Hi all,
With the trio of planets of Jupiter, Saturn and Mars approaching their opposition shortly, there is one planetary feature that will show the ultimate quality of your optics - Saturn's Encke Division. Also called the Encke Gap.
You will need to push the scope's magnification to close to its limit for this exercise.
But the Encke Division (0.045") is smaller than the angular resolution of my scope!
Actually, no it's not!
The quoted angular resolution given for scopes refers to the scope's ability to resolve two stars - but stars ARE NOT pinpoints of light, but actually disks, the Airy disk, a diffraction pattern in reality - we are not actually seeing the disk of the stars. And the resolution limit is the ability to distinguish between two similarly brilliant stars to be able to make out a pinch between the two Airy disks.
When it comes to extended objects, such as the Moon and planets, the actual resolution capability of a scope can be 10 to 20X finer than the Rayleigh or Dawes limit. When it comes to extended objects, there is no diffraction pattern at play, no Airy disk - the possible diffraction pattern is totally disrupted, and the possible resolution limit is much, much finer.
Just remember, the Cassini Division (0.65") was discovered in 1675 by the French astronomer Giovanni Cassini using a 2.5" scope - the Cassini Division is much thinner than the "traditional" resolution limit for a 2.5" scope - 2.19" Rayleigh limit!!!!
I have seen the Encke (0.045") division in 7" Maks. I have also not even come close to resolving it in 10" scopes. Photos of Saturn using a 16" scope have also not shown it - could also be that the imager didn't know about the gap and then went about eliminating it!!!
Heck! By strict resolution definition, a 7" Mak SHOULD NOT be able to resolve the Cassini division either! Yet the Cassini division is not even a test for an 80mm scope... Get the picture?
The testing doesn't stop there!
If you are able to do a side by side test of a couple of 6" and larger scopes, and both scopes can resolve the Encke, the second test is seeing related. If seeing is
GOOD,
but not excellent, then the image will be a little "fluttery", like a flag gently wafting in the breeze. If the image degrades with a scintillation or shimmer in one scope, but not the other, it is the second scope other scope that has the better optics as the photons are being much more tightly controlled to go where they are supposed to. In the first scope, there are some photons that are "stray" and it is this slightly less than perfect optics that has the image scintillate or shimmer instead of waft and stay sharply focused. But you don't need a scope scope to prove this. If seeing is good or excellent, but you also see a scintillation along with the gentle wafting, then the optics are a little lacking.
Happy hunting,
Alex.
PS: When I saw the Encke Division in the 7" Maks, Saturn was a little smaller in angular size than it is right now (20th of April 2020), so you can start doing your scope bashing now!
Alex.