Re: Starter scope for 9 year old: StarBlast or SkyQuest?

On Fri, 22 Dec 2006 20:16:29 GMT, AstroApp <AstroApp@xxxxxxxxxxx>
wrote:


The Orion scopes come with a simple little sight for
this purpose, and my Orion refractor even came with a little cheshire
tube (though it was plastic and certainly cruder than, say, a
Tectron.)

I should also add that the Newtonian reflectors sold by Orion have
handy little black rings mounted right in the center of the primary
morror; by following the instruction booklet, and using the little
center-hole reflecting adaptor that will be included, it is easy as
could be to line up the secondary, primary, and light path thru the
focuser.

Now, the StarBlast scope has a focal ratio of about f/4 and the
collimation is a bit more critical than some of the other Orion
starter scopes with a higher focal ratio. f/6 to f/8 scopes aren't
quite as critical -- at least for a non-expert. With f/4 if the
primary is tilted enough so that the center ring shows up way off
axis, you'd get a bit of a flare on planets, and brightest stars.
BUT...

A Newtonian with an f/4 optical system has a lot of coma (distortion
of point source images that makes them elongate rather like little
comets, the farther one goes from the center of the FOV.) This coma is
not objectionable at the higher powers one might use, but at the
lowest power star images are a bit rough, even right at the center of
the field, and the eye positioning is somewhat critical. A scope with
a higher focal ratio, such as my f/8 Orion SkyPro refractor, has much
nicer, cleaner star images. It is a much more costly scope and
requires a vastly larger, heavier mount. So, a simple Newtonian like
the Orion starter models may have star images that are intrinsically a
bit coarser off center axis, but the cost is lower, as is the learning
curve.

You can reduce coma effects in a "fast" (f4 to f5 Newtonian) by
raising the magnification and narrowing the FOV, particularly by
employing a Barlow eyepiece. Some eyepieces actually have built in
Barlows.

For instance: the Orion 6 and 9 mm Expanse eyepieces apparently have
such an element, and have a pretty nice even field of view with stars
that are fairly clean; but the Expanse 15 and 20 mm eyepieces don't
have the element, and the star images are extremely coarse in the
StarBlast, except rght in the center.

Simpler, narrow field eyepieces such as orthoscopics may have cleaner
stars in such a simple, "fast" reflector scope. Plossls are also
somewhat better. If the star images are not to your liking, raise the
magnification somewhat. I have even used 200x on our StarBlast for
lunar and planetary observing: but it's not easy!

All of these details will not be terribly important to almost any 9
year old that one can think of. They'll get so much more out of a 4
inch scope like the StarBlast than they would out of a 50 or 60 mm
refractor that the intrinsic coma is inconsequential.

The StarBlast mirrors seem to be made exceptionally well; even
advanced observers like Rod Mollise like theirs and have had many good
things to say about them.

I have also tried numerous of the Orion starter level reflectors and
find tem really good bargains, and probably good
recommendations,despite the warnings about collimation that are
sometimes given.

There are thngs that I can see with my StarBlaster that just aren't
all that apparent in the narrower field of view of my 10 and 11 inch
scopes: large angular diameter nebulae, for instance. My C-11 cannot
get the great Andromeda galaxy in an eyepiece field; one only looks at
PART of it at a time. In the StarBlaster the whole galaxy is nicely
set off against a wide field of space. I have even found that some of
the dark nebulae in Cygnus and Sagittarius are much more easily seen
in my StarBlaster, particularly the dark regions around the "Cocoon"
nebula and some of the Barnard objects. The whole Pleiades star
cluster is a joy to look at in the StarBlaster (using an eyepiece that
minimizes the coma.) So, even an advanced observer can find many uses
for this scope.

In that regard, a wide field "fast" Newtonian with small aperture is a
good complement to a more "powerful" large aperture but higher focal
ratio scope, with its intrinsically narrower field of view.

I would not think that worries about collimation are more significant
than the virtues of such a scope. And, as I said, collimating it is
easy to do -- if it's needed.

AstroApp

.

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