From the Monthly Notes of the Royal Astronomical Society (1965) Vol. 216 pp 411-427

Mersenne-Schmidt Telescopes

By:    R. V. Willstrop
Institute of Astronomy,
The Observatories,
Madingley Road, Cambridge CB30HA
Accepted 1965 April 4.
Received in original form 1964 June 28.

       A three-mirror telescope is described which covers a field 3° in diameter with images smaller than 0,5 arcsec. in extreme spread: over the central 2deg. of the field the images are smaller than 0.1 arcsec.. The field is flat, which may be a significant advantage i some applications.
      Compared with the three-mirror telescope previously described, the images are worse, though still very much smaller than the seeing usually experienced in a ground-based telescope, the tube length is nearly 25% longer, and the secondary mirror is more aspheric and therefore more difficult to figure.
      Methods of polishing and testing the large and deeply aspheric mirrors required, and of checking the optical alignment, are described. With optical fibres the large field of seeing-limited definition could be used to great advantage, possibly by independent observers simultaneously.

The second article is the 3 mirror survey design.

Mon. Nat. R. astro Soc. (1984) 210, 597-609
The Mersenne-Schmidt: a three-mirror survey telescope.
Accepted 1984 March 20, Received 1984 February 10.
      The Schmidt camera with an achromatic corrector is an excellent survey instrument, but is impractical in sizes much larger than the Palomar and UK Schmidts, because of the size of the dome required. Other one and two mirror systems also suffer from limitations of focal ratio, field or tube length. Three mirror systems show more promise, and some previous designs are described. A new design of survey telescope is presented which is more compact, has a larger relative aperture, and has a larger field that most earlier designs. A large central obscuration is its weakest feature.
Index of Contents of the Articles
The Flat-Field Mersenne-Schmidt Telescope
1. Introduction
2. The flat-field Mersenne-Schmidt
3 Comparisons with the curved field Mersenne-Schmidt
4. Practical details.
The Mersenne-Schmidt: a three mirror survey telescope
1. Introduction
2. Three-mirror telescopes; previous work
3. The present design
4. Comparisons with other telescopes and Applications

      These articles were retyped from the originals which were imaged from the Institute of Astronomy article posted on the web. Items of interest are of the description of various testing and the layouts of some Mersenne-Schmidt telescopes.

Also note that these are very widefield instruments, having a FOV of 4º which is a good 3-4 times what is normal for amateur instruments. Thus, the secondary obstruction, which looks bad at the value of 40% or more of the primary, can easily be understood as it is there so that the scope can see such a wide FOV. I suspect (although I haven't inspected the design) that there can be a 4th mirror put in to direct the output to the side and thus get the instruments to where you can view the image. The reason that I suspect such is that the quaternary mirror would be in the secondary shadow with no real problems of obstructing the image from the secondary.