Observing in the Astronomical Twilight

25 June 2018, 23:30 – 01:00 BST

Deep-sky observing (of faint objects at least) is limited in the few weeks either side of the summer solstice, but fortunately the moon and the planets were on show. After a week of observing (and imaging) Jupiter (and properly seeing the moon’s Hadley Rille for the first time despite less than ideal seeing conditions), Saturn finally took centre-stage. While waiting for it to emerge from behind the neighbour’s tree, I looked at the following:


Epsilon Lyrae (The Double Double)
At 133x the stars showed obvious elongations, but no clear split. At 240x both pairs were resolved, but the stars had a spiky appearance, sometimes interfering with each other’s diffraction patterns.

Mu Cephei (The Garnet Star)
I wouldn’t go so far to describe it as red, but it definitely had a rich orange hue. Would be interesting to gauge the effects of different magnifications and seeing conditions on the perceived colour.

Saturn
The rings were wide-open and bright (with opposition only a few days away). Despite the low altitude (it barely cleared the fence) and variable seeing (a feature of the evening), this was by far the best view I’ve ever had of Saturn. The Cassini Division was easily resolved almost all the way around; the inner B ring was clearly brighter than the outer A ring. Saturn’s globe was well defined against the rings, giving it a 3D appearance; the usual darker equatorial cloud belt was also visible. Multiple moons seen despite the glare from the rings; according to the Sky & Telescope online app I saw Dione, Tethys, Rhea, Titan and (fleetingly) Enceladus.

Nature Note:
Another reason to tread carefully while moving the telescope from inside to outside (and vice versa): I’ve shared my recent observing sessions with dozens of tiny froglets which emerge from the pond every evening after dark.