GPU Optical

Central Obstruction - Decreasing contrast

We believe this effect of central obstruction is generally over-stated by most authors. Central obstruction would NOT be a real problem at all for planetary telescopes - if we would be living on the Moon, or on a planet with no atmosphere. But Earth does have a dense and usually turbulent atmosphere, and this is what makes central obstruction a problem for mirrors (as we will see it soon). Central obstuction becomes a really painful problem only in connection with Earth’s turbulen atmosphere, NOT in itself!

The other problem caused by central obstruction, is the effect of changing the energy distribution of the diffraction pattern of the telescope. In an unobstructed system, most of the energy is in the central disk of the diffraction pattern, called Airy disk. Central obstruction transfers some energy from the Airy disk, into the diffraction rings, so, the contrast of the telescope suffers.

Though, a telescope with central obstruction performs about similarly as an unobstructed system, with slightly smaller aperture. It is widely accepted, that if we substract the diameter of the secondary mirror from the diameter of the main mirror, we get about the aperture of a refractor that is equivalend with the above mentioned reflector. And theoretically this is true ... but unfortunately, ONLY on a planet without atmosphere. But Earth is NOT a planet like that!
sunset Scientists have examined the effect of atmospheric turbulence on telescopes, and they derived the following results:
  • Under a given sky, the amount of wavefront error added by atmospheric turbulence to the telescope's own wavefront error is proportional to the aperture of the telescope.
  • The theoretical aperture of the telescope that suffers an average of lambda/6 PV error added to its optics by atmosphere might be referred as "coherence diameter"
  • Under a standard sky is, the coherence diameter is roughly 40-100mm (1.6" - 4"). Though, some places (e.g. Hawaii's Mauna Kea) with exceptionally good seeing might feature a coherence diameter exceeding these values. Naturally this value changes night by night, possibly from hour from hour. When we call the "seeing" better, the coherence diameter is larger.
  • The image is judged as "enjoyable" by most observers, until the Strehl ratio of the telescope system (atmosphere + thermal problems + all other optical errors together) is above roughly the 0.4 value.
This latter value will probably surprise most readers today, when most vendors market their scopes as optics reaching a Strehl ratio above 0.95 (95%). But there is no contradiction: even if a 4" the scope does have even a 99% strehl ratio in itself, on an average night (standard seeing, coherence diameter about 60mm), atmosphere adds more than lambda/4 wavefront error to the (otherwise excellent) optics. Still, the Strehl ratio under this sky will easily and seriously exceed the 40% value, so the image of a good quality 100mm telescope is excellent, most of the time.
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