AP의 유저중에 한사람이 AP 사장 Roland Christen의 트라페지움 사진을 보고 다른 사진과 달리 어떻게 그렇게 잘 찍었느냐고 질문했습니다.
이글은 Christen의 답변중에 하나인데,  차폐, 정밀도등이 사진에 어떤 영향을 미치는지에 대한 진짜 오랜~ 경험담입니다. 참고로 이사람은 광학설계자의 한사람이며 제작자이고 사진도 가끔 잘~ 찍습니다. ^^

From: chris1011@aol.com
Subject: Re: AP 10" Mak Cass vs RC Optical 12.5 & 14

My answer to the original question is going to go against the grain of what
everyone has been saying on various newsgroups. We have heard from many that
central obstruction is not a factor, that it does not affect resolution or
contrast.

However, in this case, we have 4 very bright stars close to some very faint
ones. In order to resolve the faint ones near the bright ones, the optical
system has to have very high contrast. This means smooth accurate optics, and
small obstruction. Zero would be ideal, but this is impossible in a Cassegrain.
The larger the obstruction, the more energy gets thrown out into the diffraction
rings surrounding the central Airy disc. Theoretically, the light spreads out
ad infinitum from the central spot, but realistically it will go below the
noise level of your detector at some point. The brighter the star, the further
out this point is from the center. Also, the larger the obstruction, the
further out this will be. Finally, the rougher the optics, or the less accurate they
are, the farther out this energy will spread before going below the noise
level. Therefore, in order to image a very faint star close to a very bright one,
you will need to have optics with as small as possible obstruction and high
as possible wavefront accuracy.

Because of my own experience with imaging, I cannot believe that large
obstructions have negligeable effect for any given aperture size. Of course, you may
be able to achieve this type of high resolution with larger obstructions if
you have a larger aperture. Therefore, it might be quite possible to image this
detail with a 20" aperture with a 40% obstruction, but not with a 10" @ 40%.

Roland Christen