The 8.75" f/6.7 Mirror Grind

[Chris]

With this thread I'll be outlining my journey as I attempt to grind, polish, figure, and test an 8.75" f/6.7 telescope mirror.

I chose this size mirror hopefully as an upgrade to my 6" f/8, yet not so big as to be over challenging for a first time mirror grind. Yep.. this is the first time I've attempted to do anything like this, and I expect to make plenty of mistakes! but hopefully I'll have successes too

 I chose f/6.7 because 1) Slow mirrors are supposed to be easier to make than fast mirrors, 2) An 8.75" mirror with an f/ratio of 6.7 will give a focal length in the region of 1500mm... I think this will give a good eyepiece height at zenith when the mirror is used as part of a Dobsonian telescope, and 3) slow mirrors work well with basic eyepieces and I own a humble set of Plossls. As I plan to build the scope around the mirror, I really don't mind if it turns out to be f/6.3 or 6.9....f/6.7 is just a rough guide to work towards.   

I'm writing this thread for two reasons:

1) So I'll get help along the way with any issues I run into.

2) I've already received so much advice that I really want to give something back and hopefully encourage others to give it a go, just as I've been encouraged by the great threads I've read. I'll try my best to pass on any info I've learnt so far  

In preparation for this exploit I started a mirror grinding advice thread:

This thread enabled me to source all the materials and knowledge required to get to this point in time...the start!  If you too fancy making your own mirror, please have a flick through the link above first as it may explain anything I've missed. 

I was very lucky and managed to source much of what I needed from fellow forum members  I really do owe a big thank you to John Nichol aka @Glasspusher for supplying the Pyrex mirror blank, and Ron aka @barkis and Damian aka @mapstar for supplying grinding grits of varying grades, polish and pitch. Plus lots of advice from all the aforementioned plus Nigel aka @astrobitz. 

A big thank you from me 

 

 

[Chris]

Once the mirror blank had arrived, the first thing to do was to determine which side I'm going to use. Pyrex glass can commonly have air bubbles trapped inside, one side of the blank has a few air bubbles towards the edge, these are half hatching out the surface so they would never grind out, thus by default I'll be using the other side. 

The side to be used is free from air bubbles which is great. The surfaces on both sides of the blank are not perfectly flat which I believe is the norm for non ground blanks. Any 'topography' that's not of advantage (e.g. hills in the center) needs to be ground away. The side to be used on this particular blank has a hill in the center the peak of which is flush with the outside edge of the blank, a 'moat' or annular depression 1.8mm deep, 28mm wide resides about 60% out towards the edge, which then raises up again towards the edge.

The above was determined by placing a steel ruler across the mirror and sliding things of known size e.g. feeler, gauges, twist drills, coins under the ruler where the gaps are.

[Chris]

A jig was needed to hold the blank or grinding tool steady for grinding. for this I found a redundant piece of pine shelve, placed the mirror blank on it as a template, then screwed in a few of those plastic furniture blocks around the edge of the blank, making sure they held it steady but not too tight.

because the bottom of the blank isn't perfectly flat, I cut out a round piece of carpet tile to place under the blank to take out some of the unevenness.

I then simply G-clamped the pine shelve to my kitchen table...jig complete! 

I can also attach the shelve/jig to my work mate for when I'm not allowed to grind in the kitchen!

[Chris]

Probably the next thing to do is to work out the satiga, which is the depth that the center of the mirror blank needs to be ground out to. 

The formula for working out the Sagita is:

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.

The Sagita (S) for my 8.75" or 222mm mirror when aiming for around f/6.7 is pretty much bang on 2mm.

 

 

 

[Chris]

Now what to use as a grinding tool to 'hog out' a rough spherical curve to the Sagita depth?

If you read the thread linked in my 1st post, you'll see there is more than one way of going about grinding a mirror. You can use a glass tool the same size as your mirror blank, or cast a morter/dental cement tool the same size as your blank and stick glass mosaic tiles to it using fiber glass resin, or you can use a iron barbell weight roughly 50% diameter of the blank.

After much deliberation and going back and fourth, I decided to try and hog out my Sagita depth using barbell weights I have sitting around. 

I have one that is 54% the diameter of the mirror blank, and one that is 39% the diameter of the mirror blank. I decided to start with the 54% diameter weight and see how things go?

