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StarMax 127 question



 
 
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  #1  
Old July 15th 03, 07:18 PM
Skip Freeman
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Default StarMax 127 question

"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
  #2  
Old July 16th 03, 03:04 AM
GoldfishPanda
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Default StarMax 127 question

Thanks for the detailed information. Actually my scope is a 9 or 10 on that
list. I would have to look at it agin to be sure if it is a 10 or not. But,
its pretty close. I just didn't expect to see the "circles" around the star,
since you don't really see them with the naked eye.

Anyway, overall I think I have a sharp scope. I haven't used it in many
months until this week, so that is why I thought about it again.

"Skip Freeman" wrote in message
m...
"GoldfishPanda" wrote in message

...

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



  #3  
Old July 16th 03, 05:08 AM
tony hoffman
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Default StarMax 127 question

Last weekend, I tested the collimation on my StarMax 127. I had been
concerned that it might be misaligned, because images tended to go "soft" at
~100x. There's nothing in Orion's instruction manual about collimation; it
wasn't until I read Rod's book that I realized I could collimate it myself.
I called Orion, and they pointed me to a handout that I downloaded. As it
turned out, the alignment was near perfect (and now, I'm quite aware of the
diffraction rings)--which led me to ponder what the true nature of my
problem was. I knew the mount wasn't terribly steady; I finally found the
one thing I hadn't thought to tighten, and it seems to have made a big
difference. Previously reluctant doubles split more easily, and early Monday
I got my first look at Mars with it; the air was rather steady in a "cirrus
haze" and Mars near its highest, and I was able to go up to 300x without it
becoming hopelessly fuzzed out. I had almost written the StarMax off for any
high-power or planetary work; my verdict is still out, but I feel much more
favorably about the scope than I did a few days ago.

Clear (and hopefully not too light-polluted) skies,

Tony

********************************************
http://home.earthlink.net/~tonyhoffman/astronomy.htm

Amateur Astronomers Association
of New York: (www.aaa.org)



"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.

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

As a side note, I've done the star test, and it looks great even with a

7mm
lens (the circles are even)

Panda




  #4  
Old July 16th 03, 01:58 PM
Dave Jessie
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Posts: n/a
Default StarMax 127 question

"tony hoffman" wrote in message:

snip

"the one thing I hadn't thought to tighten"

snip

Tony


And that one thing was...???

Dave Jessie


  #5  
Old July 16th 03, 04:03 PM
Skip Freeman
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Posts: n/a
Default StarMax 127 question

"GoldfishPanda" wrote in message ...
Thanks for the detailed information. Actually my scope is a 9 or 10 on that
list. I would have to look at it agin to be sure if it is a 10 or not. But,
its pretty close. I just didn't expect to see the "circles" around the star,
since you don't really see them with the naked eye.


Acutally, that chart was really a demonstration of how to evaluate
seeing conditions, ie) the atmospheric conditions present on a given
night. The airy disc and diffraction rings used in the examples on
that chart do happen, I think, to pretty much be the way they should
ideally look, so if your's look like that under good seeing, then it
would seem your scope is good. However, if they look like a 9 or 10
one night and a 3 or 4 the next, that doesn't mean your optics have
gone to pot, it just means the atmospheric conditions have.

-Skip
  #6  
Old July 16th 03, 04:04 PM
Skip Freeman
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Posts: n/a
Default StarMax 127 question

Well? What exactly did you tighten?

-Skip
 




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