Mirror Grinding - School Project

[saac]

A few months ago I mentioned that I was planning to start a mirror grinding project with my school astronomy club. Well, after what seamed like an eternity the mirror blank has arrived and everything is now in place to start. So I hope you all don't mind but I'd be really grateful if I could draw on the expertise here on the forum to help keep us on the straight and level, or should that be parabolic!

We are working with a 12inch plate glass blank with a full size tool (supplied by Galvoptics). We're aiming for an F5 mirror to take its place in our Dobsonian telescope which is also under construction.

So here is the first question. For the grinding setup I have a table top secured to a 45 gallon drum (plastic). I've filled the drum with water and while it is heavy its not as rigid as I thought - there is a slight wobble as the water sloshes around in the barrel. Is this a problem, does the table on which we grind the mirror need to be absolutely rigid? I'm considering replacing the water with heavy blocks or sand. Grateful for any thoughts.

[saac]

Here's a picture of the barrel setup.

[Peter Drew]

I wouldn't worry too much about stability, random effects are one of the keys to reducing errors.

[barkis]

You ought not to have filled the barrel so full. Keep the weight at the base of the barrel. Also, the wooden base that supports the mirror should be cut on two sides, just enough to allow you to drill some holes in the top of the barrel, which will allow you to sluice spent grinding agent down into the holes, and hence into the barrel.

It goes without saying, that each succeeding grade of grit, must not be at risk of contamination by any previous one, due to poor cleaning up. This must be a priority as you get into the finer grinding stages, otherwise, you may have to go back to a coarser grit, to rectify any damage from contamination..

Sorry to labour this point, as I'm sure you will be aware of it if you are working from a mirror making book, such as Texereau's.

Good luck with the project, I'm sure the pupils will enjoy creating a very nice optical mirror. It's leaping ahead a bit, but they will be astonished by first light, especially if it's Saturn they see first.

By the way, there are some very helpful guys on SGL, who are very expert at this work, and I'm sure they will contribute as and when you need them.

Best Wishes .

Ron.

Edit: I always used a metal oil drum. More rigid.

[saac]

Peter, Ron thanks for the input. I had hoped that the random effect from any movement would in a way be beneficial. In fact I was initially concerned about having the grinding done by a number of pupils rather than one pair of hands but I ended up being contented that the different hands would bring a further degree of randomization to the process. I can easily drain out some water to see if that reduces the wobble - thinking about it now it was more rigid in the garage when I did the trial setup and I only had less than half filled the barrel.

Good reminder about care over contamination - I'm intending on removing the table between grits for thorough cleaning but some drainage channels would help to keep the work area clean during grinding.

I have a metal drum at home as a duplicate setup. To speed the process up I'll be grinding at the weekend - can't let the kids have all the fun!

Thanks,

Jim.

[Glasspusher]

Best of luck with the project Jim, looking forward to following your progress.

Regards,

John

[saac]

Cheers John, thanks for the words of encouragement. Our Astro club meets on a Thursday so I hope to be posting a weekly update on our progress. Stand by for lots of questions as well!

Jim

[pete_l]

A few months ago I mentioned that I was planning to start a mirror grinding project with my school astronomy club. ...

We are working with a 12inch plate glass blank with a full size tool (supplied by Galvoptics). We're aiming for an F5 mirror to take its place in our Dobsonian telescope which is also under construction.

I'm not a grinder myself, but I've heard it said in mirror-making circles (groan - pollies!) that "it's quicker to make a 6 inch mirror then an 8 inch mirror, than it is to make an 8 inch mirror"

Now that may sound enigmatic, but it means that the experience you gain from making a small, introductory mirror will make the second one much faster than if you try to learn the art and make a biggun all in one step.

[Glasspusher]

A 12 inch mirror is certainly doable as a first attempt as evidenced by one of my mirror making students who has recently completed such a mirror.

John

[Gina]

Roughly how many hours of work would be involved in making an 8" mirror say? Is a 6" one much quicker?

[Tiddles]

Please keep this post updated I will be following with interest.

