The Refractor Project
I considered a bold new project for the ASGH Mirror Workshop for 2005.
That was a good refractor lens that could be built by a moderately experienced
person under the guidance of the workshop environment.
The result was a 6 inch f/15 refractor objective made from readily available
optical glass with standard mirror making techniques, little special equipment,
and WOW! Does it perform.
So if you have ever wondered if it was possible to build your own refractor lens
that performs well, the answer is YES you can. The following pages will highlight
how the refractor lens is made and what special optical tools are available in the
ASGH Mirror Workshop. (These pages are NOT intended to provide detailed
Refractor lens glass is available from many sources. Newport glass offers blanks
with tools with pregenerated curves ground in both of the blanks and the tools.
The sizes offered by Newport are 4 inch, 6 inch, and 8 inch. I used the 6 inch
diameter. The 6 inch comes available in two designs. One design is a cemented
pair and the other design is an airspaced Fraunhoffer. I used the airspaced
Fraunhoffer to avoid cementing complications then had the design modified.
I wanted a design that would be excellent optically, but be accomplishable in our
workshop. The Fraunhofer has the disadvantage that all curves are a different
radius, therefore qualifying them would require at least two test plates. (Test
plates are concave curves in a separate glass piece that is the same radius as the
convex radius of the lens element.) With the assistance of an experienced Z-max
(optical design software) specialist our research revealed an airspaced design with
almost equal internal radii. This would mean that the inside convex radius could be
tested against the concave radius in the rear flint element. This airspaced design
had an airspace much larger than the few thousandths airspace of the Fraunhoffer. It
further turns out that the increased airspace eliminates the ghost image problem
caused by the two surfaces in close proximity of the Fraunhoffer. We then optimized
the design with Z-Max. In the end, we came up with a really superbly performing
design that needed no test plates.
Four special tools are necessary to make this outstanding refractor lens. The next
few paragraphs show you the special optical tools I made that are available in the
A spherometer with a gage capable of measuring to one ten thousandth of an inch.
The radius of each curve on each lens element should be correct within 1/20% of the
design value for best results. This is achievable with a spherometer that can measure
to one ten thousandth of an inch. The digital gage shown on this spherometer meets this
accuracy and is available for approximately $60.00 from industrial supply catalogues.
The base was machined.
A fixture and gage to measure perimeter thickness of each lens element.
It is important to insure that the curves on each side of each lens element are properly
centered. This is accomplished by measuring the thickness of each lens element near
the edge in several places around the circumference. The thickness should be equal
within a few ten thousandths of an inch.
Rods for measuring the exact radius of the concave radius in the flint element.
A precision spherometer is a good starting point, as discussed above, but it is important
to check the spherometer against a radius that can be measured accurately to great
precision. A way to accomplish this, but not the only way, is to directly measure the
distance between the concave radius and the knife edge when tested like a mirror.
An autocollimation test flat
On optical flat is used to test both lens elements together as a system mounted in their
cell. The test uses a point source of light and Ronchi screen at focus similar to testing
for mirrors. For testing refractors the flat does not have to be aluminized nor does it
need to be perforated in the center.
|Making the Lens
Making the lens is similar to making a mirror. There are two glass elements therefore 4
surfaces to make, however the glass works faster than pyrex and all the surfaces are
spherical. Since the lens elements are relatively thin we protected the non working surface
with masking tape.
Then we taped the lens to the opposite tool with electrical tape so the glass was supported
during grinding and polishing. The electrical tape can be unwrapped and rewrapped many times
to avoid waste. I used the same piece of tape for all four surfaces.
(left) Jim P is fine grinding one of his lens elements. The process is exactly like grinding a
mirror at this stage. The only difference is that the first job is to work the wedge out of
each piece. Correction for wedge is done by favoring pressure along the thick side of the element
for some of the grinding. Also it is important to watch carefully the segitta and overall
thickness of the glass.
(right) Jim is checking the wedge of his crown element after doing some fine grinding.
(left) After grinding and polishing the concave radius of the flint element, that radius is
measured very carefully with our rods. This photo shows the lens element on the stand to the
right. The light source and knife edge are at the left end of the rod positioned precisely at
the knife edge null. The combination of the rods and micrometer movement of the knife edge stage is
used to measure the distance from the lens to the knife edge. The optical quality of the spherical
suface is qualified by the knife edge test itself.
