First mirror grind advice

[mapstar]

10 hours ago, Chris Lock said:

Back from work and look what's arrived! Thanks John!  Quite excited now, what a chunk of glass! 222mm diameter and 38mm thick! It weighs a fair bit, can't imagine what Damian's 22" must weigh! 

Pound coin for size reference, not bling

A quick look over the mirror shows a definite side to work with, one side has bubbles close to the surface and the other side is bubble free as far as I can tell. It may be the glass of wine talking but it's a thing of beauty!

Inside the rather lovely looking piece of glass is a portal on the universe waiting to get out.

Very special 

[Astrobits]

Hi Chris,

You need the mirror on top to produce the concave curve. I don't think that putting the tool on top would get you out of the mismatched curve scenario. Whatever is on top gets the centre worked more than the edge and the tool on the bottom gets the edge worked more than the centre. I think ( but stand to be corrected ) that the tool needs more glass removing from the edge than the mirror needs from the centre hence the mirror deepens faster than the tool can keep up. You only want the tool on top if you have produced too deep a curve such that the focal length is shorter than your target.

I said it is theory but it is based on my own deductions from observation and the solution worked for me at least. I think that the key observation was the behaviour of those bubbles. Many of the older books recommended sticking some sort of handle to the back of the mirror and many mirror makers like to grind the back flat before starting the mirror and both these actions obscure the behaviour of those bubbles.

I recently was given the optics from an old telescope and the mirror has a large defect where the grinding has broken into a bubble. The maker did not chose the right side to grind

Nigel

 

[Chris]

1 hour ago, mapstar said:

Inside the rather lovely looking piece of glass is a portal on the universe waiting to get out.

Very special 

You're a true 'space poet' Damian lol, I'll have to remember that, I like it!

[Chris]

1 hour ago, Astrobits said:

Hi Chris,

You need the mirror on top to produce the concave curve. I don't think that putting the tool on top would get you out of the mismatched curve scenario. Whatever is on top gets the centre worked more than the edge and the tool on the bottom gets the edge worked more than the centre.

Hi Nigel, so in order to hog out a sphere, I will be almost exclusively mirror on top to deepen the centre of the the mirror to produce said sphere, unless I over shoot the sagita depth, then I will go tool on top, is this right? It's been a week or so since I've watched vids on hogging out so have already partially forgot what I saw people do lol I guess it will all gel when I actually do it  

I do remember seeing people using a 50% diameter iron barbell weight to hog out, and I'm not sure how I'd go about having that on the bottom? I do have these weights availabe so it is a thought to hog out with them, I don't want to do the broken glass tool for health and safety reasons, but could still ask galvoptics for a matched Pyrex glass tool.

1 hour ago, Astrobits said:

I said it is theory but it is based on my own deductions from observation and the solution worked for me at least. I think that the key observation was the behaviour of those bubbles. Many of the older books recommended sticking some sort of handle to the back of the mirror and many mirror makers like to grind the back flat before starting the mirror and both these actions obscure the behaviour of those bubbles.

I will keep the back of my blank un ground so I can watch those bubbles

1 hour ago, Astrobits said:

I recently was given the optics from an old telescope and the mirror has a large defect where the grinding has broken into a bubble. The maker did not chose the right side to grind

Can you grind the other side if there is a sphere/parabola on the reverse of the mirror at the end, or is it done for, Nigel? I'm going to look very carefully before I start mine, but I can already see that one side has bubbles right at the surface so it will definitely be the other side which at first glance looks much better

[mapstar]

56 minutes ago, Chris Lock said:

You're a true 'space poet' Damian lol, I'll have to remember that, I like it!

Tbh Chris I'm just passing on what was said to me  

[Astrobits]

------------

Quote

 

-Hi Nigel, so in order to hog out a sphere, I will be almost exclusively mirror on top to deepen the centre of the the mirror to produce said sphere, unless I over shoot the sagita depth, then I will go tool on top, is this right? It's been a week or so since I've watched vids on hogging out so have already partially forgot what I saw people do lol I guess it will all gel when I actually do it  

 

 

That's right, Chris, Mirror on top provided you are using a tool the same diameter as the mirror.

