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First time mirror grinding - 6" F4.5ish


DuncanC

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Firstly, fix the edge as that will be the more difficult of the two options.

Just a thought, are you curling your fingers over the edge of the mirror? If so you might be heating up and thus expanding the edge area which then gets polished off and shrinks when cooled to leave a turned edge.

Nigel

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If there's no improvement in the tde I'd try a longer W stroke for 15 minutes to see it's effect on the edge. It should remove the central hill rapidly.  How quick are your strokes? Would slowing down improve the surface roughness? Does the lap have good contact to the edge?

Parabolisation will absorb a slight tde.

David

 

Edited by davidc135
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Thank you both! I really appreciate the comments.

I've been working to remove the TDE ahead of sorting out the middle of the mirror. I ended up doing a total of 1hr 20mins in 3 sessions of this stroke as suggested on a cloudy nights thread:

fixing a tde on a primary mirror

This did a good job of fixing the edge. I've since moved onto to sorting out the centre without too much luck. Tried a few 20-30min sessions of 1/6d centre over centre MOT slow with no extra pressure. Tonight I tried 20mins of the same, but with a slight w to see if I could smooth things. This is the mirror as of this evening (Ronchi Inside focus):

image.png.ccfc44f457f853c74695cc8415b152e0.png

 

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Im sarting to wonder if the strange central shape is due to an issue with my lap. Maybe I don't have enough offset to my channels. I might try some 1/4d coc strokes until the channels close again, then recut slightly more offset.

image.png.fe12ccc7154c6d648a9bc4ee4b73171d.png

Edge is a bit wonky, I might need to trim the overhang a bit.

I think the reason for my TDE was either the lap being a bit soft, or that it's a little undersized. I made the lap on tool which is 152mm diameter, where as my mirror is 155mm. I guess it will help when figuring!

Nigel, It's possible the TDE is caused by finger curling as you say. I try not to do this, bit is difficult with big hands and a 6" mirror!

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Yes, it is not easy to get your hands comfortable on a 6" mirror. Try a 4" mirror, there you are down to fingertips🤪.

If you are doing nice random strokes and rotating everything randomly then it it irrelevant where the channels are cut. If you are not randomising everything then it is irrelevant where the channels are cut, you will get zones. ( yes, the location of the channels is irrelevant whatever you are doing ). This is where doing it by hand is better than a machine. Machines tend to be repetitious in their action ( by their very nature ) and can produce zones much more easily than you can by hand.

When doing the Foucault test you should see that very close to the knife edge there are some diffraction lines, akin to the airy disc and diffraction pattern of a star. These can help you to determine the extent of any turned edge as they are 1 wavelength apart. These should be visible with the Ronchi screen as well but as I haven't ever used a Ronchi screen I cannot confirm that. ( A Ronchi screen is simply a series of knife edges and sliding a simple knife edge across the beams gives me the same info).

When you are happy with the edge you will need to lengthen the stroke and widen the "W" to parabolise. ( Doing a 10" f/5 I needed to push the centre of the mirror to the edge of the lap to get the parabola deep enough ). I would try 1/2 dia strokes to start and see how the centre deepens then go further as needed.

Nigel

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  • 2 weeks later...

Thank you Nigel. I think I understand what you mean by the diffraction rings, but will research further. When I have the knife edge throwing one side of the mirror in shade, I can still see the highlight round the edge of the mirror. Is this a similar thing?

Another few hours done to get the mirror spherical. I'm defianetly finding a small side to side action in addition to the Normal CoC stroke is needed otherwise I create rings on the surface. Example below. This is after 1/2hr of pure 1/3D CoC strokes, MOT. Knife edge coming in front right.

image.png.a8f55e3cf2d8ffe897927b837fb73721.png

 

It's very close to spherical now, just trying to get the last bit of the centre done without damaging the edge. I might try a few W strokes for maybe 10mins to see If I can smooth the general surface and take the last bit of the middle. This is inside RoC

image.png.027902303a10f724f80a95061b01962b.png

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Nice steady progress in a number of ways inc the edge. It's worth trying a bit of W to see it's effect on the residual centre zones as well as overall correction. 

As you imply a little roughness has crept into the last Foucaultgram not that it's a big deal at this stage. Perhaps the W stroke variation will help smooth the surface or are the strokes too fast or jerky?

David

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Thank you for the interest in my project. Been a bit up and down recently! I started a few W strokes and realised I'd gone a little past a sphere and dug a whole in the centre so decided to keep going into a parabola. It didnt go well! I still have this issue with a ring about a third of the way out not being polished. Tried a few different strokes (chordial and circles centred at this point) to remove but nothing was working. I've given up on that and decided to go back to a sphere.

Also had a few issues with my testing. My ronchi grams have never been great and a grating printed on acetate was no better, even trying different light sources. I tried manual measurements using Foucault and a pin stick for the zone but found it very hard to see the zones. I've since added a dial gauge to my tester aligned it better and started using Foucault unmasked which seems to work well, though I want to do a test to see how repeatable it is.

This is where I was before going back to a sphere. Getting rid of the peaks just wasn't working as I expected.

image.png.381b009e20d03d097c03e593bd0d4cf5.png

Unfortunately I picked up a two inch scratch at the weekend. I'd hoped it would polish out but further polishing has introduced a few more (though small). The first scratch does seem to have reduced from the polishing done so hoping I don't have to go back to fine grinding. I've decided to pour some more pitch on top of my lap as it's down to only maybe 4mm thick. THis should also seal in any grit causing the scratches and maybe stop the problem with the ring being created. After that I'll do a bit more polishing to a sphere and do  very slow (5min a session) parabolising sessions with testing in between to try and get a good figure.

