Re: 4 inch reflector can't resolve Mars at all

On May 10, 9:20 am, Chris L Peterson <c...@xxxxxxxxxxxxxxxxxx> wrote:
On 10 May 2007 03:17:25 -0700, wsnel...@xxxxxxxxxxx wrote:

Here is how a newbie might interpret this subthread:

USENET User 1: "Dobsonians don't track and at high power it's hard to
keep an object dead-center in the field of view, and if you don't,
coma will ruin the image."

USENET User 2: "That's not coma, that's astigmatism. I had to use a $
$$ eyepiece to get rid of the astigmatism, so that I could even see
the coma."

USENET Lurker/Newbie thinks to himself: "Wow, Dobsonians have so many
problems. No wonder they don't cost much. Maybe I should try
something else. Maybe this hobby is too expensive."

Well, here's how a newbie _should_ interpret things:

If I'm interested in critical, high power observation of planets, I need
to be aware that a Dob doesn't track, and keeping the planet in the
center of the field requires some effort. That may be more effort than
I'd like to spend, and may indeed interfere with my ability to observe.
I should consider evaluating a Dob for planetary observation before
purchasing one.

I should also be aware that typical uncorrected Newtonians display
several off-axis aberrations, and while these are usually considered
fairly minor, they are significant enough to visibly degrade planetary
images as the target leaves the center of the field.

Nothing here says that a newbie shouldn't buy a Dob, or that they have
lots of problems. It only says that (like all scopes) they have
limitations that should be considered by a potential purchaser, and that
those limitations happen to be a little more obvious to planetary
observers than to deep sky observers.


To obtain tracking with all but the smallest scopes often seems to
double the cost of the instrument, conservatively. Or for the same
outlay you might get about half the aperture with a scope that
tracks.

Comparing a clock-driven 4-inch with an 8-inch Dob, by the end of the
night, you will have seen more detail in the larger scope. No matter
how long you stare at the perfectly centered image in the smaller,
clock-driven, scope, you will not, ultimately see as much as you would
through the larger Dob.





.

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