Posted 24 November 2013 - 12:26 AM
The obstruction of the diagonal is one of the most overemphasized aspects of reflector type telescopes and one that yields itself more to myth than reality. Increasing a diagonal (relative to the primary) does not necessarily translate to a poorer image that is obvious through the eyepiece. To talk of the obstruction creating a large enough amount of shadowing on the primary and interference that would severely affect the quality of the image, one would be looking at obstruction ratio's over 0.4. Few if any companies routinely design telescopes with such a large diagonal relative to the primary mirror. Most of the time, the obstruction ratio for most designs is between 0.15 and 0.3. Both are entirely acceptable, and it would take a keen eyed observer to notice much of a difference. John Isaacs, a long time and prominent visual astronomer here uses predominantly fairly low focal ratio telescopes (and thus with so called large secondaries/diagonals) for all kinds of observations and I have never heard him complain his 12" f/4.5 is throwing up a poor image!
The most important aspect of the secondary size is that it is not too small, i.e. effectively stopping down the telescope aperture. Using some very simple design rules, it is possible to avoid this completely. Too large a diagonal can affect the image contrast and slightly dim the object, a conservative approach aiming to fully illuminate a .5" diameter field nearly always results in a very good all round telescope. Most, if not all, major brands/makers hit the .5" - 1" mark and this nearly always results in an aesthetically pleasing image.
A 3" f/10 Newt can be designed to accommodate a .75" diagonal, yielding an obstruction ratio of 0.25, which is respectable and would compare very well with a 70mm f/10 achromat. The refractor in this case would not give a major improvement in image quality or contrast (a slight improvement noticeable in the eyepiece perhaps to a keen eyed observer, but certainly not major). There is no particular advantage of the newtonian over the achromat, although an achromat could potentially throw up some false color on bright targets.
I personally would go with the 70mm achromat for a beginner scope. It will come on an easy to use ALT AZ mount and will be good to go out of the box.