[saac]

I'm not a grinder myself, but I've heard it said in mirror-making circles (groan - pollies!) that "it's quicker to make a 6 inch mirror then an 8 inch mirror, than it is to make an 8 inch mirror"

Now that may sound enigmatic, but it means that the experience you gain from making a small, introductory mirror will make the second one much faster than if you try to learn the art and make a biggun all in one step.

I'm sure there is some truth in that Pete but as FD Roosevelt said “the only thing we have to fear is fear itself". Why do I know that will come back to bite me! Anyway in true engineering style I've already built the mirror cell so the mirror has to be 12 inches!

Not sure of the total time Gina for this one; I'm following the instructions provided by Galvoptics which suggest about 3-6 hours for the roughing out phase. I’m using this as a learning experience for the kids so the process, (research, assembly, making test instruments etc) is going to be as important as the end product. No doubt we will get some bits wrong but I’m sure we’re going to have some fun along the way. As mentioned above I'll look at a weekly update on our progress.

[Demonperformer]

Roughly how many hours of work would be involved in making an 8" mirror say? Is a 6" one much quicker?

I'm no expert, but comparison-wise, bearing in mind that you are dealing with a volume of glass that has to be removed, it would seem reasonable to think that an 8" mirror would take roughly 2.5x as long to rough grind (64/27) for a similar focal ratio, and that the fine grinding/polishing would take a bit less than twice as long (16/9) as these are more concerned with the surface area you have produced. But, as I said, I am no expert and maybe completely wrong about that.

[Trull]

Good luck on the project!

I've just bought a dial test indicator to make my foucault tester around, and hope to start my own 6" project soon.

Ill be using a metal drum 2/3rds filled with water, pierced top to drain grits through (and it keeps the mud off your clothes, which might be something to consider with your plastic design - you could route a drainage channel in...)

Plastic drums are very floppy - which would annoy me.

Anyway - good luck - and please keep us posted with progress.

[Gina]

Interesting - thank you

[Glasspusher]

How long does it take to make a telescope mirror of a particular size? There are simply too many variables to be able to make an accurate prediction. Are you starting with a flat blank, are you going to grind the back flat? Are you using a sub diameter tool for hogging in the curve, or a full sized tool? What sequence of abrasives are you going to use? How many times will you have to re-grind because a scratch was picked up? What is the focal length of the mirror? Arguably a shorter focal length will take longer to grind and longer to figure. The behavior of pitch is notoriously difficult to predict adding another variable to an already complex issue. There are many stories about some of the world’s largest mirrors suffering delays in production as a result of unforeseen circumstances. Yet mirrors get made. What I am really saying is don’t think about time when making a mirror, enjoy the fulfilling experience of making and learning about optics. Like any worthwhile activity mirror making is not easy, and as a result it is a worthwhile and rewarding experience. It will enhance the enjoyment had from observing, nothing beats using your own optics.

John

[Gina]

Yes, I expected that answer really It is something I have at the back of my mind to try later. I'm a real DIY type of person and nothing gives me more pleasure than making and using my own stuff. To build my own telescope would really be the "holy grail"

I know most people like to go for a large aperture, short focus scope usually with a Dob mount. Now while that would be nice, I think it would be something for later on having gained experience with a smaller mirror and higher focal length. I'm thinking in terms of a scope for planetary work. My ED80 is great for first skirmishes with DSO but I think a longer FL would be better for planetary and a small aperture would be OK due to the brightness of the planets (which are currently tending to overload my webcams).

Would 6" be a good starting size or would a 4" be easier? Or are 4" blanks even available? Clearly I have plenty of research to do I'll have to check measurements but I think my obsy would take something like 1500mm FL. 1000 or 1200 would certainly fit. So I could get at least double the focal length of my present scope (600mm). I think I've read that a smaller aperture helps with seeing by using a smaller lump of atmosphere to look through.

[Demonperformer]

Or are 4" blanks even available?

low expansion dobsonian mirror, pyrex mirror blanks

But my understanding is that a 6" mirror is manageable by anyone with aptitude (which you would appear to have but I definitely don't:D]

[Glasspusher]

I think I've read that a smaller aperture helps with seeing by using a smaller lump of atmosphere to look through.

