The Astro-Physics 8.1" f/12 Mak-Cassegrain
-- A Test Report

by Thomas Back



Updated 6-29-03

Roland Christen, the founder, president, designer, and chief optician of the specialty high end telescope firm Astro-Physics Inc., has been making apochromatic refractors since the early 80s. With each iteration, the optical and mechanical quality has improved to the point that his current production telescopes are among the best in the world. The same can be said of his matched accessories and computer controlled mountings.

The next step in Astro-Physics' evolution is the Mak-Cassegrain telescope. Roland has studied the intrinsic problems of the Mak-Cass design and set out to solve these problems. They are: large central obstruction, which reduces the MTF (Modulation Transfer Function) at low to medium spatial frequencies. In simple terms, this reduces low contrast planetary details. Another problem not often mentioned is how a large central obstruction makes seeing sensitivity much greater compared to a small obstructed or unobstructed aperture. This is due to the energy distribution of the Airy pattern into the first diffraction ring. During poor to average seeing, the first ring often merges with the Airy disk, and this causes the apparent seeing, contrast and resolution to suffer. Other problems like image shift, extreme thermal cool-down problems, poor wavefront quality and limitations of designs (higher order aberrations such as 5th order zonal and spherochromatism) have plagued various Mak- Cassegrains. Roland was determined to reduce or eliminate these problems, otherwise there would be little point in bringing out another "Me Too" Maksutov. The first chance I had to use and test the Astro-Physics 8.1" f/12 Mak-Cass was at the AstroFest Telescope Convention, September 1998. This was a prototype, but was finished as a production model. The tube was painted a white, pebbled, non-glossy finish. Tube assembly weight was only 16 pounds, and the scope has high efficiency mirror and corrector coatings. A 2" eyepiece holder (with moveable primary focusing), a very fine focus adjustment, with the focusing knob placed on the right side of the rear tube assembly. The first thing I noticed was the total lack of image shift while focusing. This was a first for me, as even the excellent Questars can have this problem. The quartz primary (for quick cool-down, excellent thermal stability, and smooth surface finish) was visible from the rear, in an open, ribbed structure. This also aided in a quick cool-down, which I estimated to be about 30 minutes, quicker than my AP 7.1" f/9 EDT apo refractor, which I had setup not more than 10 feet away. This Mak- Cass is really a modified "Simak" design, named after the eccentric but optical genius Mike Simmons. This design controls the aberration residuals much better than any production Mak-Cass design. Spherochromatism is under the diffraction limit though out the entire visual, photographic and CCD spectrum. Also, coma can be controlled in this design, unlike the all-spherical Gregory Mak-Cassegrains.

On to the testing. During the late night hours of Saturday night, the seeing became excellent. For 6 to 8 inch apertures, it was rated a 7 to 9 during the best moments -- the best seeing I have experienced at AstroFest in the last 12 years. Jupiter and Saturn were well placed and the views through the 7.1" f/7 EDF and my 7.1" f/9 EDT were superb. Jupiter had the kind of detail that is only seen on the Hubble shots (with less image scale and lower contrast, of course). Super low contrast polar region detail, detail within the GRS, numerous white ovals, and blue festoons (and at the best moments, the detailed "swirl" and the very bright white "plume" under the festoons could be seen). Saturn was even more impressive. It was absolutely "etched." The rings were not just sharp, nor was just the Cassini division visible all around, but the Encke Minima was visible at a glance, and the most impressive sight of all -- "spokes" were seen on the east ansa of the B-ring! All of the above was visible in the two 7.1 inch apos and the 8.1" AP Mak-Cassegrain. The images were very similar, with only the Mak giving up a tiny bit of contrast, and very slightly less stable images. Considering the portability, weight, and mount considerations (not to mention that Astro-Physics has stopped producing 7.1" apos), the prototype Astro-Physics 8.1" Mak-Cassegrain was most impressive.

The dream of an affordable (relative to Questar), large aperture Mak-Cassegrain, with the highest levels of performance -- without all the pitfalls of previous Mak-Cassegrains -- makes this telescope irresistible.

Sadly, this model was never put into production. It still stands optically as the best Mak- Cassegrain I have ever tested, and I have tested all the best contenders.

Sincerely,

Thomas M. Back
TMB Optical


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