Have been grinding and polishing a 6" blank for the past few months. Have got to the stage of being able to test the mirror and found a lot of turned down edge. Have been able to get rid of some of it as the Ronchi images show. The progression is from left to right. There is still a fair amount of TDE in the final image. Am looking for advice on how to proceed from here - continue to move towards a sphere, or try to parabolise. Was aiming for about f5, but have ended up at around f4.7.
Hi David, the dreaded turned down edge. You also seem to have a slight hill at the centre as well but that is pretty easy to fix. What works for me is mirror on top, 1/4 strokes centre over centre, no sideways movement. I do this for about 10 minutes. Leave the mirror on the pitch for 15 minutes, test the mirror, and put the mirror back on the pitch for 1/2 hour to let the pitch catch up to the mirror. It's a slow process but the pressing of the lap is essential. If the hill in the centre gets bigger ,as it probably will, because by doing shorter strokes you are making the centre less deep do a couple of turns of 1/3 strokes but make sure that they are no longer than 1/3 strokes. I have removed turned down edges by doing this just proceed slowly, (patiently), and remember lots of pressing. This could take a while but it's worth it. Let me know how you go. Go to this link http://cmabar.googlepages.com/home click on the mirror making link and see the paragraph obove polishing for a trick that I use. The website is a work in progress.
Regards Cristian
Last edited by cristian abarca; 21-01-2007 at 08:51 PM.
Reason: wrong link
I forgot one important thing David, make sure that you trim your lap a little. About 1-2 mm smaller than your mirror, basically it's about a 2mm bevel.
Thanks Cristian, the technique I have been using is to press the outer 1/3 of the mirror(i.e. about 1" from the outer edge)with a ring of paper between the mirror and tool, then doing 1/3 strokes so that the outer edge of the mirror never rides over the edge of the lap. However, it is difficult to get the very edge of the mirror sorted out with this method. Pressing of the lap is something that I probably did not do enough of in the early part of my polishing, and I suspect that this has generated the TDE. I was also quite a way into polishing before I worked out how to use the tester properly, so did not have enough feedback to correct my polishing method quickly. At this point, I am quite glad that I have only a small mirror to fix
Hin David. The method that you are using works well all you have to do is keep adjusting the size of the ring to match the edge. The edge you have is not that bad and you could figure from what you have because when you parabolise the edge is somewhat turned anyway. Being F4.7 the parabolised curve is somewhat pronounced. I would still play it safe and remove a little more of the turned down edge. Here is a picture of a 10 inch F4.55. Compare the curve to you TDE.
Yes, comparing the two images it would appear that a bit more tidying of the turned down edge is warranted. Your suggestion of extending the ring a bit more is good too, as I don't have a great deal of confidence in my ability to reliably polish with 1.5" strokes as opposed to 1.75" strokes, unless I go very slowly. At this point, however,I have just pressed my lap back to being a full
6", so I will give the shorter strokes a try, just to see how it goes.
Thanks for your advice, it is a great help to have access to someone who has done it before.
Yes, comparing the two images it would appear that a bit more tidying of the turned down edge is warranted. Your suggestion of extending the ring a bit more is good too, as I don't have a great deal of confidence in my ability to reliably polish with 1.5" strokes as opposed to 1.75" strokes, unless I go very slowly. At this point, however,I have just pressed my lap back to being a full
6", so I will give the shorter strokes a try, just to see how it goes.
Thanks for your advice, it is a great help to have access to someone who has done it before.
Regards,
David.
David,
You have excellent advice from Cristian. If however, all that fails, your lap might be too soft and the edge area is burrowing in slightly with each stroke. I have found when making my mirrors that laps can be of ideal hardness when you make them, but as the weather warms up they tend to go soft on you. I ended up making 3 laps over 6 months polishing my last mirror, a 10 inch. Polish for short intervals (less heat generated in the lap) and press heaps.
Good luck; patience and determination will get you there.
OK, have done about 3x 10 min intervals of polishing with pressing in between, and the result is in the following image. Looks to have corrected the TDE quite successfiully, maybe a little too much. How should I proceed from here? Maybe just normal parabolising (1/3) strokes with the same sort of pressing?
Just do 1/3 strokes but take it easy. You are nearly ready to start parabolising. Try to get the lines alittle straighter and could you send a photo inside and then outside the radius of curvature. When the lines look the same in and otside ROC your mirror is spherical and ready to be parabolised.
Thanks for your help so far Cristian, and for your encouragement Don. I have done some more polishing and got to a point I hope I can use as a start for parabolising. The following image shows inside, at, and outside ROC.
