A Eulogy for Russian Mak-Newt Telescopes Part One – MN76 reverse-engineering
Introduction
A now-defunct Moscow-based company, Intes-Micro, produced a line of Maksutov telescopes in the late 1980’s and 1990’s in Cassegrain and Newtonian styles[1]. One popular model was a 7” (180mm aperture) Mak-Newt tube assembly, of focal ratio F6. I bought one years ago for visual planetary observing, and recently considered whether it could be a worthwhile alternative to a 6” APO for short focal length deepsky imaging. This blog summarises my investigation.
[1] NOTE: The images of this instrument, no longer in manufacture, were copied from the orphaned Russian website http://www.astronomer.narod.ru/Telescope/IntesMicro/MN_76.htm
I hope the owner does not object to my use of this information, but if they wish to object, please contact me at towens@shibumi-astroengineering.com
Background
APM Telescopes GmbH (Markus Ludes) began marketing Russian telescopes in the EU then the US in the late 90’s. Many of these were Maksutov designs, and many originated from the same small groups of skilled optical engineers. Due to the very severe recession associated with political turmoil following the breakup of the USSR, there was interest in selling telescopes into high-end Western markets. Ludes and other Western dealers facilitated this, providing marketing and distribution. The instruments, Rumak Maksutov-Cassegrain’s and Maksutov-Newtonians caused a stir in Western markets. While the optics were better than almost anything available to the consumer market at that time, the mechanics, though highly functional, seemed somewhat home-made. Terms like ‘APO-killer’ were coined in the hobby press to try and describe the experience of using these instruments.
The reality was that the ‘collectives’ of skilled people who designed and produced these instruments were accustomed to producing scientific and military instruments. They had access to materials like LZOS optical glasses, 7000-series aluminium alloys (a series of ultra high strength aerospace grades), military-spec coatings and finishes – but not to CNC machine tools, consumer plastics technology, marketing resources etc. Consequently, the products were authentic fruits of the USSR’s defence activities. They were built in small batches by experienced optical technicians, robust, conservatively-designed, no-nonsense, from components manufactured using R&D machine tools with little regard for cost-efficiency, and extremely capable. But they were not visual works of art. Even after import tariffs and margin-taking by middle-men, pricing was attractive. Never before had the consumer astronomer had access to equipment with this level of raw optical quality .
The minimum standard of wavefront error was between ¼ and 1/6th wave P-V irregularity. Most instruments were significantly better than this, and were interferometrically tested in autocollimation. At the urging of Ludes, some lightweighting was done to optical structures, and more exotic primary mirror substrates (Astrositall and fused silica) were offered. Polishing standard was very high, and coating were of acceptable quality. Early models had simple MgF antireflection coatings on corrector plates, later upgraded to contemporary broadband antireflection technology. Design features never before seen in consumer telescope such as the signature light baffling designs, combined with the high standard of polishing, proved their worth by minimising veiling glare in planetary observing.
Reverse Engineering
The optical prescription for the MN76 was published by the manufacturer, omitting details about the corrector. Only the corrector plate needed to be reverse-engineered, along with confirming some of the manufacturer’s given dimensions. Corrector ROC values were obtained by crude profilometry using a machine tool equipped with DRO’s and a dial gauge.
The MN76 was an F6 instrument. Tube length was only 106 cm, extending to 127cm after adding the dewcap/light baffle, which is essential.
The standard instrument delivers a tiny fully illuminated field (3mm diameter at the image plane) equipped with the standard diagonal mirror, giving exquisite high magnification lunar/planetary images, used visually or with a planetary camera. Obscuration is around 21%, there is no spider vane diffraction and light baffling is superb. Hence the reputation for ‘refractor-like’ views.
The 180mm aperture stop is located at the corrector meniscus, and this, together with the slow optics, small diagonal and only 8mm oversized primary mirror, has the effect of limiting the extent of off-axis aberration. Best image surface radius is aound 4000mm, concave towards the primary. In other words, field curvature is not noticeable visually or with small chip cameras.
It is important in Mak’s used for high resolution imaging that the corrector be accurately centred and squared to the optical axis. In a relatively slow medium aperture F6 instrument, corrector decentration should not exceed 0.5mm, while corrector tilt error should not exceed 0.5 degrees to the reference axis through primary mirror and corrector cell aperture. The effect of combined decentration and tilt is easily modelled using lens design software.
In the Intes-Micro instruments, these tolerances were managed by fitting the corrector cell closely to the ID of the tube, edging the corrector lens to a fairly close clearance fit into its cell, and then shimming and rotating the corrector to give a good test result in double-pass interferometry.
I suspect that the all-spherical surfaces involved, the high-quality polishing, not to mention the habitual high working standards of the people involved in the assembly process mean that most of the instruments produced use essentially identical components, that differ mainly in the efforts made to set up OTA’s for best performance.
Paper radial shims are inserted between the corrector and its mounting cell to control decentration. There is no explicit tilt adjustment provided, nor should this be necessary, given the simple mechanical design.
Note the filtered vent holes provided around the circumference. These are unlikely to be very effective at venting tube currents.
The primary mirror is spherical and simple to make to a high standard of irregularity and polish. The maker chooses to mask off the outer 2mm of the edge, which has the advantage of removing slight turned edge.
The mirror is supported radially and axially on three silicone rubber blocks, equally spaced around the circumference. This seems to work well. The primary is fixed with minimal strain, and accurately located on the C/L of the mechanical structure.
Tip/tilt screw pairs control mirror collimation. Stability is good in the face of vibration, aided by the use of shoulder screws to minimise lateral slip.
The mirrors are aluminised and SiO2 overcoated. For extended IR performance, protected silver coating can provide a substantial boost in reflective performance at IR wavelengths with minimal loss at visible wavelengths. (It may not be easy however to source a coater willing to coat small optics at a sensible cost, however)
What we see is a typical R&D-oriented instrument design, hand-made, costly to produce, with many components and some critical tolerances. The basis of the reputation for quality performance and optical stability is evident.
