To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 


Lap is Willman-Bell pitch, unadulterated.  I'll try to take a picture and
post it tonight.  I did about 5 minutes last night, avoiding any pressure. 
I  appear to have dug a little hole again in the center (too much 1/3
stroke and not enough 1/4 stroke), but I was too tired to take
measurements.  I don't think I went too far.

I agree about taking small steps, so I think I will limit myself to 2
minute sessions, then let the mirror sit for an hour (I always do that
before taking measurements, though I do peek at it qualitatively right
away), then 20 minutes or so for measurement.  I can see more structure
than a 3 window couder mask can reveal, so I am tempted to make a pin-stick
and do both pin-stick and couder.

I am convinced that rouge is the way to go for final figuring -- it works
so smoothly and *slowly* that I won't go far wrong.  And it hasn't been all
that much of a cleanliness problem -- I just keep a tub of water and a bar
of soap right next to the polishing setup and wash my hands.

I think I will follow your advice and continue in small steps until one
step takes me in (hopefully only slightly) the wrong direction. If I'm
still pretty good (Strehl>0.9 perhaps?) then that'll work for this
mirror.  As I said, I've another mirror right behind it that doesn't have
anyone waiting for it, so I can take my time and really get it nice.  The
next mirror, a 4.25" f6 is practice for a 10" f6.

Thanks for your excellent advice.

