape of the glass. The mirror and tool to polish have to be exact by this stage. On larger mirrors, you can make a tool that'll give a great polish. It replicates the strokes you would by hand
@@BornAdventures so for optical device better polish it manually?
What does the focal length depend on?
Hi there. The focal length is determined by the depth of the curve or 'sagitta'. Basically, the deeper your sphere is on your mirror the shorter the focal length.
@@BornAdventures Thanks ! There is any tabels which determine it?
How thick should be mirror for 250 and 300 mm?
@@grzegorzciszewski5204 I haven't found any tables online, but Stellafane has some nice tools to help out. Here's one that will do the trick stellafane.org/tm/atm/mirror-refs/sag2fl-calc.html. They also have some great resources and explain the whole process incredibly well.
@@BornAdventures Great ! Thank you very much :D
the curvature of the mirror
Can anyone please tell me where I can buy a mirror to grind
Which country do you live in?
Ouch, that music...
It's pretty terrible.
what really surprises me is that no one is using, any kind of cnc lathe to make parabolic mirrors. you could take regular spherical mirrors, and knock the outer third of the mirror down to convert to parabolic. yet it seems no one wants to do it, as long as they can still charge a fortune for a chunk of glass!.
Hey David. Could you elaborate? I'm keen on hearing your idea. I think a lathe can definitely be used to get the approximate spherical shape, but getting it to a perfect sphere and parabola would need a special polishing machine. We had a guy at our club use a grinder to hog out the inside of his mirror to get the bulk of the glass out, so a lathe could definitely work for that leg work.
lathes typically have tolerances of around 1/1,000th of an inch. Maybe 1/10,000th. telescope mirrors require tolerances of 1/4 of a wavelength of light minimum, preferably less. 1/4 of a wavelength of light is around 100 nanometers, which is a few millionths of an inch. If you imagine shrinking yourself down so that the defects on a telescope mirror come up to your knee, the defects on a lathe ground surface would be like 10 story buildings.
Also, you can buy mirrors with "pre generated" curves, which just means the rough grinding step has been done. But at that stage, it isn't terribly meaningful to talk about the difference between a spherical and a parabolic surface, as a rough ground spherical surface is much farther from either an ideal sphere OR ideal parabola than the ideal sphere is from the ideal parabola.
the tolerances are simply way too small. you need to go down to hundreds or even tens of nanometers for your telescope to be diffraction limited
@@marklogan4079 Great analogy!
@@marklogan4079 yeah, thats the thing with optics, nice explanation about the lathe ,and now all know how hard /worktime tike to make a near perfect parabolic mirror (and thats sound money too me)
try spin-casting epoxy directly into parabolic mirror shape/mold
I haven't worked with epoxy before. But, I've heard of a technique like this before to create the basic shapeon much larger mirrors. It's a very interesting idea and I'd love to see the results. I'd guess that you'll get to a circular shape which you could figure later into a parabola. And by changing the rotation speed you could also adjust the focal length.
@@BornAdventures you get a direct parabola bottom mold, to press/cast against, whichever way you choose to use this type of mathematical shape/mold making
So do you mean that you pour in epoxy in a circular container, spin it and as it cures it'll produce a parabolic shape? That's really neat! I'd love to try it out!
And does this need refining to bring it in shape or is it already good enough to produce an image?
@@BornAdventures well check out the parabolic liquid metal astronomic mirrors, the idea came from there, but instead of it staying liquid, its cast from epoxy (or anything that will be solid after the operation) and yes basically
Definitely NOT the longest running experiment. THat would be the 50 year test to seef if silver clampt to gold would stick permanentyly after the clamp was taken off in the required time. The two metals STUCK as expected.
Hey there. The pitch drop experiment began in 1927, so almost 100 years now.
@@BornAdventures OK, well, now I know. Thx.
It’s useless to make such a small mirror, 20 to 25 cm is optimum for a manually polished mirror
that music is annoying af
Yeah. It sounded nice when I first was putting the video together. A bit ghastly now