Taking Longer Exposures

So we have now progressed to the point where we have an image with a reasonable level of detail and an appearance which is representative of the night sky without colour casts etc.

Our previous result. f4, 8 seconds, ISO 1250 with Astronomik Light Pollution Filter and Custom White Balance

The initial images we have taken have not been subject to any real thought about the correct exposure length. We have simply chosen a shutter speed which is the longest we can practically use for the focal length of our lens without getting star trails using a fixed tripod.

To progress further with our image we need to expose for longer. To do this we must use a tracking mount which compensates for the Earths rotation. We will cover tracking mounts specifically in another tutorial.

Taking longer exposures allows us to collect more photons of light. This will provide more detail in the objects we can see and begin to define those objects we either can’t or that appear very faint. This does not mean though that we just keep the shutter open for as long as possible. For the type of image we have been taking, the main elements are Stars but we also have a Nebula in the image. This is a gas cloud of varying brightness. To overexpose would cause this feature to burn out. Even the Stars are subject to being overexposed as Stars vary in colour. Betelgeuse, the Star in the upper left corner of Orion is Red so we must try to preserve this. In a later tutorial we will cover how to maximise the dynamic range in an image (the dynamic range is the range of tones from light to dark), for now though we will just concentrate on taking a single image of longer exposure.

In practical terms then it is always good practice to make a test exposure and then check the Histogram on your camera. Typically a peak about a quarter to a third of the way from the left on the Histogram is a good starting point. Check however that details like Nebula are not getting burnt out.

We also have to consider other influences such as light pollution. Even with a custom white balance and light pollution filter the cumulative effect of a long exposure in a light polluted area will begin to have a detrimental effect. While a light pollution filter does a good job of removing the main culprits of light pollution such as sodium and mercury street lights, it does not block all visible wavelengths as this would not be practical. As exposure times increase there will be a ‘creep’ into the image from the ground up of sky glow. If at all possible go to a ‘Dark Sky’ site where there will be little or no light pollution and exposure times can be increased still further.

For the image below we kept the camera set to ISO 1250 but increased the exposure to 60 seconds. The aperture was reduced from f4 to f5.6. This was done because the image quality at f5.6 is further improved and any degree of ‘blooming’ caused by a larger aperture will become more obvious with a longer exposure.

Our new image 50mm lens, f5.6, 60 seconds, ISO 1250 with Astronomik Light Pollution Filter and Custom White Balance. Note the bottom of the picture has a colour cast caused by sky glow but that there is much more Star and Nebula detail 

We can see that by exposing for longer a lot more detail in the sky has been revealed and that features such as the Nebula have become better defined. Unfortunately we can also see that despite the use of a light pollution filter a colour cast has begun to appear at the bottom of the image.

This is not a huge problem though and something we can correct by processing the image. A specific tutorial on imaging will discuss different methods for different subjects etc. However for the purpose of this image some basic techniques will be applied.

There are many software packages available but I have grown up with Photoshop so any references will apply to that package. Most image processing packages use the same terminology so if you are already comfortable with one then I suggest you stick with it. One advantage to using Photoshop however is that other software companies tend to support it with plug-ins. One plug-in I can highly recommend is ‘AstroTools’. This is a set of ‘Actions’ designed for editing Astrophotography images. Actions are a work sequence which is then saved as a single command. The previous image was subjected to an Action called 'Soft Colour Gradient Removal' . This was used to correct for the colour cast at the base of the image. The result of this then had an action called ‘Reduce Space Noise’. This smoothed out the image and got rid of some noise. ‘Actions’ can be reapplied for greater effect but this should be done with some caution. Finally a tweak to the Levels of the image was applied to improve the overall sky appearance.

These presets will save a lot of time in image processing and can be seen as a work process. This has the added benefit of teaching you what was done to achieve the improvement. I recommend them.

Our processed image. Note the colour cast  has now gone and we have a much more natural look to the image but have retained the increased detail

So now we have an image which due to the longer exposure has much more content and details like the Nebula can be clearly seen.

Finally to finish off our image of the Orion Constellation, we can give it a further processing tweak to define the main stars of the Constellation. In this example with have defined these with star spikes. Star spikes are often seen in published astro images. They are actually called diffraction spikes and are caused by certain types of reflector telescope which have supports for their secondary mirrors. They are a nice touch to a Constellation image so we have added some.

A bit of fun really. The Constellation is defined with some 'Star Spikes'

In our next tutorial we will increase our light gathering still further. However we will now be looking to ‘stack’ images which improve noise reduction and help to improve our image definition. We will also show the benefits of a ‘Dark Sky’ site.