8 inch Aperture Telescopes part 3 – Maksutovs and other Catadioptric Telescopes.

Schmidt-Newtonian

A Schmidt–Newtonian telescope is a catadioptric telescope that combines elements from both the Schmidt-Cassegrain and the Newtonian telescopes.  In this telescope design, a spherical primary mirror is combined with a Schmidt corrector plate, which corrects the spherical aberration and holds the secondary mirror. The resulting system suffers from less coma  than a Newtonian telescope with a parabolic mirror (which is free of spherical aberration but not free of coma) and, as the secondary mirror is supported by the corrector plate, there are no diffraction spikes caused by a spider.  They have typically one half the coma of a Newtonian of the same focal ratio so may not need a coma corrector when imaging.   A range of 6, 8 and 10 inch, f/4, telescopes were manufactured by Meade in very heavy steel tubes but these can now only be found second hand.  I have the 8 inch version and it makes a very good imaging scope. 

Maksutov Telescopes

The Maksutov  Telescope is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus corrector lens normally at the entrance of the telescope tube.  The design reduces the problems of off-axis aberrations such as coma and was patented in 1941 by the Russian optician, Dmitri Dmitrievich Maksutov.  There are to two variants; the “Gregory Maksutov” and the “Rutten Maksutov”.  In the former, the secondary is a silvered spot on the interior of the corrector plate and so must have the same radius of curvature.  This limits the focal ratio to values above f/14 so they will have long focal lengths for their aperture.  In the latter, designed by Harrie Rutten, the secondary mirror is mounted on the interior of the corrector plate and so can have a different radius of curvature so allowing  shorter focal ratios to be used.  Neither type will produce diffraction spikes. 

As the corrector plate is quite thick, the cost and weight goes up quickly  with aperture so these are rarely found with apertures greater that  ~200 mm.  This thickness also tend to increase the tube interior’s cool down time so these telescopes take the longest of all telescopes to cool down.  All using full aperture corrector plates focus by moving the primary mirror so there can be some focus shift.

The lowest cost Maksutov in this aperture range is the Sky-Watcher “SKYMAX 180mm Gregory Maksutov”  at ~£1,060  having an f/15 focal ratio so giving a focal length of 2,700 mm.  This is the longest focal length of all the telescopes covered in these articles and perfect for observing or imaging small angular diameter objects such as the planets, globular clusters and planetary nebula.  A colleague has one and is producing some wonderful close up lunar images.   

At the other end of the cost scale are the Intes-Micro, Rutten Maksutovs; their 178 mm aperture, ALTER M703, has a focal ration of f/10 (similar to a Schmidt-Cassegrain) and so a focal length of 1,780 mm whilst the ALTER M715 has a focal ratio  of F/15 thus a focal length of 2,670 mm.  These incorporate cooling fans behind the primary mirrors and are superbly made.  However they cost ~£3,000.  Their central obstructions are a little less than an 8 inch Schmidt-Cassegrains (34%) at 32% (M703) and 27% (M715) – so making the latter a superb planetary scope.  Sadly, I do not have one of these but do have the smaller M500 – an f/10, 127 mm aperture, Rumak Maksutov and can attest to its build and image quality.

A Sub-Aperture Maksutov – the Vixen VMC200L is made in Japan.  In this design a small meniscus corrector lens is located directly in front of the secondary mirror so that light passes through it twice.  As a result, dewing problems are alleviated  and the telescope tube weighs less than a Maksutov with a full aperture corrector.  With its open tube,  it will also cool down faster than other Maksutovs.  It has a focal ratio of 9.75 so a focal length of  1,950 mm –  so is very similar in specification to an 8 inch Schmidt-Cassegrain.   The cost is also very similar at ~£1,500.  A really nice feature of this telescope is the fact that it has a rack and pinion focuser behind the primary mirror so there will be no focus shift when focussing.  I do not have one, but I do have the “sister” telescope VC200L, an astrograph which uses a 6th order elliptical primary mirror and, like the Celestron Edge HD Schmidt-Cassegrains, incorporates a triplet lens assembly in the primary baffle tube to give an aberration free flat field – perfect for astroimaging.  They are beautifully made and the optical performance should be a little better than a Schmidt-Cassegrain. 

Maksutov-Newtonians

The Maksutov Newtonian design is a combination of the Newtonian reflector and Maksutov correcting lens.  Standard Newtonians suffer from coma around the edge of the field of view. The Maksutov corrector plate corrects this coma so producing coma-free stars across the field with around one quarter of the coma exhibited by an equivalent  Newtonian.  It also corrects the spherical aberration of the primary mirror.  Some have said they are like observing or imaging with an apochromat refractor!  They are superb imaging scopes.

Perhaps the best, value for money, version is the Sky-Watcher EXPLORER 190mm Maksutov Newtonian DS Telescope at a cost of ~£1,300.  This has a focal ratio of f/5.26 giving a focal length of 1,000 mm with a secondary obstruction of 33%. 

At the other end of the cost scale is the  Intes-Micro ALTER MN 76 Maksutov-Newtonian Telescope at ~£2,800 from the “Widecreen Centre”.  The telescope has a 178 mm aperture so, with a focal ratio of f/6, gives a focal length of 1,068mm.  The  secondary obstruction is just 20%.  Its superb quality optics give a ~2 degree field of view when imaging.  Having a closed tube, it will need time to acclimatise to ambient temperature.  In order to minimise the secondary obstruction these use a very low profile focuser at the focal plane which is very close to the telescope tube edge.  Having only a 20% secondary obstruction it makes a superb telescope when using a Barlow Lens for planetary observing and imaging.  Given one has a sturdy mount this would be a wonderful telescope to own.