After grinding and polishing the rear convex surface of the crown element, that radius is measured
very accurately with respect to the flint concave radius by counting interference fringes. The
lenses are nested together and placed under a monochromatic light source so interference fringes
can be produced. The shape of these fringes is also to qualify that this surface is a good sphere.
|Making the Cell
The final test of the refractor is accomplished with both lens elements together testing the pair
as a system. The best way to do this is to mount the lenses in the lens cell that will be used on
the telescope. Although acceptable lens cells for home made refractors can be made out of wood, I
choose to machine a metal cell out of aluminum. I started with a bar of 8" O.D. by 5" I.D.
aluminum stock which was sliced up onto rings.
(right) Then the rings were machined on a lathe into a lens cell. Use of machine tools is not
available to Workshop participants, however lens cells may be machined here for a very fair charge.
Or participants may obtain their cells from other sources.
The lens cell is finished shown here with the lenses installed, properly spaced, and ready for
|The Final Tests
The lens assembly is put on the test stand for autocollimation testing. The light source is beyond
the picture to the left. Shown is the lens cell with the lens elements in it mounted on a board
(left). Behind the lens is the autocollimation flat.
In autocollimation testing a lens, the light passes through the lens then to the flat then back
through the lens to be refocused for viewing.
The autocollimation test returns a null condition when there is no spherical aberration. We used
a Ronchi screen and the null condition is straight parallel Ronchi lines.
When doing the autollimation test with a lens it is important to note that the spectrum of the
light source will be separated in this double pass test. For best results, a single frequency
(color) light source provides the best Ronchi lines. Also, refractors are designed to minimize
spherical aberration at a light color for its intended use. Other colors might not be corrected.
For visual observing, the lens is optimized for green light which is the color the human eye is
most sensitive to. Therefore the test should be done with a single color light source and that
color should be green. A Green LED worked very well for both reasons.
Recall that the two inner radii were qualified by the traditional knife edge test and interference
fringes. The two surfaces not qualified are the very front surface (front surface of the crown) or
the very rear surface (rear surface of the flint). These surfaces should be spherical if
traditional mirror making techniques are used. If the autocollimation test does not reveal a null
condition, either of these surfaces may be worked to achieve a good test. Since the curve of the
very rear surface is shallow, chances are that is the surface that did not come out figured
spherical. Best guess is to work that surface to achieve a good autocollimation test. That is what
was done for this refractor.
If working the rear surface does not produce a satisfactory autocollimation test after reasonable
effort using sphere producing strokes, then some exploration may be need to isolate whether the
front surface or the rear surface needs continued work. It may be necessary to make a test plate
to qualify the front surface. In this refractor, working the rear surface was all that was necessary.
The final step is to install the lens in an optical tube and test it on astronomical objects.
|Finishing the OTA
After the OTA has been used to conduct star testing, the OTA can be completed. Don't forget to make
and install light baffles in the tube to minimize tube wall reflections. For a refractor, a baffle
assembly is quite easy to make. Shown here are baffle rings made from thin plywood. They are connected
by stringers similar to a model aircraft so the entire assembly may be slipped easily into the tube
and proper spacing maintained.
Shown here is the OTA with the baffle assembly painted in flat black and ready to be inserted into
the tube. The tube has also been painted white.
This refractor exceeded all my expectations. It was easier to build than expected and performs
magnificently. The design proved itself making the Z-max work quite worth while. This refractor
produces a perfect star test both inside and outside focus (with a green filter). Images of the
planets are clean and sharp with excellent contrast. This refractor provides all the advantages
discussed in the literature for refractors (high contrast, sharpness, black sky). There is no
noticeable color separation, or halos around planetary images. There is some purple halo around bright
stars, but that was expected. Experienced observers who have looked through this refractor have stated
that this telescope out performs most other refractors they have looked through that had achromatic
doublets for the objective.
I hope you enjoyed this story and become interested in making a refractor yourself.
- Dick Parker
P.S. Since posting this story and in talking with people around the community, I have received many
inquiries about the lens cell design. Initially, I didn't have any drawings. But I have recently
gone back and made up a concept sketch. I hope you find this useful.
Photos of the Completed Telescope