Advantages to long-focus Newtonians: 1. Larger diffraction-limited area at focal plane 2. Smaller secondary mirror for a given area of 100% illumination (less total diffraction) 3. Faster grind, easier to figure 4. More forgiving, easier to collimate (relates to #1, above) 5. Excellent performance using Plossl, ortho, and other simple eyepieces Advantages of short-focus Newtonians: 1. Short tube. This is only an advantage in large apertures, where a longer scope would necessiate using a ladder to reach the eyepiece. 2. Wider field and lower power using the same eyepiece as a longer-focus scope. This is more of a "trait" than an outright advantage. "Wide field" has been used as a marketing term across the board in visual astronomy, and it's not always desireable. It's an aesthetic quality only and valued differently by different observers. A planetary observer has little use for a 90-degree AFOV eyepiece. The biggest disadvantage of a short-focus scope is that coma increases alarmingly as the focal ratio drops below about f/5 in a Newtonian. The "sweet spot", or diffraction-limited portion of the focal plane, drops below an 1/8" circle at f/4. So if your wide-angle 2" eyepiece has a field stop that's about 47 or 48 mm across, only the very center of your field is tack-sharp. The way to combat this is with expensive eyepieces such as Naglers and others that are specifically designed to correct coma. Another option is a coma corrector that allows most any eyepiece to be used, as well as correcting the focal plane for imaging. This is what I personally use, a 20-year-old Lumicon coma corrector. The disadvantage to using expensive "prescription" eyepieces, apart from cost, is reduced throughput from six, seven, eight or more elements in the eyepiece. Be sure to buy eyepieces with good anti-reflection coatings to minimize this. An enhanced coating on the mirrors themselves also maximizes throughput. That said, reduced throughput really not much of an issue with large-aperture scopes (10" and larger). Brent knows that at f/8 or higher, a Newtonian becomes an incredibly sharp telescope and the views can be amazing. Going shorter means trading sharpness across the entire field for convenience. On Wed, Jan 7, 2009 at 11:04 AM, Jim Gibson <jimgibson00@yahoo.com> wrote:
When I took the class from Brent Watson he wanted all of us (about 5 or 6 in the class) to make f8 or better scopes.