12 inch f4.9 Telescope Mirror Part 4
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- เผยแพร่เมื่อ 18 ส.ค. 2015
- Watch Gordon prepare pitch polishers and set up his fixed-post polishing machine for spin polishing on the 12-inch mirror. See the fastest way to pour pitch, and the trick to avoiding having to facet the polisher! See the mirror spinning at 60 rpm's as the rough polishing gets underway on this telescope mirror.
- วิทยาศาสตร์และเทคโนโลยี
So much fun to watch. I've learned so much about what it takes to make a premium mirror. Great stuff Gordon...thank you for sharing!
I know nothing about telescopes, glass or mirrors but have sat through three hours of your videos quite mesmerised. You're a natural presenter, thank you.
Thanks, you are very kind!
What an absolute cracking set of videos. Thanks for these.
I never thought producing a precision mirror could be so mesmerizing. Cool series of videos you made.
Very few videos out there with this much detail. I'm very appreciative of your efforts.
Gordon, please accept this in the spirit it is intended.
I lead the Stellafane optics workshop. I have made hundreds of pitch laps both for myself and for students. I have made the same mistakes that you are making, indeed in my videos, I make them. I have learned the hard way that pitch is more than just the liquid that flows in the devil's river Styx , it is downright dangerous.
We never heat pitch on an open flame. The risk of igniting the volatiles is just too great. We always keep a fire extinguisher handy as well, just in case.
Long sleeves and gloves should be the order of the day. One of my students working at home severely burned himself (third degree burns) working with pitch earlier this year. We try to keep pitch at around 145 degrees, just above the melting point. It will stick to you and transfer heat to your skin for a long time.
Goggles are also required. One of our previous students was an opthamolgist. He liked to say that an ounce of prevention is worth a pound of inoculation. In short, safety should be paramount.
Warm regards,
Francis J. O'Reilly
Francis OReilly wise words, at work we have the cold water constantly running when working with pitch and work right next to it. Means even the smallest splash can be instantly cooled. We have an electronically temperature controlled pitch pot to make working with it as hazard free as possible.
I would add that pitch is also very toxic in a bad way. The volatile materials in pitch are every bit as dangerous as smoking and that includes from skin contact. Keep the work area well ventilated and avoid those fumes.
yes can be dangerous hence from now on i am heating it in my glass kiln i can set the temperature exact
warm regards...was that a pun? :)
Is that Spirit, good or evil?
Apart from the amazing insights into making these mirrors, this guy just sounds like a top bloke.
What attention to detail! A true perfectionist. This guy’s work must be in huge demand.
Well, the long waited part4 of this series is finally here! thanks!
It is a pleasure to see you working and very surprising for me the kind of techniques that you use because, until your videos, my only source of information was the Jean Texereau book.
I wish you a good end in this work and hope to see a part5 soon with a beatifull foucault test like the one you shared in 2010.
A local workshop taught me and helped me make a 6" reflector. Watching you was great fun.
+Michael Carter Thanks, Michael! Glad you had both fun and success on your 6" reflector. There is nothing better than taking that first look through a telescope with a mirror you made yourself. Congratulations! -Gordon
Hi Gordon. I have worked with lasers and interferometry for many years and I greatly appreciate you sharing your skills.
I’ve worked with laser pointers for many years.
Why did I see the whole 4 videos on this telescope mirror? WHY? I didn't even know this was the way they are done. And the pitch, I didn't even knew that word. WHY DID I SEE THESE VIDEOS? WHY WERE THEY SO INTERESTING? Lol.
"Air currents" Wow, this guy accounts for so many factors in mirror grinding...
These videos are as good as they get!
Thanks, Dan, glad you enjoyed them!
I would like to have seen a part 5 or 6 to see how you form the parabola finish on the mirror.
Philip Collins We were just getting to the good bit 😢
Everyone wants to see the end, This is the third howto on mirrir making that goes unfinished.
