Star issue but only on one side! - Forums

Topic starter

Sep 29, 2024

Hello,

After removing the clips on the primary miror of my 300 F/4 I got rid of the 3 shadows at 120° but I have now noticed this, only on the brightest star of this image!

300s with ASI533MC Quattro 300P/F4, no miror clip...

Any idea what the issue is?

Thanks,
Cedric

Dark Matters Astrophotography

Sep 29, 2024

Could be caused by a turned down edge of the mirror. If you can, install an aperture mask of maybe 1/2” to test and see if it goes away.

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Tony Gondola

Frequent contributor

Sep 29, 2024

Before you go masking the mirror and reducing aperture why not do a simple star test. Here's a page that shows you how. The turned edge case is at the bottom of the page.

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Oskari Nikkinen

Sep 29, 2024

Does the focuser drawtube obstruct the light path when in focus? This shadow could be from this as well.

Fitting a primary mirror mask will hide much of the halo you have around this bright star, and might hide the shadow you have here now. The 3 shadows you saw before getting rid of the clips were there because that part of the mirror was hidden, so the turned down edge effect was hidden at those points. Using the same logic a primary mirror mask that reduces the aperture so that all of the edge is covered by the same amount the clips were covering would clean up much of that halo. Definitely recommend fitting one.

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Cédric Thomas

Topic starter

Sep 29, 2024

Thank you all for the good advices. The focuser drawtube doesn't obstruct the light path at all. I will try adding a miror mask and see if it fixes the problem.

Let see if I can find a model that fits my miror.

Clear skies,
Cédric

Willy Pöge

Sep 29, 2024

Bright stars can also show artifacts if your focus is slightly off, especially in fast scopes like your F/4. You might want to check and fine-tune your focus to see if that helps.

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Tony Gondola

Frequent contributor

Sep 29, 2024

Again, It's very unlikely that a turned edge is the cause. It really looks like a diffraction effect. You could get this from a rough edge, or one that wasn't smooth before the coating was applied. Yes, a mask would fix this if it has a clean edge. Just remember that most of the light collecting area of a mirror is near the outer edge. You could easily reduce your area by a significant percentage with what looks like a narrow mask. If you do apply a mask make sure it has a smooth edge and intrudes into the mirror area as little as possible.

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firstLight

Sep 30, 2024

Tony Gondola:
Again, It's very unlikely that a turned edge is the cause. It really looks like a diffraction effect. You could get this from a rough edge, or one that wasn't smooth before the coating was applied. Yes, a mask would fix this if it has a clean edge. Just remember that most of the light collecting area of a mirror is near the outer edge. You could easily reduce your area by a significant percentage with what looks like a narrow mask. If you do apply a mask make sure it has a smooth edge and intrudes into the mirror area as little as possible.

Hi Toni,

in your first message in this thread you said "Here's a page that shows you how." ... but you didn't provide a Link for "Here" ... I'm just curious ;-)

CS Frank (firstLight)

Tony Gondola

Frequent contributor

Sep 30, 2024

Tony Gondola:
Again, It's very unlikely that a turned edge is the cause. It really looks like a diffraction effect. You could get this from a rough edge, or one that wasn't smooth before the coating was applied. Yes, a mask would fix this if it has a clean edge. Just remember that most of the light collecting area of a mirror is near the outer edge. You could easily reduce your area by a significant percentage with what looks like a narrow mask. If you do apply a mask make sure it has a smooth edge and intrudes into the mirror area as little as possible.

Hi Toni,

in your first message in this thread you said "Here's a page that shows you how." ... but you didn't provide a Link for "Here" ... I'm just curious ;-)

CS Frank (firstLight)

Opps! Here it is: https://astunit.com/astunit_tutorial.php?topic=startest

firstLight

Sep 30, 2024 · Edited: Sep 30, 2024

Tony Gondola:
Tony Gondola:
Again, It's very unlikely that a turned edge is the cause. It really looks like a diffraction effect. You could get this from a rough edge, or one that wasn't smooth before the coating was applied. Yes, a mask would fix this if it has a clean edge. Just remember that most of the light collecting area of a mirror is near the outer edge. You could easily reduce your area by a significant percentage with what looks like a narrow mask. If you do apply a mask make sure it has a smooth edge and intrudes into the mirror area as little as possible.

Hi Toni,

in your first message in this thread you said "Here's a page that shows you how." ... but you didn't provide a Link for "Here" ... I'm just curious ;-)

CS Frank (firstLight)

Opps! Here it is: https://astunit.com/astunit_tutorial.php?topic=star

That is an interesting read - thanks!