Mirror figuring advice needed

[DavidH]

Hi All,

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.

Thanks,
David.

[cristian abarca]

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

[cristian abarca]

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.

regards Cristian

[DavidH]

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;)

Thanks for your help,
David.

[cristian abarca]

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.

[DavidH]

Thanks Cristian,

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.

[DonG]

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.

[DavidH]

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?

Regards,
David.

[cristian abarca]

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.

Regards Cristian

[DavidH]

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.

[cristian abarca]

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.

regards Cristian

[DavidH]

Hi Cristian,

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. :shrug:

Regards.
David.

[cristian abarca]

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.

Regards Cristian

[DavidH]

Hi Cristian,

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.

Regards,
David.

[cristian abarca]

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?

Regards Cristian

[DavidH]

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.

Regards,
David.

[cristian abarca]

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.

Regards Cristian

[DavidH]

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.

Regards,
David.

[Garyh]

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

[DavidH]

Thanks Gary and Cristian, here are some photos of my tester and lap.... :rofl: 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.

Regards,
David.

[cristian abarca]

David the Ronchi tester look OK. That's a good idea with the Webcam. It looks much better than mine. Mine looks like something out of the Flinstones. As for the lap I use a hard pitch, and with deep channels. I don't have to repour or rechannel even after 10 hours of polishing and 2- 3 hours of figuring. The down side is that you have to press continuosly, which is not a bad thing. Keep going and remember to shave your lap a couple of mm smaller than your mirror.

Regards Cristian

[DavidH]

Thanks Cristian, have had a play with the tester this afternoon and found that if I put a diffuser on the led to increase the size of the light source and use the Ronchi screen instead of a slit, the tester works as advertised with more lines both sides of the ROC.

Have also had some success getting rid of the turned up edge, but now have a central hill to remove. Probably this won't be too difficult.

I find having the Toucam mounted permanently on the tester a great help particularly when lining up the mirror, because you can see what is happening at the tester while standing at the mirror and turning the slow motion controls.

Regards,
David.

[DavidH]

BTW Cristian, where do you get your grit and pitch etc?

[DavidH]

Looks like I may be getting somewhere at last. Still have a slight hill I think.

[cristian abarca]

Looking good David. If the first picture is IRC, then you have a slight hole in the middle. You can try a little longer to try and remove it or you can parabolise from here, I would parabolise. If the first picture is ORC then you have a hill and I would try to correct before parabolising. Let me know which it is and I will give you a tip on parabolising. I get my stuff from my astronomy club but you can get it here too. http://www.telescopes-astronomy.com.au/telescopes036.htm I have used their powders before and no problems at all. The secondary mirrors I get from Andrews and you can get the mirror silvered by C Quin Pty Ltd. Ph(0412957809), there are afew others in Sydney as well.

Regards Cristian

[DavidH]

Thanks Cristian,
The order of images is IRC,at RC,and ORC. What sort of strokes should I be using to parabolise the mirror? Should I be using a set of computer generated Ronchi images from here or should I change to a Focault test? I think that this will be the fun bit.:)

Regards,
David.

[cristian abarca]

Hi David. To parabolise a mirror you basically use the same technique as for polishing (W strokes and walking around), except that the up and down movement and the side to side movements are larger. Work with the mirror on top. Mark the centre of the mirror (the back). From the edge measure 6 cm and place a dot, do this all the way around and then join the dots to make a circle, use this circle as a guide. As you do your strokes the circle you have marked should reach the edge of the lap at every turn, (sideways and up/down). Where the circle reaches the edge of the lap, is the 80% zone which is where you need to be at every stroke. Do this for about 15-20 minutes and test the figure. Remember to press in between each session. Don’t do too much at a time, take it nice and slowly. Also make sure your strokes are smooth. Download this program http://members.aol.com/ronwin20/ and go to this site, scroll down to figureXP download and download another program, you will need it later. http://bi-staff.beckman.uiuc.edu/~melockwo/ATMlinks.html . For the moment use Ronchi for windows. I have also adde some drawings explaining parabolising. Sorry for the quality but I hope the make the explanation simpler. Good Luck

Regards Cristian

[DavidH]

Thanks Cristian, I had used your method of drawing the circle on the back of the mirror later in my polishing. It definitely made it easier to get even strokes.

