[ATM] Fw: parabolizing

Mon Mar 28 10:50:11 JST 2011

 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 
> >>> 
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