Industry secret
He’s pulling a George Lucas on us. :-)
This is a true craftsmen, amazing work.
I agree with the other commenters, really nice series on how you ground this mirror. In an ideal world, you should have well over a million subscribers.
I wish!!!
I’ve always wanted to try this.
Happy New Year Gordon!
+Obishman Thanks! I hope it is a good year for everyone!
Great and informative video. Is there any substitute I can use instead of pitch?
Nice video!
Love your videos. Thanks for the info.
Hello Mr. Waite
I’d like to know if you make the pitch-lap tool for all of your mirrors like this one in the video, where you “scratch” the pitch-lap or if you make them in the conventional way, heating and melting the tool. I’m asking this because I’d like to know if this method would work for a 6 inches mirror, which is going to be polished manually. Or should I use this technic only with appropriate tools?
Thanks for sharing your videos, they are of an excellent help.
Sergio, Brazil.
Nice. As long as the work table is level, and the tool substrates are even thickness, the pitch thickness will be nice and even. Otherwise, it could cause troubles, tho the spin polishing seems like it would bypass pitch flow/contact issues.
all of these videos about how to make a mirror and ive yet to see a single reflectve surface ;(
Any links to vids that show the end product too?
+K Roon Check out: th-cam.com/video/D36pfEHq83Y/w-d-xo.html It is a little strange, but I make the mirrors, then send them to the coater for aluminization. The coater sends them on to the client. So it is very seldom that I have a coated Waite Research mirror on hand.
I'd hate to get into a fight with Gordon, he's deadly with that razor.
where part 5 pls
What is pitch? Do you make it or buy it?
Hi, Gordon! Do you have a "How to" or "Walk thru" video about your turntable? It's pretty impressive.
+magdanoz88 Hi! I don't really have a video just about the turntable, but I'll try to put one together. I'm getting help now from my son, Emerson, so we'll be doing videos a lot more often now. Thanks for your kind comment!
Thanks for an interesting tutorial. About forty years ago, I ground and polished my first (and only) six inch F8 mirror. I had no mechanised tools nor a pre-cut blank (it was a borosilicate sight glass from a chemical reactor), so it took literally months of my spare time to get it spherical using the old Foucault knife edge optical method.. Anyway, I was interested to see your use of petroleum pitch for the polishing and parabolising laps. I used laps of wood pitch and beeswax. Has wood pitch lost favour these days or is it simply a question of cost?
Actually, the most popular pitch is Gugolz, and it is still made from wood resin. Nobody uses beeswax with it any more. Gugolz pitch runs about $40 US per kg. Synthetic pitch costs about $33 per kg. Not a big difference. I use Gugolz most of the time, but keep Acculap on hand for special projects.
olá gostaria de saber se posso usar essa mesma ferramenta em um espelho de alumínio???
RIP RadioShack 12:52
I love machine
I have been using your method for trimming pitch laps since I started using Gugolz (my first two videos were with Burgundy, which is horrible for polishing and was replaced after it got stuck to my mirror) and it has worked extremely well for polishing my 6" f/8, which is now being figured. The only difference is that I don't have a brush so I instead rinse the mirror in water to remove the chips.
Hi! You should acquire a brass brush, and always brush the lap with it before rinsing it with water, before every polishing session. The brushing gives the polisher the best texture for polishing the mirror. It puts thousands of little scratches on the polisher, and these help tremendously. A brushed lap has a wonderfully smooth, silky feeling when rubbed over the mirror, much better than the sticking, slipping and jerky motion so often found with a polisher that has not been brushed.
I will go and get one for my next mirror. Thanks for the tip!
What would be the best size and focal ratio mirror for an absolute beginner to try to grind?
