After observing Jupiter and Saturn with my FC-100 yesterday night, and before finalizing the observing session, I went on a small tour among some of my favourite double stars. To spice things up a little, I reduced the aperture on the FC-100 to around 51 mm to hopefully challenge my eyesight and my scope. As things turned out I was quite impressed by how many classic doubles are actually possible to see with very modest aperture and 133x magnification. Here are the highlights of the observing session:
- Epsilon Lyrae (the "double-double"): First pair easily split, second pair barely split in moments of clear seeing.
- Epsilon Bootis (Izar): The B-component was barely resolved just inside the diffraction ring of the primary. In moments of steady seeing the B component was very clearly evident with its bluish color separating it from the primary component's first diffraction ring.
- Iota Cassiopeiae: This classic triple was very easily resolved and A, B, C components identified.
- Polaris: This visual double is a challenge to small apertures not because of the separation (which is wide) but because the secondary is much fainter (magnitude 9) than the primary (magnitude 2.1). Observing this pair requires good optics and good seeing and used to be considered a good test for a 3" aperture. But with 51 mm of perfect aperture Polaris was easy.
- Mizar: This classic, bright and widely separated double was of course easy.
I am impressed by how much one can actally see with modest, high quality aperture. Amateurs of long-gone days fortunate enough to own a high-quality 50 or 60 mm achromat would indeed have been able to see many classic double stars. Of course, the observer's level of experience plays a role as well. In my experience, once you have seen some targets clearly in a 100 mm scope they seem to become much easier to see in significantly smaller apertures too. Ironically, this also means that experienced observers - while perhaps wanting more aperture - actually need less aperture to see the same things.