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Re: First mirror!

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  • John
    Ronchi is fine for checking spheres and for checking paraboloids via a null test as it s fairly easy to see that lines are straight. On the other hand the same
    Message 1 of 69 , Mar 4 2:00 AM
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      Ronchi is fine for checking spheres and for checking paraboloids via a null test as it's fairly easy to see that lines are straight. On the other hand the same straight fringe test can be done with a Foucault tester so why bother. It's better to spend the effort on a decent tester.

      The problem most people seem to have using the foucault test on a spherical mirror is that once it really is spherical a null needs very delicate knife edge movement. As that is needed for later testing anyway it's good thing to practice on.

      There is only one book that goes into Foucault testing reasonably thoroughly as far as I'm aware. It can be downloaded for free here

      http://archive.org/details/HowToMakeATelescope

      Info on all sorts of aspects of the test can be found on the web. One that always amuses me is slitless testers to try and avoid the shadows being too deep and dark. Well the book explains how to set up a slit so that that sort of thing is less of a problem. It also explains how to get a Foucault tester to produce fringes that can be used to check spheres. There are also instructions on making the kit needed. It doesn't need much in the line of tools. There isn't any need to copy it slavishly only the general principles. I use an elastic band rather than a spring and fitted a micrometer spindle rather than use a screw. I also adjust the tilt of the slit rather than the knife edge. It's a very well thought out design even down to being able to remove the slit to make setting up easier. :-) Lacking a Ramsden eyepiece I use a 5x or 10x eyecup type loupe.

      You can of course use a block carrying the knife edge and marks on a piece of paper for many mirrors. It's still worth looking at the book and using the same style of light source he uses maybe even illuminated with a led. The method of making it and adjusting it are the important points. Aluminium should be suitable as an alternative to brass and probably easier to obtain.

      There is another book on there by some one called Howard that has some useful ideas on improving and making laps too. I find channels are best cut with a saw though.

      John


      --- In atm_free@yahoogroups.com, starguy@... wrote:
      >
      > Mike,
      >
      > My recommendation for the polishing and early parabolizing stages is to build a Ronchi tester. You can get and inexpensive Ronchi grating from Willman-Bell and the rest (LED, battery holder, variable resistor, etc) from Radio Shack. The Ronchi is much easier to set up and you can easily see what is going on with your mirror. Once you get things close (parabolically speaking) then use a Foucault or even build a Bath Interferometer.
      >
      >
      > Greg Furtman
      > 27426 County Rd H
      > Webster, WI 54893
      >
      > http://home.centurytel.net/starguy/
      >
      > "My goal in life is to become the person my dog thinks I am.
      >
      > From: Mike Malone
      > Sent: Monday, February 25, 2013 2:02 PM
      > To: atm_free@yahoogroups.com
      > Subject: Re: [atm_free] First mirror!
      >
      >
      > I'm a little bit ahead of you on my first mirror and it is very exciting. I have a 6in Pyrex blank that I've ground to about f/7 and am close to polishing it out. I think. I'm now trying to build a Foucault tester, but that in itself is a whole new set of skills that I'm trying to learn. I understand what I should see with the tester but I'm not sure what kind of a light source I need to use. Any help is always appreciated. Will post more later when I figure it out. Mike
      >
    • Jerry
      I usually use a fixed knife, moving light source on my Foucault tester. I have played with having them move together, both with slit and slit-less sources, but
      Message 69 of 69 , Mar 15 7:54 AM
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        I usually use a fixed knife, moving light source on my Foucault tester.
        I have played with having them move together, both with slit and slit-less
        sources, but I prefer the moving slit source and fixed knife.
        Slit or slit-less can be done either way, but of course a slit-less tester
        with one fixed edge can't use the same edge for the half slit edge and the
        knife.
        With the use of a small mirror I was able to use a halogen lamp for the
        source and still get the slit and knife to 1/4" separation. Now with a
        bright LED it is much easier to keep them close.

