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Saturday, February 19, 2011


The Tarantula Nebula (and FITS Liberator)

Left Image: Tarantula Nebula imaged using GRAS 14 instrument at Global Rent a Scope with 3 x 120 sec Red filter FITS images, 3 x 120 sec Green filter FITS images and 3 x 120 sec Blue filter FITS images. Each series of individual filter runs stacked using in Image J using Z projection of maximum intensity, then stacked and assembled into a RGB composite using ImageJ. Right Image: Single 120 sec RGB filter FITS images stretched using FITS liberator and ArcSinh(x) stretch. Stretched files stacked and assembled into a RGB composite using ImageJ. These are smaller frames of the nebula cropped form the whole image. You really, really need to click on these images and embiggen them (and compare to this APOD image which is 31 hours of exposure).

As people know, I'm a fan of low cost astroimaging. For GRAS remote telescope images, my workhorse is the freeware ImageJ. This works pretty well, especially for stacking and aligning images. However, astronomical images can have a large dynamic range from the very faint to the almost overexposed. How to bring out faint structure without turning the bright sections into glaring blobs.

This is where FITSLiberator comes in, it's a freeware app (also there are photoshop plugins), which makes it very easy to adjust the dynamic range of your images for later assembly in other image processing programs (You can do something similar in ImageJ but it's not as easy). As you can see above, with FITS liberator I was able to adjust the dynamic range of my Tarantula Nebula images to almost match the brightness of the multi-stacked images, without the central regions getting overexposed or the background being over-coloured.

Obviously my Tarantula Nebula won't win the astrophotography contest, and I still have a lot to learn, but I'm going to get a lot more out of my images now.

The FITS Liberator website comes with links to a whole range of Hubble raw images, and step by step instructions so you can create your own version of classic Hubble pictures. The PDF user manual is here.

Whole frame of the Trantual nebula and bits of the Large Magelanic cloud taken with GRAS-14. Click to embiggen (warning 2 meg of image)

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