Re: ATM Spherizing after rough grinding : when tp stop

[astillma@nit.AirTouch.COM (Anthony Stillman)]

Guillaume,


I assume that you have used a coarse grip to hog out the glass, say 80 grit.  The next n 
steps are designed to remove the pits and scratches you put into the glass with the 80 
grit, and oh yes, make it more or less a sphereoid.  For a 6" you will need to grind for 
one hour at 120 and 220.  For 320 and above 45 minitues should be enough.  I usually go a 
bit longer than the recomendation just to be on the safe side.  You can also look at the 
pits with a 50x glass and /or measure them with a reticle.  Pits tend to be 1/3 to 1/4 
the grit particle size.  If there are a lot of pits larger than this grind a bit more at 
the grit you are on.  Don't calculate the grit size, measure it.  Grit size varies a 
great deal.  I find I can't read pit size above 400 and just go by time.  Incidently, 
that time, is time spent grinding, not getting a cold drink, not changing grit, not 
adding water, not rubbing sore muscles, not answering the phone,... 

Some people dig a scratch in the glass that they know is a bit deeper than the pits they 
have, others use permanent markers to mark the bottom of the pits they have.  I've never 
done either,  I'm not sure I could bring myself to intentionally scratch the glass.

As for spherocity (I've probably spelled that wrong), the best way to test this is to use 
a spherometer in the center and at the edge (hard with a six inch, since your average 
amature model has a 2 inch radius) another way is to look through the mirror (wet it) at 
the air bubble which forms at the center while your are grinding.  It will get smaller as 
you become more spherical.  You'll be suprised how sensitive this is.  But facts be told 
if you work center over center with a 1/3 stroke and flip the mirror and tool now and 
again and keep turning your mirror, even when its on the bottom, you'll get a good 
sphere.

As for the questions, telescope making is a noble venture, and most TNs (Telescope Nuts) 
are glad to help.  If you are working by your self as it seems, my hat is off to you.  I 
started this way, and followed "Telescope Making" to the letter.  Boy, if I knew then 
what I know now...

anyway
Good Luck

Anthony 

PS Remember the most important thing in mirror making, is sticking with it.  Also, after 
completing your first mirror, you are automatically an expert and entitled to all the 
rights and privleges afforded.