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Chip on edge of oversized secondary

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#1 mklosterman1

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Posted 29 September 2022 - 12:08 PM

Hi y'all,

I am once again upgrading my 18" f4.3 EAA dob. I had a great deal on a 4" used secondary mirror. My current secondary is a 3.5" with dialectric coating. But after calculations and visual inspection, it is clear that I am not capturing the full image field of the primary with my 3.5".  The great deal on the 4" was due to the fact that there is a small chip on the edge about 1/8" wide. I am wondering if that will cause any significant problems imaging? Could I somehow mask it? Anyone know how to make an elliptical mask?

 

As an unrelated side-note- I also just purchased the Starizona Nexus .73x coma corrector/reducer to make my setup an f3.1. Also got a Pegasus falcon rotator that I haven't put through its paces yet but should allow me to use it as a "de-rotator" to allow me to go much much longer on my live stacking in Sharpcap Pro. I can't wait but don't worry the forecast was already supposed to be cloudy this weekend even before I purchased the secondary so don't blame me for the sky conditions.

 

Mike

 

Attached Thumbnails

  • 4inSecondarywith chip.jpeg
  • 4inSecondarywith chip2 .jpeg

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#2 emflocater

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Posted 29 September 2022 - 12:29 PM

I doubt very much the image cone would even come close to that area of the chip. Lets see what other experts here think.

 

Cheers

Don


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#3 steveincolo

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Posted 29 September 2022 - 01:55 PM

Hi y'all,

I am once again upgrading my 18" f4.3 EAA dob. I had a great deal on a 4" used secondary mirror. My current secondary is a 3.5" with dialectric coating. But after calculations and visual inspection, it is clear that I am not capturing the full image field of the primary with my 3.5".  The great deal on the 4" was due to the fact that there is a small chip on the edge about 1/8" wide. I am wondering if that will cause any significant problems imaging? Could I somehow mask it? Anyone know how to make an elliptical mask?

 

As an unrelated side-note- I also just purchased the Starizona Nexus .73x coma corrector/reducer to make my setup an f3.1. Also got a Pegasus falcon rotator that I haven't put through its paces yet but should allow me to use it as a "de-rotator" to allow me to go much much longer on my live stacking in Sharpcap Pro. I can't wait but don't worry the forecast was already supposed to be cloudy this weekend even before I purchased the secondary so don't blame me for the sky conditions.

 

Mike

Re your unrelated side-note -- I'm really looking forward to your images with the Nexus. The Starizona SCT Corrector has been superb.  


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#4 bmcclana

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Posted 30 September 2022 - 10:01 PM

The effect of that chip will be vanishingly small. You should probably just forget about it.

On axis I doubt it will do anything. But that depends on how much of the 2ndary is used to fully illuminate the center of the field.

Off axis…IF the beam from the primary hits that spot it will give a little extra diffraction spike smeared out one side of the stars. And because it is on one side of the mirror, only one side if the field would be effected, the other side whose light beam didn’t hit the chip wouldn’t show any effect. How much of the field it will effect will again depend on how much of the field is fully illuminated and how much of the field actually would use that part of the mirror for any illumination.

#5 alphatripleplus

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Posted 01 October 2022 - 07:27 AM

+1 on those that say it will be hard to detect any impact. You can always do a test with a thin mask and compare with and without the mask. My guess is you'll prefer the unmasked view without the mask because of the better off axis illumination with the slightly larger secondary aperture.


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#6 mklosterman1

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Posted 03 October 2022 - 07:19 PM

Thank you! From my calculations for fully illuminated field, my 3.5” was a bit too small so I bought the 4” for a fully illuminated field. I also had the correct offset .29” built into an astrosystems diagonal holder so we shall see. Waiting for clear skies (hopefully Thursday since my weather app seems to indicate cloud-free skies)…


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