[ATM] parabolizing

Mon Mar 28 01:57:19 JST 2011

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
  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
  >>
  >>
  >>
  >
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