Will be easier to control when you finish your tester Rich.

All heading in the right direction, well done and good luck.

It is possible that using a full diameter tool ( tool on top ) you are causing the centre of the tool to become depressed and hence lessening the polishing of the centre of the mirror. Try going to sub-diameter tools. I would never consider a full sized tool for a 20". All my 20" mirrors were made, tool on top, with 13" and smaller tools.

Have you felt the warmth of the mirror and tool after a bit of polishing?

Nigel

There is no need to achieve a perfect sphere before starting figuring. If you have a raised centre then work on the centre only. After all, you have to work on the centre of a sphere to get a parabola. If the centre is depressed ( but not too much ) then some of the figuring work has already started!

Finish the polishing first then work on the areas that your testing shows are not in the correct parabolic zone using smaller tools as necessary. It might be wise, before you finish polishing, to cast some smaller tool bases so that they will be ready for you when you need them.

Nigel

I am using a sub diameter lap,it's about 17" :)

I'm hoping it's quite spherical when I'm rid of the depressed centre but time will tell

I'd go for a smaller lap. 17" is a bit too close to a full dia lap for my liking. But whatever you use make sure that the first priority is a full polish and the figure is secondary until that full polish is reached.

I never tested my mirrors until they were fully polished. That way I avoided the temptation to call a parabolic figure with pits "done".

Nigel

 

 

I never tested my mirrors until they were fully polished. That way I avoided the temptation

 I totally agree,

 you only want a first look ( which I opted out of) to insure you have not ground a really bad zone in the

glass, testing glass before completely polished out only tend to make things worse, polish more worry

less Rich, for what its worth, that to me is not stig, its a zone on a glass that is not polished out as of yet.

can not post a photo of astigmatism here, will put one on my log,  polish on,once your sure she is done

and polished out then test, keep everything random and you should have no problems.

Rick M

I am going to keep on polishing as we are pretty sure the problem is that I've not been polishing enough through centre.I'm just going on what John told me after looking at my mirror and asking what strokes I had been doing

Nigel

Not sure if you've got confused but Rich like myself is doing a 22" mirror.

Mine is definitely under polished and 2hours of long centre through strokes today has helped a little

Damian

My test set up is pretty much compete but I'm currently sunning it up in the Algarve so no time to do anything.it's weird not being able to see ursa major at all!!

Jrm-yeah reading back I agree that it's just the centre not polish d out that caused it hence why I'm going to polish through centre