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Lockie

The 8.75" f/6.7 Mirror Grind

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If you can get 1-2 hours polishing every day it could be fully polished within a week or probably less. Parabolising will take a bit longer than polishing, I expect. After polishing and figuring sessions wash and dry the mirror and wait for about 1/2 to 1 hour for the mirror to thermally stabilise before doing any critical testing - rubbing the surface of the mirror generates a surprising amount of heat. After testing the mirror the pitch lap will probably need 10 minutes or so pressing against the mirror before continuing, to make sure of good contact.  Keep the mirror in the wet bag and in contact with the pitch lap between sessions.

Back in the 1980's I was a member of a club that would make a 6" f/8 mirror during a 1 day outreach event. Start with two discs of glass and grind, polish and figure before the day was finished. And then view the moon with it, unsilvered, that evening. Much more difficult with an 8.75" mirror at f/6.7 though.

Nigel

 

 

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Thank you Nigel :) Your post is a good bullet point reference as to how to proceed with the pitch lap polishing/figuring. The time scale sounds doable if I plan my time correctly also.

This may be a dumb question but do I start testing the mirror at the same time as polishing, or wait until I start figuring the mirror after the initial polishing? I'm sure I could research this on the net, but it's simpler to just ask yourself as you have the experience :) 

I take my hat off to that club member! To do all that in one day sounds incredible, but I can totally believe it if you know what you're doing. I've basically faffed about quite a bit, but I guess I feel fairly confident with what I've learnt so far. I think I could reach this stage quicker if and when I make another mirror :) 

Edited by Lockie

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I don't start testing until it is fully polished otherwise I feel that there is a risk that you get an acceptable figure before the mirror is fully polished. It was quite common to see mirrors that were not finished as when aluminised the incompletely polished edge ( it's nearly always an edge - the most important part of the mirror ) shows up grey.

I would therefore recommend that you first test only when you are happy with the polish, particularly at the edge as that will have very little, if not no, work done on it during figuring.

The activity to make that 6" mirror was a club effort with several members taking it in turns to do the work. That said, however, when I started making mirrors I was told that to do it commercially then you would need to do a 6" in one day and that would be a one man job but shortcuts can be had where a repetitive procedure is undertaken. 

Nigel

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Just a quick thought--- I can't remember (too lazy to go through previous posts here:happy7:) if it has been mentioned that when you start to prepare the polishing lap do it outside. The fumes and smell of molten pitch will not endear you to everyone else if you do it in the kitchen. And get a cheap saucepan just for this application.

Nigel

P.S. I have that book only a 1950 edition. V. useful if a bit dated and from an American perspective.

Edited by Astrobits
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lol Thanks for the thought Nigel, I'd read about the acrid smell of molten pitch a while ago but kind of forgot about it, thanks for the reminder! I'm also slightly concerned about dying the kitchen and myself red with the jewelers rouge I have, so may look into alternatives. 

On a positive note, guess I'm showing mirror making can be done with a small kitchen as a venue. I wouldn't fancy trying a 20" in there though!  

Edited by Lockie
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I have a copy of ATM Book 1 first edition from 1928, it is in very poor condition. It was given to me by an ex-employee of Grubb Parsons who told me that it was consulted regularly during the making of some of the companies large telescopes, it certainly bares the scares of heavy usage.

I have to say that I love the smell of molten pitch!!!!

Keep up the good work with the mirror Chris.

John

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1 hour ago, Glasspusher said:

I have a copy of ATM Book 1 first edition from 1928, it is in very poor condition. It was given to me by an ex-employee of Grubb Parsons who told me that it was consulted regularly during the making of some of the companies large telescopes, it certainly bares the scares of heavy usage.

I have to say that I love the smell of molten pitch!!!!

Keep up the good work with the mirror Chris.

John

Thanks John :) Nice to own a book with so much history behind it, in terms of use as well as being 1st Edition! 

I'm intrigued now, I'll let you guys know what I think of the smell when I melt the stuff.  

