About this earlier incarnation, Guntram Lampert writes,
"The 110mm (4.25") Kutter served me well until the Brunn
was finished. Later, I hardly used it anymore."
Last summer, a question arose: How does the four-mirror system, when stopped down to the same 110mm aperture, perform compared to the two-mirror Kutter?
Dick Suiter had done a ZEMAX analysis of the two-mirror system based on the design published in Telescope Optics, and came up with a Strehl ratio of 90% for that design. The four-mirror system is expected to operate in the range of about 97% or better (can only guess at this value because I do not have software that can calculate a Strehl number as an indicator of design quality).
There are many critics of four-mirror designs here, who are of the opinion that the many reflections would gobble up the better correction. So I tried to find out.
Location: Our roof terrace, Target: The red LEDs on a webcam on a mountain restaurant more than 3 kilometers away.
The result: The four mirror Brunn delivered textbook-like looking images while the Kutter presented images decidedly softer. Not really bad, but much more stray light. And not the pure coma one would expect from the spot diagrams.
End of story: A knife edge test of the primary seemed necessary. The commercially
produced simple spherical surface turned out to be rough and loaded with defects from what
seems bad support during polishing. A professional optician tested the mirror
interferometrically for me and found a Strehl ratio of 84% at 532nm. A picture of an artificial
star, taken at very high magnification, is included.
I then refigured the primary, ending up with a much smoother surface. Only the edge is unfortunately worse than the original surface.
The secondary was left in its original state.
I decided to construct a new tube for the optics. I was not entirely happy with the focuser, and the open construction did allow way too much dirt to get easily on the mirrors. By the way, I am not one of those guys who are too concerned about a bit of dust or a scratch on their optics.
The main mirror was situated directly over my head, and sometimes acted as a duct for warm air rising from my head, hardly improving the image. My head often bumped into the ends of the collimation screws - Ouch!
I settled on a closed tube, mainly to further reduce the already small level of scattered light. This telescope is also intended for solar observation. I installed three louvers to promote ventilation. They can be opened or closed, just as is needed.
The mirror cells are designed to allow airflow around the backside of the mirrors. The cells are now much more stable than their predecessors. The screws that support the mirror cell are held tightly by threaded inserts in the baseplate. Otherwise, the cell plus the mirror would sag considerably as the orientation of the telescope changes, therby compromising collimaton. Only now and then, a slight tweak is necessary on the primary, despite the scope is often taken around. Tweaking the collimation takes half a minute to two minutes, once you know what to look for.
Speaking of the collimation, I would be glad to offer advice on collimating such an instrument. Countless books, magazine articles and webpages deal with the collimation of the more common types of telescopes. Very little detailed information on collimation is available for Kutter Schiefspieglers and other 'Weird Telescopes'.
The photographs show the interior arrangement of stiffeners and stray light baffles. The suppression of stray light is so good that it made no visible difference if the tube was painted black inside or not, even if used in broad daylight. Based on my experiences, I am convinced that well placed baffles do much more for keeping stray light down than any black paint possibly can. The geometries of many tilted-component telescopes offer good possibilities in this direction, especially when the tilt angles become larger. The tube weighs in at 7.1kg or 15.65lb without binocular viewing attachment.
The closed tube is more sensitive to wind load. Though this is ok for my location, it is a point worth considering for others.
I am pleased with this instrument now, both optically and mechanically. The temporary mounting is a different story, could be better and will be reworked - some day.
More photos here. .. not available as of this time ..
Author's note: A variant of this design comes canned with Winspot. The filename is Schief. It uses surfaces of different radii and has even better correction than the classic Kutter two-mirror design. Recommended!
Postscript: One year later.
About one year has passed since this telescope was made. It has evolved further.
At first, I was indeed happy with the refigured primary, and in many ways I still am. In the course of making the optics for a large TCT, I learned to use the Ronchi test. Curious, I tried the Ronchi grating on this telescope.
It turned out that the edge of the primary is severely turned down. Should it become necessary to recoat the primary, I´ll definitely try to improve the edge.
It soon became evident that the Dobsonian mount on the tripod was less than ideal. I nearly broke my neck when I tried to observe close to the zenith.
Rather than using a star diagonal, I decided to make use of a diagonal from a Newtonian I once received as a gift from my friend Peter Weber, an optical designer in Switzerland.
The focuser was removed, and the diagonal was installed in the telescope tube. I obtained an inexpensive 2" rack and pinion focuser.
These two modifications proved to be the key to many rewarding nights under the stars. Now, objects high or low in the sky are easily observed while seated. A 2" eyepiece with a 46mm field stop provides low power views, with an exit pupil of 5mm at 55x. The disk of the sun or the moon fit into the field of this eyepiece with room to spare. This comes as a surprise to many who expect narrow fields from this f/ 24 system. Best of all: The good field correction and low focal plane tilt of the anastigmatic Kutter design produce images that are really sharp to the edge, undoubtedly the best off-axis performance I have ever seen!
The downside is that the images are now mirror reversed, and I´m not enthusiastic about that.
It turned out that my binoviewer would not reach focus on celestial objects with the new arrangement. After much thinking, and trying out various schemes in Winspot, I came up with a radical solution: I would repolish the primary.
Lengthening the radius of the primary by 70mm (about three inches) would provide me with all the back focus necessary for any application, without having to make modifications to the optical tube assembly! And doing so would also provide an opportunity to improve the edge of the primary. The coater would charge a small fee only for this smallish 110mm mirror.
Everything worked out as planned, and the photographs show the scope in the usual mode, and, without the two focus adaptors, in "bino-mode".
The telescope was tested interferometrically recently and found to deliver a Strehl ratio of 87%. This is pretty close to the 90% predicted by aberration theory for this design. Most of the wavefront error is the result of the coma inherent in the design. The measured coma agrees perfectly with the residual coma predicted by aberration theory.
The comparatively large measured spherical overcorrection came as a surprise. The most likely cause is the commercially manufactured secondary mirror. The primary is a decent sphere, I know for sure, and Peter would never have accepted a less than perfect flat.
The last photograph is a deliberately strongly overexposed photograph of the exit pupil of a 20mm Plössl eyepiece used on the Kutter. Such photographs reveal sources of stray light.
The only weak source of stray light is a reflection from a part of the focuser extensions that was not covered with light absorbing self-adhesive black velours. This ring of light is so weak, I discovered it only on the photo. It is hard to see even in daylight.
To further reduce tube currents, I covered the sky-facing parts of the tube with self-adhesive aluminium tape. This measure reduces radiative cooling, and keeps the parts of the instrument that "see" most open sky closer to the temperature of the surrounding air.
Users of this instrument frequently remark on the good contrast.
I really like this instrument in its fourth incarnation, and use it a lot.
I would like to express my deep gratitude to David Stevick for discussing the material presented in this article, and doing the editing, layout, and html code.
Curator: Hartmut Frommert
[Schiefspiegler Home] [SEDS]