Quote

I do remember seeing people using a 50% diameter iron barbell weight to hog out, and I'm not sure how I'd go about having that on the bottom? I do have these weights availabe so it is a thought to hog out with them, I don't want to do the broken glass tool for health and safety reasons, but could still ask galvoptics for a matched Pyrex glass tool.

If you use a sub-diameter tool then it can be used on top to generate a curve on the mirror. All my bigger mirrors ( 12" and bigger ) were done ( grinding, polishing and figuring ) with mirror on bottom. Big mirrors ( 1,2,3+ meters dia. ) simply cannot be done with mirror on top. Just dig out some pics on the internet of really big mirrors being made and all of them are mirror on bottom. Metal grinding tools don't wear down like glass ones and therefore won't take up the matching sphere of the mirror but they do work much faster than glass tools so are very useful when making multiple mirrors with the same configuration, but scratch easily on the finer grades of abrasive. For your mirror I think that using a full sized glass tool is the best for you on a 222mm dia. mirror.

Nigel

 

[Chris]

Thank you Nigel, ok, bar another equally as good way but cheaper, I'll get in touch with galvoptics after the weekend, and see if they can provide me with a Pyrex glass tool. They seem to recommend a slightly sub diameter tool though, they were going to give me a 205mm tool for a 210mm mirror, but I guess this is pretty much full size as to make no difference.

[Astrobits]

Plate glass is perfectly adequate for the tool, in fact even preferable. It is cheaper and does not have any bubbles to trap abrasive particles and works about the same as other types of common glass. I think that Galvoptics generally offer plate glass for the tool.

Nigel

[Guest]

I've done one 8.5 inch mirror so I'm just a beginner but there were a couple of things which caught me out.

1. When doing the rough grinding you have to hog out the middle and then extend it to a spherical curve which goes completely from edge to edge smoothly. Use the pencil test to check. I found it was easy to think I'd done this when I hadn't. Get a good spherical shape all the way edge to edge before you go on to the finer grades of grit because it's a pain in the ass to correct at a later stage if you don't.

2. The books will say use a 1/3 or 1/2 stroke for various figuring and corrections. I found it was important to do this accurately. Although it says in the books you need some randomisation to get a smooth shape there's no need to try. Even if you try to do the strokes accurately there will be enough randomness. When it says 1/3 stroke that means the centre of the mirror has to move a total distance of 1/3 the diameter of the mirror. That means there will be half that distance overhang on each side. If you have a 6inch mirror then the 1/3 stroke will be 2 inches which means that the mirror will overhang by one inch on each side at the ends of the stroke. I made out some sheets with circles on at the correct distances and placed them under the tool when grinding or polishing so I could correctly judge the distance.

My mirror was pyrex but I used a plate glass tool which seemed to work out fine.

 Cheers

Steve

 

[Chris]

4 hours ago, Astrobits said:

Plate glass is perfectly adequate for the tool, in fact even preferable. It is cheaper and does not have any bubbles to trap abrasive particles and works about the same as other types of common glass. I think that Galvoptics generally offer plate glass for the tool.

Nigel

Ah I didn't realise this! Someone said they wore out a plate glass tool trying to get anywhere with a harder Pyrex mirror and didn't really make a dent on the mirror. I wish I could remember who said it or where I read it? They switched to a Pyrex tool to continue and seemed happy with that, so until this point I thought you needed to match glass types.

Good to know that plate glass is good for this purpose  

[Chris]

4 hours ago, woodblock said:

I've done one 8.5 inch mirror so I'm just a beginner but there were a couple of things which caught me out.

1. When doing the rough grinding you have to hog out the middle and then extend it to a spherical curve which goes completely from edge to edge smoothly. Use the pencil test to check. I found it was easy to think I'd done this when I hadn't. Get a good spherical shape all the way edge to edge before you go on to the finer grades of grit because it's a pain in the ass to correct at a later stage if you don't.

