Clouds over Omicron Velorum
I've been taking photos of star clusters for a while now. I've been trying to demonstrate how much people with small telescopes in the city should be able to see. This one didn't go so well.
Clusters are not only pretty, and most don't need lots of magnification. They're also largely immune from the city's light pollution.
So I went out a week or two back to the dark sky site with my largely autonomous equipment all programmed to take photos of five different clusters. I set up, polar aligned, sorted out the inevitable technical problem that cropped up (urghh) and then got started on the first of the five targets.
The night was pretty good, with the Moon setting in the west and not being too much of a problem (remember star clusters aren't badly affected by stray light). The first two images went without a hitch, and so I decided to leave the telescope doing what it was programmed to and I retired to my tent.
As I slept, a thin, whispy layer of high cloud crept in. My faithful telescope, guided beautifully by PHD's new multi-star guiding, never missed a subexposure, but all the images were affected to an extent.
Have a look at these. I use a monochrome camera, so all the subexposures are black-and-white. I shot through a red filter first, and they weren't badly affected as the cloud hadn't thickened up at that stage. However, the blue and green filter subexposures were badly affected. You can see a horrible halo around the bright stars in the images.
So what can be done about this? Well, to be honest, not an awful lot. Essentially, it's destined for the cutting room floor. I recombined the colours anyway, putting the red image onto the red channel in Photoshop, etc.
Looking at the component images, it's no surprise that the haloes look a blue-green colour.
Compare the photo at the top with those of the Pleiades, which is a star cluster that's actually meant to have blue nebulosity (and which I've never actually photographed).
I mean, it's not ugly.
If only it were meant to look like that!