Oops! Believe it or not, I hadn't intended to say
that an SCT was a flat-out better choice than a
newt. I personally love newts. I also love fast
newts. Heck! I even like the StarBlast!

My point, admittedly OT, was simply that SCT's
aren't all bad. I mentioned it in this thread, only
because the thread brought the point to mind.

Fact is, I'd have a 10 or 12 inch dob right now
if my wife would abide another large piece of
astronomical hardware in the house.

Peace With Clear Skies,
-Larry Curcio

Jon Isaacs wrote:


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