Photographing the Night Sky

Filters and Cameras

DSLR cameras have a filter fitted in front of the sensor – to adjust the spectral sensitivity to match that of the eye. This compensates for the increased sensitivity of the sensor towards the red end of the spectrum. The filter also cuts out IR and UV, which can cause focussing issues.

Typical sensor chip sensitivity

Many nebulae are brightest at hydrogen alpha frequencies due to light emission at the specific frequency - 656nm. But this frequency is heavily attenuated by the filter in front of the sensor. Thus H alpha emission nebula will be difficult to image.
However, the filters can be removed or they can be replaced with a Baader filter which screens out the UV and far IR, but lets the H alpha light through. It is possible to remove the cut-off filter yourself and there are plenty of guides on the net on how to do this. I wasn’t brave enough and paid ‘Astronomiser’ to modify a Canon 1100D for me !! Most Canon DSLRs from the 400D onwards actually have two filters in front of the CCD, These dual filter cameras have one filter that cuts out IR and UV and a second filter underneath that compensates for red sensitivity. Leaving the IR/UV filter in place avoids issues with focussing and helps protect the sensor. Thus only the red filter is removed.

Issues for continued land usage

Having modified your DSLR, if you wish to use it for land usage then there are two issues to consider. Firstly with autofocus lenses, removal of the 0.8mm thick filter will alter the light path - this is down to the difference in the focal point when considering the refractive index of glass vs the refractive index of air. In practice this is very small and the difference is practically unnoticeable when taking everyday snaps. However, when using very 'fast' lenses or ones with a very short focal length you may need to manually focus. Without the cut-off filter in place, the white balance of the camera will be wrong. Removal of the this filter will cause the camera to be more sensitive to the red end of the spectrum and images thus look pink. This can be corrected quite easily using the custom white balance on the camera or by inserting an EOS clip-in filter.

Finally if you cant face modification, then Canon now sell a DSLR especially for astronomical use, the 60Da. This has the the filter ‘removed’ and is ready to use.But note it is expensive >£1100.

The benefit of camera modification is shown below. The first image is with an unmodified DSLR - this is a 36 minute exposure through 80mm refractor using a Canon 450D. The second one uses the same scope setup but with just a 20 minute exposure but using modified Canon 1100D. I am sure you will agree the difference is impressive !

     California Nebula - unmodified DSLR

     California Nebula - Modified DSLR
Image Processing

There are many image processing programmes available. One of the most popular is Adobe Photoshop, but this is expensive, typically more than £400. However, a ‘lite’ version called Photoshop Elements can be bought for around £70. The main competitor is Corel Paint Shop Pro – this is available for around £60 and in my opinion (for astronomical use) has more functionality than Photoshop Elements. Many bespoke astronomical image processing programmes are available – I like Astroart. Personally I use Astroart to do initial processing and then finish off in Paint Shop Pro. There are free image processing programmes on the net; one of these is GIMP.