Short Focal Ratio Dobs

I think it's clear that newtonians with their smaller central obstruction,
quicker cooling and larger aperture for the same dollar provide some
compelling advantages. I bought an SCT (N11, my second Celestron SCT) for
several reasons. I wear glasses and don't like to observe with them so I
wanted goto (I have a hard time star hopping taking my glasses off and on
all of the time). I like to observe sitting down (the Starmaster hybrid
14.5 is a very tempting option), I wanted tracking. I know I can get
tracking and goto in a Starmaster 14.5 but the cost has now increased
dramatically. I've looked through about half a dozen Starmasters and have
always been impressed with the construction and mirrors. All but one I've
looked through was miscollimated to some extent. I believe folks just want
to set up and observe quickly and maybe aren't as picky. My N11 holds
collimation unless I four wheel with it to get to a dark sky/camping site
(which I've done) or tried to tweak it a bit more sometimes not improving it
(which I've also done.). You pay your money make your choice, lots of
great options out there including refractors ....

"Jon Isaacs" wrote in message
...

Yes. Not as good as an f4 with a reducer, but a lot better than
an f10 there. In addition, and this was my point, albeit poorly
expressed, an SCT tends to give better views, at f10, than an
f4 newtonian barlowed up.

Not to start a war but....

Some things to consider:

1. A Newtonian is a simple device, on flat mirror and one parabolic
mirror.
Both can and are routinely made to close tolerances. Basic Asian DOBs are
spec'd as to having a minimum of an 1/8 mirror and premium mirrors are
much
better. Probably likely that commercial SCTs are not this good.

2. Collimation. A standard Newtonian can be optimally collimated because
there
are sufficent adjustments to both the primary and the secondary. The
collimation of a commercial SCT is a compromise because it only has one
adjustment, tilt of the secondary while it has three optical elements that
need
alignment.

3. A fast Newtonian can be used with a Paracorr to correct for coma. The
Televue unit has a 15% increase to the focal length. Now add a Barlow as
you
suggest and the effective focal ratio will be 9.2.

When it comes to dealing with the aberations in a less than perfect
eyepiece,
this will perform like an F9.2 scope which is to say that the long focal
length
of the SCT is not advantage in this regard. Also apparently a Newtonian
has a
flatter focal plane and there are no chromatic aberations caused by the
corrector plate.
---

So what this tells me is that certainly a fast Newtonian can provide views
that
are superior at high magnifications that an normal SCT.

As I understand it, Todd Gross's list of the best planetary scopes he has
tried
includes several fast Newtonians. And of course there is always Mike
Spooner
with his 18 incher up the Page. AZ where he likes to view Saturn at
something
around 860X.

My personal experience is that on-axis, F4 works amazingly well, it splits
doubles and provides good planetary detail as well as contrast. Add a
paracorr
and it provides nice views all around.

I think F4.5 or F5 is a better choice but I choose F4 because it makes an
OTA
that fits in a small car.

So, I think SCTs do provide nice high power views and since they most all
have
tracking, so they are well suited for this. But optically I think there
are
good reasons to think a fast newtonian can do as well and probably better.

jon