Lunar Occultation of Saturn, 21st August 2024

Having waited 17 years for another lunar occultation of Saturn, and with the next one against a dark sky not until 2036, I was very keen to observe this event. As is often the case though, "against a dark sky" implies getting up rather early in the morning! In this case the occultation began about 4:30am so I set my alarm for 3:45am and hoped for the best. In fact I was quite optimistic that skies would be clear as weather forecasts for that time in the morning had consistently shown no cloud cover. I thus did a reconnaisance drive the day before to find a suitable viewing site, as I was concerned that the buildings and trees close to my house would compromise viewing from home and my usual "astro site" - the allotments (!) - had a good horizon view to the North rather than to the South-West as I required on this occasion. The edge of a recently harvested wheat field with a pull-in for my car close by seemed ideal. It would have been difficult to mark out in advance the positions I would need to set out the legs of my equatorial tripod to ensure an accurate tracking of the Moon, hence the early start to give me plenty of time for this.

The alarm going off at any time, let alone 3:45am, is always a shock but a quick look out of my bedroom window was encouraging - no clouds! I thus quickly dressed (only a little more warmly than usual, given that it was still late summer) and headed for the car, which I had loaded up with all the required equipment the day before. Once I had a clear view of the sky I looked for the Moon and noted that I could also see Saturn, slightly to top left, which was even more encouraging. On arrival at my chosen location I quickly set everything up and was soon able to see both Moon and Saturn on the viewing screen of my camera. Yes, camera this time rather than telescope, mainly because using a camera is so much easier. The main problem with the telescope is that its webcam must be fed into a computer in order to capture images and that adds a further layer of complication. Although my laptop battery is fairly reliable it would be just my luck if everything shut down just at the vital moment, with no access to mains electricity to get it going again. The downside to using a camera is that, despite its 1365mm "hyperzoom" lens, it cannot get even close to the magnification available with the telescope and so extreme close-up views of Saturn near the Moon are just not possible. On the other hand, the "full disc" views it gets of the Moon are much closer to what can be seen with the naked eye and are thus perhaps more representative of the as-seen experience.

Anyway, once was set up I took two early test shots of the scene before me, one exposed for the Moon and one for Saturn. As ever when trying to capture both Moon and planet, it can be difficult to capture both in one image because of the great brightness difference between them. And so it turned out - there was no obvious sign of Saturn on the image exposed for the Moon and the Moon was grossly over-exposed when exposing for Saturn. I thus decided I had to take longer exposure shots to ensure I captured Saturn and then deal with the over-exposure issue later in the digital equivalent of a darkroom! See the Technical Info page for the details of how I achieved this.

Having taken a couple of shots leading up to the disappearance of Saturn behind the Moon I noticed with alarm that it was beginning to disapear of its own accord! Annoyingly, a cloud bank was drifting up from the South. Not thick enough to obscure the Moon but quite capable of blotting out Saturn when illuminated by the Moon's glow. I thus took a number of speculative shots through slight gaps in the cloud in the period leading up to the actual moment of occultation but was pretty certain I had been unable to capture anything useful. Typical! I thus had no option but to return to my car, turn on the radio, get out my reading material and wait for Saturn to re-appear.

I went back to my tripod and re-aligned the camera a little before the appointed time but quickly saw that conditions were much the same as before - just enough cloud to be thoroughly annoying. I was able to get a couple of shots as the Moon moved away from Saturn but soon realised I was wasting my time after that as the cloud was getting thicker rather than clearing. Ho hum. Nothing for it but to pack everything away and return home for a bit more rest and a late breakfast.

But what had I actually managed to capture, and how did I make what I had got suitable for viewing? Click the links below to find out.

  Pictures of the occultation        Technical Info