One thing I read to bare in mind when using a sub diameter barbell weight to hog out your curve, is that it is unlikely to generate a perfectly spherical curve, which is what you want initially. Apparently you therefore need to hog out slightly deeper than your calculated Sagita so that when you switch to your next tool and start to grind down the edge of the blank to form the required sphere, the satiga then evens out to the correct depth (if you follow me?). 

[mapstar]

Good start to the grind Chris.

With a sagitta depth of 2mm the hogging out won't take too long. I reckon four to six hours with 80grit would get you very close.

Firstly you need to bevel both front and back at 45° then flatten the back. You can then make a start on the hogging.

Well done for getting it on the forum and we're all watching with great interest.

 

[Chris]

First grinding session! 

I opened up a bin liner and placed it over my kitchen table to catch the slurry from the grit used to grind the glass, then clamped my jig/pine shelve to the table with a couple of G-clamps. I also placed a bowl of water and sponge behind the jig ready for washing down the blank and tool. Other things include a pot of 100 grit which is the most course I have, a spoon for sprinkling the grit on the blank, a washing up liquid bottle full of water to wet the grit on the blank (don't grind dry, it will give you Silicosis!), and a wet stone/tool sharpening stone for maintaining the beveled edge on the blank (if you don't maintain a beveled edge you can create clam shell fractures to the edge of your mirror blank). Last but not least the iron barbell weight.

Firstly I spent 10 minutes with the wet stone making sure I had a good beveled edge on my blank...it already had a beveled edge but I wanted to make sure. 

I placed the glass blank in the jig and sprinkled a heaped tea spoon of 100 grit onto the blank. I then wet the grit using the squeezy bottle, and ground for several minutes pushing the barbell back and fourth through the center of the mirror blank so the weight just went past the edge of the blank. whilst I was doing this I was turning the weight clockwise and moving around the table as far as I could go one way, then back again. This is all to create as much randomness as possible in order that it all averages out to form a nice symmetrical curve....that is the hope.

After a few minutes of grinding the deep grinding noise gave way to a higher pitch noise which meant that the grit had been ground away or of the side, so I wiped the blank with a the wet sponge and repeated this stop over and over for close to an hour.

I cleaned everything down using my outside tap as I read the slurry will soon block your kitchen sink if you wash it all indoors!

Once the blank was clean and dry I repeated some basic measurements with the steel ruler placed across the the middle of the mirror. I used my feeler gauges to judge the gap under the steel ruler and therefore see how much glass had been hogged out.

I could now get the 0.4mm gauge under the ruler at the center of the blank, but when I turned the ruler through 90 degrees I could only just get my 0.15mm gauge under.

This made me look at the edge of the blank for discrepancies. I lay the ruler at all angles around the mirror dragging my 0.15mm feeler gauge as far to the edge under the ruler as I could, then I marked this position. Once all the way around blank I connected the dots which showed a off center circle. I have discussed this with an experienced mirror maker since, and it's to be expected when the cast blank doesn't have flat surfaces. It should grind out symmetrically in time. 

   

 

[Chris]

Another hour of grinding. Now measuring 0.6mm deep in the middle one way, and 0.4mm when the ruler is turned 90 degrees. There is still an off center circle/rim, possibly slightly more off center then before.

Those shiny kidney bean shapes are starting to go though 

I'll do another hour tomorrow and re-assess.

[Daz69]

I would absolutely LOVE to build my own telescope. I sat and watched this a few weeks back and was totally amazed.

Perhaps one day...............

I will keep an eye on this build @Chris Lock for sure. Good luck buddy.

 

[mapstar]

Good start Chris.

Once you have flattened the bumps out on the front I would move onto flat the back off as this will make sure there is no rock whilst you grind in the curve. You could possibly grind in an offset curve and then the mirror wouldn't be a figure of revolution.

Also a flattened back will sit on a cell easier if you are going to go the floating route. At this size you could loose bond this due to the thickness of the blank I should imagine. 

[Chris]

11 hours ago, mapstar said:

Good start to the grind Chris.

With a sagitta depth of 2mm the hogging out won't take too long. I reckon four to six hours with 80grit would get you very close.

