Polishing and Figuring Tips for an 8-in Telescope Mirror
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- Опубліковано 13 лют 2014
- In three segments, Gordon Waite gives tips for polishing and figuring an 8" telescope mirror. Gordon demonstrates the standard polishing stroke. Plus he gives tips for corrected a turned-down edge, and for reducing a hyperbolic figure.
- Наука та технологія
Hi Gordon, when you make your cerium oxide mixture, what concentrations do you use?
Also, I noticed that your strokes are very fast, I've read that this is detrimental, but it's obviously not in your case since your mirrors are excellent quality... Any thoughts?
Finally, when I polish, the slower movement of my strokes helps me detect if I have good contact or not, how do you ensure you have good contact while polishing?
Carlos Paris Hi, Carlos. When I'm rough polishing, the concentration isn't too important. I usually put about a teaspoon of Cerox in 8 or 10 ounces of water. For figuring and finishing, I thin that out quite a bit. Probably about four or five times as much water. Stroke speed isn't as important as pressure. Too much pressure will roughen the surface. The trick is at the turnaround point on the stroke. That's where you will leave a ring if you are too fast or too hard. Also, using elliptical or circular strokes helps a lot to smooth out the surface. I always mold my polishers on the mirror, then cut and brush them and do a good pressing before first use. In subsequent sessions, a short cold press is all that is needed for good contact. Of course I always brush the lap with a brass brush before every session. Basically, I can just feel when the contact is right. It's a smooth, dragging feeling that you easily can read, once you know what it should feel like. No grabbing or skidding at all. Contact isn't too much of a problem when you are using sub-diameter polishers. Much more difficult with full-size pitch laps.
Mr. Waite. I am new to telescope making and I chose to start with an 8 inch blank. I made a tile tool on which to grind. I figured my sagita and started with the coarsest grit (I'm assuming #80) included in the mirror kit. I exhausted that grit and moved on to the next size smaller grit (120, I think). The grits are just marked 1,2,3... I've reached my target sagita and I have a good sphere, I believe, as I have checked it with a spherometer that I built per your instructions. My problem is that I have a flaw on the mirror about 3/4 inch from the outer edge that looks like a scratch or a fracture. It appears to extend below the surface about 1/8 of an inch. It is about 1/2 inch long in total and in a "c" shape. I can't tell whether it is clear through to the surface but if I shine a pen light through the edge of the mirror at the flaw, it lights right up. I don't know whether this was a flaw I didn't notice before I started or something I did while grinding on the tool. I don't think further grinding is going to get rid of it. My question(s) are: (1) Will this be a fatal flaw ? (2) Can I actually turn the blank over and grind a curve on the reverse side (basically starting new) ? And (3) can I use the same tool that I am now using or do I have to construct a new tool ?
Hi, Donald. Sorry to hear about your spot of bad luck with your mirror. The traditional fix is to flip that blank over, and start again on the other side. I would recommend starting with a new tool, as the convex curve on the tool won't be easy to use on a flat blank. You should be able to get more #80 and #120 grit from places like gotgrit.com, which will sell you SiC in small 4, 8, or 16 oz packages. Let me know how it goes!
Thank you very much for your quick response.
when working on the turntable, after staring at the tool with the work spinning, if you look away do you ever get vertigo? seeing the room spin?
MrPinbout Yes, all the time!
Hi Gordon -
I love your videos. Did you make one that showed how to polish an elliptical secondary face-down on a larger pitch lap? It said to use pitch as hard as will pour, and not to warm press it. I saw it somewhere, but now I can't find it. If it wasn't you, maybe you know of it anyway? Many thanks. Brian.
I always make a giant pitch polisher when I'm making a flat, and a giant flat to press it. Much easier polishing a flat on a larger polisher, you are right.
