[ATM] Fw: parabolizing

Richard Schwartz richard1941 at gmail.com
Tue Mar 29 01:28:19 JST 2011 

A better support is two pieces duct tape at 50% zone on a horizontal
diameter.
On Mar 27, 2011 8:01 PM, "Mel Bartels" <mbartels at bbastrodesigns.com> wrote:
> As discussed many times on the forum, the benefit of two supports at
exactly
> 90 degrees as compared to other edge supports is that astigmatism is
> cancelled. The bending in one point is blocked by the bending at 90
degrees
> from the other point.
>
> Try to bend a piece of paper into two cylinders at 90 degrees - it can't
be
> done. See Wilson Reflecting Telescope Optics II page 264, and Schwesinger.

>
> For an amateur calculator, see
> http://www.cruxis.com/scope/mirroredgecalculator.htm
>
> Mel Bartels
>
>
>
>
> -----Original Message-----
> From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf
Of
> Jerry Hillman
> Sent: Sunday, March 27, 2011 7:33 PM
> To: Guy Brandenburg
> Cc: atm at atmlist.net
> Subject: Re: [ATM] Fw: parabolizing
>
> 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
,ak66666
> 6 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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