Hi David. Can you verify that the pictures you sent are in the right order. Doing shorter strokes centre over centre creates a turned up edge and a hill at the centre which is what you have. The photos seem to be in the wrong order that's all. Not to stress this is easily fixed. Just a couple more 1/3 strokes centre over centre should bring it back. Try 2-3 minutes if there is no movement go for 5. You can also do slightly longer strokes than 1/3, but I wouldn't go this way it may backfire. I know it's tedious but the closer to spherical you get the easier it will be to parabolise. The idea is to reach the parabola without defects. Also I find it easier to parabolise using only one stroke all the way through without having to change to something else in order to repair a defective parabola, it makes for a better mirror.
Thanks for the reply. I have to confess to trying some parabolisation strokes on the mirror before taking these photos, and I guess that this is not what you are recommending. Just to confirm - you recommend going back to a spherical surface, then parabolising from there with a single type of stroke in order to get that best figure on the mirror. Also, my tester is not showing more than about 3 bands outside the ROC. Is this normal, or should I try inceasing the slit size or some other remedy. I have to admit this mirror figuring is turning out to appear a bit like sorcery to me.
Hi David. Yes to your answer. You can try parabolising from where you are now but you might have to end up correcting the parabola and I think you should stay away from that. What parabolising strokes did you use? Try going back a little bit further till you get more lines, if this doesn't work increase or decrease the slit, experiment. But if you are showing lines inside then you should be showing lines outside. Also are the photos you took in the right order? Send a photo of your tester.
Have been doing some more polishing, but the mirror appears to be returning to a figure similar to that in my post on 24/1. Have been considering doing a short stint with tool on top to see if I can get it to move to a more spherical figure. At present I don't have an image available. Will try to get one tonight. My tester is pretty much a standard Focault/Ronchi carriage with a green led for a light source. Am still unable to get more than about 3 lines outside ROC. The width of the slit image appears to be quite narrow past ROC.
Hi david. By all means try it. Just remember that putting the tool on top will make the mirror less deep, you'll bring the centre up. Do it but make sure that your strokes are 1/3. That is that the distance travelled up and down is a total of 1/3 diameter not 1/3 up and 1/3 down. If in doubt make it a touch shorter. That is why I mark my mirrors so that I can keep the dreaded turned down edge in check. As for you only getting 3 lines outside ROC is strange. I don't think that it is a figure problem but more a mechanical problem (set up). Is the light source parallel to the grating or close enough to parallel?
Haven't done a great deal of polishing this week. Here are the images of the current state of affairs. At the moment, have stopped to resurface the lap with resin. It was getting very thin.
Looks like I have managed to get the surface a bit uneven.
Hi David . Firstly can you post a picture of your lap. Secondly just keep doing 1/3 strokes, slowly and evenly, and thirdly here is a picture of the mess I had with my first mirror. It's the same mirror that I am using at the moment so don't get discouraged and keep going. Also a picture of the moon I took with the same mirror once I finished it.
Thanks Cristian, I am encouraged by looking at your mirror Ronchi image and how it turned out. Don't really know why the unevenness in the surface of mine has occurred, but the lap was getting very hard and thin, and maybe that had something to do with it. Will post a picture of my tester and lap shortly.
Hi David,
you are nearly there!!
This was the stage I sort of got a bit stuck with too. Christian pointed me in the right direction here.
Took me quiet a few goes to get it close to a sphere ended up more of a oblate spheroiid. then took me about 5 goes of parabolizing before I was happy with the final shape about 1/10 wavefront.
Don`t have far to go before its done!
With my mirror I had a small TDE and found that a good half hour or longer of MOT 1/3 W or shorter 1/4 W got rid of it but my lap was rather hard and getting thin. everytime I did some TOT I would start getting that TDE again...
But keep it up, Cristian is a great help and I am sure you will have a ripper of a mirror soon..
I have posted quiet a few images taken with this mirror and I am very happy withe the final results...I shall make a slightly bigger one soon!!!
Cheers Gary
Thanks Gary and Cristian, here are some photos of my tester and lap.... pauses for laughter to subside.
The tester is built on the carriage of an old printer. The Ronchi "grating" was printed on a laser printer transparency. The black box is my modified Toucam which looks thru the tester window. The laser pointer is for lineup. I have the mirror mounted on an old Alt-Az tripod, and use the slow motion controls to line it up.
The lap has just been resurfaced, channelled, and pressed.
pauses for laughter to subside. 
