Polishing advice needed....

[rochler]

Heya folks!

I've just started polishing out a 152mm mirror & have done about an hour to kick things off. It already has a 'superficial' polish but clearly not to the edge & a laser reveals that the surface still has a long way to go (no surprises there!)

However, I put it on the tester just to take a peek at the figure - and now regret doing so. It showed a rather severe turned down edge (TDE) with the ronchi bands hooking in a lot in the last 10% towards the edge (inside focus).

On reflection though (pun intended), could this simply be because of the different amount of polishing between the zones? It looks to me like the apparent TDE fairly closely matches the 'less polished' area on the mirror.

Therefore my question is: Should I just continue polishing out & check if the TDE diminishes or would I be wasting my time & better off going back to fine grinding? I'm not sure since this is my first mirror.

Cheers & thanx.... :help: :D

[Satchmo]

Its hard to tell at this early stage weather the problem is left over from grinding or uneven polishing. In my experience most first time mirrors don't have the pits from earlier grades ground out properly on top of polishing problems. At this stage do 75% of your polishing with lap on top 1/4" W strokes.

If the edge doesn't catch up fairly quickly then it may be a grinding problem. Some pits at the edge won't seriously marr your images , but greyness of the polish near the edge usually looks far worse after aluminising. Don't let it get you down, the thrill of looking at the sky with your own mirror will far outweigh the cosmetic defects.

I wouldn't recommend a laser as having such coherant light they allways show up so many specs of dust. Use a high power focussed beam from a $10 LED head torch and a low power eyepiece in reverse as a magnifier. With a black background under the mirror it will be obvious what is dust and what is pits. Hope this helps.

[rochler]

O.k. I'll try using some other kind of torch & see what the surface looks like. I'm fairly sure that I haven't got a problem with pits though - I was very careful to grind out & inspect closely under a bright light.

The edge area doesn't have 'pits' it's just a bit more hazy than the centre area. After another hour of polishing it looks a lot better so I'll test it again to see if it is reducing.

My question was really whether or not the apparent TDE is real or just an effect of uneven polishing?

[Satchmo]

A TDE is a TDE :)

[Garyh]

Very true, my LED hand torch has a real tight beam and works well for this..
Borrowed a 20mw pointer the other day to give it a go as I have read on the net to use a laser pointer. If you can`t see it on the surface its polished out they say.......:lol:.......maybe 20mw is too much, could see it in the air, front surface, through the glass and out the back..Tried it on lenses and other optical surfaces with the same result. Not much use at all!
Another 5 hours of polishing should see things differently!...:thumbsup:
cheers...

[rochler]

I'll do more TOT with w strokes & see if the edge catches up. If I don't make any real progress then I'll go back to find grinding.

Thanks for the tip about the LED torch, I'll give that a go as well.

I'll let you know how I get on later.... :P