[ATM] Fw: parabolizing

Jerry Hillman truckeratm at sbcglobal.net
Mon Mar 28 11:32:38 JST 2011 

Dale,
I don't entirely agree, and Bob May devoted a web page to the pictorial proof that I supplied concerning a pair of pegs supporting the mirror and a plop cell supporting it.  I just checked Bob Mays web site and the web page is still there after all these years.
Jerry B. Hillman

--- On Mon, 3/28/11, Guy Brandenburg <gfbrandenburg at yahoo.com> wrote:

> From: Guy Brandenburg <gfbrandenburg at yahoo.com>
> Subject: [ATM] Fw:  parabolizing
> To: "atmlist" <atm at atmlist.net>
> Date: Monday, March 28, 2011, 1:50 AM
> 
>  Guy Brandenburg, Washington, DC 
> http://gfbrandenburg.wordpress.com/
> http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
> ============================
> 
> 
> 
> ----- Forwarded Message ----
> From: Dale Eason <atmpob at yahoo.com>
> To: Guy Brandenburg <gfbrandenburg at yahoo.com>
> Sent: Sun, March 27, 2011 9:05:53 PM
> Subject: Re: [ATM] parabolizing
> 
> Guy,
> 
> I'm sorry to say that is not quite true about the 2
> pegs.  In fact if the two 
> pegs are 90 deg apart as measured by the radius lines to
> each peg and if they 
> support the mirror on the center of gravity of the edge
> then the mirror top of 
> the mirror will not flop over as you describe.  That
> has been shown to be the 
> case many times over.  It is what I use when doing
> interferometery on thin 
> mirrors.  With interferometry one can easily see the
> astig that is caused by 
> other support methods or even incorrectly application of
> the two peg support 
> when not on the center of gravity of the mirror.
> 
> Many times with the larger mirrors it is not possible to
> lean them back far 
> enough for the PLOP designed cell to start working. 
> That is because 2 times the 
> focal length is much too long and coincides with a point up
> past the ceiling.  
> 
> 
> Dale Eason
> 
> --- On Sun, 3/27/11, Guy Brandenburg <gfbrandenburg at yahoo.com>
> wrote:
> 
> > From: Guy Brandenburg <gfbrandenburg at yahoo.com>
> > Subject: Re: [ATM] parabolizing
> > To: "Paul A. Valleli" <valleli at rcn.com>,
> foreilly at bestweb.net,
> "surinder singh" 
> ><surindersingh1938 at yahoo.com>,
> foreilly at verizon.net,
> atm at atmlist.net, 
> >ak666666 at hotmail.com
> > Cc: PParkerrs at cs.com,
> mghasemi at nigceng.ir,
> Shane at ct-astropnomer.com
> > Date: Sunday, March 27, 2011, 6:09 PM
> > I was reminded by someone that it's
> > really important with a thin mirror like 
> > this to support it properly when testing. If you just
> prop
> > it up on two pegs, as 
> > we do with a full-thickness mirror, then the top of
> the
> > mirror will sag forward 
> > and you will see astigmatism. The only way out is to
> lean
> > the mirror back some 
> > 20 to 35 degrees from the vertical and to use PLOP or
> its
> > equivalent to design a 
> > multi-point cell, with a sling. 
> > The good thing about this is that this
> multi-point/sling
> > cell will work just 
> > fine in the telescope as well, once it's all done.
> >  Guy Brandenburg, Washington, DC 
> > http://gfbrandenburg.wordpress.com/
> > http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
> > ============================
> > 
> > 
> > 
> > 
> > ________________________________
> > From: Paul A. Valleli <valleli at rcn.com>
> > To: Guy Brandenburg <gfbrandenburg at yahoo.com>;
> > foreilly at bestweb.net;
> > surinder 
> > singh <surindersingh1938 at yahoo.com>;
> > foreilly at verizon.net;
> > atm at atmlist.net; 
> > ak666666 at hotmail.com
> > Cc: PParkerrs at cs.com;
> > mghasemi at nigceng.ir;
> > Shane at ct-astropnomer.com
> > Sent: Sun, March 27, 2011 12:57:19 PM
> > Subject: Re: [ATM] parabolizing
> > 
> > 
> > Surinder,
> > Inaddition to the recommendations of the other
> experts:
> > 1. rotate mirror and tool frequently
> > 2. use carpet cut into 5 to 8 cm pads and arranged in
> > concentric circles. They 
> > can be squares, trapezoids, or circles.
