Gamma Leonis and Airy Discs?

[Sunshine]

To be honest, airy discs were never on my list of things to look out for when looking at stars, I know little about said optical phenomenon or whether they occur in all three popular optical designs or not. Having said that, this night I was introduced to the phenomenon in the most pleasant way by way of an awful lot of magnification on the beautiful Gamma 1,2 Leonis. It would be an injustice to speak of this beautiful pair without a short introduction. They are a wonderful pair, a gold colored duo at a distance of 125 light years from earth. What really hammers home the incomprehensibly vast distances of celestial objects is the fact they're separated by 170 AU! to quote SkySafari, 4 times the distance between Pluto and the sun. It sure is mind boggling, the superlatives involved yet, these distant suns several times the mass of our star appear as two tiny dots which barely manage to excite a few cones and rods in our eyes. It seems there has been a planet discovered around Gamma A in 2010, a world approximately  9 times the mass of Jupiter though this is a very rough estimate as the mass of Gamma A is not accurately known.

It's been a while since I have trained my gaze on this pair, my 9mm revealed them easily but, the magic happened with my 5mm at 163x. Both appearing as sharp goldish discs which were so well defined in fact, they reminded me of paper discs a hole punch produces. My 2.5x powermate was begging to be used on this pair but I really didn't expect much, thinking I would destroy the pair and turn them into mush with 408x. Seeing was pretty good, those two stars at that ridiculous power still looked etched, it was a treat for the eyes but, what are these two beautiful rings around them? both looking like targets. So this is what they really look like! each star with its own perfect ring with black sky between them, a pair of owl's eyes looking back at me came to mind. Many here are experts when speaking of these airy discs, I can only describe how neat these two stars looked with these rings round them, Are they typical in all designs or is it a refractor thing? I must say, rather embarrassingly that it is the first time I have ever seen them so well defined that they actually made me take note and enjoy them. Then again, never have I hit stars with so much power, it took for me to develop a love affair with doubles that I began really hitting stars with some serious magnification.

[Nik271]

Yes, Gamma Leo (Algieba) is great to look at isn't it! Airy discs are present in all optical designs, a consequence of the waveform nature of light. In fact this is the reason that each scope has a limiting resolution, it's the width of the airy disc. There is a lot of nice maths and physics involved describing the diffraction pattern in each optical design.

If you want to learn more try this classic book by H. Suiter

https://www.firstlightoptics.com/books/star-testing-astronomical-telescopes-a-manual-for-optical-evaluation-and-adjustment-second-edition-book.html

By the way the airy discs look a bit different in reflectors, then you also get 4 diffraction spikes as in classic star photographs. I rather like these in photos. 

[Stu]

Nice report Sunshine. When seeing is good, high power views of doubles showing airy disks and their diffraction ring/s are very beautiful. Each of the scope designs will show them, but my understanding is that unobstructed scopes (refractors) put more of the energy into the disk versus the diffraction rings than other designs ie with SCTs, Maks, Newts etc so the first diffraction ring will be brighter and the disk a little smaller in these other designs.

Airy disk size is also related to aperture (a consequence of the improving resolution you get with aperture), so actually airy disks can be much easier to see in smaller scopes because they are bigger! This is one of the attractions of small scopes to me, observing doubles through them gives very beautiful bullseye airy disks which I love seeing. You can’t resolve as tight doubles with small scopes but aesthetically the views are lovely. With larger scopes the airy disk is that much smaller so you need higher power to see it and it is more often hidden in mushy seeing.

[chiltonstar]

Algieba is a superb pair indeed, with an amazing gold colour.

As Stu says, you get diffraction rings as well - you should see sharp diffraction rings around a bright star if your scope is fairly decent optically, and in focus. Difficult with some scope designs I believe which can be a bit mushy. The size of the Airy disk, and the brightness of the diffraction rings around can set practical limits to resolution of double stars.

This is an example ring pattern.

Chris

[Sunshine]

4 hours ago, chiltonstar said:

Algieba is a superb pair indeed, with an amazing gold colour.

As Stu says, you get diffraction rings as well - you should see sharp diffraction rings around a bright star if your scope is fairly decent optically, and in focus. Difficult with some scope designs I believe which can be a bit mushy. The size of the Airy disk, and the brightness of the diffraction rings around can set practical limits to resolution of double stars.

This is an example ring pattern.

Chris

Great info from all above! chiltonstar, this image is exactly what I saw, thanks for posting!

[John]

Great report !

I love Gamma Leonis - first double star that I ever observed with a scope. My old Tasco 60mm refractor many, many years ago.

Gamma Virginis is also a lovely one, rising later in the evening here. A yellow-white pair of mag 3 stars a bit closer together than the Gamma Leonis pair.

 

Edited March 21, 2021 by John