M63: Sunflower Galaxy

 

RA: 13hr 15min 48sec   Dec: 42 02' 00"   Mag: 8.6   Distance: 24 million light years   Constellation: Canes venatici

This photograph of the Sunflower galaxy is one of the longest exposures I have ever attempted - and considering it was taken without Polar alignment with all the problems of field rotation, it was quite a feat of patience.  I spent an entire night's observing (about 4 hours worth) to obtain just under an hour's exposure on the 8th of June 2005.  Through this time, the field had rotated so much that after stacking the pictures, the outlines of the later frames were almost at ninety degrees to the first frame!  But it was worth it.  Knowing from previous reference photographs that the arms of this spiral are multicoloured from the red end of the spectrum all the way to the blue, I tried to allow for this by shifting the DSI bias (red in my case) towards blue for 2 of the raw images using the CMY gain balance in Envisage during the exposures.  Otherwise I think the arms of the galaxy would only have come across as shades of red, orange and yellow.

5 raw images were combined ( 21x 20 seconds, 21x20, 25x30 - red bias, 25x30, 26x30 - blue bias) with a total exposure time of 52.6 minutes.  The combination was carried out with MaximDL and the file saved as an 8-bit tiff.  Histogram stretching was carried out with Photoshop using the levels command in each of the R, G and B channels.  After midtone enhancement mainly in the Blue and Green channels using the curves command, final histogram stretching to darken the sky background (alas, causing the loss of some fainter stars) was carried out in the luminosity channel (RGB).  The core of the galaxy was selected out (Feather = 7) during midtone enhancement to avoid further overexposure of the galaxy core.

Problem: the severe field rotation meant that the sky had to be darkened considerably to avoid seeing the stacked photo edges as well as the elongated circumferential streaks of peripheral stars.  Large star to the right of the galaxy was used as the alignment star by Envisage during the taking of the exposures and so, has no elongation as a result of the field rotation.

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