What was the largest telescope available for lunar-surface observations in the 60's to 70's.

The 200-inch Hale telescope on Mount Palomar was the largest optical telescope in the world at the time. It was used for lunar observations for NASA prior to the Lunar Orbiter missions.

Yep, and it was the biggest until 1975 when the Russians built their 238 im (6m) telescope in the Caucasus Mountains.

I often get asked by interested parties why the Hubble can't see the Apollo landing sites - after all, it can see objects millions and even billions of light years away. The answer is of course those objects are much bigger, and cover a wider area of sky than the landing sites. For those who push me for more information, I give them the math...

The resolution of the HST is not sufficient see anything that small. The theoretical resolution of a telescope is calculated using the formula....

R = 11.6 / D

where R is the the angular size of the object in arc-seconds and D is the diameter of the mirror in centimeters. The HST mirror is 2.4 meters (240 cm), so we can calculate that its theoretical resolution is 11.6 / 240 = 0.05 arc-seconds. (For comparison, the diameter of the moon as viewed from the Earth is half a degree, about 1800 arc-seconds

Due to something called Nyquist's Theorem, factors involving interference patterns and the wavelength range of visible light have to be taken into account and so the smallest resolvable object is reduced to about half the theoretical best resolution. You can read about Nyquist here...

http://en.wikipedia.org/wiki/Optical_resolution#Sensor_resolution_.28spatial.29So effectively, the HST's resolution is about 0.1 of an arc-second. The biggest things the astronauts left on the moon are the lunar descent stages. They are 4 meters across, and since they are about 400,000,000 meters away, their angular diameter is only 0.002 arc-seconds. You can calculate this for yourself. The formula is...

d / D x 206265 = R

where d is the actual size of an object in meters, D distance to that object in meters, and R is the resulting angular size in arc-seconds. The descent stages are 50 times too small to be detected by the HST. They would have to be a lot bigger to be seen at all. In fact, if you do the math and work backwards (set Hubble’s resolution to 0.1 arc-seconds and the distance to 400,000 kilometers) you see that Hubble’s resolution on the Moon is about 200 meters! In other words, even a football stadium on the Moon would look like a dot to Hubble. So why not use the 6m Hale telescope on Mount Palomar, you would think that would be better, right? In fact its worse. HST operates in a near vacuum, while the Hale Telescope operates in atmosphere, reducing its best resolution to about 0.5 arc sec. You’d need a mirror 50 times bigger than Hubble’s to see the descent stages at all, and we don’t have a 100 meter telescope handy.

This fact surprises a lot of people. They’re used to seeing the detail in HST images, galaxies and wisps of gas in beautiful nebulae. But those objects are far, far larger than Moon craters. The HST's resolution is 0.1 arc-seconds no matter how far away an object is. Those wisps of gas appear to be finely resolved, but they’re billions of kilometers across.

All this puts into perspective just now big some of those "small" craters are that can be seen in photos of the lunar surface taken from Earth based telescopes. If you can see a crater at all, its hundreds of metres across!