Benjamin,
I have owned an FS-102 and it was a wonderful scope. The contrast and sharpness was excellent. At the same time I owned a 7” Maksutov. I enjoyed the refractor for quick looks, the Moon, M45, the Double Cluster, and planets, but the Mak just killed it on most every deep sky object. The difference was huge. Over time I sold the FS-102 because it was not seeing much use.
Years go by and so did a few more 7” Maksutovs including a very fine Russian I-M 715DX. Today the Maks have been replaced by a Takahashi 250 Mewlon. While I have not owned an M180, my 10” cools down very quickly. The open tube design also acts like a giant dew shield. Even on nights with fog in the air, the Mewlon keeps working (finderscope not withstanding). I would be very surprised if it took more than an hour to stabilize an M180 unless you were going out in the depths of winter.
I missed the wide field view of my old FS-102, so now I have a 6” Tak to complement the long focus Mewlon. Where am I going with all this…
A high quality 4” Apo refractor will do some amazing things with bright objects such as the planets and the moon. The FS-102 will deliver uncanny contrast and color. It will also make for a nice imaging platform in many applications. Double stars are to die for. However, the second you compare something like M13 in the 4” and the 7”, do not be surprised if your hart sinks. The cluster will go from a fuzzy patch to a sea of stars. Objects such as M42, M52, and M81 will take on a whole new dimension.
If you plan to stick to the Moon and planets, an FS-102 is beautiful scope that will deliver stunning images. Just be sure that you are not going to have an interest down the road to punch things up on M15.
Because all three are very fine scopes, if you have not considered it, a very high quality diagonal and oculars will be the key to maximizing your system’s performance. I had the chance to try some imported diagonals with a strong reputation against a 2” Zeiss prism and an AP diagonal with my refractor on Saturn during a night of very good seeing. When I replaced the ‘popular brand’ units with the AP or the Zeiss, it was like taking a thin layer of plastic wrap off the scopes lens. The extra detail and contrast was significant.
A few notes about the Mewlon.
My Mewlon delivers images that are quite sharp, with very high in contrast, and wonderful color. There is a secondary, so it is not quite what you get out of an APO refractor, but it is surprising close. This winter my M250 held up to 600x on Mars under Wisconsin skies. The surface detail and color was memorable.
The Mewlon image has a different character compared to the Maks I have owned and looked through (including some bigger TECs). I think this has something to do with having only 2 very high quality optical surfaces. To me a good Mak is a great instrument, but the view is a bit more clinical, where the Mewlon is a bit more “open. ” This is hard to put into words, so that is the best I can do.
Some have commented about coma in the Mewlons. This has not been an issue for me. Even with my 2” eyepieces, all I see is a little field curvature at the very edge of my 55mm TV Plossl.In practice this is never a problem.
Best of luck with your research and your decision.
Clear skies,
Paul
