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Hi everyone,

I'm trying to figure out the cause of these weird star shapes I'm getting with my 130pds. I had them before, but someone narrowed it down to being pinched optics. So I loosened the primary retention clips, which seemed to fix the problem. Now that I've been able to get back out for another night of imaging, they seem to be back again! I made sure that the primary retention clips are still loose - should they be touching the mirror or not? I've also made sure that the primary collimation locking screws aren't done up too tight as well. Collimation through my cheshire looks to be spot on to me. Here's my image of Andromeda with the odd stars in question:

AndromedaGalaxyFinalHDRCrop.thumb.jpg.c7d576d58f1045887b4cad05a85f8860.jpg

 

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6 minutes ago, JamesAstro2002 said:

Hi everyone,

I'm trying to figure out the cause of these weird star shapes I'm getting with my 130pds. I had them before, but someone narrowed it down to being pinched optics. So I loosened the primary retention clips, which seemed to fix the problem. Now that I've been able to get back out for another night of imaging, they seem to be back again! I made sure that the primary retention clips are still loose - should they be touching the mirror or not? I've also made sure that the primary collimation locking screws aren't done up too tight as well. Collimation through my cheshire looks to be spot on to me. Here's my image of Andromeda with the odd stars in question:

AndromedaGalaxyFinalHDRCrop.thumb.jpg.c7d576d58f1045887b4cad05a85f8860.jpg

 

Its probably tube droop on the focuser causing tilt and that is variable according to the direction that the scope is pointing in. The color fringes on the stars towards the edges are inherent to the Baader Field Flattner as it suffers from some degree of radial CA.  

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1 minute ago, Adam J said:

Its probably tube droop on the focuser causing tilt and that is variable according to the direction that the scope is pointing in. The color fringes on the stars towards the edges are inherent to the Baader Field Flattner as it suffers from some degree of radial CA.  

Okay, thanks for the response. I should probably mention i'm using the Skywatcher Coma Corrector.

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Just now, JamesAstro2002 said:

Okay, thanks for the response. I should probably mention i'm using the Skywatcher Coma Corrector.

Similar two element design, to prevent that issue you need a three or even four element corrector. 

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1 minute ago, Adam J said:

Similar two element design, to prevent that issue you need a three or even four element corrector. 

Oh right, okay. I've tapped an extra thumbscrew into the focusser as well, so I would've thought that would help with any tilt that may occur.

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14 minutes ago, JamesAstro2002 said:

Oh right, okay. I've tapped an extra thumbscrew into the focusser as well, so I would've thought that would help with any tilt that may occur.

I have done the same, I still have issues. I think that nothing short of a moonlight focuser will totally solve it. 

The other thing is that the secondary mirror can shift about if you have not got the spiders sufficiently tight (just dont distort the tube!). 

Adam

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11 minutes ago, Adam J said:

I have done the same, I still have issues. I think that nothing short of a moonlight focuser will totally solve it. 

The other thing is that the secondary mirror can shift about if you have not got the spiders sufficiently tight (just dont distort the tube!). 

Adam

Yeah, a moonlight would be great.

Hmm, interesting. They seem perfectly tight to me already, though.

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40 minutes ago, JamesAstro2002 said:

extra thumbscrew into the focusser

Hi

Yeah. The SW focuser is hopeless. If it doesn't tilt, it slips. The best way we found is with one of the older built-like-a-tank- 62mm rack and pinion focusers. A drop in replacement to any sw baseplate. OK, no fine focus but hey, it it just works.

Cheers

Edited by alacant

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2 hours ago, alacant said:

Hi

Yeah. The SW focuser is hopeless. If it doesn't tilt, it slips. The best way we found is with one of the older built-like-a-tank- 62mm rack and pinion focusers. A drop in replacement to any sw baseplate. OK, no fine focus but hey, it it just works.

Cheers

Yeah, the focuser can be a pain at the best of times, lol. Guess I just have to play by it's strengths.

