Re: question about chromatic aberration

>
>Partially, but if so, why would fast medium sized achromats be 
>so popular today, costing more than an inexpensive 150-200mm newtonian?

I am not quite sure.  I owned a 120mm F8.3 Orion Skysomethingorother refractor
for a year or two.  It gave decent views, quite good on double stars.  And
there was no problem with cool down or collimation (once I fixed the diagonal
that is.)

So the simplicity and ease of use may be a big factor.  

But I sold it because the views IMHO just did not compare to my Orion 8 inch
DOB, especially on Saturn and Jupiter.   Of course the DSOs were much better in
the Newt though I did get one good night of seeing the mountains and got some
great views my favorite, the Swan Nebula.

My guess is they are popular because they do a reasonable job and outside
gettng the focuser to work properly they are nice to use.

>A 150mm f/15 achromat is no more expensive to make than an f/8 achromat,
>just more expensive to mount. And that seems to be its primary drawback.

A 150 mm F15 scope will be about 90 inches long, about 8 feet.  While this
really does take a substantial mount, it also takes a substantial vehicle to
move the thing.  SInce these days, people are more moblie and light pollution
is getting more serious by the day, portability is a big factor in choosing a
scope.

It seems to me that one of the places where Orion and Celestron along with
Synta have been wise is in designing scopes which are easily transported.  That
6 inch F8 refractor has a 48 inch OTA length which just happens to fit nicely
across the back seat of even a small car and the trunk of most cars.  

Add the mount required to handle a 90 inch OTA and it is pretty easy to see why
the 6 inch F8 is a popular scope.

jon