+Nick R Hi, Nick! For an adult, I usually recommend an 8" in the f/6 to f/8 range. I think making an 8" is a little easier than a 6" because that size just fits a little better in my hands. For a youngster, the 6" in the same focal ratios would probably be easier. Basically, I don't recommend building bigger mirrors. Your chance of getting a nice, usable mirror is way higher for 6" or 8" projects. A 10" might be practical if you are comfortable with making things in general. But a 12" mirror is quite a bit more challenging. I think it is better to start with a less expensive project with a higher probability of success. After all, if you produce a great 8" mirror, there is nothing stopping you from making a nice 12" or even larger! Best of luck with your first project.
Oh man, another hard on that left us just wishing.......... i wanted to see the end of your work with this piece of glass... the coating can be seen in other videos around but this is the most interesting part :-(
What kind of pitch you are using.... I have a pitch but when it gets hard it feels like rubber not like hard fragile ...
I always use Gugolz pitch. Mostly 55, 64 or a mixture of those two.
Hey Gordon!
It's been a lot of time since your last video, but: how do you make the mirrored surface?? I didn't get it, coz at the final part there's no reflective surface as a normal Newtonian telescope mirror...
Shouldn't it be a 5th video??
To make the mirror reflective, I send it to a coating shop in New Jersey. They put it in a vacuum chamber and evaporate aluminum onto the surface, which turns the clear glass into a mirror.
Thats what I want to see, How do you go from spherical to parabolic?
Will try to do a video on this in the future. It is often requested, but hard to make!
Nice video sir. Where is the 5th part of polishing?
Sorry about that! A project yet to see the light of day! Just got too busy...
hi Gordon, your work is fascinating. I also want to grind a mirror, from where I can buy a blank glass? and is this glass should be a any special type, because I saw from somewhere tempered glass are not suited for grind.
+Sudheera Lakmal Hi! no, you cannot use tempered glass to make a telescope mirror. Two commercial places where you can buy mirror blanks are United Lens, and Newport Glassworks.
Tempered glass goes like this when you try to cut or remove more than a tiny chunk of it th-cam.com/video/EqRNv8q3h3c/w-d-xo.html.
If I were you, I might take some liberty to try some Chinese calligraphy when scraping the pitch or scribbling on the mirror at the first step.
Erm, abrupt ending though. I was hoping to follow this mirror to completion, but it looks like things end here for this one?
At the very end of the video, you said you would rotate the mirror on the turntable to prevent astigmatism from being polished into the mirror. What is astigmatism, and why does it help to reposition the mirror on the turntable? Is it to cancel out any eccentricity in the setup?
Astigmatism is where different diameters of the mirror have different focal lengths. Very bad. It generally comes from having some kind of defect in your physical grinding and/or polishing setup. By changing the mirror's position on the turntable frequently, you average out the tendency toward astigmatism in any one or two spots.
1. What is the material that you use to make this pitch polisher? I searched many places but I could not find it. 2. These ceramic tiles which are used in making grinding tools, are they always used on the ground or any other type of tiles?
For the polisher I use "Gugolz pitch" which you should be able to find via Google. The ceramic tiles are uncoated pool tiles, made for use around swimming pools, or for flooring.
Is it possible to make a mould of a finished mirror so that new glass blanks could be poured and be almost to finished shape?
They already mold mirrors, but generally only the deepest ones, to save roughing time and a little glass expense. Molding isn't anywhere accurate enough to make a difference in mirror making if you are looking at the final shape. Evening the best molding of glass is still going to require fine grinding, polishing and figuring to make a parabolic mirror.
Gordon,
Why couldn't you do the 9" lap the same way you did the 3.5" ? Seems like it would work...perhaps someone will educate me...
+Skeptic100100 The method I used on the 3.5" lap only works well on smallish polishers. If you try to do it on a bigger polisher, the center will be too hot, or the edges will have cooled too much, and you won't have the time and the ability to mold the pitch around. The method I used on the larger lap gives you a perfect surface and is great for most laps, as the majority of laps are bigger than 3" or 4" in diameter.
Where to get a 12 inch glass to start the whole project?
