Advice for figuring a f/4.6 mirror

[Garyh]

Hi guys,
Well my 8" f/4.6 mirror is now all but polished out and just needs some slight correction for a small TDE before I try to parabolise the mirror.
My 6" mirror was slower and therefore easier to do. So before I attempt figuring does anyone have advice that might be helpful?
Would I stick with my full size lap for this? and would I need to modify the figuring stroke? and how long for a first session?
Most info I find seem more related to a slower mirror or large mirrors using small figuring laps..
All info/advice will be very appreciated....
cheers Gary

[DaveGee (Dave Gault)]

Originally Posted by Garyh

does anyone have advice that might be helpful?
Would I stick with my full size lap for this? and would I need to modify the figuring stroke? and how long for a first session?

Hi Gary,

I've recently finished a 10"f4. thew! fast mirrors are sure harder to figure!

I used a 60% tool, working TOT. I started out with a 30 minute session of W strokes concentrating mainly CoC and the closer I got to the paraboloid, the shorter the time. The penultimate last step was only once around of no more than 2 minutes and the very last session was 2 minutes with my thumb dipped in CeOx to deepen the very centre.

figure-test-figure-test-fig..... you get the idea!!!!

It was an experience not to be missed IMHO.

[Garyh]

Hey thanks Dave! a 10" f/4 would be hard work! Did you get a great figure in the end?
I will try a full size lap first and see how it goes..
Knocked the TDE off last night and have now got very close to a sphere. Just have a slight depression in the centre. As a rough guess I have a 1/6-1/8 wave sphere.
Might try a 20min session at parabolising and then test. I think the slight hole in the centre will work in my favor!
I shall start a new thread about this and my progress!
cheers Gary

[madtuna (Steve)]

as odd as it sounds..I'd kill to watch one of you guys at work on your mirrors.
I find it facinating and one day may work up the balls to have a go myself lol

[DaveGee (Dave Gault)]

Originally Posted by madtuna

I'd kill to watch one of you guys at work on your mirrors.

Well you don't need to go that far...

There are heaps of movies of people making mirrors on YouTube. Here is a link to Francis J O'Reilly's videos... http://www.youtube.com/user/foreilly1958

There is a mountain of info about the art and you can spend hours (years) reading, watching and wondering but 30 seconds with abrasive on that first mirror will teach you more.

[madtuna (Steve)]

thanks mate
will take a browse. am very tempted to give it a go

[DaveGee (Dave Gault)]

Originally Posted by Garyh

Did you get a great figure in the end? I shall start a new thread about this and my progress!

It wasn't too bad. A 1/20th wave optic it ain't but the numbers came up OK. It'll be good enough.

[pelu]

I do not like to parabolise with small or modified tools . Is
possible that with mirrors of 16" or more can be more comfortable a 2/3 tool but up to 12" I always parabolize my mirrors with the Texereau book (how to make a telescope) open in front of my eyes.