2. The books will say use a 1/3 or 1/2 stroke for various figuring and corrections. I found it was important to do this accurately. Although it says in the books you need some randomisation to get a smooth shape there's no need to try. Even if you try to do the strokes accurately there will be enough randomness. When it says 1/3 stroke that means the centre of the mirror has to move a total distance of 1/3 the diameter of the mirror. That means there will be half that distance overhang on each side. If you have a 6inch mirror then the 1/3 stroke will be 2 inches which means that the mirror will overhang by one inch on each side at the ends of the stroke. I made out some sheets with circles on at the correct distances and placed them under the tool when grinding or polishing so I could correctly judge the distance.

My mirror was pyrex but I used a plate glass tool which seemed to work out fine.

 Cheers

Steve

 

Hi Steve, thanks for all the tips!  Nice of you to clarify what is meant by 1/3 stroke, I didn't have that fully sunk in but now do. Particularly like your tip about the cut out circles so you can get the overhang dead right each stroke, I might have to copy that  

So what f/ratio did you settle on for you 8.5" Steve? Ever fancy doing another?

[Guest]

I started my mirror when I was at school about 50 years ago as an f7. I ground it and polished it but at the end I decided it had a serious defect called a turned down edge. Then I had to stop because of other priorities but I always kept it. Last year I finally got around to finishing it by regrinding and polishing again. It took ages and I made lots of mistakes but I got it there in the end. However at the end I decided that I had learned so much I'd like to start all over and do it better so I ground it out again as an f5. It's now ready for polishing again but it will have to wait until the summer. I work in an outhouse which is rather cold this time of year and the pitch gets very hard.

To be honest I enjoy the technicalities of it more than anything else. I built a Foucault tester which was ok but then I made a better one. I have started to make a laser interferometer which I hope will make testing more accurate. There's lots of stuff on the internet about that.

That 1/3 stroke is a neutral stroke. It keeps the shape more or less unchanged and you are unlikely to impose any defects on the mirror using that. Once you have the right shape almost all of the fine grinding, smoothing and polishing is done with 1/3 stroke.   During the fine grinding and smoothing it's recommended to alternate mirror-on-top/tool-on-top every few wets and that also helps to keep the focal length the same.

There are two other things I thought of.

1. Don't try to work too fast. Make sure each stage is completed thoroughly before you move on. 

2. During the final stages of fine grinding and smoothing there is a tendency for the tool and mirror to stick together. You wouldn't believe how hard they can get stuck. Keep them wet and keep them moving. It's happened to me a couple of times and the way I got them apart was to soak them in very hot water and then transfer them quickly to a cold bath. If that doesn't work then I understand that the next step is to clamp the mirror and clout the tool with a big piece of wood. I haven't had to do that so far.

There's quite a lot of information about mirror and telescope making dotted around on the SGL and I wondered if it would be a good idea to have a special subforum on the subject. I don't know how other people feel about that.

Good luck

Steve

 

[Chris]

On 1/21/2017 at 19:41, woodblock said:

I started my mirror when I was at school about 50 years ago as an f7. I ground it and polished it but at the end I decided it had a serious defect called a turned down edge. Then I had to stop because of other priorities but I always kept it. Last year I finally got around to finishing it by regrinding and polishing again. It took ages and I made lots of mistakes but I got it there in the end. However at the end I decided that I had learned so much I'd like to start all over and do it better so I ground it out again as an f5. It's now ready for polishing again but it will have to wait until the summer. I work in an outhouse which is rather cold this time of year and the pitch gets very hard.

To be honest I enjoy the technicalities of it more than anything else. I built a Foucault tester which was ok but then I made a better one. I have started to make a laser interferometer which I hope will make testing more accurate. There's lots of stuff on the internet about that.

That 1/3 stroke is a neutral stroke. It keeps the shape more or less unchanged and you are unlikely to impose any defects on the mirror using that. Once you have the right shape almost all of the fine grinding, smoothing and polishing is done with 1/3 stroke.   During the fine grinding and smoothing it's recommended to alternate mirror-on-top/tool-on-top every few wets and that also helps to keep the focal length the same.

There are two other things I thought of.

1. Don't try to work too fast. Make sure each stage is completed thoroughly before you move on. 

2. During the final stages of fine grinding and smoothing there is a tendency for the tool and mirror to stick together. You wouldn't believe how hard they can get stuck. Keep them wet and keep them moving. It's happened to me a couple of times and the way I got them apart was to soak them in very hot water and then transfer them quickly to a cold bath. If that doesn't work then I understand that the next step is to clamp the mirror and clout the tool with a big piece of wood. I haven't had to do that so far.