That focusser does look a bit of a tank!

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I'm still thinking that my star shape problem is because of the mirror being pinched. Do the clips need to be touching the mirror or not ? Because I have loosened them but they still touch the mirror. 

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8 minutes ago, JamesAstro2002 said:

Do the clips need to be touching the mirror

No. At least a sheet of paper's clearance. 

HTH

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12 minutes ago, alacant said:

No. At least a sheet of paper's clearance. 

HTH

So I just tried looseing the clips more, but no mattter how much I loosen them they still come in contact with the mirror. Almost looks like there should be a spacer to raise the clip so it doesn't come it contact with the mirror.

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1 minute ago, JamesAstro2002 said:

looseing the clips more

Remove them completely. Replace and wiggle them as you tighten. Stop when you feel contact then back off.

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5 minutes ago, alacant said:

Remove them completely. Replace and wiggle them as you tighten. Stop when you feel contact then back off.

Thanks. Just tried that, but still, they fall back and touch the mirror.

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Actually, I just thought. Could I take off the black plastic bit on top of the clips, then put them underneath so they act as a spacer so the clips won't touch the mirror? If that makes sense, lol.

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9 hours ago, JamesAstro2002 said:

they fall back

Ah, I think I see what you mean.

They are loose, yes? IOW, they are not held against the mirror by the retaining screws? You can move them easily? If so, that's it; the screws do not hold the retaining clip tightly, allowing the clips to move and there is a gap so that the sheet of paper can fit between the clip and the mirror...???

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4 hours ago, alacant said:

Ah, I think I see what you mean.

They are loose, yes? IOW, they are not held against the mirror by the retaining screws? You can move them easily? If so, that's it; the screws do not hold the retaining clip tightly, allowing the clips to move and there is a gap so that the sheet of paper can fit between the clip and the mirror...???

Yeah the clips still touch the mirror, I could only slide a sheet of paper between the clips and the mirror if I lift them up, otherwise they fall back and just rest on the mirror's surface. I'll just send some pictures to show you what I mean.

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20190904_113850.thumb.jpg.54629ec8e320425f7ee7db360d1d8c81.jpg

Hopefully this shows what I mean. No matter how loose I make the screws, the clip will always drop down and rest on the surface of the mirror. Which I'm guessing is wrong, seeing as you said I should be able to slide a sheet of paper between the clip and the mirror.

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56 minutes ago, JamesAstro2002 said:

the clip will always drop down and rest on the surface of the mirror

That's exactly how it should be.

1 hour ago, JamesAstro2002 said:

I could only slide a sheet of paper between the clips and the mirror if I lift them up

Perfect.

 

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1 hour ago, alacant said:

That's exactly how it should be.

Perfect.

 

Oh right, okay. Thank you. Hopefully there should be a patch of clear skies tonight so I can give it a test.

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11 minutes ago, JamesAstro2002 said:

give it a test

OK, but I don't think the mirror clips were the problem. More, the limitations of the focuser and the cc.

Good luck anyway.

Edited by alacant

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6 minutes ago, alacant said:

OK, but I don't think the mirror clips were the problem. More, the limitations of the cc.

Good luck anyway.

Right. Have you got any other ideas in mind I could try before getting out under the skies for a test?

Thanks for taking your time to help me out so far.

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58 minutes ago, JamesAstro2002 said:

ideas in mind I could try

Hi. No. I think it bet to restrict the number of variables. So far you have the mirror clips and your third grub screw. 

Put back the mirror, restore collimation and report back.

Hopefully you'll have done enough:)

 

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Yeah, true.

As I was doing the collimation with the cheshire, I noticed that as I reeled out the focusser, collimation slowly went out!

This is with the focusser all the way in:

In.thumb.jpg.7d93c1c6c0dc9b5a3912ea26075a3774.jpg

And all the way out:

Out.thumb.jpg.424bbe5672761730bd92f4f649abd999.jpg

I'm guessing this is due to some tilt in the foccuser or something?

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