Gina, under poor seeing conditions a small aperture may perform as well as a large aperture, under good conditions aperture will always win. Don't forget that the top imagers like Damian Peach are using 14 inch telescopes to amazing effect.

John

[Gina]

low expansion dobsonian mirror, pyrex mirror blanks

But my understanding is that a 6" mirror is manageable by anyone with aptitude (which you would appear to have but I definitely don't:D]

Thank you for the link It would certainly seem cost effective to go for a 6" rather than a 4" as there only seems to be about a tenner difference in the price, though I guess you might need to get more grinding and polishing stuff. A kit looks like a good idea.

[saac]

Thank you for the link It would certainly seem cost effective to go for a 6" rather than a 4" as there only seems to be about a tenner difference in the price, though I guess you might need to get more grinding and polishing stuff. A kit looks like a good idea.

The kit I purchased comprised mirror blank, full size tool, 3 grades of grinding/polishing powder and pitch. I also bought the Galvoptics guide on mirror grinding; the guide is very comprehensive and is well written. The blank and tool were provided already beveled. All told it was a pretty painless exercise, albeit delivery time was longer than I expected. To be fair to Galvoptics I believe there was a delay in their supply chain for the pitch. If anyone is thinking about similar I can recommend the company but I would phone rather than use e-mail - this is small stuff and is not their core business. The chap I spoke to (forgot his name) could not have been more helpful.

Jim

[saac]

Well we had our first grinding session today. We started with 180 grit with the mirror on top (MOT) using a chordal stroke pattern, or at least what we think is a chordal stroke pattern! The total amount of grinding time was just over 1 hour and we went through 5 wets. Not sure if that's too fast or slow or about right so grateful for any comments. Really hard to judge how much grit and water to use, guess it really is trial and error. As the wets progressed there was a noticeable change to the resistance and we could feel the mirror sticking to the tool so signaling time for another wet.

Once we finished I cleaned the mirror and out of curiosity I checked to see if we had any measurable depth. I placed a steel rule across the mirror diameter to measure the depth cut out. I didn't really expect to see much after such short period of time but sure enough I could slide a thin sheet of paper under the rule (I reckon about 0.1 mm thick); I'll check with feeler gauges tomorrow. On a pleasing note, the minor surface scratches that were present on the blank have now all but gone. The frosting appearance on the mirror face appears generally uniform but it does fade out (patchy) about 5mm from the edge in certain places around the circumference - not sure why this should be, perhaps just needed more time.

Anyway, it feels really good (emotional in a way) to have made a start, I've been waiting a few years to get to this stage. Grateful if those with experience could take a look at the video and perhaps comment with any concerns on the technique. Thanks in advance.

Jim

Hogging Out - Chordal Stroke - YouTube

[Glasspusher]

Jim, What you are doing looks OK to me. A couples of suggestions, work with your hands on top of the mirror so that you can apply more pressure to speed up grinding, you could also work a little quicker with your hands on top. As soon as the load grinding noise subsides it is time for another wet. Good luck!

John

[RogerTheDodger]

This thread (and that video) is very inspirational indeed, I really want to give this a go at some point

[saac]

Jim, What you are doing looks OK to me. A couples of suggestions, work with your hands on top of the mirror so that you can apply more pressure to speed up grinding, you could also work a little quicker with your hands on top. As soon as the load grinding noise subsides it is time for another wet. Good luck!

John

Thanks John, I wasn't 100% sure that I had interpreted the instructions correctly re the chordal stroke. I see what you mean about the hand position; we were finding it difficult to put even pressure on the blank. At one stage we placed some weights on the blank but this made it difficult to see what was going on underneath. I'm planning on doing some more work tomorrow using the drum at home; I hope with a bit more confidence now that the first cut has been done. Thanks again.

Roger give it a go for sure. I think projects like this are what the internet was made for. I could not have started this without the benefit of advice and encouragement from others on forums like this. Share the knowledge!

Jim