8 replies to this topic

[kingsbishop]

If we were in space or perfect seeing what size telescope would you need to see the first diffraction ring on Sirius roughly as in bad seeing it is easy but what about in perfect seeing what size telescope would you need?

[Astrojensen]

What? Any size telescope in perfect seeing will show a first diffraction ring on Sirius... That's how optics work. 

 

 

Clear skies!

Thomas, Denmark

[maroubra_boy]

As Thomas said.

Multiple diffraction rings should be visible too, getting fainter the further out from centre if the optics are good enough. Though the larger the aperture the more evident the fainter outer rings become too.

No need to be in space for this either. Tierra firma is plenty enough for this.

Edited by maroubra_boy, 25 May 2024 - 03:48 AM.

[Tony Flanders]

If we were in space or perfect seeing what size telescope would you need to see the first diffraction ring on Sirius roughly as in bad seeing it is easy but what about in perfect seeing what size telescope would you need?

This is all precisely backward. In bad seeing it's impossible to see diffraction rings at all; they're only visible when the seeing is good. And the smaller the scope, the easier it is to see the diffraction rings -- assuming there's enough light to see anything at all.

 

Sirius being as bright as it is, I'm sure a 5-mm scope operating at 10X would do the job nicely.

 

Sirius is gone from my skies now, but this might be a fun experiment to try on Arcturus now or on Vega soon. All that's needed would be a 5-mm mask for one of my refractors. Though a 10-mm mask operating at 20X would no doubt be easier.

[Sebastian_Sajaroff]

You can see it on any telescope as long as you pump up the magnification enough.
On my 3", it becomes obvious at 120x.

[Migwan]

In excellent seeing, I've noticed the first diffraction ring down to magnitude 3 & sometimes even 4 stars, with the 80ED.  This, at relatively low magnifications.  In such instances, I don't see a second or third ring on the lower mag stars at lower magnifications.  Even on brighter stars, the second an third rings are barely visible.   

 

I don't have anything smaller, but will try an aperture mask next time I'm graced by such seeing.

 

In the C11 in excellent seeing, I've seen the first diffraction ring a number of times on the Trapezium stars.   Second and third rings were not visible at the relatively lower magnifications.   On brighter stars, the second and third rings were just visible at lower magnifications.    As I up the magnifications on brighter stars, more light spills into second, third and even fourth rings, making them more obvious. 

 

Hope that makes sense.

[Asbytec]

...what size telescope would you need to see the first diffraction ring on Sirius...?

The Airy disc scales inversely with the size of the aperture. So, as Tony said, a small 5mm aperture will show it at 10x because the Airy disc is very large. Since it scales with the aperture, a 50mm aperture will show the same at 100x, a 100mm aperture will show the same at 200x, etc.

 

Each is operating at 50x per inch, which is plenty of magnification, and is true for most bright stars, too. You may be able to see the Airy disc and first ring at 25x per inch, too, but it becomes increasingly difficult at increasingly lower magnifications.

 

Attached is my sketch of Sirius in very good seeing through my 150mm aperture at around 300x. I could see about 7 rings, best I could count them. The "Pup", Sirius companion double star, is in there somewhere. I have a sketch of it somewhere, too.

 

Edited by Asbytec, 25 May 2024 - 09:03 AM.

[happycamperjohn]

I’m wondering if the OP has misunderstood that the diffraction ring isn’t a physical phenomena that occurs “at the star”, but is an artifact “at the telescope”.

[luxo II]

I’m wondering if the OP has misunderstood that the diffraction ring isn’t a physical phenomena that occurs “at the star”, but is an artifact “at the telescope”.

The more I thought about this one, the OP's question isn't so silly. As you correctly point out, diffraction is an artifact of the telescope.

The interesting thing is that reducing the aperture increases the diameter of the diffraction pattern, as well as dimming the star.

Given Sirius is easily seen naked eye in the evening sky and even daylight if you know where to look, this means the eyes aperture (1-3mm) should suffice. But the eye alone doesn’t have enough resolution (barely 1 arc minute).

But if you shrink the aperture to say a 0.5mm pinhole you should still discern Sirius, and a diffraction ring.

 

Alternatively use a larger aperture with a little magnification, such as a 8X finderscope with an aperture mask stopped down to say 5mm.

Edited by luxo II, 26 May 2024 - 05:48 AM.