14" scope project - update.

Hi Folks,

Just a quick update on the 14” Folded Stevick-Paul project…

I polished out the entire surface some time ago and measured the radius of curvature at around 10,600mm, 100mm too long (curve too shallow) so I employed a deeply unsubtle polishing action, mirror centre over tool edge with loads of pressure on the overhanging portion of the mirror. It definitely had the desired result – in less than an hour I had the RofC down to 10,400mm-ish and a hideously disfigured surface. I was well pleased! A few more hours of polishing and I had the R of C at the centre of the mirror at exactly 10,500mm (the desired design spec) and could start bringing this rather wobbly looking surface under some sort of control. Optical surfaces, like unruly children, sometimes have to be beaten into submission… and then coaxed and cajoled into making tiny little changes to their behaviour!



Here’s a Foucaultgram of the mirror as it was last night. The centre 50% is broadly spherical (there is underlying ripple still to eliminate) and I am working half-hour spells on successive zones between testing and pressing. The paper strip across the TV screen / Foucaultgram is marked with the different zones I am keeping an eye on - zoom in for detail.



In this shot, you can see that I have marked out a series of concentric circles of dots. 25% of the diameter, 35%, and 50%. These assist me in avoiding working anything inside these zones – they are Verboten! Working from the centre of the mirror out towards the edge, once the null zone expands outwards, I then do not work that zone any more.

More as I progress…

Phil.

Hi Philip,

Looking good

Later,

Jed.

Nice Phil, really starting to come together!! Now just two big flats and that convex spheroid to do right? Oh yeah and then there is a scope to build around it all!

Looking good!!

Keep them photos coming!
Time-wise, about how long does it take to completely gring a mirror of that size?

I'm well impressed Philip,
I wouldn't have the perseverance or know how to plough on with that.

Keep them photos coming!
Time-wise, about how long does it take to completely gring a mirror of that size?

It took approx 6 - 10 hours of rough grinding to get the curve to spread right across the disk, and to the correct depth. It could have taken a bit less if I hadn't been such a cheapskate... I used the left-over washings from rough grinding my last mirror as my 1st grade of grit instead of using fresh abrasive! It was nominally 80 - 120 carbo, but contained a lot of finer stuff too! This is a trick you can ONLY do when you are rough grinding. To save to washings from finer grades of carbo is asking for trouble (in the form of scratches.)

I moved on to 400 carbo for fine grinding, which took just a couple of hours to remove the pits from rough grinding and to refine the curve and sagita.

Smoothing continued the process using 303 emmery, and took just 10 wets (about 5 - 10 mins per wet) to remove the 400 carbo pits. This was astonishingly fast!

I figure (pun intended that it will take me another 6 - 8 hours of polishing to refine the figure and control the radius of curvature before I can call the mirror done... then yes, the flat comes next (tested against the primary for a perfect null and then by interference against the other two flats for greater accuracy), then finally the two remaining spherical surfaces (convex and concave) which will be tested against eachother interferometrically.

Patients is a virtue in this game, but beer, a radio and no social life to speak of also help!

Phil.

PS - I'm away until next wednesday - chat more then.

PPS - Just in case anyone is worried about the sizes of my pics - I shrink them down to between 3 and 5Kb. Don't think that's going to break the bank!