Parabolizing and avoiding a Turned down edge

[Draconid]

Hello,

Currently I am working on my first mirror, a 200cm F5 and so far it went quite ok.

Since this is my first mirror, I made some mistakes during grinding, but managed to resolve them.

Now after polishing the mirror out (checking the surface with a laser) and reached a good sphere with a good edge, I started with parabolizing the mirror.

Using classical "W"strokes, Mirror On To[,  I managed to make a nice beginning of the parabola (initial check with a ronchi screen) , but to my dismay, the edge got turned down quite a bit. I am still wondering how this happened, since I was cautious not to have the mirror cross to much over (using a 33% circle on the back of the mirror as guidance).

It took me some time to polish the TDE out and now have again a nice sphere. Does anybody around here have suggestions about how to avoid a TDE while parabolizing?

[Glasspusher]

One method is to use a 50% diameter pitch lap together with a 'W' stroke with minimal overhang. Test often and press between figuring spells.

John

[Astrobits]

As John says with tool on top.

Don't forget that having got a sphere your first reference point for the parabola had already been defined--the edge of the mirror. The edge needs NO FURTHER WORK. You only need to deepen the centre and smooth it out to just touch the edge. Using a full sized lap with MOT you will inevitably do some work on the edge which you would need to minimise thus a long W stroke ( if you go that way ) as much more work is needed in the centre zones of an F/5 compared to the outer zones. And TEST;TEST;TEST you don't want to overdo the centre.

Perhaps you have a local club with some experienced mirror makers that could give you some help?

Nigel

[Draconid]

Thanks for the advice, I am making this mirror as part of a mirror making course so I do have some experienced people to consult.

After looking at my logbook, I think I worked to long without testing and intermediate pressing. So I will be more cautious the next time and test more to see how the shape is developing.  I will post more in the future how its going.