        What John says about making it so you can easily get your eye to the knife
        is important, and it is not difficult to do. In this regard, consider that
        you have a "sight eye" that is the eye you will use for testing. There are
        left eye testers and there are right eyed testers. Unfortunately there are
        also cyclop testers that place the knife in the middle of a nose board that
        makes it impossible to get your eye to the knife without bumping your nose
        into the tester. If you are right eyed the knife should be to the left side
        of the tester. If you are left eyed the knife should be to the right side of
        the tester.
        Whichever side you put the knife on consider ergonomics of getting your hand
        to adjusting screws while your eye is at the knife. If you can't turn the
        screws and look at the same time you will have a hard time testing.
        Comfort means a lot. You will spend quite a while behind the tester to do
        precision testing. Every discomfort is a temptation to sacrifice precision
        of the readings for speed of getting the readings.

        With either knife or light source fixed it is worthwhile to incorporate some
        mechanical method to keep the knife and slit parallel as one is moved rather
        than having the typical tilt in/out movement that swings the knife into the
        light returning from the mirror.

        Some weight is a good thing, to keep the moving parts firmly seated on the
        journals and to reduce movement when you lightly touch the screws that move
        the knife or source.

        Don't forget that a good solid footing for the bottom of the tester is
        important. Three widely separated solid feet on the bottom will work. Of
        course the stand between floor and tester should be stable.

        Alignment for testing is important. The knife and slit should parallel to
        each other and squared to the direction of movement toward the mirror. The
        entire tester should be adjusted (pitch and yaw) so that the movement of the
        knife or slit (whichever one moves) is aligned as precisely as possible to
        the optical axis of the mirror.
        Source and knife should be as close together as is reasonable. For a
        slit-less tester the return beam to the knife should be as close to the
        source as is reasonable.
        If you fix either the source or knife the tester should be positioned to
        have knife and slit at very nearly the same distance from the mirror (beside
        each other).

        Most of all this is no more complicated than thinking about how to arrange
        the part that you put your eye to (the knife) and the x,y movement parts so
        that you can operate the tester with your eye comfortably at the knife.


