First time mirror grind

[dark star]

I have just started grinding my first telescope mirror. It is a six inch mirror blank, pyrex, and I am aiming for f/5. I am using a telescope weight about a third of the diameter of the blank to start grinding, using 80 grade grit. I am starting in the center and will work my way out wards.

My first question is, I have done a bit of grinding and most of the glass in the center is now frosted, but there is a small area off center which is clear. Does this mean I need to concentrate my grinding on this bit until it is also frosted?

The sagitta should be 0.0075, according to a calculator I found on the internet, however is this mm or inches! It is am American site. I was aiming to make the depth at the center around that of a penny, would this be about right?

I would like to grind a 14 inch mirror eventually, but have a lot to learn before I get to that stage! I attach a couple of pictures of the mirror blank. Any advice gratefully received.

David

[Rustysplit]

Hi David,

Well done on starting your mirror grind. I am nearing completion of my first mirror, so am in no sense experienced. What stroke are you using to rough grind and does the weight you are using have a raised rim? I am not too sure, but I think it may be a little small for rough grinding. I believe it may need to be just over 1/2 mirror diameter.

If I understand correctly, the stroke for curve generation with a subdiameter tool is the 1/3 centre though centre. Have a look at nicholoptical.co.uk and look for the atm links. Lots of really good info there.

[Glasspusher]

David, What is the url of the sagitta calculator that you are using? The non frosted area represents an area lower than the rest of the glass, you will need to remove glass across the whole blank to reduce it's level to that of the non frosted area. Be careful that you do not exceed your required curve depth in doing this.

Nice to see another miror grind in progress, there are lots's of folk here who will give good adice.

Good luck with your project.

John

[Astrobits]

Hi David, Keep going.

I also used an approx 1/3 dia tool for the initial grind on my big mirrors and then switched to a larger, 60-70% dia., tool for the finer grinds but that was all tool on top. For a 6" it will be best to use a full size tool once you get to the next grit.

The sagitta for an f/5 6" mirror is 1.88mm. a new penny I have just measured is just under 1.7mm and that would give you an f5.5 ish curve. I cannot see in your pic any sign of the edge of the blank having been bevelled. Bevelling should always be done as the first job as it protects the edges from chipping.

Nigel

[dark star]

Hi, I am pretty sure that the edges on both sides of the mirror are beveled. Terry Pierce at Vacuum Coatings Scientific Mirrors in London (not far from where I live) kindly beveled the other side of the blank, in case I wanted to grind that side, but one side already had a bevel, in other words a smooth edge. I will post another picture.

I have measured the metal tool and it is 3 inches in diameter, so it is half the size of the mirror blank and not a third! Do I still need to switch to a full sized tool when I go to the next grit?

I am going in a rotating motion round and round the center of the mirror and then around the other way, trying to copy the Sky at Night Mirror Making video which is on Youtube-40 minute version. I will check out some websites regarding the correct strokes.

I have a question about measuring the Sagitta with a penny. The Sky at Night video shows the center of the blank being rough ground first and a penny under a metal ruler used to measure the depth. Then the blank is gradually ground out towards the edge, and also the depth measured with a penny.

What I don't understand is that surely this will result in a flat blank with a penny depth of glass removed from the whole surface and no curve? In other words, how do I measure the depth of the curve out from the center as obviously it should be less and less deep as you get nearer the edge? I possibly haven't watched the video enough times! But I haven't seen an explanation of how to measure the curve away from the center of the blank anywhere.

I have done some more grinding and got the blank evenly frosted, I think the problem was that my floor slants, I have now used a spirit level and put some small blocks of wood under the legs of the table to get it level.

David

[Glasspusher]

David, looking at the pictures of your mirror there is still a fair bit of rough grinding to be done. I would continue with the 50% diameter tool working centre thru centre strokes with minimal overhang, use the coarsest abrasive that you have. After rough grinding your mirror will have a shorter focal length in the centre than at the edge, going to a full size tool will result in a focal length somewhere between the two. For this reason you need to grind a little deeper than you think you need when roughing in the curve with a sub diameter tool. I would rough grind until a 2mm drill just fits under a straight edge placed across the glass. Hope this helps.

John

[Astrobits]

It is easy to miss when a part of the edge of the mirror is getting too thin when it is not ground to a bevel. If you leave it as cast it will become a source of specular reflection ( it will get coated with aluminium so will be just as shiny as the mirror surface ) on the finished mirror and could give image problems with bright objects just out of the field of view. I suggest that you put a bevel on it asap during the first coarse grinding stage.

Nigel

[dark star]

OK, I will bevel the edge and do some more grinding at the weekend. I thought as the edge was smooth it was already beveled! I am doing circular strokes around the center with the metal tool, I am planning to get the center to the right depth and then move out. Would I be better doing straight back and forth strokes through the center? The Sky at Night video shows circular strokes.

I have found videos on the Stellafane website showing the strokes to use with a full sized tool made from plaster and tile, so once I progress to this I will know what strokes to use.

David

[Glasspusher]

David, straight strokes through the centre are much more efficient. At this stage it is all about removing glass as efficiently as possible. As NIgel says bevel the edge ASAP and re-new it if it begins to disappear.

John

[dark star]

One more question, I am still confused how I avoid rough grinding the blank flat rather than spherical!

Do I do more strokes through the center and not reaching the sides and fewer that reach the sides? There is something on the Stellafane site that explains this with a full sized tool but not with a half size metal tool.

David

[Astrobits]

When you make 1 stroke through the centre with a sub diameter tool you remove the same amount of glass all along the stroke, centre and edge. The next stroke is at an angle to the first so you will take glass from the centre, again, but now from a different part of the edge. So the centre gets glass removed every stroke but the edge only gets it every 4th or 5th stroke. Thus the centre looses much more glass than the edge and becomes deeper. If you were using the full size tool the mirror would have to be on top to produce the concave curve.

Nigel