I buy a lot of glass from Swift Glass in Elmira, NY
Pls tell me the pitcg specification..so that i can buy
I generally use Gugolz pitch, either 55 or 64 or a mixture of the two.
Unable to get more then 12mm thick glass during lockdown can we use 12mm glass in 8" diameter?
Only if you have made 20 or 30 mirrors in the past. The 16:1 ratio is too much. Difficult to work and eliminate 'stig. Really 18mm is a good practical limit for having an easier fabrication with a high probability of success in a reasonable amount of time.
Interesting series. I feel like I got just the tip....
Hello sir,can you please tell me what size of grits used for rough grinding,fine grinding & for polishing the 6 inches mirror
For rough grinding, if you need to hog out a lot of glass, you can start with #60, then move on the #80. For a shallow mirror, you can start with #80. There are many sequences you could use. I like to go from #80 to #120, then #280. These are all SiC. Then I switch to Microgrit aluminum oxide, and use 25-micron and 9-micron. This is for machine-assisted grinding. If you are working strictly by hand, I would probably add steps for #220 in rough grinding, and 12-micron during fine grinding.
@@GordonWaite Does it matter where you get the rough grits from? For example, could I just use grit from amazon, like www.amazon.de/Siliciumcarbid-Schleifpulver-verschiedene-K%C3%B6rnungen-F8000/dp/B01LWVCFYE for 60-320 grit (so down to about 50 micron; www.gemsociety.org/article/gem-cutting-abrasives-grit-mesh-microns)? It is way cheaper than the stuff from microgrit, which anyway I have no access to (super expensive shipping abroad).
How do you get the mirror on the glass when your finished grinding?
Hi, Shane. After the mirror is finished, I usually send them to Jeff Decker at Majestic Coatings. He cleans the mirror and puts it in a high-vacuum chamber. He pumps the chamber down, and then heats up pure aluminum until it evaporates and coats the mirror with a very thin layer of highly reflective aluminum. An over-coat of SiO is also evaporated after the aluminum to protect the soft coating.
Thank you sir, this is very interesting and it sure looks like you know what you're doing.
this is like alchemy , never realized grinding a mirror was so involved , i am not sure exactly what pitch is , is it a tar like compound ?
+HALE BURNSIDE MD Yes, it is much like tar, only more solid. Optical pitch is usually made from natural wood resin.
radio shack?
you should use a ladle and drip cloth under it for the pitch pouring
Think of my pitch pan as a ladle without a handle! If you actually dipped a ladle into a pan of pitch, it would make a mess unlike anything I'm willing to clean up. Every try to get pitch off of anything? Especially the bottom of a ladle? I've been pouring pitch out of a pan like this for nearly 50 years. No mess, no fuss. Also, I try to keep the amount that I melt equal to the amount that I need for the session, so pretty much the pan is empty after the pour. If you use a ladle, you will have to melt a lot more pitch than you will need. Plus you will probably need a bigger pan. Every watch John Dobson pour a polisher? He would heat up pitch in a coffee can, grab it with his hand and pour it out. He started making telescopes a few years before I did, in the late '70's. We old geezers are set in our ways! ;)
Are you doing this in your actual kitchen, or do you have burners set up somewhere in the shop?
When this was made, that was my kitchen. Now we have a nice shop.
GordonWaite I noticed in the Dobson documentary that he was definitely heating his coffee cans for a whole mirror making class worth of pitch laps in what looked very much like his 1980s home kitchen. I guess the fact that these days an electric 2 burner unit is practically free (I mean, 20 to 50 bucks, maybe 100 if super fancy) makes it a lot easier to do things like that away from food prep areas.
These days I heat up pitch in an aluminum bread pan, over a little electric hot plate. The hot plate costs maybe $20, the pan is $1 for 3 at the Dollar Store.
Can I get this sized mirror in India and price of India rupees????