I really hope the revised lap will allow me to get more predicable figuring and I can get rid of the scratches. If I get a chance I'll upload a Foucault of the latest session of polishing back to a sphere to see where I am.

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On my way back to a sphere I thought. I just seem to be making ring even worse. Very confused at this point. I am doing slow, steady 1/3 centre over centre strokes MOT. Hopefully the new lap will be better. Big scratch quite evident in the Foucault.

image.thumb.png.2ea1c0cd447f62f7890974750944349f.png

image.thumb.png.0ce4616e9c487bc0ccf72d22e73db702.png

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Even if you know that scratches have little effect on performance it's still disheartening to discover one, although much of it would be behind the diagonal.

Hunting around for explanations for the stubborn zones I'm not at all sure but wonder if the lap's softness may be an issue. The first pictures of it show a lap that has closed after only 11/2 hours. However the outer part has hardly closed at all indicating there was relatively little contact although that may have been the initial situation only. The lap in the second photo looks pretty good- maybe there should be a little more offset.

I'd cold press with weight and with the lap face down to encourage good contact to the edge. 

How long are your wets and do you feel any difference in the smoothness of polishing resistance between the beginning and end of a wet? I wonder if the soft pitch is distorting and losing some contact during the wet if it's overlong.

I'm assuming that the matching surface of the tool supports the lap? Yeah, it's got to be.

David

 

Edited by davidc135
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If it's only lasting 1 1/2 hours then the lap seems to be a bit soft to me. I always used quite hard laps which would last throughout polishing and figuring ( and do for the next mirror!).

The pitch needs to be thick enough to conform to the mirror and if it gets too thin then this is hampered and can lead to difficulty in figuring. Perhaps the scratch was caused by the soft pitch thinning and exposing a hard particle?

I don't know if you have tried to feel the mirror surface immediately after a polishing session. It can feel quite warm from the friction of polishing. That won't help a soft lap stay in shape.

I would continue with the strokes you are using, maybe reducing the W a little, if at all at this stage. Your centre looks to be on the right profile, you just need to take it down slowly and the zone should slowly disappear.

Nigel

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Thanks for the encouragement. Had another "interesting"couple of days! Decided to try and pour more pitch on the lap to increase the thickness a bit and cover any grit that might be stuck in it. Mirror was wet when I pressed but I hadn't put any cerium oxide on...  Ended up with it stuck hard to the lap. Managed to get it free with lots of pressure and progressively hotter water. Got it clear in the end then had to melt off as much of the pitch as I could befpre wiping with turps to clean the rest.

Had a second attempt to repour the whole lap again. Went better but I hadn't let it cool enough before pressing so had some seepage out the sides. Had made it pretty thick so think I got away with it. Channels didn't last though so ended up redoing with the soldering iron and scraping of the excess earlier. Did a long press (no weight) for a few hours and 1/2hr with a counterweight before doing a 1/2hr 1/3d centre over centre stroke mirror on top. Seemed very smooth without sticking etc.

I could probably be guilty of a lot of your suggestions, especially through the initial polishing sessions. I was almost definately using too much pressure, strokes too fast, sessions too long between pressing, maybe rocking the mirror during the stroke, making the wets too long / cerium too watered down etc. I did note some warmth generated during polishing back doing the inital polish, so that won't help. As I resued the pitch and had a couple of attempts it should be a little harder than before, buy could still press a finger nail in a cold lump of pitch.

From now on strict 1/2hr max sessions, with 1/2 press between them. Light pressure being carefully not to rock the mirror, very slow smooth strokes keeping to 1/3d (+/-1"), 1:7 cerium to water mix, well shaken. Hopefully will fix my issues.

Hopefully get a couple of sessions done tomorrow and will give it a chance to equalise before testing. Wish me luck!

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Today's progress: 4x 1/2hr 1/3D CoC sessions Mirror on Top, with at least 1/2hr cold press after each one. Also did one 1/2hr session Lap on Top with shorter stroke as I have a slight turned down edge I think. Looking a lot better, nearly at  a sphere.

Current images (mirror cooled for a couple of hours). Note the scratch is reduced a fair amount:

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Note the sling I mount my mirror in for testing obscures the edge a bit over the lower 150deg or so, hence the wonky shape. It's just the sling masking the image a bit.

Going to do a couple more sessions with the shorter strokes to improve the edge, and hopefully be nearly ready to attempt parabolising again.

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1/2hr ~1/4d TOT and 1/2hr 1/4d MOT and I think the overall shape is pretty good. scratches remain though obviously.

I still think the edge is a bit turned down. Would appreciate anyone else's thoughts on this . Plan at the moment is to try and get the edge better before starting parabolising. Probably another hour of 1/4d strokes in a couple of sessions.

 

Foucault at ROC:

IMG_20210706_122531.thumb.jpg.ea1671aae0ef9f0639a80b8e0a72be5b.jpg

 Ronchi (Inside focus):

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IMG_20210706_122307.thumb.jpg.5bc9a2859fc09bbfb754469cb1db0eb4.jpg

 

 

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The figure is looking much better with faint zones remaining. Some tde and a bit of surface roughness. Another hour of the same as you say. An option might be to reduce each spell to 15 minutes with a max of 30 there and back strokes per minute or slower- really just to see if there's an effect.

Even if the turned edge stays put but gets no worse some of it may be absorbed by parabolising and anything remaining could be chamfered off if it's narrow. Better to remove it during figuring, of course.

David

 

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