> > 3. The support backing plate should be reasonably flat
> and
> > stiff to 0.25mm 
> > (0.010-inch)   - if using plywood,
> check it
> > with a straight edge and sand down 
> > any high points.  If more glass is available for
> a
> > support plate, stick two 
> > disks together with buttons (spots) of pitch
> > 4. Thin mirrors are much easier to make with less
> problems
> > with astigmatism with 
> > mirror on top - MOT
> > 5. Parabolizing is fastest using a 1/3rd diameter
> (6-inch)
> > pitch lap on top -TOT
> > 6. 4&5 are contradictory, but if you rotate both
> tool
> > and mirror frequently ( 
> > not fast, just often)
> > then you will achieve an F/5 paraboloid much faster
> than
> > MOT
> > 7. Be sure you have made a true sphere first, checking
> with
> > the Foucault Knife 
> > Edge Test before starting to parabolize. The Ronchi
> test is
> > harder to detect 
> > astigmatism.
> > Good Luck and Best Wishes for success,
> > Starman Paul
> >  
> >  
> >  
> > ---- Original message ----
> > 
> > 
> > >Date: Sun, 27 Mar 2011 07:26:21 -0700 (PDT)
> > >From: Guy Brandenburg <gfbrandenburg at yahoo.com>
> > >Subject: Re: [ATM] parabolizing
> > >To: foreilly at bestweb.net,surinder
> > singh 
> >><surindersingh1938 at yahoo.com>,foreilly at verizon.net,atm at atmlist.net,ak666666 at hotmail.com
> >m
> > >
> > >Cc: PParkerrs at cs.com,mghasemi at nigceng.ir,Shane at ct-astropnomer.com
> > >>Hello, Surinder,
> > >>
> > >>I myself am working on a 16.5" (41 cm) mirror
> > (probably Pyrex or something like 
> > >
> > >>it) about 20 mm thick, so I'm doing something
> > similar. I'ts a very big job for 
> > 
> > >>me, and yours is even bigger.
> > >>
> > >>I agree with Francis' suggestions about being
> sure
> > to support the mirror 
> > >>properly. A piece of clean carpeting can help,
> or
> > else many sheets of clean 
> > >>newspaper. However, it will be very very
> important
> > to rotate the mirror 
> > >>periodically (say, every two rotations of
> your
> > mirror, or every 4 or 5 minutes 
> > 
> > >>of polishing and/or figuring) with respect to
> the
> > carpeting or newspaper, in 
> > >>order to prevent astigmatism from forming.
> > >>
> > >>It's not always needed to use a full-size
> pitch
> > lap. You may consider making a 
> > 
> > >>smaller one as well, say, 10 inches (25cm) in
> > diameter, which is about what I am 
> > >>
> > >>using. 
> > >>
> > >>Try a variety of different strokes with this
> > smaller tool and carefully note 
> > >>their results. By that, I mean the following:
> > >>
> > >>* center over center with TOT, with the center
> of
> > your tool going to within 3" 
> > 
> > >>of the edge; within 2" of the edge; within 1"
> of
> > the edge.
> > >>* similar strokes but with a W stroke that
> has
> > different side-to-side amplitudes 
> > >>
> > >>and different lengths.
> > >>* a figure-8 stroke of various amplitudes and
> > lengths.
> > >>* an oval stroke of various amplitudes and
> > lengths.
> > >>* all of the above strokes, only changing to
> Mirror
> > On Top.
> > >>
> > >>All of these strokes will have different
> effects.
> > It's going to take a lot of 
> > >>careful experimentation and taking of notes
> in
> > order to discover techniques that 
> > >>
> > >>work effectively for you. Be sure to work in
> > increments of a full revolution, or 
> > >>
> > >>you will cause astignmatism as well.
> > >>
> > >>Since your mirror is, I guess, not a
> low-expansion
> > glass, then you may need to 
> > 
> > >>have a tub of room-temperature water handy in
> which
> > you can immerse your mirror 
> > >
> > >>for a few minutes, so that its temperature
> will
> > equilibrate to that of the room, 
> > >>
> > >>before you test it. The reason for this is
> that all
> > of the rubbing and friction 
> > >
> > >>we cause by polishing can heat up various
> regions
> > or zones on the mirror. Be 
> > >>sure to wipe the mirror dry as fast as
> possible
> > once you take it out of the 
> > >>water, particularly if you are in a very
> > low-humidity climate or season of year, 
> > >>
> > >>or else its surface will be cold - and hence
> > distort your ronchi readings in a 
> > 
> > >>different direction.
