[ATM] Rechanneling, Malacara

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Bob May wrote:
> My choice for rechanneling is to just go and put the lap in some
> hot water until it gets soft and then take a metal stick and
> press it rapidly into the pitch, rocking as needed to get all of
> the surface grooved. Press the mirror onto that lap and go for
> it.  When polishing, a crude lap will do fine and produce a nice
> surface.

The method mentioned above, and the method mentioned earlier that used 
a heated bar of metal to melt grooves into the pitch, both work.  I 
have used both.

The methods involving pressing things into the pitch will work, but 
they distort the lap and this requires pressing time to correct.

There is also a very real possibility of introducing contamination 
into the pitch by pressing objects into it.  I worry about this 
especially with pieces of metal.

I find that the method involving the least work (for me) is to simply 
carve with a razor blade.  Once you get good at it you will devise a 
method to keep the mess to a minimum.  I have a dedicated enclosed 
desk for carving laps that keeps most of the chips in it.

Bob May wrote:
 > Malacara writes more for the professional than the amageur.  As
 > such, he is thus not really that interested in doing surfaces
 > other than spherical as only reflector telescopes tend to use
 > other than spherical surfaces..  We amateurs are off in a
 > different world for a bit of this with our strange surfaces so we
 > adapt interferometers to our strange needs and do such tests as
 > the Ronchi and Foucault tests.

I disagree with that - it may appear that way on the surface, but it 
is not.  Amateurs generally use a very narrow set of tests while 
Malacara must cover those tests AND all of the other types of useful 
optical tests.  So, that makes our Foucault and Ronchi tests appear 
underrepresented.

Malacara and professionals care quite a bit about aspheric surfaces. 
If anything, Malacara highlights many ways to make the testing of 
aspheric surfaces easier.

While the percentage of Malacara's book dedicated to Foucault/Ronchi 
testing is relatively small, the testing of flats and windows is 
higher.  However, relatively few ATMs ever attempt to make or fix such 
optics.  I find that about half of the secondary mirrors I test do not 
satisfy reasonable standards.  Think about that after you have 
sacrificed blood, sweat and tears on your primary mirror.

	Mike Lockwood

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