Below is a comparison of the Point Spread Function of f8.0 systems. This is at the edge of a 2.0" diameter field. RMS spot size (in microns) at the 2.0" diameter edge is given for each PSF just below the image.


f8 Classical Cassegrain

f8 Ritchey-Chretien Cassegrain

f8 Newtonian




The illustrations above show that the Newtonian offers the smallest RMS spot size. The f8 Newtonian also has, by far, the smallest central obstruction. The two Cassegrain types require secondaries that equal 37.5% central obstruction. The Newtonian only requires a secondary that is 20.1% central obstruction. Compounding the central obstruction problem for an open truss Cassegrain is the requirement for additional baffling coming off the secondary. For the Cassegrain's this puts their final central obstruction at 46.9%. The Newtonian's secondary diverts the focuser position to a perpindicular arrangement and therefore requires little to no additional baffling on the secondary itself. The area behind the secondary, as viewed from the focuser position, needs to be as dark and baffled as possible but this area can be outside the optical path.

Because the secondary on the Newtonian is so much smaller and lighter, it doesn't require spider vanes as thick as those needed for the larger secondaries of the Cassegrain types. Thinner vanes means slightly less pronounced diffraction spikes. This is a very small advantage for the Newtonian, more a point of difference between such systems.

The Newtonian is far easier to both collimate and hold collimation than the Cassegrain styles with their f3 primaries.

The parabolic primary mirror on the Newtonian is one of the least expensive mirror figures to have produced. The secondary is also a "simple" flat. From a cost/optics standpoint the Newtonian is by far the least expensive route.

The downside to the Newtonian in this focal ratio and size is its length. At 160" of focal length the telescope itself would be roughly 13 feet long. The other downside to the Newtonian is the position of the focuser. For visual use, especially at zenith, the focuser could only be reached with a tall ladder. This is less than ideal for public use. The Cassegrain design offers a lower/more accessible focuser location.

There are numerous other focuser locations for both Cassegrains and Newtonians designs. A folded Newtonian can lower the height of the focuser position. Cassegrains can use a Nasmyth focuser position. Combining an alt/az mount with a Nasmyth focuser position on a Cassegrain can yield a fixed focuser height that only rotates in azimuth, not altitude. For imaging purposes though the latter design would need a field derotator.

Specifications for the 20" f8 Newtonian: 

system focal length

optical axis to focal plane

 100% illumination diameter

M2 minor axis