Firstly you need to bevel both front and back at 45° then flatten the back. You can then make a start on the hogging.

Well done for getting it on the forum and we're all watching with great interest.

 

 

1 hour ago, mapstar said:

Good start Chris.

Once you have flattened the bumps out on the front I would move onto flat the back off as this will make sure there is no rock whilst you grind in the curve. You could possibly grind in an offset curve and then the mirror wouldn't be a figure of revolution.

Also a flattened back will sit on a cell easier if you are going to go the floating route. At this size you could loose bond this due to the thickness of the blank I should imagine. 

Thanks Damian  well after 2 ish hours I'm down to about 0.5mm so I think you're close with your time line prediction. It may take just a bit longer as I'm using 100 grit, and I'll need to go slightly deeper than 2mm because the Barbell weight won't produce a perfect sphere. I'll have to attack the edges with a tile tool after the barbell, this will hopefully bring the sagita back upto 2mm. The barbell is just really to hog out as fast as possible and generate a rough curve to cast the tile tool against...well this is what I've been told  

The blank arrived with quite a large rounded bevel on both sides which is great, but after reading your thread Damian I'll certainly be keeping my eye on it! Before each grinding session I'll be spending a bit of time looking at, and maintaining the bevelled edge with a wet stone  I've probably done 20 minutes edge maintenance so far.

I must admit I'm a bit confused about flattening the back. I've been told that it's very much needed for a large mirrors like yours, but not a smaller one like this. I am a bit concerned about the blank rocking as you point out though, and it did do so slightly in the jig until I added the carpet underneath. I can't feel any movement with the carpet in place which is promising, but I'll be carefully checking for this each session I do.

Looking at the vid you kindly sent me of John Nichols griding an 8", he keeps the back of the blank untouched so he can check how good the contact is when making the pitch lap. If I grind the back it will be frosted so I wont be able to check this. Nigel also said to keep the back of the blank clear so you can see the bubbles, which can give indication of the tool about to stick. I do take your points on board Damian, they do remain of concern if I don't flatten the back, hhmm?

Can I just check what you mean by your last sentence? 

"At this size you could loose bond this due to the thickness of the blank I should imagine." 

  

[Chris]

"I would absolutely LOVE to build my own telescope. I sat and watched this a few weeks back and was totally amazed.

Perhaps one day...............

I will keep an eye on this build @Chris Lock for sure. Good luck buddy."

 

Thanks Daz  hopefully I will inspire you rather than put you off hehe!  

[mapstar]

1 hour ago, Chris Lock said:

Can I just check what you mean by your last sentence? 

"At this size you could loose bond this due to the thickness of the blank I should imagine." 

  

I've seen mirrors up to 12" bonded with silicone to a plain board so I think it should be o.k. as your glass is quite thick. 

If you do grind the back flat you can smooth it with 400grit whichwould still leave it frosted but if you then wet it you can see through it to the other side to check the contact. Probably not as critical on mirrors of this size.

I think John in his video is using plate glass which is uniform in thickness and also flat on both surfaces so flatting the back is not required. 

[Chris]

5 hours ago, mapstar said:

I've seen mirrors up to 12" bonded with silicone to a plain board so I think it should be o.k. as your glass is quite thick. 

Ah I see, I once bought a second hand fast f/4 Newt which had terrible astigmatism due to it being bonded with silicon. I didn't know what the issue was at first so sold it really cheap with full disclosure and the buyer simply removed the silicon and it was completely cured...haha live and learn. Yes this blank is getting on for 40mm thick! so it might be safe  

5 hours ago, mapstar said:

I think John in his video is using plate glass which is uniform in thickness and also flat on both surfaces so flatting the back is not required. 

Face palm! Plate glass! off course, doh! 

5 hours ago, mapstar said:

If you do grind the back flat you can smooth it with 400grit whichwould still leave it frosted but if you then wet it you can see through it to the other side to check the contact. Probably not as critical on mirrors of this size.

That's a good solution, nice bit of problem solving  

Edit: I don't know what you think Damian about the following, I ran the idea of grinding the back by John, he said it wasn't needed due to the blank being so thick. He told me how to go about doing it if I wanted to do so though. I might not unless I notice any rocking of the blank during grinding. I'll keep my eyes peeled for this, and properly decide after hogging out.