@@GordonWaite hi bro. I very like your video but i dont know what your say in video becaous my speak english is bad. Can you write commposision in latter web. May i can read and translate. I very intrest to make a flat mirror for may hobby and school
My whatsapp 081297674586
Mr. Waite, I am stuck. I have an 8" mirror with a TDE. After achieving a sphere, I noticed the slight TDE, which was only visible far outside the RoC. I began this ToT method, and of course, see the sphere becoming oblate as expected, but no change in the TDE. Hours in, and as you said I tried "paying no attention to what's happening at the center of the mirror as it is far easier to deal with."
I now have a HUGE oblate sphere, and still not change to the edge. As prescribed, I was going to keep doing this until the TDE is fixed, but at this point the Ronchi lines are starting to be pushing to near the edges when viewing outside the RoC and I have one of the largest oblate zones I've ever seen in pictures. I've spent hours trying to correct this, and my fear is if I keep going I will only make it worse and harder to bring back to a sphere.
I have to think there's got to be more than this going on. Or am I on track? Will the mirror continue to be more oblate and eventually I will see the TDE fix on the Ronchi test? Or is there a point at which I should flip the mirror, W-stroke, do 1/3 stroke, etc? Your advice would be appreciated.
Generally, my advice is always to go back to spherical as soon as things start to go wrong. You may be confusing diffraction effects on the Ronchi with a TDE. Do 10 minutes 1/3 CoC with mirror on top, then 10 minutes 1/3 CoC with mirror on the bottom. Alternate that way for an hour or two, and you should be close to spherical, and you should have a good edge. Otherwise, I would be suspicious of your polisher or your exact stroke. Sometimes folks get a little "twirl" at the end of each stroke that can trouble the edge.
It might be worth mentioning how a turned edge can develop. With the thick full-diameter tool substrate in the video, smoothing or polishing too rapidly can cause a rocking effect that rounds the outer zone.
By the way, personally I almost always used a full-sized lap under the mirror for figuring, for mirrors up to 12", anyway. Old school, I suppose, but it worked for me!
Figuring a 12" with a full-sized lap is a lot of work! Glad you had success with it! How many mirrors have you made so far?
Several thousand! I was doing your kind of work for 25 years.
The reason why I used a full-diameter lap whenever possible is that I found that sub-laps tend to exert lap edge-effects on the mirror. I parabolized using side-overhang with the mirror face-down, to get the basic aspherical surface, and then use various smoothing strokes to finish off. Not always, but whenever possible. That resulted in a smoother overall figure ( for me).
Wow, you are a legend! I did parabolization by hand just like you described from the late 60's up through about 2005 or so. Since then I've been building more and more sophisticated grinding and polishing machines. I want to keep making mirrors for a few more years, and at my age (62) I don't have that youthful stamina. So I'm making up for it with automation. It's taken the romance out of mirror making, but even I am amazed at the surfaces I can create now. Would love to hear some of your war stories! Email at gordon@waiteresearch.com if you like. Thanks! -Gordon
Thanks, Gordon - I will email you! Just in the middle of refiguring my 4" achromatic doublet, after having hands off all glass for 18 years (I'm 67). It's like riding a bike, I have been told - you never lose it! We'll see!!
Hi Gordon - I'm trying to reach you by email. Am I getting through? bgcrabb50@gmail.com. Thanks.
hey Gordon Waite!!
i have watched almost all of your videos! I want to ask you a question. I have an 12inch mirror ready to be polished. this is a mirror thah would be used for planetary viewing. Which method do yoy recomend?
Do you have a polishing machine? If so, which type? Do you already have substrates to use to make pitch polishers?
GordonWaite no i have not any polishing machine. The pitch polishers are ready. All i am waiting is your advice!! :)
Γιώργος Γραμμ Then you will want to go the classic way to begin. Use a full-sized lap, same diameter as you mirror. Mirror on the bottom, stroke is 1/3 center over center (CoC). Polish like that until the mirror is polished out. Then figuring gets a little more interesting. With a 12" mirror, I normally figure mirror on the bottom, with sub-diameter laps. Contact me via email if you would like to start a conversation about your project! -Gordon
GordonWaite +GordonWaite Thank you Gordon!! Your advice is usefull!! I will start polishing within a few days as i have not a lot of free time (I am a high school student). If any problem occurs I would contact with you!!!
