I just got a 20 inch telescope. I’m trying to figure out what it can achieve vs my 12 inch telescope. Is it possible to actually see the disc of Saturn’s moon Titan? Or is that going to depend on the seeing conditions more than anything?
According to its wikipedia page, Titan’s disc is 0.8 arcseconds in diameter.
Dawes’ Limit stipulates that a 20 inch scope can resolve objects no smaller than 0.22 arcseconds.
So under exceptionally stable atmosphere, there’s a chance you could see Titan as more than a point of light.
Ganymede (Jupiter’s largest moon) subtends 1.2-1.8 arcsec and I’m able to resolve it as non-stellar (and occasionally see some surface coloration) using my 14" scope. But it takes exceptionally stable air, a well collimated and thermally equalized scope, and some luck. Seeing Titan as non-stellar would be at least twice as difficult, bordering on impossible. And it doesn’t help that Titan has a thick cloud deck, meaning you don’t have the benefit of surface coloration to make it obvious that you’re not just seeing a single unresolved point of light. You would need a star nearby Titan so you could compare the size of the Airy disc to confirm whether you’re actually seeing more than a point of light.
Titan’s angular diameter is about 0.85 arcsec. The Dawes limit for a 20" instrument is 0.23 arcsec.
So, technically the answer is yes. But you only get 3 Dawes limits across the diameter, so you will not be able to see any substantial detail - not that it has much detail to begin with, anyway. But yes, you will be able to tell it’s not a star, and it has an actual apparent diameter.
Keep in mind that 0.23 arcsec seeing is rare. Keep an eye on the seeing forecast for your area, and test the seeing when you begin observations.
The scope must be in perfect collimation, and must be at thermal equilibrium with the environment (a hot scope brought out of the house will underperform for an hour or so).
You will need fairly substantial magnification, several hundred for sure, maybe approaching the 1000x limit of your aperture. Seeing will be the major limiting factor most of the time.
I’m trying to figure out what it can achieve vs my 12 inch telescope.
The Dawes limit for the 12" is 0.39 arcsec. Titan would look like a fat star. Again, perfect everything is the requirement here.