I've been meaning to take a 'star trail' photo for some time and haven't got round to it, but the other day I had a practice from my home terrace, using the view across Ocean Village. I was slightly unsure as to whether I would get a good result as most advice around star trail photography recommends that you find yourself a nice dark location with as little light pollution as possible.
I started off by taking a 'base image' of Ocean Village which I took at 'blue hour' (that period of around 30 minutes starting approx 15 mins after sunset). The image was taken on a tripod and the tripod remained in the same position for the star photos which followed about an hour or so later once the sky was sufficiently dark.
The base image was taken on my Nikon D800, with Tamron 15-30mm lens at 15mm, ISO100 and f/11, with an exposure time of 15 seconds. Once dark, I then took a series of approx 100 images back-to-back, each with an exposure of 20 seconds. For the star photos, I opened up my aperture as wide as it would go (f/2.8) to let in plenty of light and upped the ISO to 200 so that the stars would be visible. As a result, the buildings and lights of Ocean Village were hugely over-exposed on the star pics, however that wasn't a concern as I had taken my base image at blue hour (properly exposed) and knew I would be blending this with the stars later on in Photoshop. As the camera remained on the tripod in the same position, there was no problem blending the images as they were already lined up and I simply needed to create a mask to blend the over exposed buildings with the buildings on the base photo.
All of the star photos were loaded in to Photoshop as a series and then blended automatically to provide the star 'trails' showing the movement of the stars in the hour or so I was taking photos, due to the rotation of the Earth.
It's a clever way of combining astronomy with photography and gives an extra dimension to what would be just an ordinary night time shot. I have put the final image below for you to have a look at.