I know it was essential for your BIG! 22" Damian, I'm guessing a mirror that massive can distort unevenly if the thickness varies from one bit to the next? I still can't believe you did a 22" as your first! 

[Astrobits]

When you get to the  approximate sagitta you will make a tile tool and then you will be using the mirror on top. There should be no further need for the mirror to be on the bottom. When the mirror is on top there is no need for the mirror's back to be perfectly flat, nor for the back edges to be bevelled.  I would not flat the back as a little rocking will not materially affect the rough grinding you are doing now. 

While your work on the table as shown in your pics will suffice for this initial grind, Once you move onto the next stage with a matched diameter tool you will need a full diameter support. I would not consider the wooden shelf wide enough for the job. Also, you must rotate the mirror on the carpet tile as well as rotating the mirror and changing your position around the grinding table. This is why it is beneficial to use something like a 45 gallon oil drum or purpose made stand. With such a set-up you can continuously walk round the stand  pushing the mirror back and forth and rotating it a little with every stroke and rotating the tool ( by varied amounts ) on the carpet every wet. Any regular interruption in this process, such as stepping between the legs of your stand, tends to show up as astigmatism in the final mirror.

There two meanings of the word "Gluing". Conventionally, we try to match the two surfaces as close as possible and then apply a thin layer of adhesive. For telescope mirrors this is NOT the way to do it as astigmatism is the result. What we try to do for mirrors is to combine a rubber buffer and gluing into one procedure by putting three ( NO MORE THAN THREE ) blobs of Silicone on the base plate and using a +/-3mm spacer ( so they can be removed when the silicone has firmed up--give it 24 hours ) so the blobs are not squeezed to a thin layer but remain as a thick rubber buffer which is now glued to both the mirror and the base plate.  For an 8" mirror the blobs should have about 1" diameter contact area on both base plate and mirror.  Sorry for the shouting but you ( and I ) have experienced this phenomenon. It is amazing how so many people will not follow instructions.  My 16" x 1" thick mirror has been glued this way ( silicone blobs approx 1 1/2" dia.) now for approx 10 years.

Nigel

[mapstar]

2 hours ago, Chris Lock said:

I ran the idea of grinding the back by John, he said it wasn't needed due to the blank being so thick. He told me how to go about doing it if I wanted to do so though. I might not unless I notice any rocking of the blank during grinding. I'll keep my eyes peeled for this, and properly decide after hogging out.

John, as with Nigel above, have both done many mirrors and their advice has much more experience than mine behind it. If it's not necessary then don't waste the time with the back. Rotating the mirror blank is essential though to prevent the dreaded astigmatism. Lots to learn. Mirror making is a lot harder than scope building so don't be afraid to have a go  

[Chris]

1 hour ago, Astrobits said:

When you get to the  approximate sagitta you will make a tile tool and then you will be using the mirror on top. There should be no further need for the mirror to be on the bottom. When the mirror is on top there is no need for the mirror's back to be perfectly flat, nor for the back edges to be bevelled.  I would not flat the back as a little rocking will not materially affect the rough grinding you are doing now. 

 

Thanks Nigel  that's settled then I don't need to flatten the back.

1 hour ago, Astrobits said:

While your work on the table as shown in your pics will suffice for this initial grind, Once you move onto the next stage with a matched diameter tool you will need a full diameter support. I would not consider the wooden shelf wide enough for the job. Also, you must rotate the mirror on the carpet tile as well as rotating the mirror and changing your position around the grinding table. This is why it is beneficial to use something like a 45 gallon oil drum or purpose made stand. With such a set-up you can continuously walk round the stand  pushing the mirror back and forth and rotating it a little with every stroke and rotating the tool ( by varied amounts ) on the carpet every wet. Any regular interruption in this process, such as stepping between the legs of your stand, tends to show up as astigmatism in the final mirror.

 

I see your point Nigel, yes the shelve is only 7.75" so there is 1/2" overhang, I'll start brainstorming an upgrade that I can use in the kitchen, maybe something with a small footprint that I can walk around then. I like doing the grinding in the kitchen as it means I can do a bit whilst I'm 'looking after the kids', multitasking  

Oh no! I haven't been rotating the glass blank! Just rotating the tool and me moving back and fourth around the table. I'll start rotating the blank on the carpet every wet from now on, thanks for pointing this out!