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That is true Damian. It always takes me back to my first visit to David Sinden's workshop......or was it my first visit to the local Indian restaurant? Either way great memories.

John

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11 hours ago, mapstar said:

I must return and do it all again soon :grin:

Go on, you know you want to do a 28" ;) 

Although, you'd probably need 28" biceps for that :grin:

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A 20" in the Kitchen? now there's a thought. Rather depends on the space and who else uses the kitchen:happy6:

Nigel

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It's not just the smell of pitch which is remembered by SWMBO 20-30 years later.

It was the melting of lead in an aluminium saucepan on a brand new gas cooker which is remembered longest.

Probably until the day you die early and are sent straight to hell for your ATM crimes against humanity. :angryfire:

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6 hours ago, Astrobits said:

A 20" in the Kitchen? now there's a thought. Rather depends on the space and who else uses the kitchen:happy6:

Nigel

I might have a go at a 12" one day if the 8" turns out well, it's not the first time it's crossed my mind. I think 12" would be pushing my current kitchens capacity though.

 A 20" would have to wait until I own a work shop or garage...that might be a long wait....I remember being sent to the stores for a long 'weight' as a 16 year old engineering apprentice :grin:

Edited by Lockie

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5 hours ago, Rusted said:

It's not just the smell of pitch which is remembered by SWMBO 20-30 years later.

It was the melting of lead in an aluminium saucepan on a brand new gas cooker which is remembered longest.

Probably until the day you die early and are sent straight to hell for your ATM crimes against humanity. :angryfire:

I can only imagine how much trouble you were in! :grin:

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21 hours ago, Lockie said:

Go on, you know you want to do a 28" ;) 

Although, you'd probably need 28" biceps for that :grin:

A 28" would be a big ask and way beyond my capabilities. Maybe another go at something smaller although I have to increase my workshop space before that. 

Edited by mapstar

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7 minutes ago, mapstar said:

A 28" would be a big ask and way beyond my capabilities. Maybe another go at something smaller although I have to increase my workshop space before that. 

You would need 28" biceps for a start! I doubt I could even lift a 28" blank net alone grind, polish, and figure one. I'm still amazed you did a 22"!  

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8 minutes ago, Lockie said:

@FLO Any news on this book, Steve? I ordered it the 25th of Jan. 

Have spoken with James (he handles our orders for products from the US). It looks like it might be another 2-3 weeks. Sorry :sad2:

Steve 

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3 minutes ago, FLO said:

Have spoken with James (he handles our orders for products from the US). It looks like it might be another 2-3 weeks. Sorry :sad2:

Steve 

Ok, thanks for asking James about it and letting me know, Steve. As long as I know what's going on I'm happy :) 

I'm a bit annoyed at myself though for not getting to the ones on the clearance thread early enough. Just missed out! Transfered the money into Paypal, went to buy the full set and it had turned to not available when I went to pay, Doh!

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Try an online secondhand book search website. Antiquarian, antiquariat, or whatever it's called these days.

I never owned my own copies but borrowed them from the library on and off for decades.

When I finally decided to buy a set they were listed secondhand online for quite a modest sum.

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You're welcome to borrow my "Handbook for Telescope Making" by NE. Howard.
The book focuses on creating a 8" f7 paraboloid, but clearly valid for other specifications too.
It is Old now like myself, but it has been a wonderful servant, and helped me through many different sized mirror objectives.
Good Luck with your project, you'll learn much, and have fun too. 
Always put any setback out of your mind, and start again. Never Surrender:thumbsup:

NE. Howard.jpg

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10 minutes ago, Rusted said:

Try an online secondhand book search website. Antiquarian, antiquariat, or whatever it's called these days.

I never owned my own copies but borrowed them from the library on and off for decades.

When I finally decided to buy a set they were listed secondhand online for quite a modest sum.

Thank you, I've already ordered but might give that a shot for any other books I fancy.

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