Inspired by Gordon's post
I thought I would have a go. Guess what, it's more difficult than I'd hoped.
My biggest difficulty continues to be getting a really crisp focus. As Gordon says, the lens needs to be set to manual focus with image stabilisation (VR) off. It really is a case of trial and error even when starting from a known good setting.
My set-up for the shots to follow is a Canon EOS 400D/XTi with an EF 24-105mm f/4L IS USM. A Hutech LPS-P2-FF Front Filter
was used to block scatter from the local sodium street lighting. The camera was set to manual, ISO 800, f/4 and daylight white balance. The lens was set to its widest zoom of 24mm. The Bulb exposure was controlled by a remote switch to stop unwanted camera vibrations. All exposures were 180 seconds for this sequence. Colour information was discarded. Three minute exposures demand that the camera counters the rotation of the Earth so the whole shooting match was mounted on top of my telescope to take advantage of its motor driven mounting like so
The telescope's optics weren't used.
software was used to process the RAW images. Here's how the shot looks like with virtually no processing except re-sizing
This and all subsequent images are click-able to allow examination of the full size originals.
I took a second three minute exposure of the Milky Way and then covered the lens and took a three minute dark frame
This shows a few stuck pixels and the effect of sensor noise.
The next shot is of this dark frame subtracted from the first image
As before, minimal processing apart from a small adjustment to the image thresholds.
The last image was produced by taking the second three minute exposure of the Milky Way and subtracting the dark frame as above. The intention was to add this to the image above to increase the signal to noise ratio. While I succeeded the result could have been better as, unknown to me at the time, the camera had rotated slightly between shots. To its great credit, the IRIS
software did a pretty good job but rotating a digital picture by just a few degrees is impossible without introducing subtle artefacts and those artefacts severely limit the degree of unsharp masking and other tweaks that can be accomplished before they become visible. Here is the result
The Milky Way here in the Northern Hemisphere can't rival the show it puts on at this time of year in the South when one is looking towards the centre of the galaxy. Even so, I am disappointed that it isn't a bit more obvious in these pictures.
That said, I am really pleased with the performance of the EF 24-105mm f/4L IS USM lens for both its sharpness and lack of chromatic aberration. I have used this lens at other zoom settings (35mm, 70mm and 105mm) as well and so far have detected no zoom creep at all despite near vertical orientation. The same can also be said for my EF 70-200mm f/4L IS USM at various zooms.
Even so, an f/4 zoom of any focal length is hardly ideal for this sort of work. In my opinion, big sky shots on a cropped frame sensor call for 20mm focal lengths or less to get the angular coverage and apertures of f/2 or faster to overcome some of the noise issues with such sensors.
That sort of glass ain't cheap and if you are that serious about astrophotography then a DSLR sensor with its Bayer pattern of colour sensors is pretty hopeless. Good glass will focus a star onto a single pixel and that could be red, green or blue
However, within these limitations there is still fun to be had. While the camera was busy doing its stuff I had time to enjoy beautiful clear skies and the occasional Perseid meteor. Without the camera I would probably have been rattling the windows with my snoring.
You don't have to put your camera on a motor driven set-up as Gordon's post
shows. If you want to try longer exposures with the camera on a fixed tripod then you can get star trails
which can be quite artistic if set against the right backdrop. Give it a go.