If we are talking about resolving the finest lunar details, the seeing will limit any telescope’s resolving potential.
“How often do nights of excellent seeing occur? At the William Herschel Telescope site in the Canary Islands, even this superb viewing location (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited* on 9 out of 10 nights.”
Some areas like Florida on occasion can get exceptional (10) seeing and on one of those nights a 10-12” telescope (give or take) “might” resolve to its full potential. In areas like the Northeast and many other locations no nights are exceptional (10) with at the most some (few) nights being very good (8) and on those nights a scope of 6-8” “might” resolve to its full potential. But on most nights in most locations the seeing is around average (5 -7) so larger scopes are not resolving to their full potential – the views might be brighter but the details will be limited to what the seeing allows and the same level of detail will be observed in a smaller scope that matches the seeing.
A large apochromatic triplet around 10” size would be ultimate lunar telescope in a location that could support such a size: less scatter, higher contrast and no obstruction compared to mirror scopes of any design.
A smaller apochromatic triplet around 5-7” size would be ultimate lunar telescope in a location with mostly average seeing: less scatter, higher contrast and no obstruction compared to mirror scopes of any design.
Of course, cost and bulk limit the availability of large apos so most amateurs turn to the more cost-effective mirror designs.
A Newtonian with a “very high quality” mirror, like from Zambuto, in a size that matches one’s seeing at F6 to F8 is more cost effective (although not cheap when combined with a high quality tube) and a potent performer.
For those that need something more compact or something that is easier to use on a GEM. A high quality folded design like the Takahashi Mewlons in 180, 210 or 250mm sizes, depending on your seeing, are excellent. High-end Maksutovs are also excellent but expensive and hard to get over 7”. High-end Cassegrains like those from CFF and other manufacturers would also be excellent, if you have the budget and the seeing that supports the larger size.
Most scopes can produce pleasing lunar views, but for the avid lunar observer wanting the best, the above would be some considerations.
Considerations for the lunar observer...
1. “Very high” optical quality
2. Scope and mount ergonomics (bulk, setup time, acclimation, etc.) that match one’s ability, location and observing style.
3. A size that matches one’s seeing on the vast majority of nights
4. All will be more important than a particular design.
With my seeing here in PA, 2 of my favorite, reasonably portable lunar scopes are…
1. My Takahashi TSA 120 refractor: when seeing is average (on many nights) or the moon is low to mid-altitude. I’m always amazed at what the Tak TSA 120 shows in average seeing conditions.
2. My Takahashi Mewlon 210: when seeing is above average (fewer nights) or the moon is higher than mid-altitude. With those conditions, the Mewlon 210 is one of my favorite lunar telescopes and will show the Alpine Rill, multiple craters on Plato that look like craters, etc., and all in extremely sharp, 3-D-like immersive detail.
Edited by bobhen, 23 August 2020 - 07:37 AM.