Mr Waite,
Where could i get a Mirrow blanke from I 'am a disabled Vietnam Vet so there isent much i have that i can do for hobby's , I have a 4 " scope with go to,on it but it ise'nt big enough,by the way it is a Meed telescope,
I want to give this one to the school or find a astronemy class to give it to.
A good place for small mirror blanks is firsthanddiscovery.com/astronomy/telescope-making/telescope-mirror-blanks.html
i actually thought about using red iron oxide for figuring. some say it produces smoother surface
Using cerium oxide will give you a very, very good surface. Much easier, too. For a person with no experience, it will be easier to get a smooth surface with cerium oxide. To use red rouge, you really need to ball mill it to remove clumps that leave streaks. And red rouge is the messiest stuff you will ever use in your life. Cerium is clean and easy. Basically everybody in the industry uses cerium oxide. Red rouge is reserved for "throw back" projects! ;)
GordonWaite good evening Sir,
i guess i just got caught up in the whole "red rouge gives smoother finish" argument. However yes i was very pleased with cerium oxide and the changes on a 12 inch f5.7 (first ever mirror) during figuring were pretty fast to me.
btw question for you sir. When figuring, even when using chordal stroke mirror on top to deepen the center i would get a small hill in a center which i then had to repair using a small subdiameter lap.
any explanation for that? to me it is unfathomabke how a chordal stroke can produce a hill in the middle?
i suspect it had to do sth with the pitch, because it was sitting for few months before i decided to finally finish the mirror.
A chordal stroke has the polisher offset from the center of the mirror. Thus, by definition, the center of the mirror isn't getting as much action from the polisher as areas farther away from the center. And so a hill develops in the middle. This one is really easy to avoid. When you are doing the chordal stroke, about every third or fourth stroke, run one stroke straight down the center of the tool. That will give the center just enough action to keep it from ending up a high zone. So think to yourself, chord-chord-chord-chord-center-chord-chord-chord-chord-center and you will get a much better result. BTW, this also is a good practice when you are hogging out a mirror with very rough abrasives using a chordal stroke. This will keep your mirror much more spherical.
@@GordonWaite thanks for the response
Pop the bubbles!
+droidika11 Too much work for too little gain. I can't say that bubbles have ever caused me any pain! Next you'll be wanting me to clean up between abrasive grade changes! ;)
I miss pitch polishing. It is high key funny to me that we can achieve sub micron surface form and nanometer or better surface finish with a spindle, dental plaster, tree juice, a razor blade, a hotplate, and some cerox.
How's it a mirror if you can see through it? Wouldn't it be considered a lens?
After you are done fabricating the glass, it goes to a lab and put into a vacuum chamber. A very thin coating of aluminum is evaporated onto the glass surface, and that is what makes the glass reflective and turns it into a mirror.
GordonWaite ah, well that makes sense. I should have waited for the answer. But wouldn’t it be more logical to CNC a piece of aluminum and highly polish it to give the same desired effect? I’ve polished metal to the point where it basically becomes a mirror. Just curious
Aluminum expands and contracts too much when heated or cooled. Borosilicate glass has only 1/6th the coefficient of thermal expansion of aluminum. So an all-aluminum mirror won't hold the precise shape needed when the outdoor temperature changes. When you use an evaporated aluminum coating on a borosilicate substrate, the aluminum layer is only a few atoms thick, and COE isn't a factor then.
GordonWaite very cool! Thank you for sharing your knowledge. I recently just got into the hobby and would love to buy a telescope, but first comes the camera and learning about astrophotography. Thank you for all your videos and information, they are outstanding
Giant Oreo biscuits thats what they are
I'm sorry for sounding silly but what is pitch what is it made of I thought you are suppose to epoxy the whole concrete with tiles on top. or am I referring to just something completely different? thnks
pitch (also called bitumen) is a residual product in petrol production. But it appears also in natural open lakes (for ex. Trinidad Tobago) . You use it normally as the binding substance in street-surfaces . Asphalt is the mixture of crushed stone with pitch. Therefor the reason that in a hot summer sometimes your car- tire sinks into the street surface when you stop your car. The sun heats up the pitch to a temperature so that it begins tu act as a fluid.
