From: Dale Eason 
To: Pat Bunn 
Cc: atm{at}shore.net
Reply-To: Dale Eason 


Better in what way?  All that is required is that the light be a slit or a
pin hole and if a slit that the return beam be stopped by a knife edge that
is parallel to the slit.  A single knife edge placed over the led performs
this function as good as anything else.  Led's are used in modern testers
because you can get one that is as bright as you need, is monocromatic
(green the color our eye is most sensitive to and what we usually figure
the mirror to) and only requires low voltage and current to power. Other
colors are available as well.

The slitless tester as expressed by the Stellafane tester is actually the
latest development.  It is a simplification of the original design that
used a second blade to make a slit however it has been shown that the
second blade is not needed and so the design has been simplified.

The only part that needs to be very accurate is the foward and backward
movement (in relation to the mirror) of the knife.  You need to be able to
measure this to .001 inch.  As you can see in some of the pictures you can
use a dial indicator that makes doing that easy.

Because of the nature of the eye and the test it is hard without practice
to get second readings for some zones that match withing .010 of the last
time you read it.  After some practice some can do it within .005.  So
using an indicator any better than .001 is not worthwhile and the data
reduction does not need that kind of accuracy as well.

If you use the indicator.  The threaded parts of the test are only used to
move it not to measure it. However even those can be used to measure its
movement to the accuracy required.  I used a micrometer to move my platform
and a rubber band to keep the platform pressed against the micrometer.
I have since replaced that with the dial indicator because it is easier to
read than the micrometer but it is not more accurate.

It is fine if you want to spend time making a prettier one but for me I
would rather spend my time and resources on the scope itself or other
things that truely need improving upon.  In my opinion there is nothing you
can do to the tester that while help you make a better mirror.  Other are
free to give thier opinion. So to get them I hope you don't mind that I
post this to the ATM list as well.

-+- Pat Bunn  wrote:
> I agree that the tester basis cannot be improved on,
> but the mechanics of
> the Stellaphane tester surely can be improved on
> especially in the design of
> the metrics . Maybe it is not necessary, but I 'd
> rather do my measuring
> with a decent threaded setup - something better than
> two "T Nuts" and a
> carriage bolt. I need the machining practice anyway
> - that is the main
> reason I am doing it a little different. What I am
> looking for is has anyone
> devised a better method of knife edge and LED ?
>
> Pat Bunn
> ----- Original Message -----
> From: "Dale Eason" 
> To: "Pat Bunn" ; "John Murray"
> ;
> "Amateur Telescope Makers" ; "Mirror
> Matic"
> 
> Sent: Saturday, August 02, 2003 12:16 AM
> Subject: Re: ATM Tester question
>
>
> >
> > Believe it or not the tester already has all the
> > accuracy required and cannot easily be improved
> upon.
> > I am always amazed that such a simple device can
> be
> > used to make  extremely accurate measurments.
> >
> > You may already be aware of the updates to the
> tester
> > but if not here is a link.
> >
> >
>
http://www.stellafane.com/atm/atm_foucault_tester/atm_tester_plans_4.htm
> >
> > --- Pat Bunn  wrote:
> > >
> > > I am just beginning to assemble a "Stellaphane"
> type
> > > tester with some
> > > machined aluminum and brass parts ( I have spent
> the
> > > last week machining
> > > these - I am not a machinist), rather than the
> wood
> > > mechanics.
> > >
> > > I am using 3/4" birch plywood with a nice
> > > polyurethane finish and a metal
> > > vernier mechanism and slide arrangement. My
> stage
> > > "faceplate"  is 1/4"
> > > delrin sheet rather than 1/4 " plywood.
> > >
> > > Any ideas or improvements on the light source
> and
> > > slit arrangement or is the
> > > published design good enough. I am trying for a
> more
> > > accurate design than
> > > the basic wood design.
> > >
> > > Pat Bunn
> > >
> > >
> >
> >
> > __________________________________
> > Do you Yahoo!?
> > Yahoo! SiteBuilder - Free, easy-to-use web site
> design software
> > http://sitebuilder.yahoo.com
> >
> >
>


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