How to find the center of a parabolic mirror in an optical bench
nutanna...@gmail.com
I have got a parabolic mirror 4" in size. I need to use it in an
optics experiment but am having trouble aligning it properly. Namely,
I have not been able to ascertain when the laser beam is hitting its
center. Would be grateful for any input.
Nutan
Helmut Wabnig
wrote:
What precision is required?
cm, mm, µm ?
w.
Harald Babucke
[...]
> I have got a parabolic mirror 4" in size. I need to use it in an
> optics experiment but am having trouble aligning it properly. Namely,
> I have not been able to ascertain when the laser beam is hitting its
One could use an aperture
on the mirror with a small hole in the center.
(An early and cheap version may be made of black paper) .
The diameter of the hole should be slightly bigger than the diameter
of the laser bundle.
If the light is going back to the source, the tilt alignment is
correct.
Harald
mp...@oscintl.com
If the collimated laser diameter is large say 2 to 6" diameter, or use
the beam from an interferometer, like a 4" diameter zygo, wyko, etc
you can place a retroreflecting sphere near the focus.
The collimated beam will reflect off of the mirror and go toward the
focus at the center of the retroreflecting sphere (or hemisphere as
you only need the front diameter to reflect the beam back).
If the beam is not centered you will see an aberrated return wavefront
in the interferometer, or simply a returned beam with an angle to the
input beam.
You could also use this method with an alignment telescope as well.
If you are using a laser pointer type small laser, as one other poster
said, poke a hole in a piece of paper and look for the returned beam
to be centered on the hole in the paper. To be clear this technique
with the laser pointer does not use a retro sphere.
You may enjoy this short course DVD on optical alignment, or others in
the industry that are delivered 1-2 times per year.
http://www.oscintl.com/Short_Course_DVDs/intro__oa_dvd.htm
Michael
nutanna...@gmail.com
> On Tue, 10 Feb 2009 00:09:11 -0800 (PST), nutannathsha...@gmail.com
> wrote:
>
> >Hi All,
>
> >I have got a parabolic mirror 4" in size. I need to use it in an
> >optics experiment but am having trouble aligning it properly. Namely,
> >I have not been able to ascertain when the laser beam is hitting its
> >center. Would be grateful for any input.
>
> What precision is required?
>
> cm, mm, µm ?
>
> w.
To Helmut;
Thanks for the response. I am using the mirror in an interferometric
setup and as such it should be better than mm.
To Harald;
Yes in theory it should work, but the distance between the mirror and
the source is small and it seems that I can have the beam go to the
source at off center positions too. Any idea what can solve the
problem.
Richard J Kinch
> optics experiment but am having trouble aligning it properly.
This is a two sided problem: detection versus control. How to you control
the alignment?
Harald Babucke
> Yes in theory it should work, but the distance between the mirror and
> the source is small and it seems that I can have the beam go to the
> source at off center positions too. Any idea what can solve the
> problem.
For the 4" = 101.6 mm diameter of an astro-telescope mirror one may
expect a focal lenght of about 1 m.
What is your focal length?
Harald
Harald Babucke
On 10 Feb., 22:52, nutannathsha...@gmail.com wrote:
> > Yes in theory it should work, but the distance between the mirror and
> > the source is small and it seems that I can have the beam go to the
> > source at off center positions too. Any idea what can solve the
> > problem.
I use a second aperture with a hole of a few millimeters on the
interferometer output (e.g. a 4" reference flat).
So I have two small apertures (one at the interfeometer, defining the
optical axis, and the other one at the mirror),
which makes it easy to to center the mirror on the optical axis.
Harald
nutanna...@gmail.com
I have translational freedom along all the axes and freedom of yaw and
pitch for rotation. Because of this the aperture concept does not seem
to work for I can always make the incident beam almost perpendicular
to the mirror as such the beam can be made to go to the source even if
it is not hitting the center of the mirror.
Helpful person
Find the center of the mirror by dropping a sphere onto it and
observing the Newton's rings. Make sure that you check the center for
different azimuths (roll) of the sphere to eliminate errors introduced
by wedge in your test sphere. View from an "on axis" direction to
eliminate errors due to the test and reference surfaces being
separated.
Once you have located the center it should be easy to align your
laser.
jeff
Rough align the system, then put a ground glass screen into the beam at
the center of curvature to create a speckle cone out to the parabola and
back. Move inside and outside the center of curvature (along the
caustic) and observe the returned pattern on the ground glass. Adjust
for a concentric pattern.
Jeff Lowe
Bob May
where the center of the paroabla definitely isn't the center of the mirror)
the assumption is that the center of the mirror is the center of the
parabola. This dimension may bary a percent or so of the diameter with a
poorly made one.
Take a laser source and beam it at the center of the mirror and look to make
the return come back to the source. You will probably have a flat in the
light path so you can see the light coming into your optical system without
any obstruction by your head, etc. At this point you basically have a
Newtonian telescope. Point this set of optics at a distant light source of
small dimensions (sunlight bouncing off of a ball bearing or Christmas
ornament many feet away) and look at the focal plane of the mirror with a
lens. If you are on-axis, the light should look like a dot. If it looks
like a comet, move the parabola so that the spot looks like a dot. The
comet points towards the center of the parabola and that means that you need
to move the comet away from the center and then realigh the optics to the
distant light source.
Doing the job fully indoors, the best way is to do the Foucault Test (google
it) and that will show where the center of the parabola is. The test is
easy to setup and do as it just requires a LED for a source of light and a
straight edge to measure the surface. Just for finding the center, you
don't need to do any precise measurements of the different ROCs of the
mirror so you can just put the LED on a post, put the straight edge over it
so you can also see the returned light back past the straight edge. You
will need ot have a mirror stand that you can tilt as needed to point the
light back to the straight edge where you can see it.
--
Bob May
rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net