[Wavytone]

John,

By the way, this would make a great high-school experiment for a physics class.

IMHO some of the old refractors should not be underestimated for resolution. While there were a lot of junk ones, some makers really did know how to produce a diffraction-limited lens.

In this respect is notable that quite a lot of the good quality 18th century refractors are about 3" aperture and I suspect this is the reason - smaller than this was not good enough to determine longitude to a useful accuracy.

While it's possible to observe the galilean moons in a 50mm - 60mm refractor IMHO the accuracy of the timings would be questionable as at say maximum 120X it really lacks the resolution to show the phenomena of the satellites well enough. However it is something worth trying and I'll see if I can rig up a small one around f/15...f/20.

A 75mm is a more likely candidate in terms of resolution, and in this respect I have a modern 70mm APO that shows Uranus nicely, so will try it for timing Jupiters moons next time I am setting up.

A 105mm is a definite yes, from personal experience with a beautiful working antique example. At school we had an 1880 Thomas Cooke 4.25" f/16 refractor and I spent many happy evenings behind this, Jupiters moons and Saturn were both fine sights and it was easily a match for modern scopes in terms of resolution. This scope made astronomy a lifelong passion for me.

What's more in year 12 I made a Ramsden eyepiece (using a motor driven spindle a 15mm one was quite easy to make) which showed that even modest eyepieces of that era performed quite well, despite the limited field of view (30 degrees).

Pre 1900 most objectives would have been air-spaced (as was the Cooke). The area where the old scopes suffer badly is transmission - with 8 air-glass surfaces (4 in the objective and 4 in the eyepiece) transmission was probably 50%. The old speculum mirrors don't seem so bad in comparison ! Using a modern (MgF coated) Kellner made a significant and visible improvement in transmission compared to my DIY Ramsden.

In this respect I can understand why some 18th century observers resorted to seemingly potty ideas like a singlet objective at f/30 followed by a singlet lens for an eyepiece, which would have had 75% transmission; the whole thing supported on a small crane.