야후에 개설된 미드 RCX400 그룹에서 퍼온 글입니다(번역은 자신 없습니다).
AP 사장이 지적하는 RCX400의 문제점은, 일단 중심차폐가 큰 것(50% 예상)이 문제고,
미드 홈페이지 광고에 나온 클래식 카세그레인의 스팟 다이어그램이 과장되어 있는 것 같다.
고전적 형식의 카세그레인일지라도 광고에 나온 것만큼 나쁘지 않으며
충분히 더 좋게 만들수 있다고 주장하고 있습니다.
막스토프 광학계라면 더 작은 차폐율로 코마없이 주변부까지 평탄한 망원경을
만들 수 있다고 주장하면서 막스토프-뉴턴식 광학계를 예로 들고 있네요.
참고하세요~.
If the CO is around 50%, then there is an immediate 25% light loss at
the focal plane. Add to that the efficiency of the mirrors and
corrector, around 80%, the light throughput will be only 60%, which
makes a 12" equivalent to a 9.3" clear aperture. The working F-ratio
will then be F10.3, rather than the stated F8. Furthermore, the peak
of the central Airy disc will contain only 50% of this energy, with
the rest going into the diffraction rings, mostly the first one. That
means if there is the slightest atmospheric disturbance, the star
size will broaden out to twice normal size, and the effective
resolution for tiny detail will be equivalent to an unobstructed 6"
instrument, however with the light grasp of a 9.3" scope.
One thing that I disagree with in their ads is the spot size of an
equivalent classical Cassegrain. One can design and build an F8
Cassegrain which will have much smaller off-axis coma. It all depends
on the F-ratio of the primary and the magnification of the secondary.
Tony Hallas, for instance, has a classical Cassegrain, 14"F8, which
does a very good job with his ST10XE camera, as well as 35mm film
format. Coma is essentially absent in any of his images. The
secondary mirror on his scope is 6" diameter, which makes it just
under 43% obstruction.
Lastly, one can design a 12" Mak-Cass with zero coma and very nice
flat field at F8 with less than 36% obstruction and still cover a
35mm format with tight star images. In fact, I built a 10" F9 with
only 30% obstruction that has been used for imaging with an ST10XE
camera by Trent Kjell. Even though theoretically it has some field
curvature, in actual use, the star images are pristine right into the
corners of the format, and bright stars do not bloat the way they
will in a 50% obstructed scope.
If you want to go further down in central obstruction, I would go for
a Mak-Newt. I recently had a chance to play with a 14" F4 Mak-Newt
built by Mathais Wirt using Russian optics. This scope had only a 20%
obstruction. The star images were very tight into the corners of my
ST10XE and with a simple field lens would easily cover a 35mm chip
size. I was able to do totally unguided imaging with this scope
because faint stuff registered in seconds. I fully expected the tube
assembly to be a brute, but was surprised that I could easily lift it
myself - weight was around 54lb. A 12" version of this scope would be
ideal for almost any kind of imaging, as well as visual. Not as
compact as the Meade, but not all that bad size wise.
Roland Christen
AP 사장이 지적하는 RCX400의 문제점은, 일단 중심차폐가 큰 것(50% 예상)이 문제고,
미드 홈페이지 광고에 나온 클래식 카세그레인의 스팟 다이어그램이 과장되어 있는 것 같다.
고전적 형식의 카세그레인일지라도 광고에 나온 것만큼 나쁘지 않으며
충분히 더 좋게 만들수 있다고 주장하고 있습니다.
막스토프 광학계라면 더 작은 차폐율로 코마없이 주변부까지 평탄한 망원경을
만들 수 있다고 주장하면서 막스토프-뉴턴식 광학계를 예로 들고 있네요.
참고하세요~.
If the CO is around 50%, then there is an immediate 25% light loss at
the focal plane. Add to that the efficiency of the mirrors and
corrector, around 80%, the light throughput will be only 60%, which
makes a 12" equivalent to a 9.3" clear aperture. The working F-ratio
will then be F10.3, rather than the stated F8. Furthermore, the peak
of the central Airy disc will contain only 50% of this energy, with
the rest going into the diffraction rings, mostly the first one. That
means if there is the slightest atmospheric disturbance, the star
size will broaden out to twice normal size, and the effective
resolution for tiny detail will be equivalent to an unobstructed 6"
instrument, however with the light grasp of a 9.3" scope.
One thing that I disagree with in their ads is the spot size of an
equivalent classical Cassegrain. One can design and build an F8
Cassegrain which will have much smaller off-axis coma. It all depends
on the F-ratio of the primary and the magnification of the secondary.
Tony Hallas, for instance, has a classical Cassegrain, 14"F8, which
does a very good job with his ST10XE camera, as well as 35mm film
format. Coma is essentially absent in any of his images. The
secondary mirror on his scope is 6" diameter, which makes it just
under 43% obstruction.
Lastly, one can design a 12" Mak-Cass with zero coma and very nice
flat field at F8 with less than 36% obstruction and still cover a
35mm format with tight star images. In fact, I built a 10" F9 with
only 30% obstruction that has been used for imaging with an ST10XE
camera by Trent Kjell. Even though theoretically it has some field
curvature, in actual use, the star images are pristine right into the
corners of the format, and bright stars do not bloat the way they
will in a 50% obstructed scope.
If you want to go further down in central obstruction, I would go for
a Mak-Newt. I recently had a chance to play with a 14" F4 Mak-Newt
built by Mathais Wirt using Russian optics. This scope had only a 20%
obstruction. The star images were very tight into the corners of my
ST10XE and with a simple field lens would easily cover a 35mm chip
size. I was able to do totally unguided imaging with this scope
because faint stuff registered in seconds. I fully expected the tube
assembly to be a brute, but was surprised that I could easily lift it
myself - weight was around 54lb. A 12" version of this scope would be
ideal for almost any kind of imaging, as well as visual. Not as
compact as the Meade, but not all that bad size wise.
Roland Christen
APKR250#3
