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Primary Mirror Aperture Mask

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#1 Nebulosity_Ireland

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Posted 11 May 2021 - 07:28 AM

Hi,

 

I'm after making an Aperture Mask for my 10" Skywatcher PDS as I had significant star flare, but at the same time I have lost some real estate which I really didn't want to do.  

Are there any better clips I can buy that provide a better solution (low refraction) than the factory supplied ones? I think the design of the current ones is pretty poor.

 

I've also seen mentioned an option to silicone the primary mirror in place and ditch the clips which will provide the full area of the mirror. Is this a bad idea?

 

Thanks,

Aaron

 

Example of the Star Flare:

 

LeoTriplet.jpg


Edited by Nebulosity_Ireland, 11 May 2021 - 10:23 AM.


#2 Poynting

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Posted 11 May 2021 - 09:36 AM

I think the real estate you would lose if you masked off an annulus just to cover the clips would be negligible. Plus you are masking off an area of the mirror that may have issues as well which is a potential plus. 

 

Adhering the mirror to the cell apparently can cause issues and warp the glass enough to cause astigmatism. It is generally stated that the mirror should be able to move freely in order to expand, contract, or shift with gravity. 

 

The clips are meant to keep the mirror from tipping over, so if you know for sure that mirror will never get close to tipping over you could go without clips. I would never trust myself with that.


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#3 Garyth64

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Posted 11 May 2021 - 09:52 AM

The mirror clips really do not reduce the area of the mirror.  It's like .005%.  Any diffraction from them would be minimal.  Changing them would be a waste of time, IMO.

What is exactly your "star flare"? 

If it is at the edge of the fov, then it is coma.  Reducing the diameter of the mirror, could help eliminated some of the coma.  But I doubt it.  A 10" f/5 stopped down to 9" becomes a f/5.5.  Not much of a change.  If you want to stop it down even further, just go buy a smaller scope.

If the star flare is from a turned edge, a mask over that area could help how the stars look.  But test the mirror to see if that is the problem.

The star flare may be from a not correctly figured mirror, and a mask may not help to fix it.

Gluing the mirror to eliminate the clips, could cause the mirror to become distorted.


Edited by Garyth64, 11 May 2021 - 09:53 AM.

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#4 Nebulosity_Ireland

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Posted 11 May 2021 - 10:19 AM

I've just uploaded an image above as an example. The flare seems to lean toward the center of the image from all stars.



#5 Poynting

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Posted 11 May 2021 - 10:28 AM

I've just uploaded an image above as an example. The flare seems to lean toward the center of the image from all stars.

Do you have an example image with the mask? What is the mask made of, how did you make it? I found masking my mirror clips on my 6" f/6 mirror with a painted black aluminum annulus solved the problem of that extra diffraction for the most part.


Edited by Poynting, 11 May 2021 - 10:35 AM.


#6 Nebulosity_Ireland

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Posted 11 May 2021 - 10:39 AM

Do you have an example image with the mask? What is the mask made of, how did you make it? I found masking my mirror clips on my 6" f/6 mirror with a painted black aluminum annulus solved the problem of that extra diffraction for the most part.

I'm only getting to test it tonight. I made it from a plant pot tray so its a matt grey color and plastic. Hopefully it will solve my issue, my only concern is light loss / vignette as I already had slightly dark corners of my image (not visible in the image above). 


Edited by Nebulosity_Ireland, 11 May 2021 - 10:39 AM.


#7 Poynting

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Posted 11 May 2021 - 10:47 AM

I'm only getting to test it tonight. I made it from a plant pot tray so its a matt grey color and plastic. Hopefully it will solve my issue, my only concern is light loss / vignette as I already had slightly dark corners of my image (not visible in the image above). 

Ok, hopefully the circle is smooth enough. Light loss shouldn't be an issue if the mask is just big enough. Light fall off is correctable with flats.


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#8 Garyth64

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Posted 11 May 2021 - 11:13 AM

My first thought is, after seeing your photograph, is that the flare is not from the primary, because it is only in one direction.  Is the flare something you can see with your eye?  It is noticeable on the bright star, if that star was on the other side of the FOV would the flare go another way?

From some of the dimmer stars there is a hint of the flare, and it seems to go towards the center of the FOV.  So now I'm wondering, what does that mean?

 

Maybe the flare is an internal reflection.  Maybe something from the secondary to the camera is causing that flair.

 

It is a very good photo even with the flare.


Edited by Garyth64, 11 May 2021 - 11:17 AM.


#9 Nebulosity_Ireland

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Posted 11 May 2021 - 06:39 PM

Quick update: I took my TS 0.95x corrector off and it has solved the issue.

I assume they will not accept sending it back, so im down 200 euro.

Plus I need to buy a new one. Planning to buy a four piece 1x corrector.


Edited by Nebulosity_Ireland, 12 May 2021 - 07:22 AM.


#10 bokemon

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Posted 13 May 2021 - 04:56 AM

What you should do is take a picture of a very very out-of-focus star, so that you can see the shadow of the secondary and the spider vanes.  This should show if there is anything in the light path.  For example, it could be your coma corrector sticking into the main body of the scope.


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