A Further Improved
4.25 Inch Unobstructed Oblique Reflector
During the late 1970's, I was attending the University of
Maryland. I used to go to the library with the good intention of studying, but
many times I would end up paging though bound volumes of past issues of
Sky
and Telescope. I was particularly
interested in Gleaning for ATM's column. I would look though that section first
to see what interesting and unusual projects people had made. One particular
article caught my attention, it was titled
An Improved 4.25 inch
Unobstructed Oblique Reflector. The "scope
was constructed by Oscar Knab based on the optical design of Anton Kutter. It
had a beautiful jeweled aluminum tube set into a mahogany mount. The "scope
looked very unusual. It appeared like a refractor except that it had a short
fat tube below the upper long skinny tube. I was fascinated by this article,
reading it over and over again. I
thought to myself that I would like to build one some day. This type of
telescope is also known as Schiefspiegler, Kutter, Tilted Component Telescope
(TCT) and others.
When I was attending the Delmarva Star Gazer's Third
Annual Mirror Grinding Weekend during March 2003, I spoke to my mentor and
friend Dave Groski about the "scope that I had read about in my college days
and how I would like to build one. Dave told me that he was in the process of
making one and that he had come up with a design that further improved on
Kutter's design. The original
design suffers from about 1ž4 wave of residue coma and Dave's design
considerable reduced that. I asked him for the plans, which he promised, to
E-mail to me. So that day, just for fun, I asked some people at the mirror
making class if they knew of anyone who had a 4.25" blank for sale. I was told
that Mike Mill of NOVAC had one that he might part with. I went straight to him
and asked him if that was true. He said, in fact, he did have one that he
bought for $1 and he was willing to sell. I bought it from him for $2. I told
Dave that I was half way there and that all I needed was a tool to make the
scope. Dave said that he had a tool that he would give to me, along with a copy
of 1961 Sky and Telescope article by Oscar Knab, other articles on TCT's that
he had collected and the information on his improved optical design.
A wish
that was born 25 years ago was now close to becoming a reality.
A few weeks later as promised, Dave sent the tool along
with all the information. The new version called for a radius of curvature of
138 inches Vs the 127 inches of the original Kutter design and some slightly
different angles that the optics would be tilted to. The new design kept the
same spherical surfaces on both the primary and secondary and the same radius
of curvature on both. Having both surfaces spherical would make for easy testing,
which is one of the major advantages of this design. The sagitta of the shallow
curve is 0.0164 inches. I decided
to use a 1/64-feeler gauge for measuring the depth of the curve during rough
grinding.
Because the curve was shallow, I started with 220 grit.
The curve was generated in about an hour of work. To accurately check the
radius, I flash polished the mirror with a polishing pad as Dave had
suggested. When I got a shine to
the mirror, I used the Foucault test to measure the radius of curvature. The
accuracy required for the curve needs to be within 1 inch of the required 138
inches and it is best to error on the long side. I found out that during
polishing the curve could change as much as 8 inches. So, if one could grind to
within 3 inches for the needed radius then this would be close enough. Fine-tuning can be done during
polishing. The mirror was then fine ground in the usual manner. One must take care to alternate the
mirror and the tool during grinding to keep the curvature the same.
After the fine grinding was done, I normally would have
used polishing pads to polish the surfaces. Due to the sensitivity of the curve
and the need to keep the radius of the primary and the convex tool/secondary as
close possible so one can use interference testing, I polished both surfaces on
slower acting pitch. I made two backing for the laps out of a 3ž4 inch
plywood disk. The disks were coated with polyurethane for waterproofing and I
then poured 1ž4 inch of pitch on top. Both the convex tool/secondary and
the mirror were pressed into their respective pitch covered plywood disks. Due
to the shallowness of the curve, the plywood can be left flat. The plan was to
fully polish the primary and then figure it a perfect sphere using the Foucault
tester. The tool/secondary would be polished next and then figured by testing
it by interference against the spherical primary.
Knab,
suggested that one could also test the tool/secondary from its backside with
the Foucault tester if one lacks the monochrome light source or as another
check of its figure. This requires that the back of the tool has been polished
and is also fairly flat, but only over the center section that will become the
finished secondary. When testing through the back, the light will come to focus
at approximately 3/4 of the true radius of curvature. I chose to use the
interference method because I felt it was more accurate, albeit a little more
work to make an interference tester and monochrome light source.
I polished the mirror in the usual manner, but one must
test often to check the progress and maintain the ROC. I learned how sensitive
the ROC was when I noticed that a very small hill was developing. In the
process of correcting it, I lost track of the ROC and it had shortened by about
8 inches. Returning the mirror back to the desired ROC was not difficult; I
just polished with the lap on top.
