"GoldfishPanda" wrote in message ...
I have a Orion StarMax 127mm, and I had a quick question.

When I focus in on a star and focus clearly, should I see little circles
around it. Sort of like a star test, but the star is in focus.

You are seeing diffraction rings. Here are some examples of what they
might look like depending on different atmospheric conditions:

http://uk.geocities.com/dpeach_78/pickering.htm

Do you see something similar to one of these pictures in your Starmax?

Someone previously told me that this means I have good optics. Just wanted
to double check.

Simply because your telescope shows diffraction rings doesn't
necessarily mean you have 'good' optics (unless 'good optics' is
defined as simply the presence of rings). To use diffraction rings to
evaluate optics, you would have to know what percentage of light is
being directed into each ring and compare those percentages to the
ideal for your particular type of scope. This ideal depends on the
size of the central obstruction of your scope.

(The central obstruction, in case you don't know, is how much of the
aperture is blocked by the round thing in the middle of the lens on
your scope. I also have a Starmax 127 but I'm not sure what percent
obstruction we are dealing with in our scopes).

But, if you have an obstruction of, say, 40% and you focus on a bright
star, you would want the Airy disc (the central, bright spot of the
star you are looking at) to have 60% of the light, the first
diffraction ring to have 26% of the light and the other rings to have
a combined total of 14% of the light.

Similarly,

25% obstruction:
Airy disc = 73%
First ring = 18%
Other rings = 9%

No obstruction:
Airy disc = 84%
First ring = 7%
Other rings = 9%

The next question is can we determine the percentage of light being
directed to the airy disc and diffraction rings in our scope? Beats
me. The heck does 18% of light look like, anyway? There may be some
some fancy optical equipment that can do this but doing it naked eye
may not be practical if you want to get a decent bead on your optics.
I'd say the star test is better for that. However, I'm not an expert
on this so maybe someone else here can shed some more light on the
subject.

-Skip

BTW: Ideal diffraction ring percentages taken from:

http://astro.umsystem.edu/apml/ARCHI.../msg00701.html