Trying LRGB Colour

Our eyes see a composite image of the world around us. If your vision is “normal” the colours you see every day are composed of four type of information: brightness, plus red, yellow, and blue information, each coming into your brain from difference cells in your retinas.

Colour images taken with large telescopes are assembled from the same four channels. Advanced cameras used in astrophotography are monochrome, or “black and white.” Colour comes into these images through combining multiple exposures, some done with red filters, blue filters, green filters, and some done with no colour filters at all. The result is an “LRGB” or Luminance-Red-Green-Blue image.

I tried this process recently by using (again) the Sierra Stars Observatory’s 61 cm telescope in California, accessed over the internet on the Sierra Stars Observatory Network (see my posting from 21 April 2013 for monochrome results). Combining monochrome images with others taken with red, green, and blue filters, using software called fitswork, and the tuning things a little in Photoshop Elements, I was thrilled to see an attractive image of Messier 51, the Whirlpool Galaxy, materialize! Might be a bit heavy on the blue channel, but the arms of M51a are known for regions of hot, young stars, and its companion, M51b, is redder because of a higher number of older, redder stars.

M51 LRGB

Messier 51, the Whirlpool Galaxy in Ursa Major, imaged using the 61 cm Sierra Stars Observatory in California. A composite colour image prepared by using monochrome images captured on 20 April 2013 and additional exposures for red, green, and blue information on 22 April 2013.

Copyright © 2013 David Allan Galbraith
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Peak at the Whirlpool (Galaxy, that is)

For International Astronomy Day (20 April) 2013, I decided to try the 61 cm f/10 Optical Mechanics Nighthawk CC06 Cassegrain telescope at the Sierra Stars Observatory in southern California to photograph M51, the Whirlpool Galaxy. M51 is relatively close to earth as galaxies go, and is a beautiful deep space object. It actually consists of two colliding galaxies.

The Sierra Stars Observatory is one of three observatories in the Sierra Stars Observatory Network, or SSON (http://www.sierrastars.com). I programmed the observations on the afternoon of the 19th of April and was delighted to see that the telescope had been able to make the photographs overnight, ready to download for the 20th.

To make this monochrome image, I took three 300 second exposures of M51 with the 61 cm telescope, and then used free software called FITSWork (http://www.fitswork.de/software/softw_en.php) to merge the three FITS-format images files. FITS files include both the image produced by an astronomical telescope camera and all of the data about the telescope’s position during the exposure. The combination of images reduces noise produced by the camera and effectively turns the result into a 900 second exposure. I then tuned up the resulting image for contrast and brightness by adjusting “levels” with Photoshop Elements 6. I feel I’m getting a little better at image processing, but I still have a lot to learn! It’s fun, though, and the remote observatory option is a way of taking your own images even when the local weather makes any stargazing impossible.

The Whirlpool Galaxy (M51)

The Whirlpool Galaxy (M51) photographed on 20 April 2013 with the 61 cm Sierra Stars Observatory.

M51 is located just south of Alkaid, the eastern-most star in the “handle” of the Big Dipper (formally named Eta Ursae Majoris). It is relatively bright and can be located with binoculars on a dark, clear night.

Because this is actually two interacting galaxies, M51 has a lot of red star-forming areas similar to the giant molecular cloud in Orion in our galaxy. Some are visible in this image as faint “knots” of light along the spiral arms of the galaxy. I’ll try imaging the galaxy with colour filters soon. These separate images can then be combined with the monochrome images I’ve already taken to produce a colour rendering. There’s a nice write-up on M51 on Wikipedia at: http://en.wikipedia.org/wiki/Messier_51.

The Whirlpool Galaxy is quite far north in the sky, and just about doesn’t set from the perspective of southern Ontario. It will certainly be on my list to see and try to photography myself once the weather and my availability make it possible.

Copyright © 2013 David Allan Galbraith