Viewing the Galaxy with Achromat

A new third class of refractors has started to pop up, called semi-apochromatic.  Semi-apo is a very subjective term.  One manufacturer's semi-apo may be quite better than another's (and there are even variations among a single company's models).  A semi-apo is really an achromat that uses special glasses the minimize (but not eliminate) secondary color.  Technically, "apo" is a strictly defined term, meaning three colors come to the exact same focus.  However, companies are beginning to use the term simply to mean "very low chromatic aberration".  While any scope termed "apo" is going to be good, there is a wide variation in quality between manufacturers.  Semi-apos and inexpensive apos are often still doublet designs (two elements) using low-dispersion glass, rather than triplet designs.  This means the secondary color is much less than in an achromat, but is not truly eliminated as is the case with a true apochromat. Refractors over 7" in aperture are also rarely manufactured and are large and unwieldy instruments.  Since most amateur astronomers own only one telescope, and use that telescope for both imaging and visual observation, aperture becomes more important than if the telescope were being used for imaging alone.  For taking pictures, aperture is of little importance, while for visual observing it is everything!  For the price of a small apo refractor, a quite large Schmidt-Cassegrain can be had.  However, for the astroimager who desires the absolute best quality, an apochromatic refractor is hard to beat.  Many imagers use an SCT for imaging smaller targets, and piggyback a small refractor on top of the SCT to image wide-field targets.