The 22" mapstar mirror

[mapstar]

Hi all,

I began this journey way back in Oct 2013 so it has taken a while to get to where I am at present but thought it about time I put my experiences down.

Wanting a larger scope than the 8" I had and after discussing with my good friend Rich (Crashtestdummy) we both decided to find out about mirror making. There is a mine field of information out there on the subject some of which is useful but not much in the way of practical experience except what I'd read about on here (Rustysplit 14" and Rick M 20")

So when a seminar was advertised on SGL by Glasspusher we both decided to book in and attend. 

As many know Glasspusher is John Nichol and he took us through a little bit of history on himself and mirror making in this country which is sadly not as prolific as it used to be, his experienced friend Jack also popped in to talk of the many years he'd spent at Grubb Parsons (

) .  

After an informative and enjoyable few hours with John we left with thoughts of big bits of glass and lots of work ahead (I mean lots) 

On returning it took us a couple of weeks to decide what size we were going to go for. Hunting for a glass supplier in this country leads to Spectraglass in Scotland and Rich ordered the two 22" blanks. Here's one with a cup for size!

The blank was 44mm thick and weighed in at 22kg! There was a small defect near the edge which I decided would be the back

Whilst waiting I'd put together a couple of grinding tables so we could make a start 

To make a start I had to bevel at 45 degrees the edges of the blank to about 5mm with a carborundum stone.  Each side took an hour! On a blank this size it is important when you turn it over you lift it completely or roll it on something flat. I didn't initially and caused some shelling of the surface. This would later be ground off but valuable lesson learnt.

Next was to flatten the back with the 80 grit carborundum. This was done using a steel ring about 12" in diameter. It was quite nerve racking due to the blank having concentric rings on the face I was flattening so each time it skipped over these really does not sound nice 

This took 11 hours in total so quite a while. I tested as I went with a straight edge and finally finished with 220grit although I think John is convinced I used 400grit as it's really smooth    Total grit usage up to this point was 2.5kg 80 grit and 0.1kg of 220grit. The blank had also lost weight, just under 1/2 kilo and 2mm from it's thickness so was 42mm.

Turning the blank over I then made a start on the curve generation using a barbell weight of approx 11". 

Damian

Edited February 21, 2015 by mapstar

[barkis]

What specifications have you decided on for the final mirror Damian?

Ron.

[mapstar]

Hi Ron

I was initially aiming for F3.9 but as I'll be detailing later this changed to nearer F3.7

It's an ambitious goal but with time and dedication its steadily progressed. There have been plenty of ups and downs

[estwing]

Wowsers mate it's really going to be a sense of achievement when this is complete!

[mapstar]

Wowsers mate it's really going to be a sense of achievement when this is complete!

Cheers Calv

Notice you added the word "when"! I'm getting there but it's slow going as I'll get to in the thread

[estwing]

slowly,slowly catchy photons!

[barkis]

Hi Ron

I was initially aiming for F3.9 but as I'll be detailing later this changed to nearer F3.7

It's an ambitious goal but with time and dedication its steadily progressed. There have been plenty of ups and downs

That will be one deep Sagitta  Damien, your lump of glass will be a bit lighter  .,

[stonemonkeylives]

Looking forward to following this Damian.

[Soupy]

Great write up Damian. Glad you have found the time to share this amazing journey.

[Stu]

Great stuff Damian. It will be a real labour of love once finished, something to be proud of.

[mapstar]

Great stuff Damian. It will be a real labour of love once finished, something to be proud of.

Cheers Stu, hopefully you'll get to see it by the end of this year!

[Stu]

Cheers Stu, hopefully you'll get to see it by the end of this year!

I look forward to it!

Is the mirror going in a Mapstar special custom truss Dob?

[faulksy]

nice one fella. look foward to your next bit. just like your big dob build off

[mapstar]

I look forward to it!

Is the mirror going in a Mapstar special custom truss Dob?

Hopefully, although it's not finished before putting a thread together unlike the scope build.

I've been on with this whilst doing the scope builds so its been a bit of a slog!

Yet to start the scope wood work but that's another thread although won't be as long as the last one ;-)

[mapstar]

nice one fella. look foward to your next bit. just like your big dob build off

Cheers Faulksy just wish it was finished like the scope was before writing.

[Astroscot2]

This is going to be a great thread . Look forward to your progress

[JRM]

Nice to see your journey on here now Damian,will be following with interest, keep us posted, every step is a learning curve in making a mirror of that size and focal length

but learn you shall, cant wait to see the final results.

Rick M

[RAC]

I'll be watching this with interest. I have my own little tough mirror project going on right now too and it's having its challenges but that's to be expected with with an 18" f3.3!

[mapstar]

Cheers Guys

I made a start on the front face by yet again flattening it making sure it was the same thickness all round. It was better doing this side as the ridges were no where near as pronounced as the other side. This time I started with 1Kg of  60 grit moving onto 80 grit to finish with. As can be seen it was a messy business!! 

This took 5 hours in total, the 60 grit cutting a lot faster. I then renewed the bevel on both sides which was another hour

Once the front was flat, I started the hogging out progress after preparing the 10Kg 11" barbell weight by grinding the middle off to leave the ring around the outer edge

 

I'd worked out that the sagitta for F3.9 was going to end up at roughly 9mm  so a lot of glass to remove. I used a centre through stroke testing with a straight edge as I went.

I needed to take the sagitta deeper as the barbell would create a non spherical curve. A tile tool is used then to bring it to a spherical curve and would grind the edge down (in theory) so lots more grinding later and another 1kg of 60 grit, 9 kg of 80 grit 12 gallons of water and I was at the depth I required. Here's a few photo's of it progressing.

A total of 41 hours later I had a depth of 12mm. Just out of interest I weighed it at 18.7Kg. Wish dieting was as easy!

Just to say hogging has to be one of the most boring things in the world!!!!!!!!!!!!! 

I re-bevelled several time's during this process

So next I was ready for the tile tool

Damian

Edited February 22, 2015 by mapstar

[estwing]

Losing weight is never interesting, stick headphones in and get dancing around that blank!

[mapstar]

Losing weight is never interesting, stick headphones in and get dancing around that blank!

Yeah blank is how your mind ends up doing the hogging. If I ever do another I'll get it pre-generated. 

P.s. the blank lost weight a lot faster than I did 

[Tiny Small]

Going to be following this with interest.

[Martin Meredith]

Inspirational stuff -- more power to your elbow! That is going to be one fast scope and not too high off the ground.

Martin

[barkis]

Sure is a brave task you have set yourself Damian, and I wish you every success.

The rough stuff is hard work, but once you have the  finely ground curve generated,

then comes the polishing and figuring stages.  It  is challenging, but exciting too.

We're all rooting for you, and look forward to the commissioning of your creation.

Best Wishes.

Ron.

[Beulah]

Guess the final height of the scope be over 6 feet. Not bad!

Will be watching this thread with interest.    Would like to have another go, one day....

Mapstar, did you hand chamfer the edges?