1.1 Meter Mirror
Updated 11-4-07
Newest info at bottom
10-4-2003
We have purchased a 1.1-meter quartz blank that was
manufactured by GE in the 60's as test blanks for
the Mayall Solar Telescope at Kitt Peak. It was rejected
for imperfections in the casting and ended up in the
backyard of amateur astronomer Norm Oberle.
For thirty years it waited to be his retirement project,
but sadly, he died of cancer before he could start
it. We have picked up the challenge and intend to
turn this blank into a fast cassegrain telescope.
Our optician has come up with an all-spherical design
that uses an 18" secondary and two corrector lenses.
This design should produce 50-micron FWHM star images
across the field of the CCD. It also offers the advantage
of a short tube length of about five feet, which is
a big bonus for this size mirror.
The all-spherical design allows us to bypass the long
tedious process of parabolizing the mirror, which
can take ten times longer than the initial spherical
figure.
May 2003
The design has evolved from an all spherical to an
ellipsoid primary with spherical secondary and corrector
lenses. This design was chosen in order to minimize
the size of the secondary down to 10 inches. We are
currently finished with the primary polishing and
are having null lenses fabricated for the final figure.
Once the null lenses are fitted to the interferometer
we should be very close to completing the final figure
and finishing the mirror.
We have started the initial design of the mount and
the non traditional concept is sparking much debate
among the team members. It is loosely based on the
Sofia airplane mounted telescope and utilizes an air
bearing at the center of gravity to support the entire
load.
The blank as it sat in the backyard of Norm Oberle.
The blank was already polished on both sides.
The grinder was purchased as government surplus and
is a 60's vintage machine in excellent working condition.
Here is a picture of us loading the 900 lb blank on
the grinder.
The optical layout - all components are spherical.
Mirror update 3/9/01
An instrumentation tower was fabricated to hold the
optical test equipment at the radius of curvature.
A hole was cut in the roof and the tower mounted to
the structural beam.
A hand grinder was mounted to an aluminum tube and
its far end attached to a pivot at the radius of curvature.
The blank was rotated while a diamond wheel cut into
the blank. This method removed well over 1000 cubic
inches of glass in two days and brought the blank
to within .020" of the final curvature.
Close-up of blank partially ground with the diamond
wheel.
Steel support frame set into the mold for the plaster
grinding tool.
Finished tool removed from mold, bottom is covered
with ceramic tile.
After four days of grinding with the 37" tool, the
blank was spherical with slightly dull finish. The
next step is creating a polishing tool and putting
on the final finish. The optical design has changed
and we will be polishing an ellipsoid into the F2.5
curve. A reflected point source test at the radius
of curvature shows no problems with the blank.
Update 3/7/02
After many months of delays due to the recession and
9-11 we finally have a smooth polish on the mirror.
We are now ready to test the mirror and start working
on the final figure. Note the strong curvature in
the wall reflection. It takes 5 gallons of water to
fill the mirror to level so we removed that much glass.
The past three months have been spent building and
debugging an interferometer. Shown here is the Palmer/Shack
design that is simple and effective. The vertical
black tube is a HeNe laser that directs the beam into
a 100x microscope objective. The microscope objective
is focused on a 10 micron pinhole that is permanently
glued in place sandwiched between the objective and
a beam splitter. The result is an objective that has
the beam splitter cube hanging off of it. The laser
light cone coming from the pin hole gets reflected
out of the splitter down to the mirror. A small percent
gets reflected off the uncoated mirror and back to
the cube. At the same time a similar amount of laser
light gets internally reflected in the cube and exits
out the top with the reflected light from the mirror.
The face of the beam splitter is polished to 1/20th
wave flatness and the combined reflected beams interfere
and demonstrate the classic fringe pattern when viewed
from directly behind. The video camera lens is right
up against the beam splitter. Click on thumbnail to
see a larger image with all the parts labeled.
Click on the image at left to see the optical layout
of the Palmer/Shack Interferometer.
This is an example of a fringe pattern on a 12" polished
blank.
simple digital picture of the fringes can be imported
into MaximDL's Quick
Fringe software. This spectacular software automatically
detects the fringes and gives a 3D map of the mirrors
wave front. It gives the peak to valley as well as
the RMS and Strehl Ratio of the optic.
First "fringe light" for the 1 meter showed a high
nulled zone about half way out from center.
First Quick Fringe analysis
of the 1 meter shows it to be out by about 2 waves
at this point. We have several additional days of
polishing in now and this zone is almost completely
gone. We are reworking the video system on the interferometer
so we don't have any new tests at this time.
The fringes in the two pictures above were later found
to be internal reflections and did not actually represent
the mirrors figure. The update below shows the actual
data.
Update 5/4/03
The fringes from mid year 2002 showing the blank out
by several waves.
After several months of trial and error we finally
worked the mirrors surface into the proper F2 spherical
shape. Originally, improper setup of the interferometer
created about 50 hours of unnecessary polishing.
After working with several of the polishing tools
we successfully came across the right combination
of tools and parameters to get a good sphere. This
interferogram in Quick Fringe
shows that the mirror is now at about 1 wave PV and
¼ wave RMS.
Quick Time movie of the grinding machine in action.
(5 meg). Movie
Currently we have designed a set of custom null lenses
for the primary and are having them fabricated at
this time. The null lenses will distort the wavefront
of the primary and will make the desired ellipsoid
look like a sphere to the interferometer. Once we
get straight fringes using the null lenses we have
a finished mirror.
Update 10/4/03
The null lenses are finally here after some extraordinary
effort on the part of our sponsor Kreisher Optics.
We now have all the pieces we need to finish the mirror.
Our largest polishing tool suffered some heat damage
this summer when the pitch delaminated from the plaster
tool. We hope to finish the final figure with the
remaining three tools.
Update 6/4/04
Or rebuilt interferometer is ready for action. The
null lenses have been incorporated into the design
along with 11 different adjustments. Focus and polarization
are now remote controlled.
Update 3/23/06
Polishing is NOT fun. We are over 100 hours into removing
material and things are just barely under control.
We are trying to get fringes all the way across the
mirror and we keep screwing things up. At this point
we have three major zones and the center is still
high by about 20 waves. We do see fringes about 3/4
of the way to the edge so there is hope...
Here we used a stainless cooking pot to bore a 10"
hole 80% of the way through the back of the mirror.
This was done to relieve stress before final figure.
Update 11-4-07
In the last year we had to take the mirror back to
a sphere three times because of a messed up figure.
The first 4 months of 2007 were spent full time figuring
the mirror before its move to Arizona. In the last
attempt, things were close but not perfect and some
high spots were taken down by hand. The mirror is
currently within 1/2 wave RMS and we don't know if
this is good enough or not. Everyone told us an F2
mirror was really hard to figure and they were right!!
The final cut from the mirror side to complete the center hole.
The secondary mirror and corrector lenses have been ordered from Tuscon Optical Research and should arrive by Jan 1st 2008. At that point we will setup the optics in a static mount and test the imaging with the full optical train.
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