Splitting the double double

[Andrew*]

There's something to be said for un-obstructed optics, that's for sure.

[Moonshane]

for me it's more about the exit pupil. my 6" f11 newtonian produces nice airy disk patterns around stars and increasing the focal ratio seems to have the effect of tightening stars into nice round dots which makes splits a lot more straight forward.

[Andrew*]

for me it's more about the exit pupil. my 6" f11 newtonian produces nice airy disk patterns around stars and increasing the focal ratio seems to have the effect of tightening stars into nice round dots which makes splits a lot more straight forward.

See this is where I am really confused, because the Airy disk gets larger with focal ratio, so the f/11 should have a large Airy disk, making it harder, no?

[Astro Imp]

Had a look last night and at 166x with my 6" f5 Newtonian just about managed a split. I spent about 20/30 minutes looking and was rewarded with occasional moments of steady seeing. Having said that Lyra wasn't in an ideal position, low down and viewed over my neighbour's house. Will give it another go when it gets a bit higher.

[alan potts]

I can't resist this must be on my revisit list. I will be trying this with the new Mak 180 when it comes though because of it being F15 I can't get to X48 even with my eyepieces. It will be one for the 115mm refractor and a very clear night and maybe the new 18mm Kasai ortho that will give me about X44, I won't be holding my breath though.

Alan

[Moonshane]

See this is where I am really confused, because the Airy disk gets larger with focal ratio, so the f/11 should have a large Airy disk, making it harder, no?

hi Andrew - I really have no idea. Maybe my terminology is problematic but what I can say is that when I create a slower focal ratio (or use my scope that has a slower focal ratio) the result is nice round flat discs with sharp rings around them. this answer in itself may well confirm your comment as the stars should be infinitely small rather than flat round dots. they look like this

maybe the reason that this effect occurs is that the exit pupil is smaller rather than the focal ratio slower? it does not happen with faster scopes/larger exit pupils.

[Stu]

I did a but of reading on this and it does appear that longer focal lengths give larger airy disks. I know from experience that I prefer the view of doubles in my f6.5 frac to my f20 mak, the stars are just nicer shapes and cleaner. I'm sure you are correct in what you are seeing Shane, it's just I don't quite understand the reasons for it! Possibly the smaller exit pupil reducing diffraction spikes and showing the disk and rings more clearly, guessing here though.

Stu