There's quite a lot of information about mirror and telescope making dotted around on the SGL and I wondered if it would be a good idea to have a special subforum on the subject. I don't know how other people feel about that.

Good luck

Steve

 

Hi Steve, wow you have a lot of history with that mirror, and by the sounds of it a lot of experience from it, you may have only done one mirror but to have done it 3 times!

Thanks for the extra tips, I'll try my hardest to make sure each step is completely finished before moving on, although I understand it can be tricky to decide when this is from the threads I've read. I'm already developing a healthy fear of the mirror and tool locking together, hopefully this will keep my senses tuned to help prevent it, but this is all new territory for me so I'll just go with the flow   

[Chris]

Hi Guys, just a bit of an update. I'm still waiting for a quote for a glass tool from galvoptics, but after watching the CDRom that Damian kindly gave me of John Nichol making an 8" mirror, where he suggested a flat tile tool for hogging out the sphere using a 1/3 stroke edge over center, I'm considering this and other alternatives.

I know the consensus was the glass tool being best for me, however, family expenses are taking there toll as they do, so I'm starting to look around for things to sell to fund my little venture. Cheaper but not so good may have to do for now! 

I've PM'd John to ask a bit more about the flat tile tool, and I also ran the iron barbell idea by him as I have these just lying around. I don't think we discussed this option in any depth here, I can't remember now without re reading the thread.

A broken glass tile tool was previously suggested and I decided against this with young children on the scene, but after talking to Damian he suggested possibly using glass mosaic tile squares instead of glazed tiles. Sounds like an idea to me.

There is also this :

Could the reverse of this be used as a glass tool?

[Chris]

John got back to me and put me onto this:

https://nicholoptical.files.wordpress.com/2016/11/curve-generation-with-a-sub-diameter-tool.pdf

I have a 1.25kg weight at 54% diameter, and a tiny 0.5kg weight at 39% diameter. 

Think I may go this way as the cost is zero, I have a grinder to remove the central raised area as suggested in the link.

Thinking about going for the larger weight as it has the better raised outer rim and more weight to it.

John suggested hogging out the curve with the barbell, then casting dental stone/cement against the curve and adding glass tiles.

 

 

[Chris]

Worked out my sagita using S=r^2/2R where little 'r' is the radius of the mirror, and big 'R' is the radius of curvature, which itself is twice the focal length (just for any other mirror grinding noobs out there  ) 

The Sagita (S) for my 222mm mirror when aiming for around f/6.8 is pretty much bang on 2mm. However hogging using an iron barbell weight doesn't usually produce a sphere so it is recommended to go a bit deeper in order to even things out once you start attacking the outer regions of the mirror with a tile tool. 

I've therefore decided I'll be aiming for a Sagita of 2.4mm 

[Chris]

I've put together a rudimentary grinding jig using some spare pine shelving and those plastic furniture support blocks. Not perfect as the pine is only 7.75" wide and my mirror is 8.75", thus there is 1/2" overhang on each side. It will do for a start, and it's a 40mm thick blank so I don't think there will be flex.

Now there is a bit of a challenge straight off the bat! I've looked closely at the mirror blank and as previously mentioned there are bubbles on one side breaking the surface so the other side must be used. The other side is lovely and clear from any bubbles but there is an interesting profile to tackle with rough grinding as there is a hill in the centre, then a moat about 70% out. Placing the rule across the mirror I was able to slide a 1p coin under off centre on both sides of the mirror. I tried with a 1.5mm drill bit and it went under easy but the 2mm one didn't quite, so I estimate the dip to be around 1.75mm, but varies from place to place. I can actually feel the dip easily by hand and I'm including a pic with my fingers placed either side of teh hill in the centre.

I'm not too far off being able to start, but how should I tackle the hill?

- use a small diameter tool and dig just the centre out to 2.4mm

- use the barbell and the usual stroke to work the centre and the edge a bit and hope that that dip isn't touched for a while.

