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Star Shape Question

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  • Robert
    Hi, All I ve been imaging using 30 minute subs. My brighter stars appeared to have little hats on them. Assuming it was a DEC tracking issue, I ve been
    Message 1 of 2 , Feb 11, 2010
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      Hi, All
      I've been imaging using 30 minute subs. My brighter stars appeared to have little "hats" on them. Assuming it was a DEC tracking issue, I've been trying to fine tune guiding, without improvement. To take tracking out of the equation, I took a 5 second image (defocused), and the star profile looks the same. The "hat" is still there, and brighter stars also have a little "bitemark" at 9:00. Image scale is 3.28 arcsecs/pixel, scope is a TMB 92ss with a .8 focal reducer field flattener.
      I'm wondering if there is an issue with the objective. Could someone take a look at the image for me? It's in the Files section and is called "starshape". Thank you.
    • Wodaski - Yahoo
      It looks like diffraction. With bright stars, even very minor physical stuff in the light path can show diffraction effects - for example, clips holding a
      Message 2 of 2 , Feb 11, 2010
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        It looks like diffraction. With bright stars, even very minor physical stuff in the light path can show diffraction effects - for example, clips holding a refractor objective, bits of dust, insect carcasses, just about anything. <g>

        Sometimes you can clean it up; sometimes it's not. I don't have any suggestions for how you might track down what it is, but you can look and see what you find...

        Ron W


        On Feb 11, 2010, at 10:58 PM, Robert wrote:

        > Hi, All
        > I've been imaging using 30 minute subs. My brighter stars appeared to have little "hats" on them. Assuming it was a DEC tracking issue, I've been trying to fine tune guiding, without improvement. To take tracking out of the equation, I took a 5 second image (defocused), and the star profile looks the same. The "hat" is still there, and brighter stars also have a little "bitemark" at 9:00. Image scale is 3.28 arcsecs/pixel, scope is a TMB 92ss with a .8 focal reducer field flattener.
        > I'm wondering if there is an issue with the objective. Could someone take a look at the image for me? It's in the Files section and is called "starshape". Thank you.
        >
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        >
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