        Jerry

        > -----Original Message-----
        > From: atm_free@yahoogroups.com [mailto:atm_free@yahoogroups.com]
        > On Behalf Of John
        > Sent: Friday, March 15, 2013 5:51 AM
        > To: atm_free@yahoogroups.com
        > Subject: [atm_free] Re: First mirror!
        >
        > This thing that gets me about those testers is that it's all sort of the
        wrong way
        > round which may be why the telescope is added. It would be more sensible
        to
        > base the tester round an L shape with the foot of the L facing the mirror
        plus
        > "webbing" on that to stiffen up the vertical. Then mount the knife and
        source
        > on the face nearest the operator or nearly so. The general idea is to be
        able to
        > get the eye close to the knife. The nice thing about the set up is that
        the source
        > and knife can be very near each other. One video on youtube doesn't make
        > much sense in that respect. The only bad aspect is that when slit and
        source
        > move together the measurement distances will be halved. On the other hand
        > that Welland book I mentioned shows that his tester moves the knife and
        > source together so I suspect it's a more precise measurement done that
        way.
        > He also mentions that the image of the slit should return precisely to the
        knife
        > edge implying that they need to be dead level with each other. The
        > arrangement in the video's ensures that. Old book as he probably retired
        in
        > the 70's but it's worth bearing in mind he worked on observatory
        instruments
        > and wrote it for others forced to do the the same thing the same way.
        >
        > As to the 1/2 ing of the measurements the easiest thing would be to feed
        some
        > fictitious perfect figures into foucault software and then change them by
        the
        > measurement accuracy you can achieve. Ideally you need a situation of
        being
        > able to measure to at least twice the accuracy that is wanted on the final
        > mirror. I doubt if you will have any problems in that direction. Fast
        mirrors
        > might. Welland measured too 0.0001in and really did make some rather fast
        > mirrors.
        >
        > It's interesting to see some one add another spring as the set up was
        > unbalanced - Texereau's extra weight in the right place is a more sensible
        > option.
        >
        > John
        > -
        >
        > --- In atm_free@yahoogroups.com, "malonemike511" <malonemike511@...>
        > wrote:
        > >
        > >
        > >
        > >
        > >
        > >
        > > Thanks John, I watched the youtube video and even though it was in
        > > Spanish I understood everything he was talking about. My base is
        > > almost a carbon copy of his. I already have a 0.0005 dial indicator
        > > and the light source and knife edge I have is a lot like his. I sanded
        > > the dome off of a spare 5mm green LED today and tomorrow will
        > > incorporate into my tester. I've followed Texeraeu to the letter so
        > > far and am pleased with my results. Next I'll finish polishing making
        > > sure I maintain my sphere and then onto figuring. Mike
        > > --- In atm_free@yahoogroups.com, "John" <a.johnw@> wrote:
        > > >
        > > >
        > > >
        > > > There is an easy to make slit and light source shown here
        > > >
        > > > http://www.youtube.com/watch?v=92_G40ijP-E
        > > >
        > > > and setting that sort of thing up here
        > > >
        > > > http://www.youtube.com/user/carluchoparis
        > > >
        > > > The source and knife edge can be very close this way.
        > > >
        > > > One shows a Plossl eyepiece and a microscope eyepiece being used as a
        > telescope - if needed which in your case I doubt. Halve a cheap plastic
        > binocular could also be used.
        > > >
        > > > These testers look far too chunky to me where as the one linked to
        here is
        > probably better for slit and knife edge moving together. The slit idea is
        sensible
        > though.
        > > >
        > > > Not much in this line on youtube and one person who followed this
        design
        > had balance problems - didn't add a bit of metal for the adjustment screw
        to
        > work against and probably didn't realise that micrometer spindles are
        often
        > extremely cheap on ebay. A dial guage can be used too of course - expense
        put
        > me off.
        > > >
        > > > One shows a bit of copper pipe used for the main slide but the same
        bits of
        > angle to run on it. Texereau's V's only need a junior hacksaw and a bit of
        > metal. Aluminium would be as effective as brass.
        > > >
        > > > I built my tester etc on a board somewhat larger than it needed to be
        so
        > that I could clamp it in place with G clamps.
        > > >
        > > > There are a few rather poor videos about on making mirrors. Noticing
        the
        > comment about sun etc if you follow Texereau's grinding and polishing
        > methods the chances are that by the time you have polished the mirror
        fully
        > out it will be a perfect sphere. One thing that helps with that is short
        varied
        > types of strokes in the finer stages of grinding and when polishing -
        mainly
        > with the tool on top. Also a lot easier with a full sized tool. Some work
        like that
        > is wise at the end stages of the coarser grits too.
        > > >
        > > > John
        > > > -
        > > >
        > > > --- In atm_free@yahoogroups.com, Mark Whitaker <tnut55@> wrote:
        > > > >
        > > > > That should be fine, I wouldn't go buy another one.  Unless you
        find
        > one of those LED's with a built in resistor, you will still need them. 
        The tester
        > described at stellafane.org gives the calculation example.
        > > > >
        > > > > The thread got so confusing, I don't remember what problem you were
        > having.  If you measure the sun at 42 inches, you need to place the
        mirror
        > approximately 84 inches from your tester for it to work.
        > > > >
        > > > > Mark
        > > > >
        > > > >
        > > > >
        > > > >
        > > > > ________________________________
