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350 newt project

paul11

By paul11
in DIY Astronomer

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Glasspusher

Glasspusher

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Paul, working with the mirror on top could result in the mirror flexing but you might get away with it. By all means start as you had planned, if the mirror shows astigmatism you can always turn it over and work face up with your full sized lap centre through centre. When workinh with a full sized lap it is often the practise to work 50% of the time with mirror on top and 50% of the time with tool on top. If you decide to do this I would start the sequence with mirror on top and end with tool on top. Sorry if I have confused you, I hope this helps.

John

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FraserClarke

FraserClarke

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Tricky subject this one -- I've heard lots of support for both positions! Putting the mirror on the bottom allows you to support it better; but it does then mean you have to support it properly. If the support structure is not perfectly flat, it will induce astigmatism in the mirror. You probably want to use some kind of 'soft' support under the mirror, that will adapt to support the mirror evenly all over its back. Bubble wrap should do the job...

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Glasspusher

Glasspusher

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If you polish with the mirror on top there is the possibility of the mirror sagging under it's own weight and becoming astigmatic. It is not possible to support the mirror if it is on top of the lap and also the pressure from the hands will tend bend the glass. The established method of polishing a large thin mirror is to do it face up as you can support the back. The glass needs to be on a flat surface, how flat? as flat as you can get it. Several layers of good quality 3/4 plywood glued together and coated should be OK. The mirror needs to be rotated every few minutes, this will lesson the effect of any small irregularities in the support surface. If you take a sheet of bubble pack and wet it's surface with cerium oxide mixture and place your mirror on top of it you will see areas of poor contact. Bubble pack is not made to any standards, some of the pockets contain more air than others. In my opinion bubble pack is the worst thing you could use to 'support' a mirror during polishing. A material such as foam backed carpet between the mirror and support will help when it comes to the regular rotation of the mirror. I often use circles cut from a yoga mat. Another important point in controlling astigmatism is to grind the back of the mirror flat before starting on the front. The back need not be perfectly flat, slightly concave is OK, the important thing is that it is regular. All of these issues are more of a concern as the mirror gets bigger assuming it is a 'thin' mirror. Certainly, for a 16 inch mirror 25mm in thickness flexure issues have to be carefully addressed. At 14 inches they are slightly less of an issue as the glass has a more favourable aspect ratio (diameter divided by thickness). Paul, please don't be put of by some of these comments, if you take reasonable steps to avoid flexure there is every chance that you will produce a good mirror. Remember, nothing worth doing is easy!

John

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FraserClarke

FraserClarke

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I agree with all your points John, though I don't think bubble wrap is quite as dangerous as you make out; yes, some cells will be at a higher pressure than others (because of having more air) -- but the same is true with foam mats because they are not a perfectly even thickness all across. The main point is that you have something under the mirror that can deform and therefore support the mirror at many points across the surface. No matter how 'flat' you make the support, if it cannot deform you will only make contact at three points on the surface of the mirror. The mirror will then deform around those three points -- and because it is on the bottom you have to worry about the weight of the mirror AND the tool.

As John says, as long as you take reasonable precautions, you'll be fine :(

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Glasspusher

Glasspusher

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Looking good Paul, time to start testing I think. Hope you manage to post some ronchigrams. Difficult to tell from the pics but that polish looks pretty even - a good sign. I don't think it will take 30 hours to polish out, at least I hope not!!! What is starting to emerge here is a really useful resource for others wishing to make their own optics, you have made impressive profress so far, well done.

John

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Glasspusher

Glasspusher

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I would start testing as soon as possible. I think you said you are using the Ronchi test, this is excellent for giving a good view of the mirrors entire surface. If possible take some pics with the Ronchi just inside RoC, at the RoC and just outside RoC. Remember to allow the mirror to cool before testing and perform the test in an area where the air is still, or as still as you can get it. In the mean while has a look at this page and download the software....The Matching Ronchi Test

John

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paul11

paul11

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Ok guys this is how it looks so far, I know the images are Rubbish they are shot at iso 3200 and hand held ¼ sec etc I may order a gorillapod to hold the camera behind the ronchi screen. Anyway the first image is IF then ROC and the last is OF, still a long way to go yet.

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post-15572-133877436991_thumb.jpg

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Glasspusher

Glasspusher

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Paul, those images look excellent. You have a basically spherical mirror which at this stage is excellent. What stroke have you been using upto the images being taken? Next time you take some images try and get about 2 Ronchi bands on the mirror, it will tell us a lot more about what is going on.

John

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Glasspusher

Glasspusher

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Paul, I have had a little more time to study your ronchigrams. You have a slightly depressed central area and a slighly turned edge. This suggests that you have been working Mirror on top and your stroke has been a little long. You could shorten your stroke slightly or work with the lap on top using a 1/3rd stroke. I assume by IF you meean inside RoC, remember we are testing at the radius of curvature which is twice the focal length.

John

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paul11

paul11

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Not so good today guys after an hour of polishing I wiped and cleaned the mirror with a kitchen paper towel and a few small and very fine scratches appeared. I think I did them myself when I dried it. They were very fine but they would not polish out so I had to go back to 600 grit to get them out, I have also done the 1000 so I am ready to start polishing again. When you have finished a polishing session how should you clean and dry the mirror?,,,,Paul.

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paul11

paul11

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Paul, I have had a little more time to study your ronchigrams. You have a slightly depressed central area and a slighly turned edge. This suggests that you have been working Mirror on top and your stroke has been a little long. You could shorten your stroke slightly or work with the lap on top using a 1/3rd stroke. I assume by IF you meean inside RoC, remember we are testing at the radius of curvature which is twice the focal length.

John

Sorry John i missed this one as i am about to start polishing again i will keep an eye on the stroke,,,Paul.

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Glasspusher

Glasspusher

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Paul are you using aluminium oxide 600 and 1000? If so I would end fine grinding with 600 grit. The scratches are probably coming from the 1000 grit, I have had similar problems myself. It is very unlikley that wiping the mirror with kitchen towel would cause scratching.

John

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paul11

paul11

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Thanks John i would never have thought of that in a million years,how is this possible?. I have just finished the 1000 are you saying i should go back to the 600 and then to the polish?. Also John i don't think the scratches were there when i started today i just don't know how they just appeared.

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Glasspusher

Glasspusher

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Yes, I would go back to 600 grit. On completion of 600 grit you need to examine the surface of the mirror VERY closely and satisfy yourself that it is free from scratches. If the sun is shining (some hope!) take the mirror outside and catch the reflection of the sun on the mirror’s surface (take care), and make a visual inspection, any scratches should show up. You can also examine the mirrors surface with a loupe or low power eyepiece the wrong way round. Shining a bright light on the mirrors surface will help show up any scratches. It is a good idea to do this between grinding grits to ensure a uniform grind across the mirrors surface. I have had problems with scratching in the past and I usually finish fine grinding with 600 aluminum oxide.

It is also possible for fine scratches to arise from pitch that is too hard. To make a rough check on your pitch hardness press your thumbnail firmly onto the pitch for 15 seconds, it should leave a mark in the pitch. If it does not your pitch may be a little too hard. Where are you leaving your lap overnight? Is it in a cold environment? If so your lap will harden and again you run the risk of scratches. It is a good idea to warm a lap up before polishing by placing it in warm water for a couple of minutes.

John

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