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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
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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 |
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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
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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
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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
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StarMax 127 question
Well? What exactly did you tighten?
-Skip |
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