Roy Troxel said:
This is probably old stuff to some of you, but after being into this hobby for the past year, I have come to the following conclusions:
Old stuff, old message, new response.
1. Reflectors can have as high a resolution as refractors of the same aperture.
They can, but resolution is a fairly constrained characterization of a telescope's discerning ability, emphasizing the ability to preserve contrast at high spatial frequencies (very small detail). This is not an unimportant ability, especially in the area of tight doubles, but there is more to optical image quality than just resolution. Telescope A can have a better limiting resolution than telescope B, yet reveal less detail at some scales.
The usual technical description of optical quality is the modulation transfer function, or MTF. It can be thought of as a graph of contrast versus detail size. In a perfect world, contrast would always be retained 100 percent, but the world is not perfect, because diffraction causes light in the image to be spread out from where it "ought" to go. As details get small to the point where diffraction matters, contrast decreases, until you get to the resolving limit of the telescope, where it reaches zero. Broadly speaking, the MTF drops continuously from 100 percent for very large details, to 0 percent at the resolution limit. There are, of course, exceptions.
Again broadly speaking, telescopes of the same size reach 0 percent contrast at essentially the same point (the same detail size). However, the way they get to 0 percent varies. Refractors, the simple beasts that they are (in principle, at least), get to 0 in a smooth curve. Obstructed designs, on the other hand, drop faster than refractors initially, and then they rejoin the refractor curve, sometimes even exceeding it a little bit (this last part is one reason why some people try adding obstructions when trying to observe tight doubles). The main difference is in that intermediate region, where details are a few times larger than the resolution limit (e.g., a couple of arcseconds in an 8-inch scope). That's where a lot of planetary detail resides, so this is not an insignificant consideration.
All that being said, the main factor is aperture. A 5-inch refractor's MTF may be above a 5-inch reflector's MTF for a large part of the range, but they are both trumped by a 10-inch reflector (assuming comparable manufacturing tolerances, of course). That's the sense in which aperture wins.
2. Refractors have better contrast, making it easier to see details on planets or galaxies. However, contrast is not the same thing as resolution.
Well, as I said, they're not the same thing, but they're not entirely distinct. Resolution is an expression of how small you can go before contrast vanishes.
3. Reflectors can have as good resolution as refractors, but you might have to strain your eyes a little more with the reflectors.
On high spatial frequency objects, such as tight doubles, there is no reason why this should be true, and I've never noticed it.
5. In order to be properly collimated, first both mirrors must be collimated by laser, then they must be star-collimated.
That might make things easier, but no, it certainly doesn't have to be done that way--obviously, since many Newtonians have been accurately collimated since before laser collimators became available.
The whole point is to get the axis of the primary to align with the axis of the eyepiece. Everything else is mechanism. Whether you do this by star-collimation or laser-collimation or X-collimation (for whatever value of X)--the only criterion is that they align well enough to provide diffraction-limited performance. That criterion is harder to achieve for faster mirrors, and it is here that lasers can help a lot, but they aren't inherently necessary.
