Re: Re[2]: [ATM] On Axis Foucault (try this instead- longish)

["David Harbour" <scarab2@cox.net>]

Richard,

Now that you make (clarify) the dimensions of what you are doing, this casts
things into a new light. You see, I was working large mirrors. Now, here you
are, working this tiny set up. If you tried to do what I recommend, there
may be "untoward" effects that I never experienced, having never worked so
small with Foucault; in fact, Foucalt may not be what you want to do. Right
now, as I rack my brains, I am trying to figure out a null test for you that
allows you to avoid testing at center of curvature at all. In fact, with
such a small mirror, you may not be able to avoid using a pellicle or a
cube, no matter what you do.

Before I make a suggestion, let me explain exactly what John and I do. When
I built old number five, (I'm going to stick a link in here so that you can
see old number five: the top picture is the best one to show (unfortunately,
not too well) the verticle bar that carries the optical guts of this thing.
I was more interested in showning the mechanical layout, and I never foresaw
that so many have not seen making ONE EDGE OF THE JAW OF THE SLIT THE SAME
EDGE AS THE KNIFE EDGE, so I didn't take any close-ups. Let me briefly
explain, as you look at the picture (left click blows it up). The reason I
am willing to explain is that, even though you are working so small, you
might be able to make this work. It worked WONDERFULLY WELL on G.
Kinderman's f/2 12.5" mirror, which I had to find grind as well as figure.
Okay:

http://www.atmsite.org/contrib/Harbour/Tester.html

picture to enlarge:

http://www.atmsite.org/contrib/Harbour/Testermed/ROLL2.JPG-3.JPG

(This is the link to the enlargement)

Okay- on the verticle bar, there are three perforations. Ignore the small,
bottom one. You will see a little black box on the back side of the bar;
this houses the lamp; I do not use dim LEDs, I use bright fiament lamps; if
you go to your hardware store they have a little chart to select flashlight
bulbs from. One (exactly ONE) of them is THREADED, to fit a threaded socket;
this little lamp requires two "D" cells (holder available at radio shack,
with  threaded socket). Advantage? MORE LIGHT THAN YOU NEED! In fact, I had
trouble seeing shadows on a COATED mirror. (I am not kidding).

Now the little black box held this little incandescent lamp, and you see
that the top is angled at a 45 degree angle; this was for a little extra
room for my FAT FACE when I was behind the knife edge, which is just above
this little black lamp box. I put a little dental mirror on top at 45
degrees, to give me some extra "face room". Now, here is the important part:
in front of this little black box, that held the little lamp, was the SLIT.
I made it by gluing (epoxying) two "Wilkinson Sword" narrow razor blades
together; what will surprise you is that I left the slit so big. In a
moment, I will tell you why. But let me finish; this slit was formed across
a hole about seven sixteenths of an inch in diameter- yes-PRETTY LARGE! But
I will tell you why it doesn't matter, in a minute. Now, here is the
important part:

Right above this lower hole, which had these two Wilkinson Sword (I collect
real, hand forged swords!) blades that formed the slit, was another hole
approximately (exactly) the same size: either seven sixteenths of an inch or
perhhaps a  half an inch in diameter, and it was drilled VERY CLOSE to the
lower hole. On the left side, the Wilkinson Sword razor blade was epoxied
across BOTH OF THESE HOLES- with its edge, of course, coming ALMOST to the
middle of the hole. Now, this blade covered both of these holes, as I said.
The lower hole had the lamp; adjacent the lower part of the razor blade, I
epoxied another, identical blade on the right side to form the slit (bevel
side up: flat side down). Now, before all of you spontaneously combust, and
become enraged at me, I will tel you I had a good reason for leaving the
gap, when I epoxied the other blade on, LARGE.

You must think about how Foucault testers work, especially SLITLESS testers,
in order to understand THAT IT DOESN'T MAKE ANY DIFFERENCE HOW WIDE THE GAP
IS: when you bring the K.E. in from the side, as soon as the aerial image of
the slit that is formed on the edge of the knife IS SMALLER THAN THE ACTUAL
DIAMETER OF THE SLIT, YOUR TESTER IS OPERATING AS A SLITLESS TESTER: ALL
TESTERS THAT USE A SLIT ARE SLITLESS TESTERS, AS SOON AS THE AERIAL IMAGE OF
THE SLIT IS SMALLER (IT'S GAP) THAN THE ACTUAL SLIT.

So to finish this kind of tester, Richard, you would epoxy the other
Wilkinson Sword on the right side, but do not extend it up across the top
hole. In other words, the right hand blade is only for making the slit.

IF YOU BELIEVE IN SLITLESS TESTERS AS AN ARTICLE OF FAITH. LEAVE IT OFF!!!!
I believe there are advantages in leaving it on. But do as you please; by
the time you advance your knife edge inside the diameter of the slit, you
are a slitless tester. I gave my slit .01" (Go ahead, laugh; I turned out
some masterpieces). The point is, either with or without the right hand
blade, by the time you get close to extinguishing the shadows, you are
operating as a slitless tester. Test my theory: get an eyepiece with a
reticle, set it up to the side, to examine the aerial image of the slit;
advance the assembly in until you can no longer see the slit in its entire
width; ONE OF ITS EDGES HAS BEEN LOST, OCCULTED, BY THE REAL BLADE.

However, all this is moot (I publish it for those who have asked me so many
times about the "detector/analyzer" bar for my old tester).

However, as I have been writing this explanation, I have figured out an EASY
NULL TEST FOR YOU!!!!

It will only take me about 20 minutes to resolve a small neighborhood
crisis, here, and then I will come back and write it up for you. I've got
your dilemma all resolved (might be one difficulty)- but when I come back I
will tell you the set up I would have used (I used the past tense for a
special reason). Look for my newer, brief message after I come back from
taking a neighbor's child somewhere.

Dave

Okay: this
----- Original Message ----- 
From: "Richard" <cnc@cncservo.co.uk>
To: "David Harbour" <scarab2@cox.net>
Cc: "ATM List" <atm@atmlist.net>; <aneves@snowsoft.biz>
Sent: Wednesday, June 15, 2005 10:26 AM
Subject: Re[2]: [ATM] On Axis Foucault (try this instead)


> Hi David,
>
> Wednesday, June 15, 2005, 3:47:48 PM, you wrote:
>
> DH> I and others (most notably John Sherman) have been preaching this for
> DH> decades; and yet we just can't
> DH> make it catch on.
>
> Not sure what it is that we can't catch onto David, but maybe
> you can draw on your experience to help me please.
>
> I have to test a 2" F/2 spherical mirror (yes, TWO inch).
> Actually it has focal length 100mm and usable diameter 50mm.
> Ideally I would like to get it to lambda/20 or as near to
> perfection as I can over the central 80%, as it will be the
> reference element in an interferometer and any error in it will
> cause errors in everything that is ever tested with the
> interferometer in the future.
>
> Could you talk me through a layout for Focault testing it
> please?. Assume ignorance as I haven't done this for many years.
> Eg, I don't know what to use for an abberation free light
> source. What I do have, that I can pull off my microscope, is an
> X-Y table with 1um/0.0001" digital micrometers. Also have a
> laser, microscope objectives, smattering of lenses, LED's,
> pellicles, cube beamsplitters, digital camera, poor eyesight.
>
> I'm happy to send it away to a calibration lab to assure it's
> final quality, but any help in achieving it would be much
> appreciated.
>
> -- 
>
> Best regards,
>   Richard in the UK
>
>
>
>


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