Sky brightness was 17.9 to 18.1 mag/sq.arc.sec., with relative humidity about 80%. Probably as dark as it gets for a clear Toronto area night. Over the past few weeks of relatively poor weather I’ve managed to put the C9.25 seriously out of collimation, so tonight was a night for slow rebuilding and only limited observing.
In collimating a Schmidt-Cassegrain scope, the aim is to get the main mirror of the scope lined up with the secondary mirror and the centre of the eyepiece holder all at the same time, so in principle the optical axis of the primary, secondary and eyepiece(s) are all colinear. Unfortunately the only thing you can adjust on an SCT is the secondary, and the adjusting screws are just enough out of reach that it’s pretty much impossible to watch what happens while you adjust the screws.
Previously I had managed to mess things up royally by using a laser collimator. It’s not impossible to align an SCT using a laser, but it’s much more difficult than with a Newtonian, and I didn’t take enough time to assemble everything I would need to do this, and made things worse rather than better. Tonight I went back to older methods which use the way a star appears in the eyepiece when it is close to focus. With the high humidity the atmosphere was fairly steady, and I think I’m much closer to collimation than I was previously. Unfortunately, there’s more to be done.
Anyway, along the way I was able to watch Europa, Io, and Ganymede dance around Jupiter. Europa transited the face of Jupiter just after midnight, but while I could see Europa, my coolimation at that point wasn’t good enough for me to distinguish Europa’s shadow.
I hopped back and forth from Altair to Vega to Jupiter and then around again, each time trying to refine the collimation. Around a quarter to one, Io came slipping out from behind Jupiter, so I had a new satellite to watch as I kept making my rounds. About an hour after that Io and Ganymede exchanged places as the Io drew away from Jupiter and Ganymede pulled in closer. I still couldn’t make out Io’s shadow, and by 2.20 it was pointless to look for it, as it was no longer on the face of the planet at all. Half an hour later I was getting glimpses of Ganymede’s shadow on the face of the planet.
However, one problem with adjusting the collimation is that you can’t collimate and use a dew shield — it’s just too much trouble to reach the collimation screws when they are deep inside the dew shield, so while I was getting hints of Ganymede’s shadow the scope was dripping wet: the dew alone was enough to mess up the view, even if I had had perfect collimation, so just before three am I called it a night.
Still, this was an enjoyable night after many days and nights of rain.