1 hour ago, Astrobits said:

There two meanings of the word "Gluing". Conventionally, we try to match the two surfaces as close as possible and then apply a thin layer of adhesive. For telescope mirrors this is NOT the way to do it as astigmatism is the result. What we try to do for mirrors is to combine a rubber buffer and gluing into one procedure by putting three ( NO MORE THAN THREE ) blobs of Silicone on the base plate and using a +/-3mm spacer ( so they can be removed when the silicone has firmed up--give it 24 hours ) so the blobs are not squeezed to a thin layer but remain as a thick rubber buffer which is now glued to both the mirror and the base plate.  For an 8" mirror the blobs should have about 1" diameter contact area on both base plate and mirror.  Sorry for the shouting but you ( and I ) have experienced this phenomenon. It is amazing how so many people will not follow instructions.  My 16" x 1" thick mirror has been glued this way ( silicone blobs approx 1 1/2" dia.) now for approx 10 years.

Nigel

 The f/4 scope I bought was glued within an inch of it's life to the cell! No wonder stars looked like lines or even figure of eights! Thanks for pointing out the right way of doing it  

[Chris]

14 hours ago, mapstar said:

John, as with Nigel above, have both done many mirrors and their advice has much more experience than mine behind it. If it's not necessary then don't waste the time with the back. Rotating the mirror blank is essential though to prevent the dreaded astigmatism. Lots to learn. Mirror making is a lot harder than scope building so don't be afraid to have a go  

Cheers Damian  I dropped the ball when it comes to rotating the blank I must admit, definitely lots to learn! I've not even got to Ronchi testing yet! That reminds me I need to look at building a Ronchi tester soon. 

[Chris]

Another 40 minutes grinding completed this evening, plus 10 minutes maintaining the bevelled edge with a wet stone. This time I regularly turned the mirror blank when grinding which is a step I was missing previously. I also decided to switch to the smaller 0.5kg barbell which has 39% of the diameter of the blank, as I just want to see how it compares to the larger 1.25kg 54% diameter barbell for a while. No preference as of yet.

Upon measuring I haven't managed to go much deeper with the satiga, maybe an average reading of 0.7mm deep.

I do think the edge of the mirror is 'maybe' starting to become more symmetrical. I did my little routine of laying the metal ruler across the mirror, placing a 0.15mm feeler gauge under it, and dragging it as far as it will go towards the edge then marking this position for lots of points around the mirror. The smallest measurement was 7mm from the edge, and the largest 14mm from the edge, which is better than the previous 5mm and 17mm respectively.

The shiny kidney bean shapes are nearly gone!

I'll keep on trucking 

[mapstar]

Good progress Chris and a pleasure to see the glass steadily grind out. The Sagitta will get there although a little slower with finer abrasive. Keep at it 

[Chris]

Thanks Damian, will do  Another 10 minutes of bevel maintenance, and 50 minutes of grinding done this evening. 

With the ruler and feeler gauge across the mirror one way it suggests I'm down to 1.15mm, then turning the ruler through 90 degrees it reads 0.95mm. I guess I'm down to 1.05mm on average which is pretty much half way to target depth!

My grinding style has changed a bit in that I'm doing plenty more blank turning. I'm maybe turning the blank 30-40 degrees every 6-7 strokes, and I'm walking around through about 90 degrees back and fourth. This is really saving my back compared to the 180 degrees stretching past obstacles I was trying to do previously. I'm starting to give it more welly now too 

The off center circle issue seems to be on the mend, now reading 6mm from the edge one side and 10mm from the edge the other side.

 

[RobertI]

Fascinating thread Chris, watching with real interest. 

[Chris]

3 hours ago, RobertI said:

Fascinating thread Chris, watching with real interest. 

Thanks Rob:) It's certainly a lot of fun so far!

[nightfisher]

Doing a great job Chris, will this be a truss or full tube, i guess this will be a dob

[Chris]

7 hours ago, nightfisher said:

Doing a great job Chris, will this be a truss or full tube, i guess this will be a dob

Hi Jules, thanks  Yes indeed a Dob. I've been thinking about this, and I'm leaning towards a truss design for easy storage and transportation. Plus I think they look cool!