4 replies to this topic

[kingsbishop]

Since the intensity of the first diffraction ring is around 1.75% of the spurious disk how many diffraction rings could you see around Sirius with an 8 inch telescope and how bright would the first diffraction ring look?

Edited by kingsbishop, 23 March 2025 - 11:09 PM.

[triplemon]

It depends on the telescope. The size of central obstruction (if any) and other optical aberations will change how bright the first ring is. You would also need pretty good seeing for a 8" scope to show truely steady diffraction rings.

 

But once this is all met, I have seen in small refractor, in focus, 3-4 rings during star testing.. Same as you would see with a much dimmer star, like Polaris. As the number of rings you can still see is about the contast, i.e. scatter in the sky, telescope and ultimately in your eyes, too. So with Sirius you won't see any more than with Polaris.

 

Edited by triplemon, 24 March 2025 - 12:28 AM.

[RichA]

With a scope with a large central obstruction like an Schmidt-Cassegrain, the first ring can appear as bright as the disk, almost.  in addition, any bright star will show more than one ring, even in a refractor.  

[kingsbishop]

It depends on the telescope. The size of central obstruction (if any) and other optical aberations will change how bright the first ring is. You would also need pretty good seeing for a 8" scope to show truely steady diffraction rings.

But once this is all met, I have seen in small refractor, in focus, 3-4 rings during star testing.. Same as you would see with a much dimmer star, like Polaris. As the number of rings you can still see is about the contast, i.e. scatter in the sky, telescope and ultimately in your eyes, too. So with Sirius you won't see any more than with Polaris.

huh how does that work the second ring has a surface brightness way dimmer than the limiting magnitude with an 8 inch with perfect optics

[triplemon]

With perfect optics you'll be hard pressed to even see the 3rd.
 

Or add a large CO and both the 3rd and 4th will get definitely be there. A occulting bar to suppress the center also greatly helps with shenanigans like this.