        > > > > From: malonemike511 <malonemike511@>
        > > > > To: atm_free@yahoogroups.com
        > > > > Sent: Tuesday, March 12, 2013 6:15 PM
        > > > > Subject: [atm_free] Re: First mirror!
        > > > >
        > > > >
        > > > > Â
        > > > > Mark, The green LED I have is 5mm, 2.1 volt, 30mA, 630mcd. I guess
        > > > > thats almost twice as bright as I need. I'll check Radio Shack
        > > > > tomorrow. With the 350mcd LED I guess I wont need any resistors?
        > > > > Mike
        > > > >
        > > > > --- In atm_free@yahoogroups.com, Mark Whitaker <tnut55@> wrote:
        > > > > >
        > > > > > A green LED works just fine.  You just need to get a 5mm dia
        one
        > that has about 350mcd output.
        > > > > >
        > > > > > Mark
        > > > > >
        > > > > >
        > > > > >
        > > > > >
        > > > > > ________________________________
        > > > > > From: malonemike511 <malonemike511@>
        > > > > > To: atm_free@yahoogroups.com
        > > > > > Sent: Tuesday, March 12, 2013 4:51 PM
        > > > > > Subject: [atm_free] Re: First mirror!
        > > > > >
        > > > > >
        > > > > > ÂÂ
        > > > > >
        > > > > >
        > > > > > Mike, your so right. I am that newbie dealing with too much
        > > > > > information all seemingly correct and well intentioned but too
        much
        > too soon nonetheless. I have a 6in Pyrex piece of glass that I think is
        about at
        > f7. I measure the sagitta with feeler gauges and by taking it out in the
        sunshine
        > and measuring that distance. They seem to be in agreement. In the sun I've
        > measured 42in. from mirror to the most consolidated sunspot I can manage.
        > I'm now trying to build the Foucault tester and I have most of the parts
        even
        > down to a 0.0005 in dial indicator. My base is built but I think my
        problem is
        > the light source and the knife edge on the carriage. I've tried everything
        from a
        > small flashlight to a green 30mA LED with a couple of 15-Ohm resistors and
        for
        > the knife edge I'm using 2 utility knife blades which form the slit but
        being so
        > new I'm not sure what works the best. I know if I saw one in action a lot
        of my
        > questions would be answered. I have the Texereau book and am waiting for
        > the Ingalls books ATM 1,2and3 so we'll see where they take me. Thanks
        Mike.
        > > > > >
        > > > > > --- In atm_free@yahoogroups.com, Mike Coslo <mcoslo@> wrote:
        > > > > > >
        > > > > > >
        > > > > > > On Mar 12, 2013, at 11:07 AM, Jerry wrote:
        > > > > > >
        > > > > > > > After reading all these helpful "First mirror!" comments I
        > > > > > > > suspect that Ed has by now given his mirror the hydrant test.
        > > > > > > >
        > > > > > > > Ed, don't be waylaid by all these comments that have nothing
        > > > > > > > to do with "First mirror!". If you get off into caustic
        > > > > > > > tests, grinding multiple spheres into your mirror for
        > > > > > > > parabolizing, CNC machines, or even simple machines I hope
        > > > > > > > you are young because you will not finish your mirror any time
        > soon.
        > > > > > >
        > > > > > >
        > > > > > > We do this from time to time.
        > > > > > >
        > > > > > > If I may digress only for a second, the Ingalls book is perhaps
        even
        > better suited for the new worker.
        > > > > > >
        > > > > > >
        > > > > > > But otherwise, well meaning people can complicate the bejabbers
        > out of something. And the amazing thing is that we are all 100 percent
        correct.
        > > > > > >
        > > > > > > Some years ago, we had some people in the ATM group suggesting
        > that a first time worker could make a relatively huge mirror, and that the
        > traditional start small and work your way up method was lame. In
        principle,
        > Yes. In practice? Unless the goal was to spend years fixing mistakes, not
        so
        > good. We had a fellow here maybe two years ago who was trying to make a
        > 16 inch very thin mirror as a first mirror. He had a scattershot technique
        of
        > trying to troubleshoot and fix multiple problems with multiple - and wrong
        -
        > solutions at the same time. He ended up disappearing from the list, I
        guess he
        > just gave up.
        > > > > > >
        > > > > > > I don't even know what Ed's mirror size is any more, but if I
        could
        > give what I consider the best advice, it would be to for a first
        telescope:
        > > > > > >
        > > > > > > 6 inch f6 or f8
        > > > > > >
        > > > > > > Grind with a glass tool of the same diameter.
        > > > > > >
        > > > > > > Check saggita with a template made via drawing with a tool you
        > make yourself out of a broomstick or board and pencil.
        > > > > > >
        > > > > > > For each grit use twice the number of wets you think you need.
        > > > > > >
        > > > > > > Pour your pitch lap on top of that glass tool. Consider using
        netting to
        > micro-facet the lap.
        > > > > > >
        > > > > > > Make a Foucault tester, and learn to use it. There is magic that
        it can
        > show you.
        > > > > > >
        > > > > > > Then after you get that first scope under your belt, you can
        move on
        > to the more subtle stuff.
        > > > > > >
        > > > > > > All of this has a purpose. You could make a tool to measure
        Sagitta
        > after calculation, but it would not show you the relationship between
        radius of
        > mirror and radius of curvature.
        > > > > > >
        > > > > > > All the brouhaha about tools and laps and special hogging
        devices,
        > yeah, they are all better. But they can get the noob bogged down in
        details
        > that will be easily understandable after they've done a mirror using some
        > simpler yet also effective techniques.
        > > > > > >
        > > > > > > - Mike -
        > > > > > >
        > > > > >
        > > > >
        > > >
        > >
        >
        >
        >
        >
        > ------------------------------------
        >
        > Yahoo! Groups Links
        >
        >
        >
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