Thanks guys! If Porrima is around 1 sec arc, it's no wonder I could resolve it in the 6", and I'll stop patting myself on the back. But it's a puzzle that it was a mess in the 11".
Rutten (Telescope Optics, p221) gives a detailed discussion arguing that a large central obstruction (SCT) can make a small improvement in resolving power on bright doubles, but a huge worsening on low-contrast planetary detail. Berry (Star testing astronomical telescopes) makes the same point, and casually throws in (p289) that
"seeing must be superb before the double star resolution test yields interesting results".
The good old Cambridge Encyclopaedia of Astronomy says "an optical telescope of 10 cm diameter has a resolution of about 1 arc sec. If that diameter were increased to 1 metre, the scope would have a theoretical resolving power of 0.1 arc sec, but ... is
spread over an angle seldom less than 0.5 sec arc by fluctuations in the earth's atmosphere". In other words, even good seeing limits the effective diameter of the scope to 20 cm. I have a dim recollection of having read that elsewhere, but can't put my hand on it.
So on a superb night from a desert mountaintop, a superb 11" SCT should actually be better than a 6" refractor. But on a good night from a back garden at sea level, the SCT should be no better than an 8", and perhaps the other night, my very ordinary SCT looking through very ordinary to turbulent air, was much worse than the 6" refractor mainly because of the fatter column of turbulent air. Attached are some images of some random doubles on random nights through the 11". It can clearly manage 1.2 sec arc on a typical night.
I'm not trying to pontificate here - I really don't know - I like galaxies and planetary nebulae and gas and dust. They're more than a pixel across, and they have personality. For me, doubles and variables and planets and asteroids and open clusters are what you do when it's windy or the moon's up. I'm out on a limb.
I really hope this discussion excites some of you to go out and look at gamma virginis, or other doubles like eta orionis (1.5), gamma CrA (1.3), xi sco (0.6), pi lup (1.4), zet boo (0.8), - with a variety of instruments, on typical and brilliant nights, and let us know what you see.
The open questions for mugs like me are:
- what can you resolve with a 6 to 12 inch scope on a typical night from sea level, vs a brilliant night from Wirruna ?
- does a central obstruction make an important difference for us?
- what is the effect of the price tag? Should I buy a 14" ACOS RC, or stick to trying to photograph with my STL-11000M through a pair of hand-held binoculars because on a larger better instrument the seeing will get me?
Let's go out and look!
Meanwhile, does anyone have any good quantitative info on the effect of poor to median (typical) to good (one night in 10) to superb seeing on resolution in seconds of arc for a large instrument?
Cheers,
Mike
but your estimate of the sep is a bit off according to either predictions.