Pitch is made from tree sap. It is the material that you use to hold cerium oxide to polish the mirror. The business with tiles is for making a tool for grinding the mirror before polishing.
The black material looks and asks like roofing tar witch is usually called pitch and its burned on to the roofs
Close to the same material. Mine is probably a little more consistent and a little more expensive! ;)
part 5 ?
+yobb89 Very soon... working on it now!
@@GordonWaite Hi Gordon - any chance to see Part 5?... Thx!
I don't get it - why spend all that time and effort making a perfect surface on the pitch polisher, when you basically remove the surface during the scoring and brushing? Is the goal to get a particular curve on the pitch? If so, doesn't scratching the pattern in remove that curve to a point?
+Hifi Fan You want the top-most layer of pitch to have perfect contact everywhere with the surface of the mirror. But cutting channels below the surface is a good thing. While you can certainly polish with an unchanneled polisher, you really need to scratch it all up to make it work right. An unchanneled polisher takes a different technique, and is a little bit more difficult, in my opinion. The channels and the scratches serve to help distribute the liquid polishing agent. Generally, if you don't have scratches and/or channels, the polisher will skip and grab and jump around, and that leads to a poor surface texture on the mirror. Also, by scratching up the surface before each session, the lap polishes more quickly and aggressively. If you can keep it under control, you can finish the mirror faster that way.
GordonWaite Thanks for explaining that ... I kinda figured that the channels were for sloughing away debris etc, I guess I was wondering more about why so much effort was made to form the pitch so carefully against the mirror in the first place. I assume now that it's more to provide a correct curve to the pitch, rather than a particular surface?
I'm confused....why didn't he just make F5.....why specifically F4.9....what does that 0.1 difference make?
Somebody please explain.
There’s little difference between f/4.9 and f/5.0, and he may have been trying to achieve a specific focal length, either for mechanical constraints (to fit an existing or predesigned structure) or to match the focal length of another mirror or to meet specific magnification parameters. One doesn’t try to “hit” an integer f/ratio “just because.” And most people are going to just refer to it as an f/5 anyway, because it’s close enough that it makes no practical difference.
@ 20:17 steel isn't harder than glass only diamond is harder than glass. am i right?
Nope! If only diamond was harder than glass, we wouldn't be able to grind mirrors with SiC and AlOx. There are several steel alloys that can scratch glass.
hmm interesting...
Sapphire can cut glass just as well as diamonds
Good lord man, pouring boiling pitch out of a tin, with no gloves!?!
+g0dbel0w It's not all that hot when it is ready to pour... maybe 140F or a bit higher. You definitely don't want it boiling, as it changes the temper too much. It also helps a bit to wet your hands before you pour, and I do keep cold water running in case I need to cool off a finger!
That is... Pitch perfect... Yeeeeaahhha
This glass looks as though it has lead in it . if that's true aberration will be great
There is no lead in borosilicate glass, nor in soda lime or plate glass. Lead is generally used only in crystal.
you keep referring to the black thing as a mirror. Confusing.
The black thing is the pitch polisher. The white thing is the mirror.
I dont know why I am watching this. I am brain dead.
Ahh man, i'd do that in an aluminum tray or something..
Ummm... ever thought about using gloves???
At the least latex gloves that you can tear off in a hurry when necessary.
I sometimes wear gloves, but not usually when pouring a lap. When I cut a lap, though, to put in channels, now that is where you want to wear gloves!
GordonWaite razor blades cutting into a slightly unpredictable spalling material? Yeah, that makes sense. Single edge razors are hard enough to use without cutting the hand that holds them when you’re cutting something predictable.
Save your money and buy a mirror.
I make mirrors for a living. So I can't save money until I've made a mirror to sell. Then I would have to buy a mirror from myself. Confusing, isn't it?
Nice video!