Any of these sound good, or is there a better way to begin? with a hill in the centre, and a 1.75mm dip in places quite far out.

   

[mapstar]

Hi Chris. 

Firstly you should start with an edge bevel on both front and back.

The back should also be ground flat but if the front needs a little flattening first then you should do that first.

The hill will disappear quite quickly 

[barkis]

I never used sub diameter tools for anything than small polishers  for remedial work.
Mirror on top always for me for the coarse, and find grinding stages. I never deemed the small Sagitta depths too
much  to necessitate using a small grinder on a face up mirror. 
For Biggies, I can see the justification, but 10" down? not for me.
You'll find your own way that suits you Chris. But as I did with Damian, I'll withdraw from the advisory board,
for fear of cluttering your head with too much information, well Intended as it is. 
You have the Information necessary to see you through the initial stages, then as things get a bit more demanding,
the help will still be there from the Guys interested in your project.

[Chris]

Hi Guys, I lay in bed last night thinking about how t tackle the 'moat' around the edge of the blank. I tried my best to imagine how my barbell weight would behave as it traveled over this terrain, and I can't help worrying that it will change it's angle slightly and dig into the 'moat' rather than slide over it grinding down the central hill and outer edge. If I run my finger over the mirror it almost feels like a shallow Sine wave, so I just feel this contour will be followed by a small tool.

I think if I was able to use the other side, I would have been happy to use John's method with the barbell weight to hog out, especially as I have them all ready, but I won't due to bubbles hatching out towards the edge,they would never grind out. I couldn't fall sleep thinking about it, but did after I decided to take a trip to the shop today to buy cement, glass tiles, fiber glass resin and hardener. (good call Damain, I'll also buy a wet stone!). 

I'll cast a full size tool against a flat surface rather than the blank, and use this larger tool to bring down the centre to 2.4mm and the edge in order to reduce the moat depth problem? I'm still worried a bit thought because it's 1.75mm deep just over 1" from the edge, so the edge will probably need a fair bit of grinding. 

 

As you say Ron, I have enough info to make a start on this now, but would appreciate any advice on problems I run into down the line, especially if anyone sees me going in totally the wrong direction 

I'll do what Damian did and start an actual grind thread rather than stick to this advice thread. I think it would be good to outline the bullet point steps I'm doing to add to the pool of info on mirror grinding. I hope this would then be good for anyone else thinking about giving a smallish mirror ago, and they can learn from both my mistakes and successes

I've just signed up to the Great East Run 13 mile in September, raising money for Ipswich Hospital. So it's looking like a busy year with family, training, and a mirror grind. What am I like!  

 

 

[Chris]

Sorry this is like a game of tennis going back and fourth, but John got in touch and suggested trying the weight for 2 hours too see what happens before jumping to a bigger tool. I can't fault the logic as I have it all here, so will give it a try before proceeding with the above.

Still going shopping to get mi tile tool bit's and wet stone for the edge  

[JohnSadlerAstro]

This is an old thread, but did you ever get the mirror ground down in the end, @Lockie?  

John

[Gina]

An interesting thread, that's for sure   I've often wondered about the possibility of making my own telescope from scratch.  Doubt I could make one good enough for imaging but observing might be a possibility.  I've done very little observing (having concentrated on imaging) but I'm thinking if giving it a go sometime.  I guess I should use the scopes I already have and see how I get on first.  I know someone who has built a scope from 3D printed parts (apart from the optics, of course) and this has a fascination ?

[JohnSadlerAstro]

Ah yes the 3d printing, lol. You ought to try printing out a 20" dob, it might be a bit time consuming though....  

I'm very tempted to try making a dob, but I'm probably better waiting a few years when I will hopefully have the money and expertise to be able to put it together. Having said that though, with an obsy and Eq8 a quatro f4 in 400mm would be fine for both visual and imaging.

John

[Gina]

Biggest SW Quattro is 300mm (12") at least on FLO.  A 400mm mirror would be great - quite a light bucket ?.  Biggest scope I have currently is an MN190 - 190mm mirror and f5.3.  A great scope for galaxies and small nebulae like M1.  I'm also thinking of trying it for planetary imaging and observing.  Probably wants a 2x or 3x Barlow - I have both.