repolishing mirrors

Is it a feasible prospect to repolish a low grade mirror (such as one
you might buy in a cheaper telescope) to give a higher quality surface
finish?

"matt" wrote

Is it a feasible prospect to repolish a low grade mirror (such as one
you might buy in a cheaper telescope) to give a higher quality surface
finish?

Yes. Would you be doing this yourself?

Only if its affects can be polished out during refiguring, but quite often
this
cannot be done and regrinding is required - more time consuming usually than

its worth on a cheap mirror. Only a test will tell. Sometimes you are better
off
just to recoat the mirror ad mask out any edges flaws, for example. That's
the
usual case in cheap mirrors.
jerry



Howard Lester wrote:

"matt" wrote

Is it a feasible prospect to repolish a low grade mirror (such as one
you might buy in a cheaper telescope) to give a higher quality surface
finish?

Yes. Would you be doing this yourself?

On 22 Mar 2004 13:35:38 -0800, (matt) wrote:

Is it a feasible prospect to repolish a low grade mirror (such as one
you might buy in a cheaper telescope) to give a higher quality surface
finish?

It depends on what's wrong with it. If it has many pits, or
astigmatism, the answer is no. TDE, zones, correction and smoothness
can all be improved with refiguring. This is true of either plate or
pyrex, as long as there is no significant strain in the glass(which
will cause astigmatism).

DC

Dan Chaffee wrote:

On 22 Mar 2004 13:35:38 -0800, (matt) wrote:

Is it a feasible prospect to repolish a low grade mirror (such as one
you might buy in a cheaper telescope) to give a higher quality surface
finish?

It depends on what's wrong with it. If it has many pits, or
astigmatism, the answer is no.

TDE, zones, correction and smoothness
can all be improved with refiguring.

That would all depend on the focal length of the mirror?


This is true of either plate or
pyrex, as long as there is no significant strain in the glass(which
will cause astigmatism).

DC

On Tue, 23 Mar 2004 23:53:47 -0600, jerry warner
wrote:




TDE, zones, correction and smoothness
can all be improved with refiguring.

That would all depend on the focal length of the mirror?

Why?

Dan

deeper mirrors (f5 and below) are harder & more expensive to refigure if
needed.
More glass to have to correct in some cases. The time and cost can be
almost as
much as doing a new mirror, and in some cases it would be easier to
start with a new pregenerated blank. Most opticians will turn those
projects down.

Yes, refiguring can release a strain if one exists in the glass, but
that's fairly rare in
my experience (unless the glass was junk to begin with - some older
plate glasses
carried strains). If a finished mirror has obvious strain in the glass
then you're
taking a risk fooling with it at all. Strains are less common in pyrex.
Strains should
be relieved with back grinding (and polishing) before fabricating the
surface. In some
cases you can back grind a finished mirror in the hope of relieving
internal strains,
but its a risk and no guarantee you wont deform the finished surface.
Something
like this is done when off axis sections are cored out of a large
finished paraboloid
in order to make smaller off axis newt primaries. Strains can be
released or created during the coring which deform part of the surface.
It's not a happy day when that happens. But I am amazed that some
current makers are able to core-away and
do it successfully as much as they do - they take every precaution. I
admire their
skill and their generous attitudes.

jerry









Dan Chaffee wrote:

On Tue, 23 Mar 2004 23:53:47 -0600, jerry warner
wrote:



TDE, zones, correction and smoothness
can all be improved with refiguring.

That would all depend on the focal length of the mirror?

Why?

Dan

jerry warner wrote in message ...
deeper mirrors (f5 and below) are harder & more expensive to refigure if
needed.
More glass to have to correct in some cases. The time and cost can be
almost as
much as doing a new mirror, and in some cases it would be easier to
start with a new pregenerated blank. Most opticians will turn those
projects down.

Yes, I know this, but the real question is 'worth it to whom'? If I
had bought a cheap f5 mirror with a lousy figure, I would have no
problem putting a good figure on it provided the aberrations weren't
resulting from strain or grinding. I have never seen aberrations polished
into a mirror that I couldn't polish out, and in less time than it would
take to polish an unpolished mirror. As for professional opticians, Steve
Swayze is known to have taking some terrible figures on fast mirrors and
turned them into excellent primaries.

Dan

and Steve also regularly turns projects down -
Thanks Dan.
jerry



Dan Chaffee wrote:

jerry warner wrote in message ...
deeper mirrors (f5 and below) are harder & more expensive to refigure if
needed.
More glass to have to correct in some cases. The time and cost can be
almost as
much as doing a new mirror, and in some cases it would be easier to
start with a new pregenerated blank. Most opticians will turn those
projects down.

Yes, I know this, but the real question is 'worth it to whom'? If I
had bought a cheap f5 mirror with a lousy figure, I would have no
problem putting a good figure on it provided the aberrations weren't
resulting from strain or grinding. I have never seen aberrations polished
into a mirror that I couldn't polish out, and in less time than it would
take to polish an unpolished mirror. As for professional opticians, Steve
Swayze is known to have taking some terrible figures on fast mirrors and
turned them into excellent primaries.

Dan