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6 inch first time mirror grind


dark star

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After a long break I have started grinding the mirror again. I read that you should grind the back flat. There were a couple of large pits which I have ground out on the back. The back is now smooth, I used 120 grit. But I got a raised center. I then ground this out and ended with a curve! What stroke do I need to use to grind it flat using a sub diameter half sized metal tool? Also, for a 6 inch f/5 mirror does it really need to be flat? If so, how flat, do I need to fine grind the back?

I have got the front of the mirror hogged out deep enough. I am now switching to a full sized tool. I am going to use a concrete and two penny tool instead of tiles. A penny tool was recommended on the Cloudy Nights forum. I am going to use a W stroke, I assume this should be across the whole mirror? Tool on top, starting with 80 grit.

I have made a Foucault tester and bought a Ronchi screen, 120 lines. I tried it out on an old Skywatcher 6 inch f/8 mirror. I can see the lines from the Ronchi screen, but it is pretty hard! I am thinking of trying to attach a small finder scope to make the lines bigger. Next I will practice the Foucault test on the mirror.

David 

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Hello David,

To prevent grinding a concave or convex surface, reverse the discs regularly Half a dozen whets one way, then half a dozen the other

By reversing, I mean, mirror on top, then change to  tool on top.

It's nice to have the  mirror back nice and flat, but don't knock yourself out over it, It's only a six inch, so you don't need much in the way of support in the cell.

3 Cork Pads will do. 120 degrees apart 1 third of the way in from the edge. The cork if thick enough, will negate any anomaly in the rear surface of your mirror.

 It's a nice  little  project making a six inch. Once you've enjoyed using it for a while, you can go on to  a larger size. A 10" perhaps ?

Good luck.

Ron.

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It is necessary to have even support across the back when you get to polishing and figuring the mirror surface. Any rocking due to a convex or rough back will make it difficult/impossible to get a good mirror figure. The finish on the back is up to your preference, it is not necessary to grind it very fine but you might feel that a coarse surface could harbour larger grains of grit when doing the mirror itself.

For a 6" I would prefer to use a penny rather than a two pence coin. If you consider it is a flat disc of metal, only the edges of the disc will be in contact with a concave surface and by the time a reasonable area has worn away from a larger disc, the edge will be very thin or gone completely. the smaller diameter penny will give you more leeway.

With a fullsize tool on top you will be lengthening the radius of curvature (roc) and the longer the stroke and the bigger the W the more this will occur. With mirror on top you will shorten the roc. If you have the roc that you want already and don't want to change it then keep the strokes short with a small w to keep the surface smooth, check the roc and work with mirror on top if necessary . After grit 220 the changes in roc will be very small.

If you use a fullsize lap for figuring then you will have to work with the mirror on top to deepen the centre of the mirror. If your mirror F ratio is 5 or longer then I would not try figuring with a sub-diameter lap on top, I would stick with the full size lap with mirror on top. The amount of glass to be removed from a 6" mirror between polishing and figuring is small compared to larger mirrors so it is usually much easier to accomplish.

Happy grinding

Nigel

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  • 4 weeks later...

I have got to the stage of figuring the mirror, parabolizing it. I have managed to solve various problems up until now. Now I am stuck.

No matter what stroke I use it seems to be correcting the middle and edge of the mirror but not the center. I am working mirror on top with a full sized lap. I have made the lap a second time, adding half a teaspoon of linseed oil as it was too hard. However, I think it is still too hard and this is possibly causing the problem.

With a Rronchi grating inside the radius of curvature the lines at the center are almost straight and those in the 70% area and edge are curved. I have tried a W stroke, a half W stroke, a stroke with the center of the mirror over the edge of the lap but nothing has much effect on the center.

I press the mirror on top of lap with a weight and hve tried pressing after heating the lap in the oven (very carefully!). Next weekend I will remake the lap for a third time and try adding 1 to 1 and a half teaspoons of linseed oil. Are there any figuring strokes that I can try which will definitely parabolize the center in the meantime?   

I have also used a Foucault Tester, but I think this is more difficult for a beginner than a Ronchi grating. I can see that the mirror is  a small amount parabolic, but cannot tell which regions need working on using a Foucault tester. 

David

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I forgot tp say that the W stroke did not seem to be parobolizing at all. So I have been using a circular stroke, starting with the mirror overhanging the lap by around half, then going in a circular motion to the center, then out again. This is working, but not on the center. As shown in the Sky at Night mirror making video. I am now going to try the W stroke again, as it is recommended in a lot of places and I can't see why it only doesn't work on the center of the mirror for me!

David  

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Pitch is usually softened with pure turpentine ( artists supplies have small bottles ). Turps substitute is not the same and only partly mixes with pitch. The advantage of Turpentine over oils is that you can boil off the excess if necessary. Use extreme care and do it in the open as the vapours are highly flammable :angry4:  I have not tried to soften pitch with Linseed oil so cannot comment on it's suitability.

To parabolize a fast mirror a long W stroke is usual. If you are only getting the outer areas to move then the contact might be poor, probably due to too hard a pitch lap.

Nigel

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David,

When I ground my 14", the biggest problem for me was also a pitch that was too hard. When I softened it, everything became far smoother and the strokes started to do what you expect of them. I softened mine by SLOWLY adding boiled linseed oil to the molten pitch and testing the result after each tiny addition. I tested by dropping a blob of molten pitch into a cup of cold water and then tried the thumbnail test. It should leave an imprint when you press your nail in. Personally I found that a slightly soft pitch worked well for my needs, but I think a lot depends on the ambient temperature of where you are working.

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