Regards,
David.

[cristian abarca]

Just remember david that the circle you draw for the figuring is different to the one for the polishing.

Regards Cristian

[DavidH]

Understood Cristian,

Have previously downloaded the two programs you mentioned. I was originally trying to use the Foucault test (with my own spreadsheet) to measure the mirror figure, as I could not get the Ronchi test to work for me. When I managed to get the Ronchi test working, I realised the extent of the TDE. This has been a very steep learning curve, but I am glad that I have had a go and am grateful for your expert advice amd patience.

Regards,
David.

[cristian abarca]

You flatter me David, but I'm no expert. I've only made 6 mirrors, I just like to help people who are interested in making their own mirrors and to pass on any tricks that I have learned along the way. I find that some literature tends to confuse people a little and since mirror pushing seems to be a dying craft, the simpler it is explained the more people will be interested in taking it up. I also find that the Focault test is a bit difficult to interpret for first timers, the Ronchi test is easier. have a look at these two sites regarding the Ronchi test. In particular the one relating to the Lumley test. This is an excellent way of getting a good figure on a telescope mirror and it will only take a little effort from there to get better than 1/10 wave using FigureXP. Besides it was by an Australian E Lumley. http://www.turbofast.com.au/astrotel/ronchi1.html

http://www.atm-workshop.com/ronchi-test.html

[DavidH]

Hi Cristian,

Here is the result after about 20-30 min of doing 80% W strokes. Could you please take a look and check if I am on the right track.

Thanks.
David.

[cristian abarca]

Hi David you are in the right track, just do a lot of pressing and try to keep the strokes even. By this I mean don't do more strokes in the middle try to do the same amount of strokes in the middle as well as the edge so that you work the whole mirror evenly. at the moment you are working the centre more than the edge but that is OK at this stage. The edges will turn automatically if you keep the strokes even. Proceed slowly and get back to me after another 1/2 hr. I would be doing 10 - 15 minute sessions, with about 1/2 hr to 1 hr pressing in between.

Regards Cristian

[DavidH]

Thanks Cristian, at this stage am trying to take it slowly, and hopefully not make too many errors, but things are happening comparitively quickly with this stroke.

Regards,
David.

[cristian abarca]

Compared to the rest of the work that goes into making a mirror figuring is rather quick, but the most critical. You're doing well.

regards Cristian

[DavidH]

Hi Cristian,

Sorry to trouble you again, but looks like I may have made a bit of a mess here. Cannot seem to get the edge of the mirror to respond. Any idea what I should do from here?

Regards,
David.

[cristian abarca]

Hi David can you give me some measurements (distances that the photos were taken at).

Regards Cristian

[DavidH]

Hi Cristian, I'll have to set my dial guage up on the tester to get the measurements. Haven't been using any measurements yet, but I guess its about time I did. At this point, the Roc may be a bit difficult to track down precisely, so it will have to be offsets from an arbitrary zero point I suppose.

Regards,
David.

[cristian abarca]

Hi David I think that you have over corrected you mirror. Have a look at the picture. This is a ronchigram of an over corrected mirror. It's a hyperbolic cassegrain secondary test plate, that I made. You can fix it by doing 1/3 strokes for about 5 minutes and pressing for about 1/2 hr this will take a bit of time but It will keep you going for a day or two. I'll get back to you tomorrow on another course of action. I'm moving house and all my astro stuff is in boxes at the moment.

Regards Cristian

[DavidH]

Mmm, sure looks as though that's what I have Cristian.

Regards,
David.