Re: [ATM] Quartz test

[Dale Eason <atmpob@yahoo.com>]

Light from a very far away point like a star becomes a
very wide parallel beam of light wider than your
telescope aperture even though it came from what
looked like a point source.  The telescope focuses
that broad beam back down into a point source.

You can make that work in reverse. Put a point source
of light at the focal point and send it back out of
the telescope as a broad collimated beam the same
diameter as the mirror.

So you can use another telescope and a point light
source (laser) to make an artificial star.  It works
as long at the other telescope you use is as least as
big and preferably bigger than the mirror you are
trying to test.

You may have heard of a collimator before.  One way to
make one is the above description.

A laser by itself does not have a wide collimated
beam.  But it can be a good approximation of a point
source when used as a point source of the collimator.

A very crude version of a collimator is a flash light.
 The bulb (bad point source) is put at the focal point
of the reflector.  That puts out a beam of light that
is roughly collimated.  Not good enough to test
telescopes with.


Dale Eason
--- GARY FUCHS <gary@rcn.com> wrote:

> Thanks Mike,
> 
> Could you explain more about the collimated 24"
> diameter beam and how that ends up being a point of
> light? There's probably something simple I'm
> missing...
> 
> Could a plain laser be used instead?
> 
> Thanks,
> 
> Gary Fuchs
> 
> 
> >
> >Gary,
> >
> >GARY FUCHS wrote:
> >> Thank you for the photos and explanation. Your
> site's been helpful
> >> to me with several things. I hope you don't mind
> some questions:
> >> For the transmission test with the 4.25" scope,
> how far away is the
> >> quartz from the scope and laser? Does it matter?
> Do you need to
> >> move the quartz to have the laser go through
> different areas? I'm
> >> assuming you don't view the laser/star image
> directly - is that
> >> correct? Also assuming the laser isn't centered -
> does it matter
> >> where it's positioned as long as the beam clears
> the spider?
> >
> >The collimator puts out a 24" diameter beam of
> collimated light, that 
> >is, light that is parallel, like star light.  The
> 4.25" sits in front 
> >of it, collecting a small portion of the beam to
> form a "star" image. 
> >  I put a ronchi grating in the focuser to get the
> ronchi image later 
> >in the page.
> >
> >The quartz was right in front of the 4.25" as an
> optical window would be.
> >
> >You can view the star image directly with a welding
> filter and an 
> >eyepiece.  It is not bright enough to damage
> vision, but it's not 
> >pleasant to view directly.
> >
> >	Mike Lockwood
> >
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



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