[gregbradley]

I thought this was a very informative post by Roland Christen of AstroPhysics fame:

Reflector - refractor, what is the difference?
Under real good seeing of course there is very little difference between reflector and refractor of same aperture size. Such seeing as we get in Chile at Las Campanas which is usually under 0.5 arc seconds. However, here in the middle of the country, we often have the jet steam above, so seeing is always compromised.

It is somewhat complicated, but I will try to make it very simple as a first order approximation. In a refractor the theoretical strength of the Airy Disc is 84% of the energy, with 16% in the diffraction rings surrounding the central Airy Disc. The first diffraction ring is the brightest, which makes the diameter of a star of long exposure to be approximately 2 times the theoretical resolution. When the image wiggles due to atmospheric instability it "paints" a star diameter somewhat larger than 2x the theoretical resolution, depending how much the star wiggles. Let's say that a 7" aperture can resolve 4.5/7 = 0.64 arc sec, so under really perfect seeing you might get 1.3 arc sec star disc, with maybe 0.9 arc sec FWHM value. The shape of the star brightness is a Gaussian distribution with a base diameter of 1.3 arc sec and a peak diameter of 0.64 arc sec. This would be the absolute limit for that aperture.

In a reflector, you always have some central obstructions. Large obstruction occur in fast instruments like the R-H F3.8 astrograph exceeding 50% by diameter, and perhaps 40% for an F8 Astrograph. The obstructions are necessary in order for the system to cover a wide field. You get similar obstructions in fast Newtonian astrographs, something that is unavoidable if you want to cover a large chip. So let's regard a 40% obstruction mirror system. The immediate effect is that the central Airy disc drops to around 60% of the energy with 40% remaining going to the surrounding diffraction rings. In fact, the second ring is now brighter than the first and has considerable energy. Thus the star image that gets "painted" in a time exposure is now some 3 times larger than the theoretical resolution limit versus 2 times for an unobstructed aperture. There is also considerable energy further out, so in terms of raw resolution you may see even in perfect seeing somewhat larger star sizes for the same aperture. When the seeing is not really good, all these rings begin to paint their own diameters and you get poor resolution.

Of course reflectors generally are larger in size, so the actual arc second resolution may still be larger than a much smaller refractor, especially when the seeing is excellent. In our case, the 12" F8 astrograph down in Chile experiences seeing of less than 0.5 arc seconds, and the best images I have recorded in tests have been on the order of 0.9 arc sec FWHM for a 10 second exposure. It might have been better but we are limited by the 9 micron pixel size of that setup.

Rolando

I think this is also why we see some star bloat with bright stars with some CCDs. Small wells mean the outer airy discs can saturate quickly given how much of the energy in some scopes goes to them. Its not a matter of pixel bleed as CCDs are usually anti blooming but merely the outer airy discs saturating the small pixels charge carrying capacity.


Greg.