=Matt

>Delivered-To: mlbrown{at}0
>From: "scottythefiddler" 
>To: "Matthew L. Brown" ,

>Subject: Re: Re: ATM Getting less edgy
>Date: Sun, 23 Mar 2003 10:40:23 -0500
>X-Mailer: Microsoft Outlook Express 6.00.2800.1106
>
>Matt,
>Your mirror is a thing of beauty.   It really is thrilling.  I am glad that
>it worked out for you, and even more thrilled to see that you have made good
>progress.   And you managed to avoid creating any zones!!  Can you post a
>pic of you lap?  What type of pitch are you using?
>
>As you have realized, your mirror is very close right now.  With 3
>two-minute spells, it has come a long way.  The mirror is changing quite
>fast.  If you can slow the amount of change with each wet, you will have
>more control over the final figure.   This is one of the toughest parts of
>ATM.  We all want to see that final parabola, and it always seems possible
>with just one more step.  If you can finish it in 4 smaller steps, you will
>have more control in that you can stop at the right time, or inch (or should
>that be nanometer) ever closer to the desired surface.
>
>You could stop right here and have a perfectly fine mirror.  There is a risk
>that the 'same stroke' might not have the desired effect, which is another
>reason to take smaller steps.  If the mirror starts heading in the wrong
>direction, you can quit and call it finished, before it gets out of hand.
>
>The question is how to achieve smaller steps.  You might limit your figuring
>spells to 'one trip around the barrel', if you are not already doing this.
>Slower strokes is another possibility.  As well, avoid any pressure on the
>mirror.  Allow the weight of the mirror, alone, to bear on the lap.
>
>Press thoroughly.   This is your only assurance that the lap will continue
>to behave in a predictable manner.
>
>One thing that has worked for me in improving a TDE is to warm the mirror
>until it is warm to touch.  Immediately put the lap on top, and press the
>cold lap for 5 to 10 minutes before starting.  As the mirror cools, the edge
>cools faster than the rest of the blank, and as a result, it contracts away
>from the lap, which results in less glass being removed at the edge.  This
>helps to 'protect' the edge, and ultimately helps to correct a TDE if given
>enough time.   At this stage of the game, you are probably not too concerned
>about the edge.  This procedure might be considered as 'protection', rather
>than 'correction' at this point.  Note that it may take several hours after
>polishing for the edge to expand back to its' equilibrium position, so any
>improvement may not be apparent in the initial test immediately following a
>figuring spell.
>
>Best of luck,
>sd
>
>
>----- Original Message -----
>From: "Matthew L. Brown" 
>To: 
>Sent: Saturday, March 22, 2003 11:55 PM
>Subject: Fwd: Re: ATM Getting less edgy
>
>
> >
> > Scott,
> >   You were completely right.  I did about 3 two-minute sessions 1/4 to 1/3
> > diameter, wide W, and it walked right toward a sphere.
> > You can see the latest Foucaultgram at:
> >   http://host67.everstrive.com/~mlbrown/ASTRONOMY/Foucaultgram22Mar8pm.jpg
> > The image shows more TDE than my eye says, and I can see a slight
> > diffraction ring on the KE side that isn't in the photo.
> > Ronchi shows the straightest lines I've ever seen, with a little apparent
> > TUE followed by just the slightest hint of TDE.
> >
> > I did some measurements, using the 3 zone mask I'd already made.
> > Sixtests says Strehl 0.862 RMS 16.8.
> >
> > Tex says 1/4.6 wave:
> >            Comments:    Optical diameter: 4.15
> >    Readings per zone: 4
> > Radius of curvature: 76.625
> >                  f/D:  9.23
> >     Diffraction disc: 0.00024328
> >
> >   1 ZONE                1          2          3
> >   2 h(x)              1.1980     1.6942     2.0750
> >   3 h(m)              0.5990     1.4461     1.8846
> >   4 hm**2/R           0.0047     0.0273     0.0464
> >   5 hm/4f             0.0039     0.0094     0.0123
> >   6 D1                0.1445     0.1507     0.1205
> >   7 D2                0.1445     0.1507     0.1205
> >   8 D12               0.1445     0.1507     0.1205
> >   9 D12 -  0.0956     0.0489     0.0552     0.0249
> > 10 Lamda c           0.0443     0.0279    -0.0214
> > 11 Lamda f * 1e5      17.30      26.33     -26.33
> > 12 Lamda f / rho      0.711      1.082     -1.082
> > 13 u * 1E6            -4.52      -6.87       6.87
> > 14 Wavefront          -3.34      -4.68       0.00
> >          Reference parabola: y = -1.44071 * x**2 + 0
> >          passing through (  0.00,  0.00) and (  2.08,  0.00)
> >
> > Maximum wavefront error = 1 / 4.6 wave at zone 2
> >
> > Results for individual sets of readings:
> >          Reading set 1: 1 /  4.6 lamda, Max Trans Abberation=1.16
> >          Reading set 2: 1 /  4.8 lamda, Max Trans Abberation=0.99
> >          Reading set 3: 1 /  4.6 lamda, Max Trans Abberation=1.05
> >          Reading set 4: 1 /  4.5 lamda, Max Trans Abberation=1.12
> >
> > So I plan to do a little more of the same wide stroke, to move it a little
> > more toward a parabola.  I'm not planning on perfection on this one -- I
> > really need to get it aluminized and in the mail so the scope can be made
> > while Jupiter and Saturn are still easily visible.  The 1/4 wave
> > measurement says I'm done in that regard, but I don't really trust those
> > numbers, not when I can still see that TUE.  I think I'll take it to
> > something like 1/8 lambda by measurement to know for sure.
> >
> > I'll be a perfectionist on my next mirror (a 4.25" f-6
waiting for polish
> > now) .
> >
> > =Matt
> >
> >
> > >Delivered-To: mlbrown{at}0
> > >From: "scottythefiddler" 
> > >To: "Matthew L. Brown" 
> > >Cc: "ATM" 
> > >Subject: Re: ATM Getting less edgy
> > >Date: Fri, 21 Mar 2003 12:02:59 -0500
> > >X-Mailer: Microsoft Outlook Express 6.00.2800.1106
> > >
> > >Matt,
> > >
> > >the hole in the center is a sign that you are reducing the hill.
>Further,
> > >the edge is better than it was in the first focogram you showed.
> > >
> > >Right now, the mirror is oblate...i.e. it has 'correction', but in the
>wrong
> > >direction.
> > >Try a parabolizing stroke, MOT, wide side overhang, 1/4
diameter, and the
> > >same overhang top and bottom.  Do 3 minute spells.  Expect the edge to
>start
> > >looking worse, before it starts looking better.  With MOT, the figuring
> > >action may take place on the outside of the TUE, which will
make the TDE
> > >appear to getting worse.  The TDE will will start to improve
when the TUE
>is
> > >almost gone.  It might take 10 or more 3 minute spells to get to a
>sphere,
> > >or it might take 2!  Take it in small steps, and you won't
overshoot your
> > >target.  Bear in mind that it might take 15 minutes to get rid of half
>the
> > >TUE/TDE, and then 2 minutes to finish it off.....
> > >While figuring a 10" mirror, it took 7 steps to reduce
the abberation in
>the
> > >middle zones by about .040", and then 1 step to reduce
it the last .015".
> > >The key is to take measurements after each step, so that you
can see your
> > >progress.   As long as there is progress, continue with the
same stroke.
> > >
> > >Best of luck,
> > >Scott Donaldson
> >
> >
> >
> >
> >

--- BBBS/NT v4.00 MP