Tuesday, 26 May 2009

An example of what a good 'flat' can do

I've decided to do my own collection of Messier object photos. Here is a photo of an object that I can barely see from my home latitude of 52.6ºN - M83, the Southern Pinwheel. It's 8.3 magnitude, but Declination 30ºS. Easily visible from latitudes such as Paris, Rome, New York, but venture back northward to the UK and it disappears into the murky brown skies we have here. It barely rises above the south horizon. When I took this photo it was about 6º up. (I also managed to get Ptolemy's cluster, M7 this night too!). Fortunately the observatory dome has a beautiful flat horizon, only spoilt by distant light pollution, which was exacerbated by moisture in the atmosphere after a hot, sunny day. During the long summer evening twilight, I managed to capture 10 flat field frames in a blank spot near Capella (at ~40% pixel saturation level) and thereafter I took 10 dark flat frames. So armed with a smooth flat I took 3x10", then a 20", followed by 16x30" exposures of M83, which individually looked like a yellowish blurry star on an intensely reddish brown background. I later got 4x30" dark frames. I spent the whole of the next day trying to stack them, but had to shrink each image by 50% so Deep Sky Stacker would pick up the stars, but once I did that, at 1 o'clock the following morning, the spiral arms finally revealed themselves! It definately needed the 16-bit image processing in DSS. After a quick cross-check using Starry Night Pro, I'd picked up PGC 48132, a 16.0 mag galaxy, which is just to the left, inside from 2 x 12 mag stars. What an achievement!

Tuesday, 5 May 2009

Lunar halo

While on my ante-meridional astronomical expedition I had the fortune to watch the gibbous moon setting behind cirrus cloud, creating a delicious opportunity for a lovely composition of a lunar 22º halo with stars above it and a very early, subtle dawn glow. Can you spot any constellations? Details: ISO 1600, 20s, 28mm f/2.8 + fish eye, rotated, cropped & colour balanced.

Good old summer sky coming back around

I recently did a midnight-'til-dawn astrononomy expedition to my solitary coastal heathland spot and saw all of those lovely summer delights at the milky way centre slowly rising in the south-east. So, I thought I would put a bit of effort into manual off-axis guiding. This involves me bent over looking at an extremely faint guide star and trying to keep it on the red crosshairs using a N/S/E/W hand pad. After the 30 second pictures started trailing because the automatic tracking was slowing, I got a 3 min 45 second picture of the Eagle Nebula, M16. I also took some 30 second darks, flats, flat darks, a 4 min dark, which roughly offset the internal camera 'glow' from the Eagle Nebula picture. After applying all the flat corrections I found my image much noisier than the raw image minus the dark. So here it is: a single exposure at f/6.3 f=1260mm 1600ISO (my manual guiding skills are great). I have used the vignetting (central brightness) to my advantage during the processing. The 'Pillars of Creation' are in the middle of the red nebula just to the right of the open cluster. An improvement on last year's multiple 30" exposures, and a surprising amount of red!