Fireworks in the sky

NGC6946 is known as “The Fireworks Galaxy”, probably because of the distinct array of colours found within, from the deep yellows of the dying old stars at its centre to the hot blue clusters of younger stars in its outer arms.  It presented almost at zenith just after the onset of astronomical full darkness last week, so I set up in my reasonably dark lay-by on the A591 and spent the whole evening – until about midnight – imaging this one target.  Immediately next to the galaxy, from our point of view, is NGC6939, an open star cluster that lies within our own galaxy.

Star cluster NGC6939 with galaxy NGC 6946.
Nikon D90 on Altair Wave 115/805
ISO 800, 10x600sec

The star cluster is about 5,000 light years away, and the Fireworks galaxy is about 22 million light years away.  That’s still difficult to grasp, looking at them together in the photo.  It’s like snapping Kendal Castle from my bedroom window and glimpsing the Taj Mahal in the background.  Crazy perspective.

Visually, the galaxy is very small, only about 10 arc minutes (one third of the Moon) in diameter.  At magnitude 9.6, it is well below naked eye visibility.  I guess I would have liked better quality from the image, but the local conditions were not helpful that night – light, humidity and an unfortunate number of cars driving by on full headlights.  There’s another obstruction, which is that this galaxy is in line with the plane of our galaxy, so the photo is taken through the permanent clouds of interstellar dust at the edge of the Milky Way.

Autumn targets

Another clear sky forecast tempted me out to the Tebay road again last night.  I have a list of zenith targets that will keep me going for a few sessions.

It is important to understand the limitations of your setup in astrophotography.  I avoid nebulae, as the red light they emit is mostly filtered out before it reaches my camera sensor: that’s one of the disadvantages of a standard DSLR.  Star clusters and galaxies are much more appropriate targets for me – along with comets, of course!

Polar alignment went like a dream this time, after the nightmare of the previous outing.  The handset reported both azimuth and altitude correct to 1 arc minute, so I felt comfortable taking exposures of 60 seconds plus.  With a telescope focal length of 805mm, I still think I should be able to take longer exposures than this without guiding, but further refinement will have to wait – probably when the clocks go back at the end of the month.

First up, a star cluster in Cygnus, designated NGC6910.  Not large, at 10 arc minutes, it is set against the background of the Gamma Cygni Nebula at the centre of the constellation of Cygnus.  That’s Sadr, the central star of Cygnus, just to the right of centre at the bottom of the frame:

NGC6910 with Sadr
Nikon D90 on Altair Wave 115/805
ISO 800, 10×57 sec

Next in line is another star cluster in Cygnus, catalogued by Messier as M39.  A very loose arrangement of stars covering 29 arc minutes, about the same visual size as the moon:

M39 Open Cluster in Cygnus
Nikon D90 on Altair Wave 115/805
ISO800, 10x57sec.

Very slightly west of straight up overhead, seventy stars forming the faint cluster NGC 6939 (mag 7.8), conveniently located right next to spiral galaxy NGC6946 (mag 8.8):

NGC6946 and NGC6939
Nikon D90 on Altair Wave 115/805
ISO 800, 20 x 57sec.

After the Moon sets at about half past eleven, I have a go at Comet C/2011 J2 (LINEAR) which has been reported recently as having split into two parts.  At magnitude 14, it is only about 1% of the brightness of the spiral galaxy in the last photo, so it’s about as ambitious as it can be for my setup.

I catch the comet (just), but no possible trace of the split.

Comet C/2011 J2 LINEAR at Mag 13.9
Nikon D90 on Altair Wave 115/805
ISO 1250, 20×57 sec.