I soon discovered that testing a 4.25 inch disk from a
distance of about 11.5-ft required a bright light source. I found that a tester
using a slitless light source and Ronchi grating provided the best method to
test. I could check the over all figure of the mirror, then try to null out the
mirror using a single line of the Ronchi grating as a knife edge. After I got a
nice null on the mirror, indicating a smooth and zone free spherical surface, I
proceeded with the polishing of the tool/secondary. I then tested the
tool/secondary against the finished spherical primary mirror by interference.
The shape of the interference line indicated the figure of secondary. I
repeated figuring until the interference lines were straight, indicating that
the figure of secondary was spherical and matched that of the primary.
I then had the secondary cut to a diameter of 2.2 inches
from the center of tool. I tested the secondary against the mirror one last
time after it was cut out just to be sure that figure had not change. Off they went to the coater.
Now that the optics is finished, I had to decide on the
details of the telescope and it's mounting. I liked the look of Oscar Knab's
telescope and decided to do the same except I make it from wood. Both tube
sections were made into a hexagonal shape. I drew a full size layout of the
'scope on my shop floor. This helped me visualize and measure the components of
the scope.
Knab had mounted his 'scope on Alt/Az mount but after some
discussion with Dave Groski it was decided that a tracking equatorial mount
would be better. With the long focal length, the 'scope would provide mostly
high magnification views, and a tracking mount would make for a more enjoyable
viewing. I soon found a CG-5 on Astromart.
Collimation of this scope has become easier then Knab's
day because of the use of laser collimators. The laser collimator is used to
trace a light path in reverse through the system. I put the laser into the focuser then adjusted/shimmed the
focuser until the laser dot was centered on the secondary then I adjusted the
secondary so that the laser dot was centered on the primary. Next I had to
adjust the tilt of the primary. I made a mask from a 2.5" X 4"
section of plywood that was 1ž4" thick with a center hole drilled
into it. The center hole was the
center reference line of incoming star light and allows the laser beam to pass
through the mask and hit the primary. From ray tracing, it was determined that
the beam off of the primary would fall 2.53 inches below the center of the
secondary. From the center of the secondary, I measured 2.53 inches down and
marked the spot on the 1ž4 inch plywood I had taped below the secondary. I
adjusted the primary until the laser dot fell on the spot on the plywood. I
then fine tuned with a defocused star. So far I have only had to adjust the
tilt of the primary a slight amount to get a perfect circular star pattern.
Finally came the moment of truth, would the effort pay
off, would the improved design real be better? The first light was the Moon. Due to the tilting of the
primary mirror, the scope does not look like it is aimed at the correct area of
the sky. I first tried aiming the scope by sighting down the tube that holds
the primary mirror, and then looking into the focuser without an eyepiece. I
was just looking at the secondary. I slowly moved the scope around until the
secondary was fully illuminated by the Moon. I then inserted a 30mm eyepiece and
looked. I centered the Moon in the eyepiece and adjusted the Telrad so that it
was centered on the Moon. I went back to the eyepiece quickly and focused the
eyepiece. Did all the work pay off! Yes, the Moon looked great. I fell in love
with this 'scope. I finished this 'scope just in time for Mars' arrival. On
some nights I have seen Mars at 290X with 10mm Plossl and it looks just like
the pictures. Contrast was high and the background was black. Saturn and
Jupiter also look great, as do double stars. The tracking mount helps a great deal with the high
magnification viewing because one can concentrate on the image.
Overall, this is a very nice planetary scope. I have
looked at a few Deep Sky Objects. M13 looks like fine grains of sugar and M57
shows nice contrast a very delicate ring. The 'scope has a narrow field of view
so only a small section of extended objects like M31 can be seen even at 73X
with a 40mm plossel. It is also very light and weighs about 8 lbs. It also
holds the collimation well. With this 'scope I no longer dread the arrival of
the Moon.
In conclusion, Schiefspiegler would be a great scope for
an ATM who is looking for an advanced project. The optics are all spherical and
of the same ROC. The mirror and the tool are rough and fine ground together.
The ROC will have to be kept within one inch of the specification but that is
not difficult. The tool is cut into a smaller diameter, which becomes the
secondary. The optical performance
compares well to high price apochromatic refractors. The star tests I and Dave
Groski have done so far shows the inside and outside image to be the same.
Again, I would like to thank Dave Groski for all of his
help. He refined the optical design and cut the secondary for me. With his help
I was able to fulfill a 25-year-old wish.
References
1.
Sky and Telescope,
October 1961, Gleanings for ATM's, An Improved 4 1ž4 inch
Unobstructed Oblique Reflector.
2. Dave Stevick's Weird Telescopes Web Site,
http://P18.S502.C10.K12.WV.US/homepage/alumini/dstevick/weird.htm
3.
Advanced Telescope Making, Vol. 1, Allan Mackintosh, Chapter 3.12.
4.
Telescope Optics Evaluation and Design, by Harrie Rutten and Martin van
Venrooij, Chapter 12.