> > >>
> > >>Good luck, and keep us posted!
> > >> Guy Brandenburg, Washington, DC 
> > >>http://gfbrandenburg.wordpress.com/
> > >>http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
> > >>============================
> > >>
> > >>
> > >>
> > >>----- Original Message ----
> > >>From: "foreilly at bestweb.net"
> > <foreilly at bestweb.net>
> > >>To: surinder singh <surindersingh1938 at yahoo.com>;
> > foreilly at verizon.net;
> > 
> > >>atm at atmlist.net;
> > ak666666 at hotmail.com
> > >>Cc: PParkerrs at cs.com;
> > mghasemi at nigceng.ir;
> > Shane at ct-astropnomer.com
> > >>Sent: Sun, March 27, 2011 9:17:32 AM
> > >>Subject: Re: [ATM] parabolizing
> > >>
> > >>
> > >>
> > >>
> > >>Surinder,
> > >>An eighteen inch plate glass blank 3/4 of an
> inch
> > thick
> > >>is a very challenging project. I understand
> that
> > 19mm thickness is a
> > >>standard size throughout Europe and Asia (and
> > probably the rest of the
> > >>world outside North America) St. Gobain is an
> > excellent manufacturer with
> > >>plenty of experience in telescope optics over
> the
> > last 100 years. The
> > >>project is not impossible, but difficult. Was
> the
> > blank designerd for
> > >>telescope purposes or converted from another
> use
> > such as a ships
> > >>porthole?
> > >>In 1862 Henry Draper made a three fifteen and
> a
> > half inch
> > >>mirrors an inch thick from porthole glass.
> Some
> > friends and I had the
> > >>opportunity to test it recently. My friend who
> led
> > the testing came to the
> > >>conclusion based upon reports of Draper's
> > observations that the mirror we
> > >>tested was not the one he used for research
> > purposes. The figure was lousy
> > >>showing astigmatism roughness and a severe
> turned
> > edge. The good news is
> > >>that you can probably do much better.
> > >>Some tips that I have learned
> > >>from people along the way about very thin
> large
> > diameter mirrors. I have
> > >>never attempted such a mirror and I probably
> never
> > will. It is just too
> > >>challenging for me. Based upon where you are
> > located, it may be the only
> > >>glass available.
> > >>Proper support is critical at every stage of
> > >>manufacture. One friend strongly suggests
> using
> > carpet under the mirror at
> > >>all phases for support.
> > >>If the glass is not properly annealed, you
> > >>will have nothing but frustration. Try to
> ascertain
> > through the use of
> > >>polarized light if there is any strain in the
> > glass. A Google search
> > >>should help to show the proper tools and
> > methodology for the test.
> > >>I
> > >>am forwarding this email to some friends for
> their
> > comments. They may also
> > >>have some suggestions.
> > >>I am aware of the political situation between
> > >>India and Pakistan, however I believe that
> there is
> > a small group of
> > >>telescope makers in Lahore. If you are
> interested,
> > I can reach out for
> > >>them and try to arange for some further hands
> on
> > assistance for your
> > >>project.
> > >>Best,
> > >>Francis
> > >>
> > >>> Dear sir, Iam
> > >>pleasently surprised at so prompt a reply
> .Thank
> > you very 
> > >>>
> > >>much.I am sure you will consider it foolhardy
> on
> > reading the vitals of the
> > >>
> > >>> mirror.It is Saint Gobain plate glass of
> 19 mm
> > thickness,fratio
> > >>is f5, 
> > >>> pich is what i get here in India;it is
> soft to
> > bite
> > >>in,though it has not 
> > >>> flowed to close the channels.As it has
> > >>given a good 
> > >>> sphere on the normal recommended
> strokes,I
> > feel it
> > >>is ok. Thichness of the 
> > >>> pich is 1cm and the polishing compound
> > >>is cerium oxide 
> > >>> II am going to work on the mirror as per
> your
> > >>recommendations and will 
> > >>> report back on the progress 
> > >>>
> > >>I live in Mohali near chandigarh.Interes in
> > astronomy is non exitant in
> > >>
> > >>> this part of the country. Most of the
> > enthusiast are from the
> > >>south of the 
> > >>> country like Banglore. S chander shekhar
> of
> > the
> > >>chandershekhar limit and C 
> > >>> V raman were from the south 
> > >>> Thank you again Surinder Singh 
> > >>> 
> > _______________________________________________
> > ATM mailing list http://www.atmlist.net/
> > 
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