Re: [ATM] Looking for information on a telescope design

["vladimir sacek" <vla@toast.net>]

Scott Milligan wrote:

> IMHO, a rather large drawback to the design is the presence of a 
> dual-field
> of view if the secondary obstruction isn't made large enough to block the
> wider field of the back end from reaching the eyepiece.

Another Mersenne drawback worth mentioning is
that the imaging element has to handle wide angles
projected from the secondary mirror. Any angle
projected from the secondary (in any two-mirror
system) is larger than the angle subtended on the sky
by a factor 1/k, "k" being the marginal ray height on
the secondary in units of the aperture radius (this is
rather obvious from the fact that the secondary is producing
linear image height identical to that of the imaginary
main aperture placed 1/k times farther away from the
final image, at the distance of the effective focal length).

So a minimum secondary that is four times smaller than the primary,
for instance (k=0.25), will produce four times larger
ray angles than those coming at the main mirror. If the
angular field is, say, half a degree, marginal ray will be
coming at the imaging element at a 2-degree angle.
This means that the imaging element in the Mersenne
arrangement will have four times more coma, and sixteen
times more astigmatism than what it would have if used alone.
In other words, it will have less than stellar off-axis performance.

Btw., this is why a two-mirror system of any given focal ratio
is harder on the eyepiece than a comparable Newtonian, or refractor.

Vlad 

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