Re: [ATM] Curious

["Donald Good" <donald.good@comcast.net>]

I found 2 videos on the YouTube ATM group
http://youtube.com/group/telescopemaking
that appear to match your description.  They are:
Polishing of a 6" amateur telescope mirror at
http://youtube.com/watch?v=pLMfa3KhmH4
and
Desbaste Fino do Espelho do Telescópio at
http://youtube.com/watch?v=R1Ux0e7k_KI

Texereau describes rough grinding (hogging) as 4/5 or 5/6 diameter stroke
COC 5 to 10 times (cycles or double strokes) then turn and rotate.  Fine
grinding is 1/3 diameter stroke narrow W 5 or 6 times then turn and rotate.
I made my mirror in a class that used this method.  It turned out very good.
Of course a mirror grinding machine is constantly turning and rotating as it
strokes, so your method probably does not hurt unless the frequent stopping
has a lasting effect.  Texereau states that stopping at the edge (end of
each stroke) should be avoided.  It may be that stopping after each cycle
imprints a pattern.

Also that much turning and rotating is probably very tiring.  And it must
slow things down, too.  (stroke, stroke, turn, rotate) instead of (stroke,
stroke, stroke, stroke, stroke, stroke, stroke, stroke, stroke, stroke,
turn, rotate).

About the video that you watched; the "simulated" walk around the barrel has
a problem.  Since it is seen in two different independent videos, it may be
an actual method that has developed (mayby for tight working spaces) and
passed on.  The problem I see is it appears that from the maker's point of
view, everything is turned one direction (say angle a1) and then the mirror
is turned the other direction (say angle a2) with the result that the mirror
has turned a1 - a2 from the maker's point of view.  It appeared that a1 and
a2 were about the same.  That means that even though the tool turned under
the mirror, the mirror was being stroked in approximately the same direction
for a long time.  That has to cause astigmatism.

Don

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Neil Booker
Sent: Tuesday, January 22, 2008 2:29 PM
To: Francis J. O'Reilly; AMT LIST
Subject: Re: [ATM] Curious

Hi Francis

> Neil,
>
> Thank you for watching my videos!
>
> My question to you is how have your two mirrors turned out?

Well they're round and concave. First attempt was a six inch F10. Got the
kit (blank, tool, grits etc.) then said what's first? Bevelling the edge. So
put a bit of a bevel on blank and tool and asked what's next? Hogging out.
So started hogging out only to look down and see that the supplier had
bevelled the other side of the blank and the tool already. Things haven't
improved much since then. I couldn't get the hang of the Foulcault test at
all so when it looked a bit like the diagrams I thought 'stuff it' that's
near enough. I now know that's was wrong and have since learned, from here,
that at F10 once you can see the shadows you've gone too far.

Second attempt is a six inch F5 (because I'd heard that the shadows were
easier to see in a short focus mirror), using the tool from the previous
mirror as the blank, that came out F5.5 as I got confused as to the number
of inches in half a foot (no I'm not sure how I managed that either) Got the
hang of the Foulcault test though but couldn't get rid of the TDE (maybe
third time lucky). Haven't managed to get the time and the inclination to
coincide and finish the latest scope so haven't used it in anger yet.

> You might consider joining a mirror making group.

Have done but cant find any near by. Heard of one three hundred miles away
in north London.

> Stellfane has a list of
> worldwide mirror making groups at their website, www.stellafane.org 
> look
on
> the webpage for the mirror making class.

Thanks I'll have a look at that.

Neil

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