Mr Waite
Could you let me know where to find a Mirror, I am looking for at least an 8" maby even a 18".
I am service connected ,disabled from Vietnam I 'am not getting around as good as when I was younger, the wife wants me to stay close to home , I want to get my grandkids involved in astronomy while they are young.
Thant You Sir.
buy one of those 8 inch spherical mirrors with secondary from ebay for around 100 to 120 bucks, and turn down the edge a little to make it a parabolic. most of these include shipping in the price or just buy one 8 inch f/6 from agena astro for 205 dollars including free shipping oh btw the agena mirrors are already parabolic so no adjustment needed!. the 8 inch f/5 are $220 and the 8 inch f/4 is $230 all include free shipping.
Woudln't a mirrored finished concave alluminium plate (round) have the same effect as a regular telescope?
I wonder because I don't see anything saying it's not good, and also saying it's good..!
NewArtConcept If it was accurate enough. Telescope mirrors are made to a very high precision, typically holding surface shape to 5 nm or so. It's hard to work aluminum to those tolerances.
Ah that's a very good answer that makes me understand the whys but, if the aluminium cylinder is worked under vaccum or electrolysis, could it reach (lower quality of course) a reflective surface enough for star gazing?
NewArtConcept It's not a problem to polish up an aluminum surface into a really good mirror. But the surface will oxidize a bit. We don't make telescope mirror substrates out of aluminum mainly because the aluminum contracts and expands too much with temperature change. Glass is a very cheap substrate, and borosilicate glasses don't have serious expansion problems like thick aluminum would.
Back in the 1700's and early 1800's telescope mirrors were made out of plain old polished metal. Examples are the Herschel 40-foot telescope and the "Leviathan of Parsonstown." These mirrors were 1.26 meters and 1.83 meters in diameter. These mirrors were made from a metal called speculum, which was roughly 68% copper and 32% tin. They had to be repolished quite often, but the process was relatively simple and mechanical, just like polishing any piece of metal. The trick, of course, was getting the shape right.
@@RiderRickMaker aluminum with polish out as bright as any coating it just takes a little work and btw the mirror doesn't have to be round!
Hi, i m a huge fan of your video, thanks, i have a question, i m figuring a 12 inch mirror and i have a couder mask with 6 zone, and i can put any correction between Z5Z6, i tried tangentiel strokes but it appear a turn down edge. Now i m like this Z1Z2 70% Z2Z3 100% Z3Z4 100% Z4Z5 75 % and Z6 have same curvature than Z4 so turn up ege. thanks a lot, JBaptiste
That's a tough place to work from. I would advise to go back to spherical and try to parabolize more evenly. Otherwise, put the right edge of a 40% polisher on the right side of the mirror, with the right edge at the Z5/Z6 boundary, and use a 3" or 4" tangential stroke. Work 5 or 10 minutes and see if you make progress. But I recommend you go back to spherical.
@@GordonWaite Thanks for your answer, now the mirror is finished, thank you!! i have could put correction on Z5Z6 doing parabolisation stroke (4/5) mirror on top.
shouldnt you be rotating the tool along with the W stroke?
You rotate the tool in the opposite direction that the mirror is rotating. I give the tool a little bump after every set or every second set.
Очень интересно!
is Gordon still alive? I know he starting to get old like me and was wondering since i havent seen any of his work on youtube lately. I hope hes still kicking because it would be a shame, to lose all that knowledge hes gained over the years.
Still hanging in there! Last video was about four months ago: ua-cam.com/video/-KDl-entYwI/v-deo.html
why couldn't you just blacken the edge with a marker, instead of having to grind the whole face down like we do for refractor telescope lenses?.
Not a professional move! They would take away my license for that. ;)