WHICH INSTRUMENT IS BEST
FOR HIGH RESOLUTION ?
A famous astronomer said: " the worse part of the instrument is the atmosphere ". This was certainly true for him and for the telescopes that he used, but actually most of the planetary and lunar amateur images are more degraded by the instrument than by the atmosphere. The telescope is the most important element, and the highest attention must be payed to it.
The type of telescope
Even if slight differences can appear between different types of telescope, any kind of instrument can produce good results in the high resolution field: refractor, Newton, Cassegrain, Schmidt-Cassegrain, etc...The optical and mechanical quality of the telescope is more important that its type.
The optical and mechanical quality
In high resolution CCD imaging, the fields are very small because of the small size of the CCDs and the large focal lengths. Therefore the telescope does not need to give a sharp image inside a large and flat field of view. The only important thing is that, on the optical axis, the quality of the image must be irreproachable.
The quality of the image given by the telescope depends on its mechanical quality, specially the maintaining system of the optics and the focusing system. If it is difficult to align the optics, if optical elements are badly maintained or if the focusing system is not reliable, good optics will never produce the results that they could give with good mechanics.
About the evaluation of the quality of the image given by a telescope, read Star Testing Astronomical Telescope (see bibliography).
The diameter of the telescope
The diffraction laws say that the size of the smallest details that a telescope can detect is in inverse proportion to its diameter (see What is a MTF curve ?) : at equal optical quality, a large telescope theoretically shows more details that a smaller one.
But the larger instrument is more affected by the atmospheric turbulence, and its efficiency is more damaged. However, this does not mean that, in case of bad seeing, the larger instrument shows less details than the smaller one, it only means that its relative efficiency is lower. The superiority of the larger instrument may be very small in bad seeing conditions, but at least it gathers more light, ensuring shorter exposure times (a very important advantage in the struggle against the turbulence !). And its superiority will become unquestionable when the seeing improves. This is why decreasing the aperture of a telescope with a diaphragm does not improve its resolution power, unless its optics are not good.
The problem is that it is very difficult to build very good large optics, and, considering the questions of mechanical reliability, thermal equilibrium and alignment, it is also very difficult to master a big instrument. After all, a good small instrument is better than a bad large one ! Diameter race has its limits...
Refractor or reflector ?
An endless debate ! Although we sometimes meet refractor ayatollahs who swear that in any case the refractor works better, it is not serious to be so categoric. Each type of instrument has its own qualities and defects.
For high resolution, the refractor has the following advantages:
1) a refractor with a long focal lenght gives a more stable image, less sensitive to the atmospheric turbulence,
2) the tube of a refractor is closed, the wind cannot rush in it and the air inside is more stable (however, telescopes like Schmidt-Cassegrain share this advantage),
3) the light beam passes through the tube only once (against twice for a Newtonian and three times for a Cassegrain or a Schmidt-Cassegrain). Air movements inside the tube, due to imperfect thermal equilibrium, have less influence on the quality of the images,
4) a (thermal or mechanical) distortion of an optical element leads to a deflection of the light ray four times smaller for a lens than for a mirror,
5) a refractor has no central obstruction due to the secondary mirror (see What are the effects of obstruction ?),
6) focusing tolerances are larger for a refractor than for fast primary mirrors like Schmidt-Cassegrain ones (see The focusing).
The absence of collimation in a small refractor is not a technical advantage, it is only a comfort element (see The collimation).
Without any doubt, at equal diameter and equal optical quality, the refractor can obtain slightly better results in high resolution, above all on the planets. But comparing refractors and reflectors of same diameter is interesting only for an optician, not for an amateur whose aim is to obtain the best possible resolution with a limited budget. At equal cost, the advantages of a reflector whose diameter is considerably larger (2 to 3 times) are:
1) the extra diameter largely compensates for the effects of obstruction and gives a better contrast and a better resolution (see What are the effects of obstruction ?). A refractor, however perfect it is, cannot avoid the laws of diffraction, its performances are limited by its diameter (see What is a MTF curve ?).
2) the extra diameter allows to collect more light, an important advantage in photography and CCD (decreasing exposure times helps in fighting against turbulence) and in visual observation (the eye needs enough light to discern low contrast features).
At equal cost, a good reflector, even if it is more demanding (careful collimation, more delicate thermal equilibrium) can give better results than a refractor in high resolution, if its owner is ready to make the effort to learn to master it. The resolution/cost ratio is better for telescopes. J. Dragesco demonstrates in High Resolution Astrophotography (see bibliography), that the highest resolution lunar and planetary images obtained these last years by amateurs (G. Therin, D. Parker, I. Miyazaki, C. Arsidi) have been made with reflectors from 200 mm to 400 mm.
The highest attention must be payed to the chromatic correction of a refractor, specially if it is an simple achromat, because CCDs are very sensible to the close infrared part of the spectrum. Careful tests must be performed and eventually an infrared blocking filter must be installed in front of the camera ; the inconvenience of such a filter is that it decreases the sensitivity of the system by a factor of about 2.